我们都知道Android使用消息机制进行UI更新，UI线程也就是主线程里有个Looper，在其loop()方法中会不断取出message，调用其绑定的Handler在主线程执行。如果在handler的dispatchMesaage方法里有耗时操作，就会发生卡顿。
我们来看下Looper.loop()的源码
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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.isTagEnabled(traceTag)) {
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();
}
}
只要检测 msg.target.dispatchMessage(msg) 的执行时间，就能检测到主线程是否有耗时操作。注意到这行执行代码的前后，有两个logging.println函数，如果设置了mLogging，会分别打印出”>>>>> Dispatching to “和”<<<<< Finished to “这样的日志，这样我们就可以通过两次log的时间差值，来计算dispatchMessage的执行时间，从而设置阈值判断是否发生了卡顿。
如何给Looper设置mLogging？
在源码中定义了私有的mLogging，但是提供了赋值方法。
private Printer mLogging;
public void setMessageLogging(@Nullable Printer printer) {
mLogging = printer;
}
我们可以自己实现一个Printer并赋值。
Looper.getMainLooper().setMessageLogging(new Printer() {
private static final String START = ">>>>> Dispatching";
private static final String END = "<<<<< Finished";
@Override
public void println(String x) {
if (x.startsWith(START)) {
//从这里开启一个定时任务来打印方法的堆栈信息
}
if (x.startsWith(END)) {
//从这里取消定时任务
}
}
});
我们设定一个阈值为1000ms，当匹配到>>>>> Dispatching时，开启定时任务，会在1000ms 后执行任务，这个任务负责打印UI线程的堆栈信息。如果消息低于1000ms内执行完成，就可以匹配到<<<<< Finished日志，那么在打印堆栈任务启动前执行取消了这个任务，则认为没有卡顿的发生；如果消息超过1000ms才执行完毕，此时认为发生了卡顿，并打印UI线程的堆栈信息。
看下定时任务的代码实现
public class LooperLog {
private static LooperLog sInstance = new LooperLog();
private HandlerThread mLogThread = new HandlerThread("log");
private Handler mIoHandler;
private static final long TIME_BLOCK = 1000L;
private LooperLog() {
mLogThread.start();
mIoHandler = new Handler(mLogThread.getLooper());
}
private static Runnable mLogRunnable = new Runnable() {
@Override
public void run() {
StringBuilder sb = new StringBuilder();
StackTraceElement[] stackTrace = Looper.getMainLooper().getThread().getStackTrace();
for (StackTraceElement s : stackTrace) {
sb.append(s.toString() + "\n");
}
Log.i("LogPrinter--", sb.toString());
}
};
public static LooperLog getInstance() {
return sInstance;
}
public void startPrintLog() {
mIoHandler.postDelayed(mLogRunnable, TIME_BLOCK);
}
public void canclePrintLog() {
mIoHandler.removeCallbacks(mLogRunnable);
}
}
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，只不过比普通的Thread多了一个Looper，对外提供自己这个Looper对象的getLooper方法，然后创建Handler时将HandlerThread中的looper对象传入。这样我们的mIoHandler对象就是与HandlerThread这个非UI线程绑定的了，它处理耗时操作将不会阻塞UI。如果UI线程阻塞超过1000ms，就会在子线程中执行mLogRunnable，打印出UI线程当前的堆栈信息，如果处理消息没有超过1000ms，则会实时的remove掉这个mLogRunnable任务。
发生卡顿时打印出堆栈信息的大致内容如下，开发可以通过log定位耗时的地方。
LogPrinter--: java.lang.Thread.sleep(Native Method)
java.lang.Thread.sleep(Thread.java:1031)
java.lang.Thread.sleep(Thread.java:985)
com.koolearn.android.CommonPperationImpl.startActivityAfterLogin(CommonPperationImpl.java:124)
com.koolearn.android.home.MainActivity.onClick(MainActivity.java:490)
android.view.View.performClick(View.java:4811)
android.view.View$PerformClick.run(View.java:20136)
android.os.Handler.handleCallback(Handler.java:815)
android.os.Handler.dispatchMessage(Handler.java:104)
android.os.Looper.loop(Looper.java:194)
android.app.ActivityThread.main(ActivityThread.java:5546)
java.lang.reflect.Method.invoke(Native Method)
java.lang.reflect.Method.invoke(Method.java:372)
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:964)
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759)
