Eclipse-Praktikum WS 12/13
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Warning

This tutorial isn't complete yet!

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.

Preliminaries

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.

Required Software

For this tutorial, we need you to have Eclipse and Git installed:

  1. Install Eclipse. For what we do, we recommend installing the Eclipse Modeling Tools, with a few extras. Our Wiki page on getting Eclipse has the details: simply follow the instructions for downloading and installing Eclipse and you should be set.
  2. You should already have obtained a working Git installation for the first tutorial.

General Remarks

Over the course of this tutorial, you will be writing a bit of code. Here's a few rules we ask you to follow:

  • All your Java code should be in packages with the prefix de.cau.cs.rtprak.login, where login 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.
  • All Java classes, fields, and methods should be thoroughly commented with the standard Javadoc 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.
  • 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 or the material of your Theoretical Computer Science lecture might be a great start.
  • 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.

Finding Documentation

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:

  • Java Platform, Standard Edition 6 API Specification
    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.
  • Eclipse Help System
    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:
    • Programmer's Guide
      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.
    • References -> API Reference
      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.
    • References -> Extension Points Reference
      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.
  • Eclipsepedia
    The official Eclipse Wiki. Contains a wealth of information on Eclipse programming.
  • Eclipse Resources
    Provides forums, tutorials, articles, presentations, etc. on Eclipse and Eclipse-related topics.

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 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.

Preparing the Repository

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. You will first have to configure your Stash account:

  1. Login with your Rtsys account information.
  2. Through the button in the top right corner, access your profile.
  3. Switch to the SSH keys tab.
  4. Click Add Key and upload a public SSH key that you want to use to access the repository.

You should now be able to access the repository. Clone it:

  1. Open a console window and navigate to an empty directory that the repository should be placed in.
  2. Enter the command ssh://git@git.rtsys.informatik.uni-kiel.de:7999/PRAK/12ws-eclipse-tutorials.git . (including the final dot, which tells git to clone the repository into the current directory instead of a subdirectory).
  3. You should now have a clone of the repository in the current directory.

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:

  1. Enter git checkout -b login_name

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:

  1. Enter git push origin login_name

You would usually have to enter git pull first, but since nobody will mess with your branch this won't be necessary. By the way, you only need to mention origin login_name with the first git push, 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 git push.

Creating a Simple Text Editor

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.

Creating a New Plugin

For our text editor to integrate into Eclipse, we need to create a plug-in project for it:

  1. New -> Project...
  2. In the project wizard, choose Plug-in Project and click Next.
  3. As the project name, enter de.cau.cs.rtprak.login.simple. 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 /path/to/git/repository/de.cau.cs.rtprak.login.simple). Click Next.
  4. As the name, enter Simple (login). 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...)
  5. 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: plugin.xml and META-INF/MANIFEST.MF. (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 org.eclipse.ui.editors and org.eclipse.jface.text. Save the editor and close it. (You can always reopen it by opening one of the two mentioned files from the Package Explorer.)
  6. 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.

Create the Main Editor Class

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.

  1. New -> Class.
  2. Package: de.cau.cs.rtprak.login.simple.editors. Name: SimpleEditorPart. Superclass: org.eclipse.ui.editors.text.TextEditor. Click Finish.

Register the Editor

For the editor to be available inside Eclipse, we will have to register it by adding an extension to an extension point.

  1. Copy the attached file 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).
  2. Open the Plug-in Manifest Editor again and switch to the Extensions tab.
  3. Click Add..., choose the org.eclipse.ui.editors extension point and click Finish.
  4. 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 de.cau.cs.rtprak.login.simple.editor, the name to Simple Text Editor, the icon to icons/turing-file.gif, the extensions to simple, the class to de.cau.cs.rtprak.login.simple.editors.SimpleEditorPart, and the default to true.
  5. Save the manifest editor.

Test the Editor

It's time to test your new simple editor in a new Eclipse instance.

  1. Switch back to the Overview tab of the Plug-in Manifest Editor.
  2. Click Launch an Eclipse Application.
    • For future tests, you can now select Eclipse Application in the run menu.
    • 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.
    • 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.
      • Make sure that org.eclipse.ui.ide.application is also selected, else you won't be able to launch Eclipse.
      • The requirements list needs to be updated when the dependencies of your plugins have changed; click Add Required Plug-ins again for updating. 
  3. In the new Eclipse instance, click New -> Project... -> General -> Project. Enter test as the project name.
  4. Right-click the new project and click New -> File... As the file name, enter test.simple. 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.)

