Android(安卓)子线程修改UI方法对比

Android 子线程修改UI方法对比

在Android 开发中经常是在子线程中获取数据或者处理数据，然后修改UI显示，修改UI一般有四种方法：
1.Handler()的handleMessage()和handler.sendMessage(msg)
2.handler.post(runnable)和handler.postDelayed(runnable,milliseconds)
3.activity.runOnUiThread();
4.View.post(runnable)和View.postDelayed(runnable,milliseconds);

原理

1.Handler()的handleMessage()和handler.sendMessage(msg)

使用时创建Handler对象：

private Handler handler = new Handler(){        @Override        public void handleMessage(Message msg) {            super.handleMessage(msg);        }    };

在子线程中handler发送message

new Thread(){            @Override            public void run() {                super.run();                Message message = Message.obtain();                handler.sendMessage(message);            }        }.start();

sendMessage()方法会最终会调用Handler.enqueueMessage()方法把Message放入MessageQueue中；

public final boolean sendMessage(Message msg)    {        return sendMessageDelayed(msg, 0);    }public final boolean sendMessageDelayed(Message msg, long delayMillis)    {        if (delayMillis < 0) {            delayMillis = 0;        }        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }public boolean sendMessageAtTime(Message msg, long uptimeMillis) {        MessageQueue queue = mQueue;        if (queue == null) {            RuntimeException e = new RuntimeException(                    this + " sendMessageAtTime() called with no mQueue");            Log.w("Looper", e.getMessage(), e);            return false;        }        return enqueueMessage(queue, msg, uptimeMillis);    }private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {        msg.target = this;        if (mAsynchronous) {            msg.setAsynchronous(true);        }        return queue.enqueueMessage(msg, uptimeMillis);    }    boolean enqueueMessage(Message msg, long when) {            ........            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;            }        return true;    }

在Looper循环中通过Looper的next方法取出Message，调用msg.target.dispatchMessage(msg);将message分发出去；

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

msg.target是Handler的一个实例，dispatchMessage()是Handler的方法；

首先判断msg.callback不为null时，执行handleCallback(msg)；

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

callback是一个Runnable对象，当通过handler.post方法发送一个runnable的时候就会被封装到这个msg中，
handleCallback(msg)方法直接调用的是Runnable的run方法

private static void handleCallback(Message message) {        message.callback.run();    }

mCallback是一个Callback接口对象，构造Handler时可以传递一个Callback 对象进来，实现 handleMessage()方法，当handleMessage()返回true时就不在执行Handler的handleMessage()方法

public interface Callback {        public boolean handleMessage(Message msg);    }

private Handler mHandler = new Handler(new Handler.Callback() {      @Override      public boolean handleMessage(Message msg) {          return false;      }  });

以上就是调用Handler的sendMessage()方法的执行流程；

2.handler.post(runnable)和handler.postDelayed(runnable,milliseconds)

handler.post()和handler.postDelayed()方法都会调用sendMessageDelayed()方法；
最后还是通过Handler的enqueueMessage() 将Message放入MessageQuene中；
getPostMessage()方法会新建一个Message将Runnable 传递给Message的callback 最后执行的时候回判断callback 不为null时执行Runnable 的run方法

public final boolean post(Runnable r)    {       return  sendMessageDelayed(getPostMessage(r), 0);    }private static Message getPostMessage(Runnable r) {        Message m = Message.obtain();        m.callback = r;        return m;    }

3.activity.runOnUiThread();
获取当前线程是否是主线程，如果是主线程就直接执行Runnable 的run方法；不是主线程就通过Handler post(Runnable )形式将message放入MessageQuene中，最后通过Handler的dispatchMessage方法执行Runnable 的run方法

    public final void runOnUiThread(Runnable action) {        if (Thread.currentThread() != mUiThread) {            mHandler.post(action);        } else {            action.run();        }    }

4.View.post(runnable)和View.postDelayed(runnable,milliseconds);

