Handler是Android中消息傳遞機制，可以將工作線程所創(chuàng)建的消息傳遞到主線程中去處理懦傍，進行UI更新或者其他需要在主線程進行處理的工作÷樱總的來說粗俱，就是線程之間的通訊。

Handler原理解析

了解Handler原理之前虚吟，我們需要先了解幾個相關(guān)的類

• Hander (主要作用發(fā)送和處理消息和Runnable)
• Message (線程間消息傳遞的載體)
• MessageQueue(消息隊列寸认，存放消息)
• Looper (循環(huán)讀取消息隊列，發(fā)送到指定Handler處理)

Handler的基本使用

//在主線程創(chuàng)建一個Handler對象
//并重載handleMessage方法
Handler mHandler = new Handler(){
    @Override
    public void handleMessage(Message){
        //需要在mHandler的線程處理的業(yè)務(wù)邏輯
        //可以根據(jù)Message的參數(shù)處理相應(yīng)業(yè)務(wù)串慰，例如UI更新
        textView.setText("通過sendMessage更新UI")
    }
}

//然后在子線程中發(fā)送消息
//通過sendMessage方法發(fā)送
new Thread(new Runnable(){
    @Override
    public void run(){
        //主線的Handler
        Message message = new Message();
        message.what = 1;
        message.obj = "可以傳遞對象偏塞，這里是傳遞字符串對象";
        //通過sendMessage發(fā)送消息對象
        mHandler.sendMessage(message)
    }
}).start();

//或者可以傳遞一個Runnable對象
new Thread(new Runnable(){
    @Override
    public void run(){
        mHandler.post(new Runnable(){
            @Override
            public void run(){
                //需要在mHandler的線程處理的業(yè)務(wù)邏輯
                //如更新UI
                textView.setText("通過post更新UI")
            }
        })
    }
})

除了上面2種方法，Handler發(fā)送消息很多種

//發(fā)送Runnable
post(Runnable)
postAtTime(Runnable,long)
postAtTime(Runnable,Object,long)
postDelayed(Runnable,long)
postDelayed(Runnable,Object,long)
//發(fā)送Message
sendMessage(Message)
sendMessageDelayed(Message,long)
sendMessageAtTime(Message,long)
sendMessageAtFrontOfQueue(Message)
sendEmptyMessage(int)
sendEmptyMessageDelayed(int,long)
sendEmptyMessageAtTime(int,long)

//我們先分析比較簡單的sendMessage和post
public final boolean sendMessage(Message msg){
    return sendMessageDelayed(msg, 0);
}
//調(diào)用成員方法sendMessageDelayed邦鲫，延時為0
public final boolean sendMessageDelayed(Message msg, long delayMillis){
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    //所以插入消息隊列的時候就是（當前時間+0）
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
//uptimeMellis就是消息傳遞到Handler的時間
public boolean sendMessageAtTime(Message msg, long uptimeMillis){
    MessageQueue queue = mQueue;
    //省略部分代碼...
    //最終通過enqueueMessage把消息插入到消息隊列
    return enqueueMessage(queue, msg, uptimeMillis);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    //把當前Handler對象引用保存在Message.target灸叼，后續(xù)通過這個target來傳遞到指定Handler去處理消息
    msg.target = this;
    //設(shè)置Message是否異步Message
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    //插入到消息隊列
    return queue.enqueueMessage(msg, uptimeMillis);
}

//MessageQueue.java#enqueueMessage
boolean enqueueMessage(Message msg, long when) {
    //省略部分代碼...
    synchronized (this) {
        //省略部分代碼...
        msg.markInUse();//標記Message為使用中
        msg.when = when;//執(zhí)行時間
        Message p = mMessages;//消息隊列鏈頭部
        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;
            //遍歷消息隊列神汹，尋找插入Message的位置
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            //插入到消息隊列
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }

        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            //喚醒
            nativeWake(mPtr);
        }
    }
    return true;
}
//sendMessage基本流程就如上所示
//sendMessage -> sendMessageDelayed -> sendMessageAtTime -> enqueueMessage

//接著看post流程
public final boolean post(Runnable r){
   return  sendMessageDelayed(getPostMessage(r), 0);
}
//調(diào)用getPostMessage把Runnable封裝為Message
private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}
//最后流程還是sendMessage流程一致

