EventThread Pattern

When designing an event-listener model, the developer has to make three decisions. First, should notifications use listeners or events. Second, should notifications be processed immediately or queued for later handling. Third, should notifications be handled in the observable's executing thread or some other. EventThread coding pattern addresses the second and the third question. Not only are both cases solved with the same simple solution, but also - instead of implementation, the caller of the object can decide which model to use. Here goes.

Event Thread Pattern

In EventThread pattern, the Listener/Observer interface has a function that allows the implementation to decide the executing environment of the event. Java (Executor) is an interface that has various default implementations (See Executors). Many models are supported. Work can be executed in current thread, executed in new thread, or placed in a work queue, or directed to an EDT (EventDispatchThread).

<syntaxhighlight lang="java"> public class MyObservableObject { ... void addListener(MyListener listener); } public interface MyListener {

void onEvent(Object sender, Object event);

/** * Get the executor environment where the event will be handled. * null value denotes that the events is handled immediately * and in the caller's thread. * * @return executor or null */ Executor getExecutor();

}

// Caller Usage MyObservable obj = ... ; obj.addListener( new MyListener() { @Override public void onEvent(Object sender, Object event) { }

@Override public Executor getExecutor() { return CURRENT_THREAD; } } ); </syntaxhighlight>

Variation

In an variation, the listener doesn't have getExecutor(), but instead the executor environment is given as argument to observable's addListener(Listener, Executor). <syntaxhighlight lang="java">

public class MyObservableObject { ... void addListener(MyListener listener, Executor executor); } public interface MyListener { void onEvent(Object sender, Object event); }

// Caller Usage MyObservable obj = ... ; obj.addListener( myListener, CURRENT_THREAD ); obj.addListener( myListener, myWorkQueue ); obj.addListener( myListener, AWT_EDT ); // or SWT_EDT obj.addListener( myListener, Executors.newSingleThreadScheduledExecutor() ); </syntaxhighlight>

Adapter

Some listeners have default implementation called adapter (For example AWT Listeners & Adapters). With this pattern it is a good idea to have a default executor in the adapter.

<syntaxhighlight lang="java"> public abstract class MyAdapter implements MyListener {

public Executor getExecutor() { return CURRENT_THREAD; } }

// Caller Usage MyObservable obj = ... ; obj.addListener( new MyAdapter() { @Override public void onEvent(Object sender, Object event) { ... } } ); </syntaxhighlight>

Event Dispatch Threads

There are many implementations supported in the Executors except the one that support GUI Framework EDTs and the current thread. You can have it with a few lines of code.

<syntaxhighlight lang="java">

public class Executors2 {

// Executor that runs in current thread public static Executor CURRENT_THREAD = new CurrentThreadExecutor();

// Async executor queues the command into AWT event queue public static Executor AWT_EDT = new AWTExecutorAsync();

// Sync executor blocks the call until the command is ran and finished in AWT Thread public static Executor AWT_EDT_SYNC = new AWTExecutorSync();

public static Executor createSWTExecutor(Display display, boolean async) { return async ? new SWTExecutorAsync(display) : new SWTExecutorSync(display); }

}

class AWTExecutorAsync implements Executor {

@Override public void execute(Runnable command) {

       EventQueue.invokeLater(command);

} }

class AWTExecutorSync implements Executor {

@Override public void execute(Runnable command) { if (EventQueue.isDispatchThread()) { command.run(); } else { try { EventQueue.invokeAndWait(command); } catch (InterruptedException e) { throw new RuntimeException(e); } catch (InvocationTargetException e) { throw new RuntimeException(e); } } } }

class CurrentThreadExecutor implements Executor { @Override public void execute(Runnable command) { command.run(); } }

class SWTExecutorAsync implements Executor {

Display display; public SWTExecutorAsync(Display display) { this.display = display; }

@Override public void execute(Runnable command) { // Don't accept work if the SWT thread is disposed. if (display.isDisposed()) throw new RuntimeException("The SWT thread has been disposed"); display.asyncExec(command); }

}

class SWTExecutorSync implements Executor {

Display display; public SWTExecutorSync(Display display) { this.display = display; }

@Override public void execute(Runnable command) { // Don't accept work if the SWT thread is disposed. if (display.isDisposed()) throw new RuntimeException("The SWT thread has been disposed"); display.syncExec(command); }

}

</syntaxhighlight>

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Toni Kalajainen