Handler是Android给我们提供用于更新UI的一套机制，一套消息处理机制。一般处理程序在UI线程中执行耗时操作，这会导致UI线程阻塞，当UI线程阻塞，屏幕会出现卡死，用户体验会变得非常差，当线程阻塞超过5s，Android系统可能进行干预，弹出对话框询问是否关闭。我们处理方式是创建一个新的线程来实现耗时操作，采用handle机制来实现子线程发送message通知主线程去改变UI组件。另外如果在线程中更新UI，会报“Only the original thread that created a view hierarchy can touch its views.” Handler消息机制由Handler/Looper/MessageQueue/Message等组成。 Handler框架代码目录： framework/base/core/java/andorid/os/ - Handler.java - Looper.java - Message.java - MessageQueue.java 当一个应用程序运行时，它会创建一个进程。这个进程就是主线程（UI线程&Activity Thread），会运行如下代码： Frameworks\base\core\java\android\app - ActivityThread.java public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper(); //注 1
ActivityThread thread = new ActivityThread(); thread.attach(false);
if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); }
if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); }
// End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); //注 2
throw new RuntimeException("Main thread loop unexpectedly exited"); } 其中注1 的具体实现如下：public static void prepareMainLooper() { prepare(false); //注3 synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } } 进一步看注3： 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)); } 从prepare（）中，确保主线程中有且唯一一个Looper对象，并保存到线程的线程本地存储区（Thread Local Storage，简称为TLS）。Looper构造函数中，创建了MessageQueue对象并在mThread中记录当前线程。 private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); } 再来看注2 Looper.loop()的实现: public static void loop() { final Looper me = myLooper(); // 注 从线程的本地存储区TLS获取当前线程的Looper对象（Looper.prepare()创建存储）。 if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; // 注 获取 Looper对象中的消息队列。
// 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 //注 提取消息队列中待处理message。 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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); final long end; try { msg.target.dispatchMessage(msg); //分发处理message。 end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (slowDispatchThresholdMs > 0) { final long time = end - start; if (time > slowDispatchThresholdMs) { Slog.w(TAG, "Dispatch took " + time + "ms on " + Thread.currentThread().getName() + ", h=" + msg.target + " cb=" + msg.callback + " msg=" + msg.what); } }
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(); } } Looper.loop()核心就是获取Looper对象的消息队列，从消息队列中取出队列头的message，并分发处理。 至此，Hander机制的Looper，MessageQueue，Message都已经出现了，还差主角Handler。 我看看一个app例子： 下面例子在子线程中发message，主线程处理message更新UI显示。 private TextView mTextView; private Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { Person person = (Person)msg.obj; mTextView.setText("name:" + person.getName()+" age:"+person.getAge()); } }; private int index = 0;
@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mTextView = findViewById(R.id.tv_text); new Thread() { @Override public void run() { try { Thread.sleep(2000); Message message = handler.obtainMessage(); Person person = new Person(); person.setName("张三"); person.setAge(18); message.obj = person; mHandler.sendMessage(message); } catch (InterruptedException e) { e.printStackTrace(); } } }.start(); } 创建了一个mHandler对象， 并重写了handleMessage（）函数。 同时在子线程中封装创建message对象，并通过mHandler对象的sendMessage（）放到Looper的MessageQueue队列中。 具体看handler的实现： 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(); //获取当前线程，也就是UI主线中的looper对象。 if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; //获取消息队列。 mCallback = callback; mAsynchronous = async; }- 再来看子线程中的操作 Message message = handler.obtainMessage(); Person person = new Person(); person.setName("张三"); person.setAge(18); message.obj = person; mHandler.sendMessage(message); Handler的sendMessage会依次调用： 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; //把handler对象赋予msg.target，消息分发的时候通过这个handler对象dispatchMessage处理。 if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); } 最终调用MessageQueue的enqueueMessage()把消息添加到MessageQueue队列中。 总结一下： APP初始化时，创建了私有成员Handler对象。其将自动与主线程的Looper对象绑定。具体ActivityThread初始化了Looper，运行在主线程，这部分对应用开发者来说透明的。 然后子线程中通过mHandler.sendMessage发送消息到队列，sendMessage会依次调用到enqueueMessage，在enqueueMessage中，msg.target会赋予handler对象，Looper队列轮询到后，执行msg.target.dispatchMessage分发到主线程的handler.handleMessage()处理。
我们知道使用Handler来更新UI，常用的方法除了上面APP的sendMessage方法，另外一种Post方法。看例子：private TextView tv_up; private String new_str = ""; /*post方法解决UI更新问题handler创建方式*/ private Handler handler_post = new Handler(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);
new Thread(new Runnable() {
@Override public void run() { new_str = "更新UI"; /*post方法解决UI更新，直接在runnable里面完成更新操作，这个任务会被添加到handler所在线程的消息队列中，即主线程的消息队列中*/ handler_post.post(new Runnable() { @Override public void run() { tv_up.setText(new_str); } }); } }).start(); }
Handler的post代码： public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); } private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; //把Runnable对象赋予Message的callback。接下来还是和sendMessage一致的操作。 return m; } 回过头来，再来看Looper.loop()的消息分发 msg.target.dispatchMessage(msg)。 具体调用handler的dispatchMessage()。 public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } 对msg.callback的判断，正是两种更新UI方式的消息处理的差异。 sendMessage方式通过handleMessage处理，post方式通过handleCallback()处理。 private static void handleCallback(Message message) { message.callback.run(); } 总结： Handler的post和sendMessage本质上是没有区别的，只是实际用法中有一点差别。 从之前例子，我们看到，都没有在APP中运行Looper.prepare()， 这是由于Activity的UI主线程默认是有消息队列的。所以在Activity中新建Handler时，不需要先调用Looper.prepare()。默认情况下一个线程是不存在消息循环（message loop）的，需要调用Looper.prepare()来给线程创建一个消息循环，调用Looper.loop()来使消息循环起作用。看个例子，很简单，不做进一步分析： private void initThead() { new Thread(new Runnable() { public void run() { Looper.prepare();//启用Looper。 handler1 = new Handler(){ @Override public void handleMessage(Message msg) { super.handleMessage(msg); } }; handler1.sendEmptyMessage( 5 ) ; Looper.loop();// Looper开始工作，从消息队列里取消息，处理消息，让消息处理在该线程中完成。 } }).start(); }
