参考：
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeEx ception("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();
}
}
原理：我们只需要计算打印这两天 log 的时间差，就能得到 dispatchMessage 的耗时，android
提供了 Looper.getMainLooper().setMessageLogging(Printer printer)来设置这个 logging 对
象，所以只要自定义一个 Printer，然后重写 println(String x)方法即可实现耗时统计了。
通过在 DispatchMessage 的方法的执行时间，来判断卡顿，管是哪种回调方式，回调一定发
生在 UI 线程。因此如果应用发生卡顿，一定是在 dispatchMessage 中执行了耗时操作。我
们通过给主线程的 Looper 设置一个 Printer，打点统计 dispatchMessage 方法执行的时间，
如果超出阀值，表示发生卡顿，则 dump 出各种信息，提供开发者分析性能瓶颈。
mBlockListener.onBlockEvent
https://blog.csdn.net/lhd201006/article/details/79044497