Last modified by cds on 2025/01/30 12:03
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1 This tutorial will teach you the basics of writing plugins that run inside the Eclipse framework. You will learn about editors, views, and extension points by creating one of each yourself. Once you're done with this tutorial, you will have an application that will look something like this:
2
3 [[image:attach:result.png]]
4
5 You may want to download [[the slides>>attach:presentation.pdf]] of the presentation explaining the basic concepts you will explore in this tutorial.
6
7
8
9 {{toc/}}
10
11 = Preliminaries =
12
13 There's a few things to do before we dive into the tutorial itself. For example, to do Eclipse programming, you will have to get your hands on an Eclipse installation first. Read through the following sections to get ready for the tutorial tasks.
14
15 == Required Software ==
16
17 For this tutorial, we need you to have Eclipse and Git installed:
18
19 1. Install Eclipse. For what we do, we recommend installing the Eclipse Modeling Tools, with a few extras. Our [[Wiki page on getting Eclipse>>doc:KIELER.Getting Eclipse]] has the details: simply follow the instructions for downloading and installing Eclipse and you should be set.
20 1. You should already have obtained a working Git installation for the first tutorial.
21
22 == General Remarks ==
23
24 Over the course of this tutorial, you will be writing a bit of code. Here's a few rules we ask you to follow:
25
26 * All your Java code should be in packages with the prefix {{code language="none"}}de.cau.cs.rtprak.login{{/code}}, where {{code language="none"}}login{{/code}} is your login name as used for your email address at the institute. From now on, this rule will apply to all tutorials. Once we start with the actual practical projects, we will choose another package name.
27 * All Java classes, fields, and methods should be thoroughly commented with the standard [[Javadoc>>url:http://download.oracle.com/javase/1.5.0/docs/tooldocs/windows/javadoc.html#javadoctags||shape="rect"]] comment format. Javadoc comments are well supported by Eclipse, providing code completion, syntax highlighting, and further features to help you. The code inside your methods should also be well commented. Try to think about what kinds of information would help someone unfamiliar with your code understand it.
28 * As you will already have noticed during the first tutorial, our tutorials use Turing machines as the underlying theme. This is partly because we're computer scientists and computer scientists are expected to choose computer sciency examples, but mostly because Turing machines work great as examples for the different kinds of topics we will be covering with you. You may thus want to take some time to read up again on the topic. [[Wikipedia>>url:http://en.wikipedia.org/wiki/Turing_machine||shape="rect"]] or the material of your Theoretical Computer Science lecture might be a great start.
29 * During this tutorial, we will be using Git mostly from the command line instead of using Eclipse's built-in Git support. This is because we've found Eclipse's Git support to be too unstable and buggy for us to trust it completely.
30
31 == Finding Documentation ==
32
33 During the tutorial, we will cover each topic only briefly, so it is always a good idea to find more information online. Here's some more resources that will prove helpful:
34
35 * [[Java Platform, Standard Edition 6 API Specification>>url:http://download.oracle.com/javase/6/docs/api/||shape="rect"]]
36 As Java programmers, you will already know this one, but it's so important and helpful that it's worth repeating. The API documentation contains just about everything you need to know about the API provided by Java6.
37 * [[Eclipse Help System>>url:http://help.eclipse.org/juno/index.jsp||shape="rect"]]
38 Eclipse comes with its own help system that contains a wealth of information. You will be spending most of your time in the //Platform Plug-in Developer Guide//, which contains the following three important sections:\\
39 ** Programmer's Guide
40 When you encounter a new topic, such as SWT or JFace, the Programmer's Guide often contains helpful articles to give you a first overview. Recommended reading.
41 ** References -> API Reference
42 One of the two most important parts of the Eclipse Help System, the API Reference contains the Javadoc documentation of all Eclipse framework classes. Extremely helpful.
43 ** References -> Extension Points Reference
44 The other of the two most important parts of the Eclipse Help System, the Extension Point Reference lists all extension points of the Eclipse framework along with information about what they are and how to use them. Also extremely helpful.
45 * [[Eclipsepedia>>url:http://wiki.eclipse.org/Main_Page||shape="rect"]]
46 The official Eclipse Wiki. Contains a wealth of information on Eclipse programming.
47 * [[Eclipse Resources>>url:http://www.eclipse.org/resources/||shape="rect"]]
48 Provides forums, tutorials, articles, presentations, etc. on Eclipse and Eclipse-related topics.
49
50 You will find that despite of all of these resources Eclipse is still not as well commented and documented as we'd like it to be. Finding out how stuff works in the world of Eclipse can thus sometimes be a challenge. However, this does not only apply to you, but also to many people who are conveniently connected by something called //The Internet//. It should go without saying that if all else fails, [[Google>>url:http://www.google.de||shape="rect"]] often turns up great tutorials or solutions to problems you may run into. And if it doesn't, Miro and I will be happy to help you as well.
51
52 == Preparing the Repository ==
53
54 We have created a Git repository for everyone to do his tutorials in. You can access the repository online through our Stash tool [[over here>>url:http://git.rtsys.informatik.uni-kiel.de:7990/projects/PRAK/repos/12ws-eclipse-tutorials/browse||shape="rect"]]. You will first have to configure your Stash account:
55
56 1. Login with your Rtsys account information.
