Android线程间通信Handler源码详解

//主线程
private val mHandler: Handler = object : Handler(Looper.getMainLooper()) {
    override fun handleMessage(msg: Message) {
        when (msg.what) {
            1 -> {}
        }
    }
}

//子线程
Thread {
    Looper.prepare()
    val handler = object : Handler() {
        override fun handleMessage(msg: Message) {
            when (msg.what) {
                1 -> {}
            }
        }
    }
    handler.sendEmptyMessage(1)
    Looper.loop()
}.start()

//方法 1
//Handler.java
public Handler(@Nullable Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Class<? extends Handler> klass = getClass();
        if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                (klass.getModifiers() & Modifier.STATIC) == 0) {
            Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                klass.getCanonicalName());
        }
    }
    mLooper = Looper.myLooper(); \\标识 1
    if (mLooper == null) { \\ 标识 3
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    MQueue = mLooper.mQueue; \\标识 2
    mCallback = callback;
    mAsynchronous = async;
}
//方法 2
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

//跟进标识 1
//Looper.java
public static @Nullable Looper myLooper() {
   return sThreadLocal.get();
}

//Looper.java
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));
}

//ActivityThread.java
public static void main(String[] args) {
    //...省略无关代码
    Looper.prepareMainLooper(); \\标识 4
   //...
    ActivityThread thread = new ActivityThread();
    thread.attach(false, startSeq);
    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }
  //...
    Looper.loop();
}

//跟进标识 4
//Looper.java
public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}