Android开发——监控造成UI卡顿的原因

0. 前言

Android只有主线程才能更新UI。如果界面1秒钟刷新少于60次，即FPS小于60，用户就会产生卡顿感觉。

Android使用消息机制进行UI更新的，如果在主线程handler的dispatchMessage方法进行了耗时操作，就会发生UI卡顿。

本文原创，转载请注明出处：Android开发——监控造成UI卡顿的原因_SEU_Calvin的博客-CSDN博客

1. dispatchMessage方法在哪

dispatchMessage()是在Looper.loop()里调用，源码如下：

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 blockif (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 loggerPrinter logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}msg.target.dispatchMessage(msg);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();}
}

所以说，第27行的代码就是可能发生UI卡顿的地方。注意这行代码的前后，有两个logging。也就是说在执行第27代码的前后，如果设置了logging，会分别打印出“>>>>> Dispatching to”和“<<<<< Finished to”这样的Log。这样就给我们监视两次Log之间的时间差，来判断是否发生了卡顿。

2. 设置logging

主要看一下21行的mLogging是什么，源码如下所示：

public final class Looper {private Printer mLogging;public void setMessageLogging(@Nullable Printer printer) {mLogging = printer;}
}
public interface Printer {void println(String x);
}

Looper的mLogging是私有的，并且提供了setMessageLogging(@Nullable Printer printer)方法，所以我们可以自己实现一个Printer，在通过setMessageLogging()方法传入即可。

public class AppContext extends Application {@Overridepublic void onCreate() {super.onCreate();Looper.getMainLooper().setMessageLogging(new Printer() {private static final String START = ">>>>> Dispatching";private static final String END = "<<<<< Finished";@Overridepublic void println(String x) {if (x.startsWith(START)) {LogMonitor.getInstance().startMonitor();}if (x.startsWith(END)) {LogMonitor.getInstance().removeMonitor();}}});}
}

当我们设置了mLogging之后，loop()方法中就会回调logging.println，并将带有“>>>>> Dispatching to”和“<<<<< Finished to”的字符串传入，我们就可以拿到这两条信息。

如果“>>>>> Dispatching to”信号发生了，我们就假定发生了卡顿（这里我们设定1秒钟的卡顿判定阈值），并且发送一个延迟1秒钟的任务，这个任务就用于在子线程打印出造成卡顿的UI线程里的堆栈信息。而如果没有卡顿，即在1秒钟之内我们检测到了“<<<<< Finished to”信号，就会移除这个延迟1秒的任务。

3. LogMonitor的实现

public class LogMonitor {private static LogMonitor sInstance = new LogMonitor();private HandlerThread mHandlerThread = new HandlerThread("log");private Handler mHandler;private LogMonitor() {mHandlerThread.start();mHandler = new Handler(mHandlerThread.getLooper());}private static Runnable mRunnable = new Runnable() {@Overridepublic void run() {StringBuilder sb = new StringBuilder();StackTraceElement[] stackTrace = Looper.getMainLooper().getThread().getStackTrace();for (StackTraceElement s : stackTrace) {sb.append(s.toString() + "\n");}Log.e("TAG", sb.toString());}};public static LogMonitor getInstance() {return sInstance;}public void startMonitor() {mHandler.postDelayed(mRunnable, 1000);}public void removeMonitor() {mHandler.removeCallbacks(mRunnable);}}

这里我们使用HandlerThread来构造一个Handler，HandlerThread继承自Thread，实际上就一个Thread，只不过它比普通的Thread多了一个Looper，对外提供自己这个Looper对象的get方法，然后创建Handler时将HandlerThread中的looper对象传入。这样我们的mHandler对象就是与HandlerThread这个非UI线程绑定的了，这样它处理耗时操作将不会阻塞UI。