57 1. Through the button in the top right corner, access your profile.
58 1. Switch to the //SSH keys// tab.
59 1. Click //Add Key// and upload a public SSH key that you want to use to access the repository.
60
61 You should now be able to access the repository. Clone it:
62
63 1. Open a console window and navigate to an empty directory that the repository should be placed in.
64 1. Enter the command ssh:~/~/git@git.rtsys.informatik.uni-kiel.de:7999/PRAK/12ws-eclipse-tutorials.git{{code language="none"}} .{{/code}} (including the final dot, which tells git to clone the repository into the current directory instead of a subdirectory).
65 1. You should now have a clone of the repository in the current directory.
66
67 You will use this repository for all your tutorial work, along with everyone else. To make sure that you don't interfere with each other, everyone will work on a different branch. This is not exactly how people usually use Git, but goes to demonstrate Git's flexibility... Add a branch for you to work in:
68
69 1. Enter {{code language="none"}}git checkout -b login_name{{/code}}
70
71 You have just added and checked out a new branch. Everything you commit will go to this branch. To push your local commits to the server (which you will need to do so we can access your results), do the following:
72
73 1. Enter {{code language="none"}}git push origin login_name{{/code}}
74
75 You would usually have to enter {{code language="none"}}git pull{{/code}} first, but since nobody will mess with your branch this won't be necessary. By the way, you only need to mention {{code language="none"}}origin login_name{{/code}} with the first {{code language="none"}}git push{{/code}}, since Git doesn't know where to push the branch yet. After the first time, Git remembers the information and it will be enough to just enter {{code language="none"}}git push{{/code}}.
76
77 = Creating a Simple Text Editor =
78
79 OK, with all the preliminaries out of the way let's get working. Fire up Eclipse, choose an empty workspace, close the Welcome panel it will present you with and follow the following steps.
80
81 == Creating a New Plugin ==
82
83 For our text editor to integrate into Eclipse, we need to create a plug-in project for it:
84
85 1. //New// -> //Project...//
86 1. In the project wizard, choose //Plug-in Project// and click //Next//.
87 1. As the project name, enter {{code language="none"}}de.cau.cs.rtprak.login.simple{{/code}}. Uncheck //Use default location// (which would put the project into your workspace), and put it into your local clone of the Git repository instead (the //Location// should read something like {{code language="none"}}/path/to/git/repository/de.cau.cs.rtprak.login.simple{{/code}}). Click //Next//.
88 1. As the name, enter {{code language="none"}}Simple (login){{/code}}. Also, make sure that //Generate an activator// and //This plug-in will make contributions to the UI// are both checked. Click //Finish//. (Eclipse might ask you whether you want to switch to the //Plug-in Development Perspective//, which configures Eclipse to provide the views that are important for plug-in development. Choose //Yes//. Or //No//. It won't have a big influence on your future...)
89 1. Eclipse has now created your new plug-in and was nice enough to open the //Plug-in Manifest Editor//, which allows you to graphically edit two important files of your plugin: {{code language="none"}}plugin.xml{{/code}} and {{code language="none"}}META-INF/MANIFEST.MF{{/code}}. (By the way, this would be a great time to research the editor and the two files online.) Basically, those two files provide information that tell Eclipse what other plug-ins your plug-in needs and how it works together with other plug-ins by providing extensions and extension points. Our new plug-in will depend on two other plug-ins, so switch to the //Dependencies// tab of the editor and add dependencies to {{code language="none"}}org.eclipse.ui.editors{{/code}} and {{code language="none"}}org.eclipse.jface.text{{/code}}. Save the editor and close it. (You can always reopen it by opening one of the two mentioned files from the //Package Explorer//.)
90 1. Tell Eclipse that the project is inside a Git repository. Right-click on the project, click //Team//, and click //Share Project//. Select Git as the repository type and click //Next//. The repository information should appear and you should be able to simply click //Finish//.
91
92 == Create the Main Editor Class ==
93
94 We will now create the class that implements the simple text editor. There won't be any programming involved here since we're lazy; instead, we will just inherit from an existing simple text editor.
95
96 1. //New// -> //Class//.
97 1. Package: {{code language="none"}}de.cau.cs.rtprak.login.simple.editors{{/code}}. Name: {{code language="none"}}SimpleEditorPart{{/code}}. Superclass: {{code language="none"}}org.eclipse.ui.editors.text.TextEditor{{/code}}. Click //Finish//.
98
99 == Register the Editor ==
100
101 For the editor to be available inside Eclipse, we will have to register it by adding an extension to an extension point.
102
103 1. Copy [[the attached file>>attach:turing-file.gif]] to a new subfolder icons in the plug-in folder (right-click the plug-in folder in the //Package Explorer// and choose //New// -> //Folder...//). You can copy the file by importing it from inside Eclipse (//File// -> //Import...// -> //File System//) or by copying it from outside Eclipse and refreshing the plug-in project afterwards (right-click the plug-in folder in the //Package Explorer// and choose //Refresh).//
104 1. Open the //Plug-in Manifest Editor// again and switch to the //Extensions// tab.
