从源码详细分析Handler 原理

终于对handler 下手了😂

先来看看源代码，要看Handler源码还是要从构造方法入手。

  @Deprecated
    public Handler(@Nullable Callback callback) {
        this(callback, false);
    }

    /**
     * Use the provided {@link Looper} instead of the default one.
     *
     * @param looper The looper, must not be null.
     */
    public Handler(@NonNull Looper looper) {
        this(looper, null, false);
    }

    /**
     * Use the provided {@link Looper} instead of the default one and take a callback
     * interface in which to handle messages.
     *
     * @param looper The looper, must not be null.
     * @param callback The callback interface in which to handle messages, or null.
     */
    public Handler(@NonNull Looper looper, @Nullable Callback callback) {
        this(looper, callback, false);
    }

默认无参的构造方法已经过时，所以从有参数的构造方法入手，一共三个，这里涉及到一个类

Looper所以简单看下Looper是个什么

Looper 类的源码还是比较简单的，主要就是ThreadLocal,MeesageQueue

其中 ThreadLocal 是静态的。可以理解为一种数据结构。他本身并不存储对象

而是通过set方法吧对象于当前线程关联起来

从下面两个方法可以看t出主要用于Thread对象创建ThreadLocalMap.而这个Thread LocalMap 的

作用就是把当前对象和T 对象用map 关联起来。在Looper 类中调用set 方法就是把 当前线程和Looper 类关联起来。

private static Looper sMainLooper;  // guarded by Looper.class
private static Observer sObserver;

都提供了静态的set方法来供外部调用

  public void set(T value) {
        Thread t = Thread.currentThread();//当前线程
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

MessageQueue 更为简单 ，是用来管理Message的。Message 是个消息队列，是链式的结构。

所以整个Looper类可以理解为构造了一个MessageQueue，并将当前这个包含了MesageQueue的Looper通过ThreadLocal关联到创建Looper 的线程。

另外Looper类中还有一个静态loop()方法，从源码来看这个方法是分发消息的，但这个方法并没有被Handler类使用。而是提供给HandlerThread 等其他类来使用的。（所以很多网上说的Hander 是使用这个方法来从MessageQueue 中取出消息分发是不对的）。

这里可以参看这篇 https://blog.youkuaiyun.com/u011578734/article/details/106241075 这个作者详细的从native层解析了Handler的流程。作者提到是用nativeWake方法来实现消息循环的唤醒操作 

/**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        if (me.mInLoop) {
            Slog.w(TAG, "Loop again would have the queued messages be executed"
                    + " before this one completed.");
        }

        me.mInLoop = true;
        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();

        // Allow overriding a threshold with a system prop. e.g.
        // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
        final int thresholdOverride =
                SystemProperties.getInt("log.looper."
                        + Process.myUid() + "."
                        + Thread.currentThread().getName()
                        + ".slow", 0);

        boolean slowDeliveryDetected = false;

        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);
            }
            // Make sure the observer won't change while processing a transaction.
            final Observer observer = sObserver;

            final long traceTag = me.mTraceTag;
            long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
            long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
            if (thresholdOverride > 0) {
                slowDispatchThresholdMs = thresholdOverride;
                slowDeliveryThresholdMs = thresholdOverride;
            }
            final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
            final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

            final boolean needStartTime = logSlowDelivery || logSlowDispatch;
            final boolean needEndTime = logSlowDispatch;

            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }

            final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
            final long dispatchEnd;
            Object token = null;
            if (observer != null) {
                token = observer.messageDispatchStarting();
            }
            long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
            try {
                msg.target.dispatchMessage(msg);
                if (observer != null) {
                    observer.messageDispatched(token, msg);
                }
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } catch (Exception exception) {
                if (observer != null) {
                    observer.dispatchingThrewException(token, msg, exception);
                }
                throw exception;
            } finally {
                ThreadLocalWorkSource.restore(origWorkSource);
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (logSlowDelivery) {
                if (slowDeliveryDetected) {
                    if ((dispatchStart - msg.when) <= 10) {
                        Slog.w(TAG, "Drained");
                        slowDeliveryDetected = false;
                    }
                } else {
                    if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                            msg)) {
                        // Once we write a slow delivery log, suppress until the queue drains.
                        slowDeliveryDetected = true;
                    }
                }
            }
            if (logSlowDispatch) {
                showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
            }

            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();
        }
    }

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

个然后看一下Handler做了什么事情。

从Handler 最常用 send Message方法入手.sendMessage方法调用了 enqueueMessage

而enqueueMessage调用了MessageQueue类的enqueueMessage。所以handler 无论是post 方法还是直接send 最终都是调用enqueueMessage。

最终都是执行的native 方法，只不过异步的方法会初始化message的callback属性。

不过native是如何调用callback的不得而知了。getPostMessge方法初始化了Message的callback 

  public final boolean post(@NonNull Runnable r) {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }
    

    private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

 public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }


  public boolean sendMessageAtTime(@NonNull 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(@NonNull MessageQueue queue, @NonNull Message msg,
            long uptimeMillis) {
        msg.target = this;//把当前Handler传给Message
        msg.workSourceUid = ThreadLocalWorkSource.getUid();

