Android 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<HandlerAction> 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。
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