105 1. Click //Add...//, choose the {{code language="none"}}org.eclipse.ui.editors{{/code}} extension point and click //Finish//.
106 1. The extension point is now shown in the list of extensions, along with an //editor// extension. Select that extension and edit its details using the fields on the right. Set the ID to {{code language="none"}}de.cau.cs.rtprak.login.simple.editor{{/code}}, the name to {{code language="none"}}Simple Text Editor{{/code}}, the icon to {{code language="none"}}icons/turing-file.gif{{/code}}, the extensions to {{code language="none"}}simple{{/code}}, the class to {{code language="none"}}de.cau.cs.rtprak.login.simple.editors.SimpleEditorPart{{/code}}, and the default to {{code language="none"}}true{{/code}}.
107 1. Save the manifest editor.
108
109 == Test the Editor ==
110
111 It's time to test your new simple editor in a new Eclipse instance.
112
113 1. Switch back to the //Overview// tab of the //Plug-in Manifest Editor//.
114 1. Click //Launch an Eclipse Application.//\\
115 1*. For future tests, you can now select //Eclipse Application// in the run menu.
116 1*. To enable debug mode for your test instances: open the //Run Configurations// dialog, select the //Arguments// tab of the //Eclipse Application// configuration, and add -debug -consoleLog as program arguments. This dumps all errors and exceptions to the console view, so you can directly see what went wrong.
117 1*. To improve performance, select only the plugins that are necessary: in the //Plug-ins// tab select //Launch with plug-ins selected below only//, deselect //Target Platform//, select //Workspace//, and then //Add Required Plug-ins//.\\
118 1**. Make sure that org.eclipse.ui.ide.application is also selected, else you won't be able to launch Eclipse.
119 1**. The requirements list needs to be updated when the dependencies of your plugins have changed; click //Add Required Plug-ins// again for updating.
120 1. In the new Eclipse instance, click //New -> Project...// -> //General// -> //Project//. Enter {{code language="none"}}test{{/code}} as the project name.
121 1. Right-click the new project and click //New// -> //File...// As the file name, enter {{code language="none"}}test.simple{{/code}}. This will create a new file with that name and open the file in your newly added text editor. (You can see that it is your editor by looking at the editor icon, which should look like the icon you downloaded and put into the icons folder.)
122
123 = Creating a Simple View =
124
125 The next task consists of creating a view that is able to display the state of a Turing Machine. We will do this using a table with one column, where each row represents an entry on the tape of the Turing Machine. The tape shall be infinite to one side, and the position of the read/write head shall be movable by two buttons. The content of the tape shall be determined by the currently active instance of our simple text editor.
126
127 {{info title="Hint"}}
128 In the following, we will be making use of the Standard Widget Toolkit (SWT) and JFace to build a user interface. It might be a good idea now to search for an introduction to SWT and JFace concepts on the Internet before you proceed.
129 {{/info}}
130
131 == Creating the View Class ==
132
133 We will start by creating a class that will define the view.
134
135 1. Create a class {{code language="none"}}TapeViewPart{{/code}} in a new package {{code language="none"}}de.cau.cs.rtprak.login.simple.views{{/code}} that extends the [[ViewPart>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/ui/part/ViewPart.html||shape="rect"]] class. (make sure that in the //New Java Class// wizard, the option //Inherited abstract methods// is checked.)
136 1. Add a private field {{code language="none"}}tableViewer{{/code}} of type [[TableViewer>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/jface/viewers/TableViewer.html||shape="rect"]].
137 1. (((
138 Your {{code language="none"}}TableViewPart{{/code}} contains a still empty method {{code language="none"}}createPartControl{{/code}}. This method will be responsible for creating the user interface components of your view. Add the following code to create the table we want to display:
139
140 {{code language="java"}}
141 Table table = new Table(parent, SWT.BORDER);
142 table.setHeaderVisible(true);
143 TableColumn column = new TableColumn(table, SWT.NONE);
144 column.setWidth(80);
145 column.setText("Tape Data");
146 tableViewer = new TableViewer(table);
147 {{/code}}
148 )))
149 1. (((
150 The {{code language="none"}}setFocus{{/code}} method controls what happens when your part gets the focus. Make sure the focus will then automatically be set to the table by adding the following code:
151
152 {{code}}
153 tableViewer.getControl().setFocus();
154 {{/code}}
155 )))
156
157 == Create the View Extension ==
158
159 We will now have to register our new view with Eclipse so that it can be seamlessly integrated into the workbench.
160
161 1. Copy the three files [[attach:tape_head.gif]], [[attach:head_present.gif]], and [[attach:head_absent.gif]]to the {{code language="none"}}icons{{/code}} subfolder of your plug-in as you did it before. (You might need to refresh your project again if you did the copying outside of Eclipse.)
162 1. Open the {{code language="none"}}plugin.xml{{/code}} file in the //Plugin Manifest Editor// and switch to the //Extensions// tab.
163 1. Click //Add// to add a new extension for the extension point {{code language="none"}}org.eclipse.ui.views{{/code}}. Right-click the newly added extension and add a new {{code language="none"}}view{{/code}} element through the //New// menu.
