[Android进阶】Handler机制原理解析_android handler的机制和原理

3.1 Handler是如何关联消息队列和线程的？
我们首先看Handler默认的构造函数：

 public Handler(Callback callback, boolean async) {
    //代码省略
    mLooper = Looper.myLooper();//获取Looper
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
        mQueue = mLooper.mQueue;//通过Looper对象获取消息队列
        mCallback = callback;
        mAsynchronous = async;
    }
    
    //获取Looper对象
    public final Looper getLooper() {
        return mLooper;
     } 

从Handler的构造函数中我们可以发现，Handler在初始化的同时会通过Looper.getLooper()获取一个Looper对象，并与Looper进行关联，然后通过Looper对象获取消息队列。那我们继续深入到Looper源码中去看Looper.getLooper()是如何实现的。

 //初始化当前线程Looper
    public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
        }
     }

    //为当前线程设置一个Looper
    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));
     } 

从上面的程序可以看出通过prepareMainLooper()，然后调用 prepare(boolean quitAllowed)方法创建了一个Looper对象，并通过sThreadLocal.set(new Looper(quitAllowed))方法将该对象设置给了sThreadLocal。

 //通过ThreadLocal获取Looper
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
     } 

通过Looper中的预备工作，sThreadLocal中已经存储了一个Looper对象，然后myLoop