在Android中解决子线程更新UI的三种方法：
-
Activity中 调用 runOnUiThread(Runnable action) {}
源码解析：/** * Runs the specified action on the UI thread. If the current thread is the UI * thread, then the action is executed immediately. If the current thread is * not the UI thread, the action is posted to the event queue of the UI thread. * * @param action the action to run on the UI thread */ public final void runOnUiThread(Runnable action) { if (Thread.currentThread() != mUiThread) { mHandler.post(action); } else { action.run(); } } -
View 调用 View.post(Runnable action) {}
源码解析：/** * <p>Causes the Runnable to be added to the message queue. * The runnable will be run on the user interface thread.</p> * * @param action The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. * * @see #postDelayed * @see #removeCallbacks */ 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; } -
传统的用法： handler.post()
源码解析：/** * Causes the Runnable r to be added to the message queue. * The runnable will be run on the thread to which this handler is * attached. * * @param r The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); }这三种方法最后都殊途同归的调用了 handler.post(Runnable r). 而 handler.post 内部却调用的是 sendMessageDelayed(Message m , 0);
所以最终的结论引入了 Handler 消息传播机制。
Handler.post(Runnbale r) 的调用
首先查看 post 源码
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
调用到 sendMessageDelayed 方法
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
延时发送消息，其实就是定时发送消息 sendMessageAtTime
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);
}
最后执行到 MessageQueue 轮循消息
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
引发的一些问题：
- Handler Looper Message MessageQueue 怎么发生联系？
- Looper是什么？
- Looper 和 MessageQueue 的关系？
- Handler 会发生内存泄露？为什么？
在handler创建的时候，handler 和 Looper 产生联系： handler 的构造函数如下：
public Handler() { this(null, false); }
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
在handler创建的时候，和handler所在线程的 looper 产生了关联。在主线程中，Activity在创建的时候就帮我们创建了 looper 和 handler ，所以在子线程中更新ui的前两种方法可以行得通。第三种更新方式，实际原理和前两种一样，只不过是手动在主线程中创建了handler.
注意： 因此在子线程中创建 handler 时候会报错，只有开启了 looper 才可以创建 handler
Looper相关知识点
Looper类用来为一个线程开启一个消息循环
默认情况下android中新诞生的线程是没有开启消息循环的。（主线程除外，主线程系统会自动为其创建Looper对象，开启消息循环。）
Looper对象通过MessageQueue来存放消息和事件。一个线程只能有一个Looper，对应一个MessageQueue。通常是通过Handler对象来与 looper 进行交互的。
Handler可看做是Looper的一个接口，用来向指定的Looper发送消息及定义处理方法。
默认情况下Handler会与其被定义时所在线程的Looper绑定，比如，Handler在主线程中定义，那么它是与主线程的Looper绑定。
mainHandler = new Handler() 等价于new Handler（Looper.myLooper()）.
Looper.myLooper()：获取当前进程的looper对象，类似的 Looper.getMainLooper() 用于获取主线程的Looper对象。在非主线程中直接new Handler() 会报如下的错误:
E/AndroidRuntime( 6173): Uncaught handler: thread Thread-8 exiting due to uncaught exception
E/AndroidRuntime( 6173): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
原因是非主线程中默认没有创建Looper对象，需要先调用Looper.prepare()启用Looper。-
Looper.loop(); 让Looper开始工作，从消息队列里取消息，处理消息。
注意：写在Looper.loop()之后的代码不会被执行，这个函数内部应该是一个循环，当调用mHandler.getLooper().quit()后，loop才会中止，其后的代码才能得以运行。
通过以上知识，可实现主线程给子线程（非主线程）发送消息。
Looper的使用过程以及和 Messagequeue 关系的建立：
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper的调用方法是 Looper.prepare() -- new handler（）-- looper.loop();
轮询器在循环执行 Messagequeue 中的消息时候调用的：
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();
}
}
当执行到其中的一条message时候， 会回调携带 message 的handler 的方法 dispatchMessage
msg.target.dispatchMessage(msg);
而在 Handler的 dispatchMessage(msg) 中又执行了 callback.handleMessage(msg),
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
在callback的handleMessage(msg)中又接着执行了 handler 的hanleMessage(msg) 方法；
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
因此在创建 handler 的时候，一定要复写其中的 handleMessage 方法.
总结
首先Looper.prepare() 在本线程创建并保存一个实例，然后该Loop实例创建并维护一个 MessageQueue 对象，因为一个线程中 Looper.prepare()只能执行一次，因此 MessageQueue 只会存在一个。
Looper.loop() 会让当前线程进入一个无限循环，不断从消息队列中读取消息，并处理，此时会回调 msg.target.dispatchMessage(msg)
handler 的构造方法会得到当前线程中保存的 looper 实例。
Handler 的 sendMessage() 方法会给msg的 target 赋值为 handler 本身，然后加入到消息队列中。
在构造 handler 时候，我们会重写 handlemessage() 方法，也是 msg.target.dispatchMessage(msg)最终调用的方法。
三者的关系如下面这个丑图：
网上这个美图更漂亮一些：
补充
- 为什么主线程中的Looper.loop() 一直无限循环不会造成 ANR?
答： ANR 的原因就是有需要处理的事情未处理，或者在规定时间内未处理！
因为Android是由事件驱动的，looper.loop()不断地接收事件、处理事件，每一个点击、触摸等等都在主线程looper.loop()的控制之下，如果它停止了，应用也就停止了。只能说对消息的处理阻塞了主线程的looper.loop() 才会导致 ANR, looper.loop()会阻塞自己的运行？？？
- ANR的时间是多少？按键或者触摸是5s，广播是10s。官网的定义：
https://developer.android.com/training/articles/perf-anr.html
