记早年的一次面试：
面试官：说说你对Handler的了解？
我：new 一个handler，然后在某个线程里发送message，在handler的handleMessage里接收message，message不要new 用Message.obtain()获取池子里的，来避免新建。
面试官：就这么多了么？
我：。。。。。。。。。。。。。。。。。
以前我对handler的理解很薄，在开发多年，深入了解多年后，细细的了解了handler
一、Looper
looper是关联message与handler的重要部分。
你在Android的子线程中new handler为啥报错，和looper关系不浅，据如下个例子，在子线程中调用handler
new Thread(new Runnable() {
public void run() {
Looper.prepare();
Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show();
}
};
handler.sendEmptyMessage(1);
Looper.loop();
};
}).start();或者在子线程直接使用Looper.getMainLooper()；
new Thread(new Runnable() {
public void run() {
Handler handler = new Handler(Looper.getMainLooper()){
@Override
public void handleMessage(Message msg) {
Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show();
}
};
handler.sendEmptyMessage(1);
};
}).start();Looper.getMainLooper()实际就是主线程的looper，主线程默认都会创建looper
创建looper时会创建messagequeue
注：每一个线程只能有一个looper，looper是线程持有
Looper实际上是给当前子线程建立的looper，为了能够遍历当前子线程的消息队列，
源码
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 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);
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();
}
}二、MessageQueue
MessageQueue是一个消息队列，用于存放message，looper来把其中的message发送到handlermessage
采用先进先出的方式来管理message，内部采用单链表来进行维护，插入删除比较快
三、Message
就是咱最常用的消息了，按照我上面的说法建议使用Message.obtain()来获取Message实例，应为message有个pool
源码
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
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