android/os/MessageQueue.java

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

import android.util.Log;
import android.util.Printer;

import java.util.ArrayList;

/**
 * Low-level class holding the list of messages to be dispatched by a
 * {@link Looper}.  Messages are not added directly to a MessageQueue,
 * but rather through {@link Handler} objects associated with the Looper.
 *
 * <p>You can retrieve the MessageQueue for the current thread with
 * {@link Looper#myQueue() Looper.myQueue()}.
 */
public final class MessageQueue {
    // True if the message queue can be quit.
    private final boolean mQuitAllowed;

    @SuppressWarnings("unused")
    private long mPtr; // used by native code

    Message mMessages;
    private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
    private IdleHandler[] mPendingIdleHandlers;
    private boolean mQuitting;

    // Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
    private boolean mBlocked;

    // The next barrier token.
    // Barriers are indicated by messages with a null target whose arg1 field carries the token.
    private int mNextBarrierToken;

    private native static long nativeInit();
    private native static void nativeDestroy(long ptr);
    private native static void nativePollOnce(long ptr, int timeoutMillis);
    private native static void nativeWake(long ptr);
    private native static boolean nativeIsPolling(long ptr);

    MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        mPtr = nativeInit();
    }

    @Override
    protected void finalize() throws Throwable {
        try {
            dispose();
        } finally {
            super.finalize();
        }
    }

    // Disposes of the underlying message queue.
    // Must only be called on the looper thread or the finalizer.
    private void dispose() {
        if (mPtr != 0) {
            nativeDestroy(mPtr);
            mPtr = 0;
        }
    }

    /**
     * Returns true if the looper has no pending messages which are due to be processed.
     *
     * <p>This method is safe to call from any thread.
     *
     * @return True if the looper is idle.
     */
    public boolean isIdle() {
        synchronized (this) {
            final long now = SystemClock.uptimeMillis();
            return mMessages == null || now < mMessages.when;
        }
    }

    /**
     * Add a new {@link IdleHandler} to this message queue.  This may be
     * removed automatically for you by returning false from
     * {@link IdleHandler#queueIdle IdleHandler.queueIdle()} when it is
     * invoked, or explicitly removing it with {@link #removeIdleHandler}.
     *
     * <p>This method is safe to call from any thread.
     *
     * @param handler The IdleHandler to be added.
     */
    public void addIdleHandler(IdleHandler handler) {
        if (handler == null) {
            throw new NullPointerException("Can't add a null IdleHandler");
        }
        synchronized (this) {
            mIdleHandlers.add(handler);
        }
    }

    /**
     * Remove an {@link IdleHandler} from the queue that was previously added
     * with {@link #addIdleHandler}.  If the given object is not currently
     * in the idle list, nothing is done.
     *
     * <p>This method is safe to call from any thread.
     *
     * @param handler The IdleHandler to be removed.
     */
    public void removeIdleHandler(IdleHandler handler) {
        synchronized (this) {
            mIdleHandlers.remove(handler);
        }
    }

    /**
     * Returns whether this looper's thread is currently polling for more work to do.
     * This is a good signal that the loop is still alive rather than being stuck
     * handling a callback.  Note that this method is intrinsically racy, since the
     * state of the loop can change before you get the result back.
     *
     * <p>This method is safe to call from any thread.
     *
     * @return True if the looper is currently polling for events.
     * @hide
     */
    public boolean isPolling() {
        synchronized (this) {
            return isPollingLocked();
        }
    }

    private boolean isPollingLocked() {
        // If the loop is quitting then it must not be idling.
        // We can assume mPtr != 0 when mQuitting is false.
        return !mQuitting && nativeIsPolling(mPtr);
    }

    Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                if (msg != null && msg.target == null) {
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                if (msg != null) {
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (false) Log.v("MessageQueue", "Returning message: " + msg);
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }

                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf("MessageQueue", "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;

            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;
        }
    }

    void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuitting) {
                return;
            }
            mQuitting = true;

            if (safe) {
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            nativeWake(mPtr);
        }
    }

    /**
     * Posts a synchronization barrier to the Looper's message queue.
     *
     * Message processing occurs as usual until the message queue encounters the
     * synchronization barrier that has been posted.  When the barrier is encountered,
     * later synchronous messages in the queue are stalled (prevented from being executed)
     * until the barrier is released by calling {@link #removeSyncBarrier} and specifying
     * the token that identifies the synchronization barrier.
     *
     * This method is used to immediately postpone execution of all subsequently posted
     * synchronous messages until a condition is met that releases the barrier.
     * Asynchronous messages (see {@link Message#isAsynchronous} are exempt from the barrier
     * and continue to be processed as usual.
     *
     * This call must be always matched by a call to {@link #removeSyncBarrier} with
     * the same token to ensure that the message queue resumes normal operation.
     * Otherwise the application will probably hang!
     *
     * @return A token that uniquely identifies the barrier.  This token must be
     * passed to {@link #removeSyncBarrier} to release the barrier.
     *
     * @hide
     */
    public int postSyncBarrier() {
        return postSyncBarrier(SystemClock.uptimeMillis());
    }

    private int postSyncBarrier(long when) {
        // Enqueue a new sync barrier token.
        // We don't need to wake the queue because the purpose of a barrier is to stall it.
        synchronized (this) {
            final int token = mNextBarrierToken++;
            final Message msg = Message.obtain();
            msg.markInUse();
            msg.when = when;
            msg.arg1 = token;

