Changes for page The Plug-in Architecture of Eclipse

Last modified by cds on 2025/01/30 12:03

From version < 22.1 >

edited by cds
on 2012/10/24 13:47

To version < 24.1 >

edited by cds
on 2012/10/24 17:48

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@@ -342,12 +342,261 @@
  = Creating an Extension Point =
--WRITE THIS SECTION
++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).
--
++== Defining a Command Class ==
--
++We will start by defining a class representing a command that will be passed to a selected head controller.
--
++1. Add a class {{code language="none"}}HeadCommand{{/code}} to the package {{code language="none"}}de.cau.cs.rtprak.login.simple.controller{{/code}}.
++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}}.
++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}}.
++1. (((
++Add the following private fields:
--
++{{code language="java"}}
++private Action action;
++private Direction direction;
++private char newChar;
++{{/code}}
++)))
++1. Add a constructor to initialize the fields.
++1. Add getter methods to access the fields.
++
++== Defining the Controller Interface ==
++
++We will now define an interface that all head controllers will have to implement:
++
++1. Add an interface {{code language="none"}}IHeadController{{/code}} in the package {{code language="none"}}de.cau.cs.rtprak.login.simple.controller{{/code}}.
++1. (((
++Add the following methods to the interface:
++
++{{code language="java"}}
++/**
++ * Calculate the next command depending on the currently seen character.
++ * @param character the currently seen character
++ * @return the next command specifying which character to write and
++ *     which direction to move the head
++ */
++HeadCommand nextCommand(char character);
++
++/**
++ * Reset the internal state of the head controller.
++ */
++void reset();
++{{/code}}
++)))
++
++== Defining the Extension Point ==
++
++We will now define the extension point that head controllers will be registered at.
++
++1. Open the {{code language="none"}}plugin.xml{{/code}} file in the //Plugin Manifest Editor// and switch to the //Extension Points// tab.
++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//.
++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.
++1. In the new editor, open the //Definition// tab.
++1. Add a new element named {{code language="none"}}controller{{/code}}.
++1. Add three new attributes to the {{code language="none"}}controller{{/code}} element:\\
++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}}.
++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}}.
++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.
++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//.
++1. Save the editor and switch back to the //Plugin Manifest Editor//.
++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.
++
++== Accessing the Extension Point ==
++
++We will now add a class that will be in charge of loading all extensions registered at our new extension point.
++
++1. (((
++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:
++
++{{code language="java"}}
++/**
++ * Class that gathers extension data from the 'headControllers' extension point
++ * and publishes this data using the singleton pattern.
++ * @author msp
++ */
++public class HeadControllers {
++    /** Identifier of the extension point */
++    public final static String EXTENSION_POINT_ID = "de.cau.cs.rtprak.login.simple.headControllers";
++    /** The singleton instance of the {@code HeadControllers} class */
++    public final static HeadControllers INSTANCE = new HeadControllers();
++    /** list of head controller ids with associated names. */
++    private List<String[]> controllerNames = new LinkedList<String[]>();
++    /** map of controller ids to their runtime instances. */
++    private Map<String, IHeadController> controllerMap = new HashMap<String, IHeadController>();
++    /**
++     * Creates an instance of this class and gathers extension data.
++     */
++    HeadControllers() {
++        IConfigurationElement[] elements = Platform.getExtensionRegistry()
++                .getConfigurationElementsFor(EXTENSION_POINT_ID);
++        for (IConfigurationElement element : elements)  {
++            if ("controller".equals(element.getName())) {
++                String id = element.getAttribute("id");
++                String name = element.getAttribute("name");
++                if (id != null && name != null) {
++                    try {
++                        IHeadController controller = (IHeadController)element
++                                .createExecutableExtension("class");
++                        controllerNames.add(new String[] {id, name});
++                        controllerMap.put(id, controller);
++                    }
++                    catch (CoreException exception) {
++                        StatusManager.getManager().handle(exception, Activator.PLUGIN_ID);
++                    }
++                }
++            }
++        }
++    }
++
++    /**
++     * Returns a list of controller ids and names. The arrays in the list are
++     * all of size 2: the first element is an id, and the second element is the
++     * associated name. The controller name is a user-friendly string to be
++     * displayed in the UI.
++     * @return a list of controller ids and names
++     */
++    public List<String[]> getControllerNames() {
++        return controllerNames;
