/*******************************************************************************
 * Copyright (c) 2005, 2008 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *     Matthew Hall - Fix NPE, more detailed assert messages (bug 210115)
 *******************************************************************************/
module org.eclipse.core.databinding.observable.ObservableTracker;

import java.lang.all;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import org.eclipse.core.internal.databinding.IdentityWrapper;
import org.eclipse.core.runtime.Assert;

/**
 * This class makes it possible to monitor whenever an IObservable is read from.
 * This can be used to automatically attach and remove listeners. How to use it:
 * 
 * <p>
 * If you are implementing an IObservable, invoke getterCalled(this) whenever a
 * getter is called - that is, whenever your observable is read from. You only
 * need to do this once per method call. If one getter delegates to another, the
 * outer getter doesn't need to call the method since the inner one will.
 * </p>
 * 
 * <p>
 * If you want to determine what observables were used in a particular block of
 * code, call runAndMonitorcast(Runnable). This will execute the given runnable and
 * return the set of observables that were read from.
 * </p>
 * 
 * <p>
 * This can be used to automatically attach listeners. For example, imagine you
 * have a block of code that updates some widget by reading from a bunch of
 * observables. Whenever one of those observables changes, you want to re-run
 * the code and cause the widget to be refreshed. You could do this in the
 * traditional manner by attaching one listener to each observable and
 * re-running your widget update code whenever one of them changes, but this
 * code is repetitive and requires updating the listener code whenever you
 * refactor the widget updating code.
 * </p>
 * 
 * <p>
 * Alternatively, you could use a utility class that runs the code in a
 * runAndMonitor block and automatically attach listeners to any observable used
 * in updating the widget. The advantage of the latter approach is that it,
 * eliminates the code for attaching and detaching listeners and will always
 * stay in synch with changes to the widget update logic.
 * </p>
 * 
 * @since 1.0
 */
public class ObservableTracker {

    /**
     * Threadlocal storage pointing to the current Set of IObservables, or null
     * if none. Note that this is actually the top of a stack. Whenever a method
     * changes the current value, it remembers the old value as a local variable
     * and restores the old value when the method exits.
     */
    private static ThreadLocal currentChangeListener = new ThreadLocal();

    private static ThreadLocal currentStaleListener = new ThreadLocal();

    private static ThreadLocal currentObservableSet = new ThreadLocal();

    /**
     * Invokes the given runnable, and returns the set of IObservables that were
     * read by the runnable. If the runnable calls this method recursively, the
     * result will not contain IObservables that were used within the inner
     * runnable.
     * 
     * @param runnable
     *            runnable to execute
     * @param changeListener
     *            listener to register with all accessed observables
     * @param staleListener
     *            listener to register with all accessed observables, or
     *            <code>null</code> if no stale listener is to be registered
     * @return an array of unique observable objects
     */
    public static IObservable[] runAndMonitor(Runnable runnable,
            IChangeListener changeListener, IStaleListener staleListener) {
        // Remember the previous value in the listener stack
        Set lastObservableSet = cast(Set) currentObservableSet.get();
        IChangeListener lastChangeListener = cast(IChangeListener) currentChangeListener
                .get();
        IStaleListener lastStaleListener = cast(IStaleListener) currentStaleListener
                .get();

        Set observableSet = new HashSet();
        // Push the new listeners to the top of the stack
        currentObservableSet.set(observableSet);
        currentChangeListener.set(changeListener);
        currentStaleListener.set(staleListener);
        try {
            runnable.run();
        } finally {
            // Pop the new listener off the top of the stack (by restoring the
            // previous listener)
            currentObservableSet.set(lastObservableSet);
            currentChangeListener.set(lastChangeListener);
            currentStaleListener.set(lastStaleListener);
        }

        int i = 0;
        IObservable[] result = new IObservable[observableSet.size()];
        for (Iterator it = observableSet.iterator(); it.hasNext();) {
            IdentityWrapper wrapper = cast(IdentityWrapper) it.next();
            result[i++] = cast(IObservable) wrapper.unwrap();
        }

        return result;
    }
    
    /**
     * Runs the given runnable without tracking dependencies.
     * @param runnable
     * 
     * @since 1.1
     */
    public static void runAndIgnore(Runnable runnable) {
        // Remember the previous value in the listener stack
        Set lastObservableSet = cast(Set) currentObservableSet.get();
        IChangeListener lastChangeListener = cast(IChangeListener) currentChangeListener
                .get();
        IStaleListener lastStaleListener = cast(IStaleListener) currentStaleListener
                .get();
        currentObservableSet.set(null);
        currentChangeListener.set(null);
        currentStaleListener.set(null);
        try {
            runnable.run();
        } finally {
            // Pop the new listener off the top of the stack (by restoring the
            // previous listener)
            currentObservableSet.set(lastObservableSet);
            currentChangeListener.set(lastChangeListener);
            currentStaleListener.set(lastStaleListener);
        }
    }

    /*
     * Returns the same string as the default Object.toString() implementation.
     * getterCalled() uses this method IObservable.toString() to avoid infinite
     * recursion and stack overflow.
     */
    private static String toString(IObservable observable) {
        return observable.getClass().getName() + "@" //$NON-NLS-1$
                + Integer.toHexString(System.identityHashCode(observable));
    }

    /**
     * Notifies the ObservableTracker that an observable was read from. The
     * JavaDoc for methods that invoke this method should include the following
     * tag: "@TrackedGetter This method will notify ObservableTracker that the
     * receiver has been read from". This lets callers know that they can rely
     * on automatic updates from the object without explicitly attaching a
     * listener.
     * 
     * @param observable
     */
    public static void getterCalled(IObservable observable) {
        Realm realm = observable.getRealm();
        if (realm is null) // observable.isDisposed() would be more appropriate if it existed
            Assert.isTrue(false, "Getter called on disposed observable " //$NON-NLS-1$
                    + toString(observable));
        if (!realm.isCurrent())
            Assert.isTrue(false, "Getter called outside realm of observable " //$NON-NLS-1$
                    + toString(observable));

        Set lastObservableSet = cast(Set) currentObservableSet.get();
        if (lastObservableSet is null) {
            return;
        }
        IChangeListener lastChangeListener = cast(IChangeListener) currentChangeListener
                .get();
        IStaleListener lastStaleListener = cast(IStaleListener) currentStaleListener
                .get();

        bool added = false;
        if (lastObservableSet !is null) {
            added = lastObservableSet.add(new IdentityWrapper(observable));
        }

        // If anyone is listening for observable usage...
        if (added && lastChangeListener !is null) {
            observable.addChangeListener(lastChangeListener);
        }
        if (added && lastStaleListener !is null) {
            observable.addStaleListener(lastStaleListener);
        }
    }
}