        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

看一下这个MessageQueue类的enqueueMessage 。就是判断消息的target不为空，不是异步的，就把当Message对象放到队列的末尾，并调用一个native方法nativeWake。 那么target指向的对象就是调用sendMessage方法的Handler 对 象。也就是Handler对应的Looper的MessageQueue的每一条Message的target都是这个Handler

盲猜一下这个nativeWake方法。应该就是调用Handler 的Callback并且把Message传递给call back了。这里为猜测是一次执行一条循环的取出。

 boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }

        synchronized (this) {
            if (msg.isInUse()) {
                throw new IllegalStateException(msg + " This message is already in use.");
            }

            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w(TAG, e.getMessage(), e);
                msg.recycle();
                return false;
            }

            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;
    }

那么如果现在就要自己实现一个类似于handler 的东西。当然就是用于整理思路，并不能真正意义的实现线程的通讯，因为回调方法仍然是在子线程被调用的。虽然能拿到主线程的对象，但是如果要在主线程执行代码，还是要调用runOnUiThread本质上还是使用的Activity 默认创建的Handler。而真正用于线程通讯的方法是native 的。实际的通讯是使用ThreadLocalWorkSource.getUid();方法返回当前执行sendMessage的线程的标识。而回调执行的线程应该是通过ThreadLocal关联到Looper。由于Handler保存了Looper 并且Message的target 是handler。所以能保证回调到创建Handler 的线程（至于怎么回调的，由于是native方法看不到源码）

那么最后就是根据我的理解，写的demo。源码放上来，看下我理解的大致的流程。

（这里后续看了loop方法后有重新模拟了一个demo在另一篇文章，修改的流程是增加了在创建handler的线程中死循环取出消息，简单的用while循环模拟了一下）

class MyHandler (looper: MyLooper,callback: ((MyMessage) -> Unit)?){
    var mLooper:MyLooper
    var mQueue:MyMessageQueue
    var callBack: ((MyMessage) -> Unit)? =callback
    init {
        mLooper=looper
        mQueue=looper.messageQueue
    }
    fun sendMessage(message: MyMessage){
        message.mHandler=this
       mQueue.enqueueMessage(message)
    }
}

class MyLooper {
    var mThread:Thread
    companion object{
        var threadLocal =ThreadLocal<MyLooper>()
        fun myLooper():MyLooper{
            return threadLocal.get()
        }
    }
    var messageQueue: MyMessageQueue
    constructor(){
        Log.e("threadLooper",Thread.currentThread().name)
        threadLocal.set(this)
        mThread= Thread.currentThread()
        messageQueue=MyMessageQueue(true)
    }

}
class MyMessage {
    var what:Int=-1
    var next:MyMessage?=null
    var mHandler: MyHandler?=null
    var workSourceUid:String?=null
}


class MyMessageQueue(isAllowQuit: Boolean) {
    var isAllowQuit:Boolean=isAllowQuit

    private var mMessage: MyMessage?=null
    fun enqueueMessage(message: MyMessage){
        var isNeekWake=false
        if(mMessage==null){
            isNeekWake=true
            mMessage=message
        }else {
            message.mHandler?.let {
                var temp: MyMessage = mMessage!!
                while (true) {
                    if (temp.next == null){
                        temp.next=message
                        break
                    }else{
                        temp = temp.next!!
                    }
                }
            }
        }
        if(isNeekWake)
        {
            wake()
        }

    }
    fun quitAll(){
        if(isAllowQuit){
            while(true){
                var temp:MyMessage?=mMessage
                if(temp!=null){
                      mMessage=temp.next
                      temp=null
                }else{
                    break
                }
            }
            mMessage=null
        }
    }
    fun wake(){
        mMessage?.mHandler?.let {
            handler->
            handler.callBack?.let {
                    callback->
                mMessage?.let {
                        message->
                    Log.e("thread",handler.mLooper.mThread.name)
                    handler.mLooper.mThread//创建handler的线程对象
                    callback（message）
                    message.next?.let{
                        mMessage=it
                        wake()
                    }
                }
            }
        }
    }
}

调用的部分，并不能真正的在UI 线程执行，只能回调UI 线程的代码 

class MainModel(binding: ActivityMainBinding, context: Activity) :
    MyBaseModel<ActivityMainBinding>(binding, context) {
    var handler:MyHandler?=null
    init {
        bindListener(binding.tvHello)
            handler= MyHandler(MyLooper(),{
                    myMessage ->
                Log.e("threada",Thread.currentThread().name)//子线程
//
                //这里要执行在主线程还是需要runOnUiThread(Runnable{})
            })

    }
    override fun onClick(v: View?) {
        v?.let {
            when(v.id){
                R.id.tv_hello->{
                    Thread(Runnable {
                        var message= MyMessage()
                        message.what=33
                        handler?.sendMessage(message)
                    }).start()
                }
                else->{}

            }
        }
    }
}