164 1. Set the view element's properties as follows: ID {{code language="none"}}de.cau.cs.rtprak.login.simple.view{{/code}}, name {{code language="none"}}Tape{{/code}}, class {{code language="none"}}de.cau.cs.rtprak.login.simple.views.TapeViewPart{{/code}}, category {{code language="none"}}org.eclipse.ui{{/code}}, icon {{code language="none"}}icons/tape_head.gif{{/code}}.
165
166 When you start the application, you should now be able to open your view by clicking //Window// -> //Show View// -> //Other//.
167
168 == Add Content and Label Providers ==
169
170 The idea of JFace viewers is to abstract a bit from the underlying widget (in our case, the table) and instead work on data models that are to be viewed. Instead of adding items to the table directly, the table viewer is supplied with an input object, a content provider, and a label provider. The content provider allows the viewer to make sense of the input object and basically allows the viewer to access the input object's data. The label provider translates each item of data into text and icons that can be used to present the item to the user in the table.
171
172 We will now create content and label providers to do just that.
173
174 1. (((
175 Create a class {{code language="none"}}TuringTape{{/code}} in a new package {{code language="none"}}de.cau.cs.rtprak.login.simple.model{{/code}} with the following fields:
176
177 {{code language="java"}}
178 private int headPosition = 1;
179 private StringBuffer text = new StringBuffer();
180 {{/code}}
181
182 Also add corresponding getter and setter methods. (You can simply right-click somewhere in the class and choose //Source// -> //Generate Getters and Setters//.)
183 )))
184 1. (((
185 Add two constants to the class:
186
187 {{code language="java"}}
188 public static final char START_CHAR = '\u25b7';
189 public static final char BLANK_CHAR = '\u25fb';
190 {{/code}}
191 )))
192 1. Add a method {{code language="none"}}getCharacter(int pos){{/code}} that calculates the tape character at position {{code language="none"}}pos{{/code}} as follows:\\
193 1*. For {{code language="none"}}pos == 0{{/code}}, return the character {{code language="none"}}START_CHAR{{/code}}.
194 1*. For {{code language="none"}}pos > text.length(){{/code}}, return the character {{code language="none"}}BLANK_CHAR{{/code}}.
195 1*. Otherwise, return the text character at index {{code language="none"}}pos - 1{{/code}}.
196 1. Add a private field tape of type {{code language="none"}}TuringTape{{/code}} to {{code language="none"}}TapeViewPart{{/code}} and initialize it with a new instance.
197 1. Create a class {{code language="none"}}TapeData{{/code}} in {{code language="none"}}de.cau.cs.rtprak.login.simple.model{{/code}} with two fields {{code language="none"}}int index{{/code}} and {{code language="none"}}char character{{/code}}, and add a constructor for initialization as well as corresponding getter methods.
198 1. Create a class {{code language="none"}}TapeContentProvider{{/code}} in the {{code language="none"}}de.cau.cs.rtprak.login.simple.views{{/code}} package that implements [[IStructuredContentProvider>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/jface/viewers/IStructuredContentProvider.html||shape="rect"]].\\
199 1*. The methods {{code language="none"}}dispose(){{/code}} and {{code language="none"}}inputChanged(){{/code}} may remain empty.
200 1*. The method {{code language="none"}}getElements(){{/code}} must return an array of objects, where each object must contain all necessary data to be displayed in a single row of the table. The number of returned objects corresponds to the number of rows.
201 1*. Suppose the input element is an instance of {{code language="none"}}TuringTape{{/code}}. The result of {{code language="none"}}getElements(){{/code}} shall be an array of {{code language="none"}}TapeData{{/code}} elements. The size of the array shall be one more than the maximum of the tape head position and the length of the tape text. The index and character of each tape data element shall be filled with {{code language="none"}}i{{/code}} and the result of {{code language="none"}}turingTape.getCharacter(i){{/code}}, respectively, where {{code language="none"}}i{{/code}} is the array index of the element.
202 1. Create a class {{code language="none"}}TapeLabelProvider{{/code}} in the {{code language="none"}}de.cau.cs.rtprak.login.simple.views{{/code}} package that extends [[BaseLabelProvider>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/jface/viewers/BaseLabelProvider.html||shape="rect"]] and implements [[ITableLabelProvider>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/jface/viewers/ITableLabelProvider.html||shape="rect"]].\\
203 1*. Add a private field {{code language="none"}}tape{{/code}} of type {{code language="none"}}TuringTape{{/code}} that is initialized from the constructor.
204 1*. Add fields {{code language="none"}}presentImage{{/code}} and {{code language="none"}}absentImage{{/code}} of type [[Image>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/swt/graphics/Image.html||shape="rect"]].
205 1*. (((
206 Initialize each image using the following code, where {{code language="none"}}path_to_image{{/code}} is {{code language="none"}}icons/head_present.gif{{/code}} and {{code language="none"}}icons/head_absent.gif{{/code}}, respectively:
207
208 {{code language="java"}}
209 image = Activator.imageDescriptorFromPlugin(Activator.PLUGIN_ID, "path_to_image").createImage();
210 {{/code}}
211 )))
212 1*. Override the implementation of {{code language="none"}}dispose(){{/code}} in {{code language="none"}}TapeLabelProvider{{/code}} to dispose both images after calling {{code language="none"}}super.dispose(){{/code}}. (Right-click in the source-code and click //Source// -> //Override/Implement Methods//.)
