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 very current 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
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). Work can be executed in current thread, executed in new thread, or placed in a work queue.
<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. * * @return executor */ Executor getExecutor();
}
obj.addListener( myListener ); </syntaxhighlight>
Null is current thread
For syncronous object, you can design your interface so that null value denotes current thread. It is more convenient to use and you can optimize the implementation with one less Runnable objects.
<syntaxhighlight lang="java"> 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();
} </syntaxhighlight>
CURRENT_THREAD
There are many implementations supported in the Executors except the one that runs in the current thread. You can have it with a few lines of code.
CURRENT_THREAD handles events right-away in current thread, which is also the most typical case.
<syntaxhighlight lang="java"> public static Executor CURRENT_THREAD = new Executor() { public void execute(Runnable command) { command.run(); } };
public class MyListener {
public Executor getExecutor() { return CURRENT_THREAD; } }
</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; } }
MyObservable o = ... ; o.addListener( new MyAdapter() { @Override public void onEvent(Object sender, Object event) { ... } } ); </syntaxhighlight>
Variation
In an variation, the listener doesn't have getExecutor, but instead the executor environment is given as argument to addListener method to the observable. <syntaxhighlight lang="java">
public class MyObservableObject { void addListener(MyListener listener, Executor executor); }
obj.addListener( myListener, CURRENT_THREAD ); obj.addListener( myListener, myWorkQueue ); obj.addListener( myListener, Executors.newSingleThreadScheduledExecutor() ); </syntaxhighlight>
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