<AWT Dev> hg: jdk7/awt/jdk: 6949936: Provide API for running nested events loops, similar to what modal dialogs do

[Artem Ananiev]

Hi, Rémi,

thanks for your feedback. See more comments below.

On 8/24/2010 2:36 PM, Rémi Forax wrote:
> Le 24/08/2010 10:57, artem.ananiev at sun.com a écrit :
>> Changeset: 7e26538596be
>> Author: art
>> Date: 2010-08-24 12:54 +0400
>> URL: http://hg.openjdk.java.net/jdk7/awt/jdk/rev/7e26538596be
>>
>> 6949936: Provide API for running nested events loops, similar to what
>> modal dialogs do
>> Reviewed-by: ant, anthony
>>
>> ! src/share/classes/java/awt/Dialog.java
>> ! src/share/classes/java/awt/EventDispatchThread.java
>> ! src/share/classes/java/awt/EventQueue.java
>> + src/share/classes/java/awt/SecondaryLoop.java
>> + src/share/classes/java/awt/WaitDispatchSupport.java
>> + test/java/awt/EventQueue/SecondaryLoopTest/SecondaryLoopTest.java
>>
>
> Hi all, nice to see that this kind of API find its way to jdk7.
>
> The example provided in SecondaryLoop don't compile.
> Here is a slightly modified version that compile.

The code in JavaDoc is not a complete test case, but rather a kind of 
stub. To make it compile, you need to declare 'loop' final or make it a 
class field, put the code with ActionListener into a method, etc.

> JButton jButton = new JButton("Button");
> jButton.addActionListener(new ActionListener() {
> @Override
> public void actionPerformed(ActionEvent e) {
> EventQueue queue = Toolkit.getDefaultToolkit()
> .getSystemEventQueue();
> final SecondaryLoop loop = queue.createSecondaryLoop();
>
> Runnable runnable = new Runnable() {
> @Override
> public void run() {
> // Perform calculations
> doSomethingUseful();
>
> // Exit the loop
> loop.exit();
> }
> };
>
> // Spawn a new thread to do the work
> Thread worker = new Thread(runnable);
> worker.start();
>
> // Enter the loop to block the current event
> // handler, but leave UI responsive
> if (!loop.enter()) {
> // Report an error
> }
> }
> });
>
> About the API design, SecondaryLoop.enter is not reentrant so it's
> an error to call it recursively. So enter() should throw a runtime
> exception
> instead of returning false. I propose to use IllegalStateException.

Well, these two approaches are roughly the same, but throwing an 
exception usually makes the code a bit less clear.

> SecondaryLoop is in my opinion a bad name, a for or a while is a loop,
> in my opinion, SecondaryEventLoop is a better name.
>
> Also, you don't provide a way to transfer any value from the computation
> done in the thread back to the event queue thread like by example
> the SwingWorker does.
>
> I propose to change SecondaryEventLoop to:
>
> public interface SecondaryEventLoop<V> {
> public V enter();
> public boolean exit(V value);
> }
>
> exit() can send a value that will be returned when enter() will end.
>
> I also wonder if the return value of exit is useful.
> exit can be called before enter() due to a race between enter() and exit(),
> why a developer should care about that.

SecondaryLoop is different from SwingWorker: it shouldn't be used to 
perform a task and obtain a value. Its only purpose is to make sure UI 
is not frozen (EDT keeps running even if the current event handler is 
blocked), but it doesn't spawn any new threads and doesn't delegate any 
work to other threads.

> cheers,
> Rémi

Thanks,

Artem