src/java.desktop/share/classes/java/awt/SecondaryLoop.java

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package java.awt;

/**
 * A helper interface to run the nested event loop.
 * <p>
 * Objects that implement this interface are created with the
 * {@link EventQueue#createSecondaryLoop} method. The interface
 * provides two methods, {@link #enter} and {@link #exit},
 * which can be used to start and stop the event loop.
 * <p>
 * When the {@link #enter} method is called, the current
 * thread is blocked until the loop is terminated by the
 * {@link #exit} method. Also, a new event loop is started
 * on the event dispatch thread, which may or may not be
 * the current thread. The loop can be terminated on any
 * thread by calling its {@link #exit} method. After the
 * loop is terminated, the {@code SecondaryLoop} object can
 * be reused to run a new nested event loop.
 * <p>
 * A typical use case of applying this interface is AWT
 * and Swing modal dialogs. When a modal dialog is shown on
 * the event dispatch thread, it enters a new secondary loop.
 * Later, when the dialog is hidden or disposed, it exits
 * the loop, and the thread continues its execution.
 * <p>
 * The following example illustrates a simple use case of
 * secondary loops:
 *
 * <pre>
 *   SecondaryLoop loop;
 *
 *   JButton jButton = new JButton("Button");
 *   jButton.addActionListener(new ActionListener() {
 *       {@code @Override}
 *       public void actionPerformed(ActionEvent e) {
 *           Toolkit tk = Toolkit.getDefaultToolkit();
 *           EventQueue eq = tk.getSystemEventQueue();
 *           loop = eq.createSecondaryLoop();
 *
 *           // Spawn a new thread to do the work
 *           Thread worker = new WorkerThread();
 *           worker.start();
 *
 *           // Enter the loop to block the current event
 *           // handler, but leave UI responsive
 *           if (!loop.enter()) {
 *               // Report an error
 *           }
 *       }
 *   });
 *
 *   class WorkerThread extends Thread {
 *       {@code @Override}
 *       public void run() {
 *           // Perform calculations
 *           doSomethingUseful();
 *
 *           // Exit the loop
 *           loop.exit();
 *       }
 *   }
 * </pre>
 *
 * @see Dialog#show
 * @see EventQueue#createSecondaryLoop
 * @see Toolkit#getSystemEventQueue
 *
 * @author Anton Tarasov, Artem Ananiev
 *
 * @since 1.7
 */
public interface SecondaryLoop {

    /**
     * Blocks the execution of the current thread and enters a new
     * secondary event loop on the event dispatch thread.
     * <p>
     * This method can be called by any thread including the event
     * dispatch thread. This thread will be blocked until the {@link
     * #exit} method is called or the loop is terminated. A new
     * secondary loop will be created on the event dispatch thread
     * for dispatching events in either case.
     * <p>
     * This method can only start one new event loop at a time per
     * object. If a secondary event loop has already been started
     * by this object and is currently still running, this method
     * returns {@code false} to indicate that it was not successful
     * in starting a new event loop. Otherwise, this method blocks
     * the calling thread and later returns {@code true} when the
     * new event loop is terminated. At such time, this object can
     * again be used to start another new event loop.
     *
     * @return {@code true} after termination of the secondary loop,
     *         if the secondary loop was started by this call,
     *         {@code false} otherwise
     */
    public boolean enter();

    /**
     * Unblocks the execution of the thread blocked by the {@link
     * #enter} method and exits the secondary loop.
     * <p>
     * This method resumes the thread that called the {@link #enter}
     * method and exits the secondary loop that was created when
     * the {@link #enter} method was invoked.
     * <p>
     * Note that if any other secondary loop is started while this
     * loop is running, the blocked thread will not resume execution
     * until the nested loop is terminated.
     * <p>
     * If this secondary loop has not been started with the {@link
     * #enter} method, or this secondary loop has already finished
     * with the {@link #exit} method, this method returns {@code
     * false}, otherwise {@code true} is returned.
     *
     * @return {@code true} if this loop was previously started and
     *         has not yet been finished with the {@link #exit} method,
     *         {@code false} otherwise
     */
    public boolean exit();

}