++    }
++
++    /**
++     * Returns the head controller instance for the given id.
++     * @param id identifier of a head controller
++     * @return the associated controller
++     */
++    public IHeadController getController(final String id) {
++        return controllerMap.get(id);
++    }
++}
++{{/code}}
++)))
++
++== Adding Support for Head Controllers to the View ==
++
++We will now have to add support for head controllers to our view.
++
++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}}.
++1. (((
++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:
++
++{{code language="java"}}
++IMenuManager menuManager = getViewSite().getActionBars().getMenuManager();
++for (String[] controllerName : HeadControllers.INSTANCE.getControllerNames()) {
++    final String id = controllerName[0];
++    String name = controllerName[1];
++    Action action = new Action(name, IAction.AS_RADIO_BUTTON) {
++        public void run() {
++            if (checkedControllerAction != null) {
++                checkedControllerAction.setChecked(false);
++            }
++            this.setChecked(true);
++            checkedControllerAction = this;
++            currentController = HeadControllers.INSTANCE.getController(id);
++        }
++    };
++    if (checkedControllerAction == null) {
++        action.run();
++    }
++    menuManager.add(action);
++}
++{{/code}}
++)))
++1. (((
++Implement the following method in the {{code language="none"}}TuringTape{{/code}} class:
++
++{{code language="java"}}
++public void execute(final IHeadController controller)
++{{/code}}
++
++The method shall have the following properties:
++
++\\
++
++* Determine the character at the current head position using
++
++{{code language="none"}}
++getCharacter(getHeadPosition())
++{{/code}}.
++* Call
++
++{{code language="none"}}
++controller.nextCommand()
++{{/code}} with the current character as parameter.
++* Depending on the action in the returned head command, either write the returned new character to the current position in text (
++
++{{code language="none"}}
++WRITE
++{{/code}}), or write the blank symbol (
++
++{{code language="none"}}
++ERASE
++{{/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.
++* 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.
++)))
++1. Copy the files [[attach:step.gif]]and [[attach:reset.gif]]to the icons folder.
++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:\\
++1*. Check whether the current head controller is not {{code language="none"}}null{{/code}}, than call {{code language="none"}}tape.execute(currentController){{/code}}.
++1*. Refresh the table viewer with its {{code language="none"}}refresh(){{/code}} method.
++1*. (((
++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:
++
++{{code language="java"}}
++Activator.imageDescriptorFromPlugin(Activator.PLUGIN_ID, "path_to_icon");
++{{/code}}
++)))
++1. Add another action with text Reset and icon reset.png which does the following:\\
++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}}.
++1*. Set the current head position to 1.
++1*. Refresh the table viewer with its {{code language="none"}}refresh(){{/code}} method.
++
++== Adding a Test Head Controller ==
++
++Before creating a proper head controller in another plug-in, we will add a test controller to check whether all this stuff works.
++
++1. (((
++Add a new class {{code language="none"}}NullController{{/code}} to the {{code language="none"}}de.cau.cs.rtprak.login.simple.controllers{{/code}} package:
++
++{{code language="java"}}
++/**
++ * Head controller that does nothing, for testing.
++ * @author msp
++ */
++public class NullController implements IHeadController {
++    /**
++     * {@inheritDoc}
++     */
++    public HeadCommand nextCommand(final char character) {
++        return new HeadCommand(Action.NULL, Direction.NONE, '_');
++    }
++
++    /**
++     * {@inheritDoc}
++     */
++    public void reset() {
++    }
++}
++{{/code}}
++)))
++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}}.
++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!
++
++== Implementing Your Own Head Controller ==
++
++We will now create a new plug-in with a new head controller:
++
++1. Create a new plug-in {{code language="none"}}de.cau.cs.rtprak.login.simple.extension{{/code}}. In the //Plugin Manifest Editor//, add {{code language="none"}}de.cau.cs.rtprak.login.simple{{/code}} to the dependencies of the new plug-in.
++1. Create a new class that implements {{code language="none"}}IHeadController{{/code}}:\\
++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}} .
++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.
++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.
++1. Register the new controller class using an extension in the new plug-in.
++1. Test your controller.
++
++= Congratulations! =
++
++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.
++
++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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@@ -1,1 +1,1 @@
--2982312
++2982316

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@@ -1,1 +1,1 @@
--https://rtsys.informatik.uni-kiel.de/confluence//wiki/spaces/WS12EclPract/pages/2982312/The Plug-in Architecture of Eclipse
++https://rtsys.informatik.uni-kiel.de/confluence//wiki/spaces/WS12EclPract/pages/2982316/The Plug-in Architecture of Eclipse

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