213 1*. In {{code language="none"}}getColumnImage(){{/code}} and {{code language="none"}}getColumnText(){{/code}}, first check whether the element is an instance of {{code language="none"}}TapeData{{/code}} and the column index is 0, and return {{code language="none"}}null{{/code}} otherwise. If the check passes, return the following:\\
214 1**. {{code language="none"}}getColumnImage(){{/code}}: {{code language="none"}}presentImage{{/code}} if the index given by the tape data element equals the current value of {{code language="none"}}tape.getHeadPosition(){{/code}}, {{code language="none"}}absentImage{{/code}} otherwise.
215 1**. {{code language="none"}}getColumnText(){{/code}}: a {{code language="none"}}String{{/code}} containing the character of the tape data element.
216 1. (((
217 Add the following lines to {{code language="none"}}createPartControl(){{/code}} in {{code language="none"}}TapeViewPart{{/code}}:
218
219 {{code}}
220 tableViewer.setContentProvider(new TapeContentProvider());
221 tableViewer.setLabelProvider(new TapeLabelProvider(tape));
222 tableViewer.setInput(tape);
223 {{/code}}
224 )))
225
226 == Use Simple Text Editor as Tape View Input ==
227
228 We will now add code to make the Tape view display the content of a currently active Simple Text Editor.
229
230 1. (((
231 Add the following methods to {{code language="none"}}SimpleEditorPart{{/code}}:
232
233 {{code language="java"}}
234 /**
235 * Returns the text that is currently displayed in the editor.
236 * @return the currently displayed text
237 */
238 public String getText() {
239 return getDocumentProvider().getDocument(getEditorInput()).get();
240 }
241 /** The listener that is currently registered for this editor. */
242 private IDocumentListener registeredListener;
243 /**
244 * Registers the given runnable as listener for changes to the text
245 * of this editor.
246 * @param runnable a runnable to register as text listener
247 */
248 public void registerTextListener(final Runnable runnable) {
249 registeredListener = new IDocumentListener() {
250 public void documentAboutToBeChanged(DocumentEvent event) {}
251 public void documentChanged(DocumentEvent event) {
252 runnable.run();
253 }
254 };
255 getDocumentProvider().getDocument(getEditorInput())
256 .addDocumentListener(registeredListener);
257 }
258 /**
259 * Removes the last registered text listener.
260 */
261 public void disposeTextListener() {
262 if (registeredListener != null) {
263 if (getDocumentProvider() != null) {
264 getDocumentProvider().getDocument(getEditorInput())
265 .removeDocumentListener(registeredListener);
266 }
267 registeredListener = null;
268 }
269 }
270 {{/code}}
271 )))
272 1. (((
273 Add the following code to {{code language="none"}}TapeViewPart{{/code}}:
274
275 {{code language="java"}}
276 /** The editor part that is currently set as input for the viewer. */
277 private SimpleEditorPart currentInput;
278 /**
279 * Sets the displayed text of the given editor part as input of the
280 * viewer, if the editor part is a SimpleEditorPart.
281 * @param part workbench part to set as input
282 */
283 private void setInput(final IWorkbenchPart part) {
284 if (part instanceof SimpleEditorPart && part != currentInput) {
285 if (currentInput != null) {
286 currentInput.disposeTextListener();
287 }
288 currentInput = (SimpleEditorPart) part;
289 Runnable runnable = new Runnable() {
290 public void run() {
291 tape.setText(new StringBuffer(currentInput.getText()));
292 tableViewer.refresh();
293 }
294 };
295 runnable.run();
296 currentInput.registerTextListener(runnable);
297 }
298 }
299 {{/code}}
300 )))
301 1. (((
302 Add the following code to {{code language="none"}}createPartControl(){{/code}}:
303
304 {{code language="java"}}
305 IWorkbenchWindow workbenchWindow = getSite().getWorkbenchWindow();
306 IWorkbenchPage activePage = workbenchWindow.getActivePage();
307 if (activePage != null) {
308 setInput(activePage.getActivePart());
309 }
310 workbenchWindow.getPartService().addPartListener(new IPartListener() {
311 public void partActivated(final IWorkbenchPart part) {
312 setInput(part);
313 }
314 public void partDeactivated(final IWorkbenchPart part) {}
315 public void partBroughtToTop(final IWorkbenchPart part) {}
316 public void partClosed(final IWorkbenchPart part) {}
317 public void partOpened(final IWorkbenchPart part) {}
318 });
319 {{/code}}
320 )))
321
322 == Create Actions to Move the Tape Head ==
323
324 If we want to add buttons to the view's tool bar, we will have to ask its [[IToolbarManager>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/jface/action/IToolBarManager.html||shape="rect"]] to do that for us:
325
326 1. (((
327 Get the tool bar manager using the following code:
328
329 {{code language="java"}}
330 IToolBarManager toolBarManager = getViewSite().getActionBars().getToolBarManager();
331 {{/code}}
332 )))
333 1. Add two actions to the toolbar manager by extending the class [[Action>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/jface/action/Action.html||shape="rect"]] and implementing the {{code language="none"}}run(){{/code}}method.
