本质：一个线程开启循环模式持续监听并依次处理其他线程给它发的消息
成员介绍
1.Handler
消息的发送者和处理者
消息发送给Looper持有的MessageQueue
消息处理发生在handler所在线程
2.Looper
消息循环器，主要将MessageQueue中的message依次出队
并将其分发给target也就是handler
3.MessageQueue
存放消息message的队列
message可以存放硬件和软件产生的消息
机制运行
1.创建Handler关联Looper,Looper关联MessageQueue
2.handler发送消息给MessageQueue 入队 ,looper.loop循环取出message
3.looper分发给message对应的handler处理消息
原理解析
三者的关联
在使用的时候我们常在全局中
private Handler handler=new Handler(){
@Override
public void handleMessage(Message msg) {//重写 自己处理消息
super.handleMessage(msg);
}
};
我们从构造方法入手看
public Handler(Callback callback, boolean async) {
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Looper.prepare()是创建当前线程的looper的函数
从这里我们可以发现handler是无法在没有Looper的线程执行的
而app进程的主入口ActivityThread的main函数中我们可以发现
public static void main(String[] args) {
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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));
}
主线程中已经创建了looper开始循环消息队列并设置了共享变量
我们在主线程创建Handler那么这个handler对象
会持有主线程的Looper和此Looper循环的消息队列
就可以进行message的分发和处理了
模拟一次操作
handler.sendEmptyMessage()//发送空消息 会创建一个空消息的message对象
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;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
boolean enqueueMessage(Message msg, long when) {
synchronized (this) {
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
MessageQueue是以链队的形式组织message
handler发出的消息会入队进入
再来看Looper.loop();
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
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();
}
可以看出来是一个无限循环 将消息队列出队一个message对象
核心代码msg.target.dispatchMessage(msg);
这里的msg.target就是message对于->messagequeue对应的->Looper对应的handler
Handler target;
接着看dispatchMessage
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
分发消息有三种情况
1.如果message有回调 先处理回调 再执行我们重写的处理方法
2.message无回调但构造handler时有传入回调 那就只执行传入的回调
3.message无回调无handler构造传入回调 只执行我们重写的处理方法
End!
总结
图解消息机制
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