消息机制——handler

android消息机制——handler

为啥要有消息机制？

android有个主线程，activityThread，可以操作UI，其他线程不能更新UI。APP开发过程中经常遇到耗时操作，如文件读写，网络请求等。如果把这些耗时请求都放在主线程上面，会发生ANR(Application Not Responding)。针对这种情况，一般的解决措施都是另开个线程去执行这些耗时操作，当这些耗时操作执行完，需要更新UI，咋办？这个时候消息机制就可以大显身手，发送message，告诉主线程，去更新UI。

ANR?

三种类型：

• KeyDispatchTimeout(5 seconds) ——按键或触摸事件在特定时间内无响应
• BroadcastTimeout(10 seconds) ——BroadcastReceiver在特定时间内无法处理完成
• ServiceTimeout(20 seconds) ——Service在特定的时间内无法处理完成

两种原因：

• 当前事件没有及时完成
• 当前事件迟迟没有被处理

Handler如何成为通讯员？

ActivityThread被创建时会创建looper,lopper的构造方法里面创建MessageQueue。

• Handler的send与post都会sendMessage，执行enQueueMessage方法，向MessageQueue中插入一条message。

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
}

• looper会不断轮询QueueMessage的next方法。

public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // This must be in a local variable, in case a UI event sets the logger
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }

            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

            msg.recycleUnchecked();
        }
    }

• 发现message，就会执行handler中的dispathMessage,dispathMessage被调用后，调用handlerMessage方法。

try {
    msg.target.dispatchMessage(msg);
} finally {
    if (traceTag != 0) {
        Trace.traceEnd(traceTag);
    }
}

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

随便提下

当 MessageQueue中没有message且没有空闲的handler可以处理的时候，通过nativePollOnce将循环阻塞与此。enqueueMessage后，再重新通过nativeWake唤醒。