Android 启动优化-IdleHandler

1、IdleHandler 的简单使用

在Android中，当消息队列没有可处理的消息的时候，就是消息队列时空的，或者消息队列里的消息还没到可触发的时间，就会执行IdleHandler。

 Looper.myQueue().addIdleHandler(new IdleHandler()
       {
              @Override
             public boolean queueIdle() 
              {
				 //在这里可以加载一些业务没有这么快涉及三方的sdk
            
      			
                 return false;  //false 只会触发一次 ，true空闲是都会返回
              }    
           });

2、系统使用的案例

我们都知道，Android系统开启应用进程的时候，会调到ActivityThread类中的main()方法，在ActivityThread类中会用到，第一次idler的时候，调了一次GC。

 void scheduleGcIdler() {
        if (!mGcIdlerScheduled) {
            mGcIdlerScheduled = true;
            Looper.myQueue().addIdleHandler(mGcIdler);
        }
        mH.removeMessages(H.GC_WHEN_IDLE);
    }
    
 final class GcIdler implements MessageQueue.IdleHandler {
        @Override
        public final boolean queueIdle() {
            doGcIfNeeded();
            return false;
        }
    }

在Instrumentation 类里边。

    public void waitForIdle(Runnable recipient) {
        mMessageQueue.addIdleHandler(new Idler(recipient));
        mThread.getHandler().post(new EmptyRunnable());
    }

   private static final class Idler implements MessageQueue.IdleHandler {
        private final Runnable mCallback;
        private boolean mIdle;

        public Idler(Runnable callback) {
            mCallback = callback;
            mIdle = false;
        }

        public final boolean queueIdle() {
            if (mCallback != null) {
                mCallback.run();
            }
            synchronized (this) {
                mIdle = true;
                notifyAll();
            }
            return false;
        }

        public void waitForIdle() {
            synchronized (this) {
                while (!mIdle) {
                    try {
                        wait();
                    } catch (InterruptedException e) {
                    }
                }
            }
        }
    }

3、IdleHandler的源码实现

当我们调用 Looper.myQueue().addIdleHandler(new IdleHandler()）的时候，会将当前的idle加入IdleHandler[] mPendingIdleHandlers，当消息队列没有可执行消息时，进入阻塞，就有回调 idler.queueIdle()

  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 (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        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(TAG, "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;
        }
    }

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