334 1*. It is convenient to add actions as anonymous nested classes.
335 1*. The first action shall have the text "L". When it is run, it shall move the head to the left (to the top in the table viewer), if the head is not already at position 0.
336 1*. The second action shall have the text "R". When it is run, it shall move the head to the right.
337 1*. You should call {{code language="none"}}tableViewer.refresh(){{/code}} after any change to the {{code language="none"}}tape.headPosition{{/code}} variable.
338
339 == Test the View ==
340
341 If you open an instance of the simple text editor and open the Tape view, the view should correctly display the editor's text on a tape, and the L and R buttons should move the tape head.
342
343 = Creating an Extension Point =
344
345 For the final part of the tutorial, we will now use the extension point mechanism of Eclipse to add some behavior to our Turing Machines. An //extension point// is basically a well-defined point where other plug-ins can register to add functionality. The extension point is basically defined by an XML Schema file that defines an interface; other plug-ins may access this interface using XML code in their {{code language="none"}}plugin.xml{{/code}} file, so-called //extensions//. Our extension point will provide an interface for classes that define behavior of a Turing Machine, and we will call them head controllers (programs that control the tape head).
346
347 == Defining a Command Class ==
348
349 We will start by defining a class representing a command that will be passed to a selected head controller.
350
351 1. Add a class {{code language="none"}}HeadCommand{{/code}} to the package {{code language="none"}}de.cau.cs.rtprak.login.simple.controller{{/code}}.
352 1. Add a nested public static enumeration {{code language="none"}}Action{{/code}} with values {{code language="none"}}WRITE{{/code}}, {{code language="none"}}ERASE{{/code}}, and {{code language="none"}}NULL{{/code}}.
353 1. Add a nested public static enumeration {{code language="none"}}Direction{{/code}} with values {{code language="none"}}LEFT{{/code}}, {{code language="none"}}RIGHT{{/code}}, and {{code language="none"}}NONE{{/code}}.
354 1. (((
355 Add the following private fields:
356
357 {{code language="java"}}
358 private Action action;
359 private Direction direction;
360 private char newChar;
361 {{/code}}
362 )))
363 1. Add a constructor to initialize the fields.
364 1. Add getter methods to access the fields.
365
366 == Defining the Controller Interface ==
367
368 We will now define an interface that all head controllers will have to implement:
369
370 1. Add an interface {{code language="none"}}IHeadController{{/code}} in the package {{code language="none"}}de.cau.cs.rtprak.login.simple.controller{{/code}}.
371 1. (((
372 Add the following methods to the interface:
373
374 {{code language="java"}}
375 /**
376 * Calculate the next command depending on the currently seen character.
377 * @param character the currently seen character
378 * @return the next command specifying which character to write and
379 * which direction to move the head
380 */
381 HeadCommand nextCommand(char character);
382
383 /**
384 * Reset the internal state of the head controller.
385 */
386 void reset();
387 {{/code}}
388 )))
389
390 == Defining the Extension Point ==
391
392 We will now define the extension point that head controllers will be registered at.
393
394 1. Open the {{code language="none"}}plugin.xml{{/code}} file in the //Plugin Manifest Editor// and switch to the //Extension Points// tab.
395 1. Click the //Add// button and enter {{code language="none"}}de.cau.cs.rtprak.login.simple.headControllers{{/code}} as the extension point's ID, and {{code language="none"}}Head Controllers{{/code}} as its name. Shorten the schema file's file name to {{code language="none"}}schema/headControllers.exsd{{/code}}. Make sure that //Edit extension point schema when done// is checked and click //Finish//.
396 1. Eclipse will now have opened the new schema file in the //Extension Point Schema Editor//, a graphical editor similar to the //Plugin Manifest Editor// that provides a way to define things that might be easier than directly editing the text files.
397 1. In the new editor, open the //Definition// tab.
398 1. Add a new element named {{code language="none"}}controller{{/code}}.
399 1. Add three new attributes to the {{code language="none"}}controller{{/code}} element:\\
400 1*. First attribute: name {{code language="none"}}id{{/code}}, use {{code language="none"}}required{{/code}}, type {{code language="none"}}string{{/code}}, translatable {{code language="none"}}false{{/code}}.
401 1*. Second attribute: name {{code language="none"}}name{{/code}}, use {{code language="none"}}required{{/code}}, type {{code language="none"}}string{{/code}}, translatable {{code language="none"}}true{{/code}}.
402 1*. Third attribute: name {{code language="none"}}class{{/code}}, use {{code language="none"}}required{{/code}}, type {{code language="none"}}java{{/code}}, implements {{code language="none"}}de.cau.cs.rtprak.login.simple.controller.IHeadController{{/code}}. This is the attribute that will tell us which Java class actually implements the controller that is to be registered at our extension point. To make sure that we know how to speak to the class, we require it to implement the interface we defined for head controllers.