            Message prev = null;
            Message p = mMessages;
            if (when != 0) {
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
            }
            if (prev != null) { // invariant: p == prev.next
                msg.next = p;
                prev.next = msg;
            } else {
                msg.next = p;
                mMessages = msg;
            }
            return token;
        }
    }

    /**
     * Removes a synchronization barrier.
     *
     * @param token The synchronization barrier token that was returned by
     * {@link #postSyncBarrier}.
     *
     * @throws IllegalStateException if the barrier was not found.
     *
     * @hide
     */
    public void removeSyncBarrier(int token) {
        // Remove a sync barrier token from the queue.
        // If the queue is no longer stalled by a barrier then wake it.
        synchronized (this) {
            Message prev = null;
            Message p = mMessages;
            while (p != null && (p.target != null || p.arg1 != token)) {
                prev = p;
                p = p.next;
            }
            if (p == null) {
                throw new IllegalStateException("The specified message queue synchronization "
                        + " barrier token has not been posted or has already been removed.");
            }
            final boolean needWake;
            if (prev != null) {
                prev.next = p.next;
                needWake = false;
            } else {
                mMessages = p.next;
                needWake = mMessages == null || mMessages.target != null;
            }
            p.recycleUnchecked();

            // If the loop is quitting then it is already awake.
            // We can assume mPtr != 0 when mQuitting is false.
            if (needWake && !mQuitting) {
                nativeWake(mPtr);
            }
        }
    }

    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

        synchronized (this) {
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

    boolean hasMessages(Handler h, int what, Object object) {
        if (h == null) {
            return false;
        }

        synchronized (this) {
            Message p = mMessages;
            while (p != null) {
                if (p.target == h && p.what == what && (object == null || p.obj == object)) {
                    return true;
                }
                p = p.next;
            }
            return false;
        }
    }

    boolean hasMessages(Handler h, Runnable r, Object object) {
        if (h == null) {
            return false;
        }

        synchronized (this) {
            Message p = mMessages;
            while (p != null) {
                if (p.target == h && p.callback == r && (object == null || p.obj == object)) {
                    return true;
                }
                p = p.next;
            }
            return false;
        }
    }

    void removeMessages(Handler h, int what, Object object) {
        if (h == null) {
            return;
        }

        synchronized (this) {
            Message p = mMessages;

            // Remove all messages at front.
            while (p != null && p.target == h && p.what == what
                   && (object == null || p.obj == object)) {
                Message n = p.next;
                mMessages = n;
                p.recycleUnchecked();
                p = n;
            }

            // Remove all messages after front.
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && n.what == what
                        && (object == null || n.obj == object)) {
                        Message nn = n.next;
                        n.recycleUnchecked();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
        }
    }

    void removeMessages(Handler h, Runnable r, Object object) {
        if (h == null || r == null) {
            return;
        }

        synchronized (this) {
            Message p = mMessages;

            // Remove all messages at front.
            while (p != null && p.target == h && p.callback == r
                   && (object == null || p.obj == object)) {
                Message n = p.next;
                mMessages = n;
                p.recycleUnchecked();
                p = n;
            }

            // Remove all messages after front.
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && n.callback == r
                        && (object == null || n.obj == object)) {
                        Message nn = n.next;
                        n.recycleUnchecked();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
        }
    }

    void removeCallbacksAndMessages(Handler h, Object object) {
        if (h == null) {
            return;
        }

        synchronized (this) {
            Message p = mMessages;

            // Remove all messages at front.
            while (p != null && p.target == h
                    && (object == null || p.obj == object)) {
                Message n = p.next;
                mMessages = n;
                p.recycleUnchecked();
                p = n;
            }

            // Remove all messages after front.
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && (object == null || n.obj == object)) {
                        Message nn = n.next;
                        n.recycleUnchecked();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
        }
    }

    private void removeAllMessagesLocked() {
        Message p = mMessages;
        while (p != null) {
            Message n = p.next;
            p.recycleUnchecked();
            p = n;
        }
        mMessages = null;
    }

    private void removeAllFutureMessagesLocked() {
        final long now = SystemClock.uptimeMillis();
        Message p = mMessages;
        if (p != null) {
            if (p.when > now) {
                removeAllMessagesLocked();
            } else {
                Message n;
                for (;;) {
                    n = p.next;
                    if (n == null) {
                        return;
                    }
                    if (n.when > now) {
                        break;
                    }
                    p = n;
                }
                p.next = null;
                do {
                    p = n;
                    n = p.next;
                    p.recycleUnchecked();
                } while (n != null);
            }
        }
    }

    void dump(Printer pw, String prefix) {
        synchronized (this) {
            long now = SystemClock.uptimeMillis();
            int n = 0;
            for (Message msg = mMessages; msg != null; msg = msg.next) {
                pw.println(prefix + "Message " + n + ": " + msg.toString(now));
                n++;
            }
            pw.println(prefix + "(Total messages: " + n + ", polling=" + isPollingLocked()
                    + ", quitting=" + mQuitting + ")");
        }
    }

    /**
     * Callback interface for discovering when a thread is going to block
     * waiting for more messages.
     */
    public static interface IdleHandler {
        /**
         * Called when the message queue has run out of messages and will now
         * wait for more.  Return true to keep your idle handler active, false
         * to have it removed.  This may be called if there are still messages
         * pending in the queue, but they are all scheduled to be dispatched
         * after the current time.
         */
        boolean queueIdle();
    }
}