通過上面的代碼，了解了Handler如何發(fā)送消息的方法古今，那么線程之間消息是如何傳遞的呢屁魏？

為什么在其他線程通過Hander發(fā)送消息后，會到Handler線程進行處理捉腥？
首先先看創(chuàng)建Hander實例時候處理

Handler mHandler = new Handler()//創(chuàng)建了Handler實例

//Handler.java
public Handler() {
    this(null, false);
}
    
public Handler(Callback callback, boolean async) {
    ...
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException("Can't create handler inside thread "
        + Thread.currentThread()
        + " that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    ...
}
//Handler構(gòu)造方法主要工作：
//獲取Looper對象
//從Looper對象獲取mQueue(消息隊列)

Looper對象獲取

//Looper.java

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
public static @Nullable Looper myLooper() {
    //通過ThreadLocal.get獲取Looper對象
    return sThreadLocal.get();
}

ThreadLoal變量氓拼，線程局部變量，簡單理解為線程私有變量抵碟，這里就不展開描述桃漾；

myLooper方法中通過sThreadLocal.get()獲取Looper，那Looper是在什么時候set進去的拟逮？通過sThreadLocal.set關(guān)鍵字搜索呈队，在Looper的prepare方法內(nèi)找到

//一般線程初始化Looper
public static void prepare() {
    prepare(true);
}

//主線程初始化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();
    }
}

//真正初始化Looper方法
private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    //創(chuàng)建Looper對象，并將set到ThreadLocal
    sThreadLocal.set(new Looper(quitAllowed));
}

//Looper構(gòu)造方法
private Looper(boolean quitAllowed) {
    //創(chuàng)建MessageQueue
    mQueue = new MessageQueue(quitAllowed);
    //記錄線程ID
    mThread = Thread.currentThread();
}

所以要創(chuàng)建Handler對象唱歧，必定要先調(diào)用Looper對象的prepare或者prepareMainLooper方法，否則在Handler構(gòu)造方法就會跑出異常粒竖，因為Looper.myLooper返回的對象為null

public Handler(Callback callback, boolean async) {
    ···
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        //獲取到Looper對象為空颅崩，拋出異常
        //報錯說明未調(diào)用Looper.prepare()
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    ···
}

那Looper.prepare方法在什么時候調(diào)用？了解過Android源碼的可能知道蕊苗，在ActivityThread的main方法中沿后，調(diào)用了Looper.prepareMainLooper，而且當前線程也就是我們所說的主線程

ActivityThread.java

public static void main(String[] args) {
    ···
    //初始化Looper對象
    Looper.prepareMainLooper();
    ···
    //開始循環(huán)讀取消息
    Looper.loop();
    ···
}

既然我們要循環(huán)從消息隊列獲取消息進行發(fā)送朽砰，那就可能到開啟循環(huán)尖滚，Looper.loop方法的作用就是循環(huán)從消息隊列讀取消息并且發(fā)送。通過源碼看看Looper.loop方法是如何循環(huán)讀取消息

Looper.java
public static void loop() {
    //獲取當前線程的Looper對象
    final Looper me = myLooper();
    ···
    //獲取Looper的消息隊列
    final MessageQueue queue = me.mQueue;
    ···
    //開始循環(huán)獲取消息
    for (;;) {
        //獲取消息隊列瞧柔，可能會阻塞
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }
        ···
        try {
            //分發(fā)到指定Handler去處理消息
            //msg.target就是前面sendMessage中設(shè)置的Handler引用
            msg.target.dispatchMessage(msg);
            ···
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        ···
        //回收Message對象
        msg.recycleUnchecked();
    }
}

知道了Message如何分發(fā)到指定Handler漆弄，跟蹤dispatchMessage看看最后如何傳遞到Hander的handleMessage方法

//Handler.java#dispatchMessage
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        //判斷Runnable對象存在
        handleCallback(msg);
    } else {
        //可以在構(gòu)造方法傳入Callback，作用和handlerMessage一樣
        //一般都直接重寫handleMessage方法
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        //最終調(diào)用handlerMessage造锅，把消息傳遞到我們實例話的Handler對象撼唾，一般我們都會重寫handleMessage方法
        handleMessage(msg);
    }
}

總結(jié)

Handler消息傳遞機制使用之前，需要先初始化Looper綁定當前線程哥蔚，再循環(huán)從消息隊列獲取消息分發(fā)到指定的Handler去處理倒谷，以上只分析了Handerl機制的一般用法，還有其他如異步消息糙箍，IdleHandlder的用法還未涉及渤愁，后續(xù)逐步進行分析