Creating a Simple View

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.

Hint

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.

Creating the View Class

We will start by creating a class that will define the view.

  1. Create a class TapeViewPart in a new package de.cau.cs.rtprak.login.simple.views that extends the ViewPart class. (make sure that in the New Java Class wizard, the option Inherited abstract methods is checked.)
  2. Add a private field tableViewer of type TableViewer.

  3. Your TableViewPart contains a still empty method createPartControl. 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:

    createPartControl(...)
    Table table = new Table(parent, SWT.BORDER);
TableColumn column = new TableColumn(table, SWT.NONE);
column.setWidth(80);
tableViewer = new TableViewer(table);

  4. The setFocus 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:

    setFocus(...)
    tableViewer.getControl().setFocus();

Create the View Extension

We will now have to register our new view with Eclipse so that it can be seamlessly integrated into the workbench.

  1. Copy the three files tape_head.gif, head_present.gif, and head_absent.gif to the icons 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.)
  2. Open the plugin.xml file in the Plugin Manifest Editor and switch to the Extensions tab.
  3. Click Add to add a new extension for the extension point org.eclipse.ui.views. Right-click the newly added extension and add a new view element through the New menu.
  4. Set the view element's properties as follows: ID de.cau.cs.rtprak.login.simple.view, name Tape, class de.cau.cs.rtprak.login.simple.views.TapeViewPart, category org.eclipse.ui, icon icons/tape_head.gif.

When you start the application, you should now be able to open your view by clicking Window -> Show View -> Other.

Add Content and Label Providers

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.

We will now create content and label providers to do just that.

  1. Create a class TuringTape in a new package de.cau.cs.rtprak.login.simple.model with the following fields:

    Fields
    private int headPosition = 1;
private StringBuffer text = new StringBuffer();

    Also add corresponding getter and setter methods. (You can simply right-click somewhere in the class and choose Source -> Generate Getters and Setters.)

  2. Add two constants to the class:

    Constants
    public static final char START_CHAR = '\u25b7';
public static final char BLANK_CHAR = '\u25fb';
  3. Add a method getCharacter(int pos) that calculates the tape character at position pos as follows:
    • For pos == 0, return the character START_CHAR.
    • For pos > text.length(), return the character BLANK_CHAR.
    • Otherwise, return the text character at index pos - 1.
  4. Add a private field tape of type TuringTape to TapeViewPart and initialize it with a new instance.
  5. Create a class TapeData in de.cau.cs.rtprak.login.simple.model with two fields int index and char character, and add a constructor for initialization as well as corresponding getter methods.
  6. Create a class TapeContentProvider in the de.cau.cs.rtprak.login.simple.views package that implements IStructuredContentProvider.
    • The methods dispose() and inputChanged() may remain empty.
    • The method getElements() 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.
    • Suppose the input element is an instance of TuringTape. The result of getElements() shall be an array of TapeData 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 i and the result of turingTape.getCharacter(i), respectively, where i is the array index of the element.
  7. Create a class TapeLabelProvider in the de.cau.cs.rtprak.login.simple.views package that extends BaseLabelProvider and implements ITableLabelProvider.
    • Add a private field tape of type TuringTape that is initialized from the constructor.
    • Add fields presentImage and absentImage of type Image.

    • Initialize each image using the following code, where path_to_image is icons/head_present.gif and icons/head_absent.gif, respectively:

      image = Activator.imageDescriptorFromPlugin(Activator.PLUGIN_ID, "path_to_image").createImage();
    • Override the implementation of dispose() in TapeLabelProvider to dispose both images after calling super.dispose().
    • In getColumnImage() and getColumnText(), first check whether the element is an instance of TapeData and the column index is 0, and return null otherwise. If the check passes, return the following:
      • getColumnImage(): presentImage if the index given by the tape data element equals the current value of tape.getHeadPosition(), absentImage otherwise.
      • getColumnText(): a String containing the character of the tape data element.

  8. Add the following lines to createPartControl() in TapeViewPart:

    tableViewer.setContentProvider(new TapeContentProvider());
tableViewer.setLabelProvider(new TapeLabelProvider(tape));
tableViewer.setInput(tape);

 

Creating an Extension Point

WRITE THIS SECTION

 

 

 

 

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