403 1. Add a sequence to the {{code language="none"}}extension{{/code}} element. Right-click the sequence and click //New// -> //controller//. Set the //Min Occurrences// of the sequence to 0, and set //Max Occurrences// to be //Unbounded//.
404 1. Save the editor and switch back to the //Plugin Manifest Editor//.
405 1. On the Runtime tab, add {{code language="none"}}de.cau.cs.rtprak.login.simple.controller{{/code}} to the list of packages exported by the plug-in. This is necessary because plug-ins that want to provide extensions for the extension point must provide a class that implements {{code language="none"}}IHeadController{{/code}}. For this to work, those plug-ins must have access to that interface; thus, we have to export the package containing it.
406
407 == Accessing the Extension Point ==
408
409 We will now add a class that will be in charge of loading all extensions registered at our new extension point.
410
411 1. (((
412 Add a class {{code language="none"}}HeadControllers{{/code}} to the package {{code language="none"}}de.cau.cs.rtprak.login.simple.controller{{/code}}. Add the following code, replacing {{code language="none"}}login{{/code}} with your login name in {{code language="none"}}EXTENSION_POINT_ID{{/code}} as usual:
413
414 {{code language="java"}}
415 /**
416 * Class that gathers extension data from the 'headControllers' extension point
417 * and publishes this data using the singleton pattern.
418 * @author msp
419 */
420 public class HeadControllers {
421 /** Identifier of the extension point */
422 public final static String EXTENSION_POINT_ID = "de.cau.cs.rtprak.login.simple.headControllers";
423 /** The singleton instance of the {@code HeadControllers} class */
424 public final static HeadControllers INSTANCE = new HeadControllers();
425 /** list of head controller ids with associated names. */
426 private List<String[]> controllerNames = new LinkedList<String[]>();
427 /** map of controller ids to their runtime instances. */
428 private Map<String, IHeadController> controllerMap = new HashMap<String, IHeadController>();
429 /**
430 * Creates an instance of this class and gathers extension data.
431 */
432 HeadControllers() {
433 IConfigurationElement[] elements = Platform.getExtensionRegistry()
434 .getConfigurationElementsFor(EXTENSION_POINT_ID);
435 for (IConfigurationElement element : elements) {
436 if ("controller".equals(element.getName())) {
437 String id = element.getAttribute("id");
438 String name = element.getAttribute("name");
439 if (id != null && name != null) {
440 try {
441 IHeadController controller = (IHeadController)element
442 .createExecutableExtension("class");
443 controllerNames.add(new String[] {id, name});
444 controllerMap.put(id, controller);
445 }
446 catch (CoreException exception) {
447 StatusManager.getManager().handle(exception, Activator.PLUGIN_ID);
448 }
449 }
450 }
451 }
452 }
453
454 /**
455 * Returns a list of controller ids and names. The arrays in the list are
456 * all of size 2: the first element is an id, and the second element is the
457 * associated name. The controller name is a user-friendly string to be
458 * displayed in the UI.
459 * @return a list of controller ids and names
460 */
461 public List<String[]> getControllerNames() {
462 return controllerNames;
463 }
464
465 /**
466 * Returns the head controller instance for the given id.
467 * @param id identifier of a head controller
468 * @return the associated controller
469 */
470 public IHeadController getController(final String id) {
471 return controllerMap.get(id);
472 }
473 }
474 {{/code}}
475 )))
476
477 == Adding Support for Head Controllers to the View ==
478
479 We will now have to add support for head controllers to our view.
480
481 1. Open the {{code language="none"}}TapeViewPart{{/code}} class and add the private fields {{code language="none"}}checkedControllerAction{{/code}} of type [[IAction>>url:http://help.eclipse.org/juno/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/jface/action/IAction.html||shape="rect"]] and {{code language="none"}}currentController{{/code}} of type {{code language="none"}}IHeadController{{/code}}.
482 1. (((
483 Add a list of registered head controllers to the view's menu (which can be opened using the small white triangle) in the {{code language="none"}}createPartControl(){{/code}} method:
484
485 {{code language="java"}}
486 IMenuManager menuManager = getViewSite().getActionBars().getMenuManager();
487 for (String[] controllerName : HeadControllers.INSTANCE.getControllerNames()) {
488 final String id = controllerName[0];
489 String name = controllerName[1];
490 Action action = new Action(name, IAction.AS_RADIO_BUTTON) {
491 public void run() {
492 if (checkedControllerAction != null) {
493 checkedControllerAction.setChecked(false);
494 }
495 this.setChecked(true);
496 checkedControllerAction = this;
497 currentController = HeadControllers.INSTANCE.getController(id);
498 }
499 };
500 if (checkedControllerAction == null) {
501 action.run();
502 }
503 menuManager.add(action);
504 }
505 {{/code}}
506 )))
507 1. (((
508 Implement the following method in the {{code language="none"}}TuringTape{{/code}} class:
509
510 {{code language="java"}}
511 public void execute(final IHeadController controller)
512 {{/code}}
513
514 The method shall have the following properties:
515
516 \\
517
518 * Determine the character at the current head position using
519
520 {{code language="none"}}
521 getCharacter(getHeadPosition())
522 {{/code}}.