这种方式会先判断一下 attachInfo 是否为null；如果不为null，会调用attachInfo.mHandler.post(runnable);AttachInfo 里的mHandler是ViewRootHandler对象，ViewRootHandler是Handler的子类；最后还是通过handler来调用runnable的run方法；如果attachInfo 为null就调用getRunQueue().post(action);

public boolean post(Runnable action) {        final AttachInfo attachInfo = mAttachInfo;        if (attachInfo != null) {            return attachInfo.mHandler.post(action);        }        // Postpone the runnable until we know on which thread it needs to run.        // Assume that the runnable will be successfully placed after attach.        getRunQueue().post(action);        return true;    }public boolean postDelayed(Runnable action, long delayMillis) {        final AttachInfo attachInfo = mAttachInfo;        if (attachInfo != null) {            return attachInfo.mHandler.postDelayed(action, delayMillis);        }        // Postpone the runnable until we know on which thread it needs to run.        // Assume that the runnable will be successfully placed after attach.        getRunQueue().postDelayed(action, delayMillis);        return true;    }

AttachInfo 是什么东西？
AttachInfo 看到这个类名，我们就知道，他是代表着绑定的信息，View.AttachInfo 里面的信息，就是View和Window之间的信息。每一个被添加到窗口上的View我们都会看到有一个AttachInfo，AttachInfo 会通过View的diapatchAttachedTowWindow分发给View。如果是一个ViewGroup 那么这个这个AttachInfo也会以引用的方式分发给所有子View。

void dispatchAttachedToWindow(AttachInfo info, int visibility) {        mAttachInfo = info;        if (mOverlay != null) {            mOverlay.getOverlayView().dispatchAttachedToWindow(info, visibility);        }        .....    }

attachInfo == null 说明view还没有显示到window上，如果View还未attach到window，将runnable放入ViewRootImpl的RunQueue中。
那么post到RunQueue里的runnable什么时候执行呢，又是为何当View还没attach到window的时候，需要post到RunQueue中。

View#post与Handler#post的区别

其实，当View已经attach到了window，两者是没有区别的，都是调用UI线程的Handler发送runnable到MessageQueue，最后都是由handler进行消息的分发处理。

但是如果View尚未attach到window的话，runnable被放到了ViewRootImpl#RunQueue中，最终也会被处理，但不是通过MessageQueue。
ViewRootImpl#RunQueue源码注释如下：

/** * The run queue is used to enqueue pending work from Views when no Handler is * attached.  The work is executed during the next call to performTraversals on * the thread. * @hide */

大概意思是当视图树尚未attach到window的时候，整个视图树是没有Handler的（其实自己可以new，这里指的handler是AttachInfo里的），这时候用RunQueue来实现延迟执行runnable任务，并且runnable最终不会被加入到MessageQueue里，也不会被Looper执行，而是等到ViewRootImpl的下一个performTraversals时候，把RunQueue里的所有runnable都拿出来并执行，接着清空RunQueue。

由此可见RunQueue的作用类似于MessageQueue，只不过，这里面的所有
runnable最后的执行时机，是在下一个performTraversals到来的时候，MessageQueue里的消息处理的则是下一次loop到来的时候。
ViewRootImpl#performTraversals：

private void performTraversals() {    // ....    // Execute enqueued actions on every traversal in case a detached view enqueued an action    getRunQueue().executeActions(mAttachInfo.mHandler);    // ....}void executeActions(Handler handler) {     synchronized (mActions) {     final ArrayList actions = mActions;     final int count = actions.size();     for (int i = 0; i < count; i++) {         final HandlerAction handlerAction = actions.get(i);         handler.postDelayed(handlerAction.action, handlerAction.delay);      }        actions.clear();   }}

也就是说，当View没有被attach到window的时候，最后runnable的处理不是通过MessageQueue，而是ViewRootImpl自己在下一个performTraversals到来的时候执行。最后还是handler.postDelayed(handlerAction.action, handlerAction.delay);执行run()方法。

Android 子线程修改UI方法对比

原理

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