523 * Call
524
525 {{code language="none"}}
526 controller.nextCommand()
527 {{/code}} with the current character as parameter.
528 * Depending on the action in the returned head command, either write the returned new character to the current position in text (
529
530 {{code language="none"}}
531 WRITE
532 {{/code}}), or write the blank symbol (
533
534 {{code language="none"}}
535 ERASE
536 {{/code}}), or do nothing. If the current position exceeds the end of the text, append enough blank characters up to the current position, then append the new character.
537 * Depending on the direction in the returned head command, either move the head to the left (but no further than position 0), or to the right, or do nothing.
538 )))
539 1. Copy the files [[attach:step.gif]]and [[attach:reset.gif]]to the icons folder.
540 1. Add an action to the toolbar of the Tape view with text {{code language="none"}}Step{{/code}} and icon {{code language="none"}}step.png{{/code}} which does the following:\\
541 1*. Check whether the current head controller is not {{code language="none"}}null{{/code}}, than call {{code language="none"}}tape.execute(currentController){{/code}}.
542 1*. Refresh the table viewer with its {{code language="none"}}refresh(){{/code}} method.
543 1*. (((
544 Note: actions don't need images, but only image descriptors. Thus, to set the action's icon to {{code language="none"}}step.png{{/code}}, you can use something like the following:
545
546 {{code language="java"}}
547 Activator.imageDescriptorFromPlugin(Activator.PLUGIN_ID, "path_to_icon");
548 {{/code}}
549 )))
550 1. Add another action with text Reset and icon reset.png which does the following:\\
551 1*. Check whether the current head controller is not {{code language="none"}}null{{/code}}, then call the {{code language="none"}}reset(){{/code}} method on {{code language="none"}}currentController{{/code}}.
552 1*. Set the current head position to 1.
553 1*. Refresh the table viewer with its {{code language="none"}}refresh(){{/code}} method.
554
555 == Adding a Test Head Controller ==
556
557 Before creating a proper head controller in another plug-in, we will add a test controller to check whether all this stuff works.
558
559 1. (((
560 Add a new class {{code language="none"}}NullController{{/code}} to the {{code language="none"}}de.cau.cs.rtprak.login.simple.controllers{{/code}} package:
561
562 {{code language="java"}}
563 /**
564 * Head controller that does nothing, for testing.
565 * @author msp
566 */
567 public class NullController implements IHeadController {
568 /**
569 * {@inheritDoc}
570 */
571 public HeadCommand nextCommand(final char character) {
572 return new HeadCommand(Action.NULL, Direction.NONE, '_');
573 }
574
575 /**
576 * {@inheritDoc}
577 */
578 public void reset() {
579 }
580 }
581 {{/code}}
582 )))
583 1. Open the //Plugin Manifest Editor// and switch to the //Extensions// tab. Add your {{code language="none"}}de.cau.cs.rtprak.login.simple.headControllers{{/code}} extension point. Add a {{code language="none"}}controller{{/code}} element with ID {{code language="none"}}de.cau.cs.rtprak.login.simple.nullController{{/code}}, name {{code language="none"}}Null Controller{{/code}}, and class {{code language="none"}}de.cau.cs.rtprak.login.simple.controller.NullController{{/code}}.
584 1. Start the application and observe how your program behaves if you change the action and direction in the {{code language="none"}}NullController{{/code}} class. You can actually change both while the application is running, but only if you have started it in the Debug mode. In that case, Eclipse will actually hot-swap your changes into the running application. Sorcery!
585
586 == Implementing Your Own Head Controller ==
587
588 We will now create a new plug-in with a new head controller:
589
590 1. Create a new plug-in {{code language="none"}}de.cau.cs.rtprak.login.simple.extension{{/code}}. (Remember to create the project in your Git repository.) In the //Plugin Manifest Editor//, add {{code language="none"}}de.cau.cs.rtprak.login.simple{{/code}} to the dependencies of the new plug-in.
591 1. Create a new class that implements {{code language="none"}}IHeadController{{/code}}:\\
592 1*. Assuming that the initial head position is 1, the controller shall copy the input text infinitely often. So if the tape initially contains the word {{code language="none"}}hello{{/code}}, the controller shall generate {{code language="none"}}hellohellohellohe...{{/code}} .
593 1*. Your class needs some private fields to store the internal state of the controller, and you may need some special character as marker. Imagine how a Turing Machine would do this.
594 1*. It is not allowed to store data that can grow infinitely, since a Turing Machine may only have a finite number of states. This means that you may store single characters or numbers, but you must not store Strings, StringBuffers, arrays, lists, or sets.
595 1. Register the new controller class using an extension in the new plug-in.
596 1. Test your controller.
597
598 = Congratulations! =
599
600 Congratulations, you just made a big step towards understanding how Eclipse works. Plus, you've refreshed your knowledge on Turing Machines along the way. Eclipse is an industry standard technology, and having experience programming against it is a valuable skill for you.
601
602 If you have any comments and suggestions for improvement concerning this tutorial, please don't hesitate to tell us about them!

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