Android Handler 原理分析
Handler一個(gè)讓無數(shù)android開發(fā)者頭疼的東西，希望我今天這邊文章能為您徹底根治這個(gè)問題
今天就為大家詳細(xì)剖析下Handler的原理
Handler使用的原因
1.多線程更新Ui會(huì)導(dǎo)致UI界面錯(cuò)亂
2.如果加鎖會(huì)導(dǎo)致性能下降
3.只在主線程去更新UI,輪詢處理
Handler使用簡(jiǎn)介
其實(shí)關(guān)鍵方法就2個(gè)一個(gè)sendMessage，用來接收消息
另一個(gè)是handleMessage，用來處理接收到的消息
下面是我參考瘋狂android講義，寫的一個(gè)子線程和主線程之間相互通信的demo
對(duì)原demo做了一定修改
public class MainActivity extends AppCompatActivity { public final static String UPPER_NUM="upper_num"; private EditText editText; public jisuanThread jisuan; public Handler mainhandler; private TextView textView; class jisuanThread extends Thread{ public Handler mhandler; @Override public void run() { Looper.prepare(); final ArrayList<Integer> al=new ArrayList<>(); mhandler=new Handler(){ @Override public void handleMessage(Message msg) { if(msg.what==0x123){ Bundle bundle=msg.getData(); int up=bundle.getInt(UPPER_NUM); outer: for(int i=3;i<=up;i++){ for(int j=2;j<=Math.sqrt(i);j++){ if(i%j==0){ continue outer; } } al.add(i); } Message message=new Message(); message.what=0x124; Bundle bundle1=new Bundle(); bundle1.putIntegerArrayList("Result",al); message.setData(bundle1); mainhandler.sendMessage(message); } } }; Looper.loop(); } } @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); editText= (EditText) findViewById(R.id.et_num); textView= (TextView) findViewById(R.id.tv_show); jisuan=new jisuanThread(); jisuan.start(); mainhandler=new Handler(){ @Override public void handleMessage(Message msg) { if(msg.what==0x124){ Bundle bundle=new Bundle(); bundle=msg.getData(); ArrayList<Integer> al=bundle.getIntegerArrayList("Result"); textView.setText(al.toString()); } } }; findViewById(R.id.bt_jisuan).setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Message message=new Message(); message.what=0x123; Bundle bundle=new Bundle(); bundle.putInt(UPPER_NUM, Integer.parseInt(editText.getText().toString())); message.setData(bundle); jisuan.mhandler.sendMessage(message); } }); } } Hanler和Looper，MessageQueue原理分析
1.Handler發(fā)送消息處理消息（一般都是將消息發(fā)送給自己），因?yàn)閔anler在不同線程是可使用的
2.Looper管理MessageQueue
Looper.loop死循環(huán)，不斷從MessageQueue取消息，如果有消息就處理消息，沒有消息就阻塞
public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } 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(); 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 Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(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(); } } 這個(gè)是Looper.loop的源碼，實(shí)質(zhì)就是一個(gè)死循環(huán)，不斷讀取自己的MessQueue的消息
3.MessQueue一個(gè)消息隊(duì)列，Handler發(fā)送的消息會(huì)添加到與自己內(nèi)聯(lián)的Looper的MessQueue中，受Looper管理
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); } 這個(gè)是Looper構(gòu)造器，其中做了2個(gè)工作，
1.生成與自己關(guān)聯(lián)的Message
2.綁定到當(dāng)前線程
主線程在初始化的時(shí)候已經(jīng)生成Looper，
其他線程如果想使用handler需要通過Looper.prepare()生成一個(gè)自己線程綁定的looper
這就是Looper.prepare()源碼，其實(shí)質(zhì)也是使用構(gòu)造器生成一個(gè)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)); } 4.handler發(fā)送消息會(huì)將消息保存在自己相關(guān)聯(lián)的Looper的MessageQueue中,那它是如何找到這個(gè)MessageQueue的呢
public Handler(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(); 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; } 這個(gè)是Handler的構(gòu)造方法，它會(huì)找到一個(gè)自己關(guān)聯(lián)的一個(gè)Looper
public static Looper myLooper() { return sThreadLocal.get(); } 沒錯(cuò)，他們之間也是通過線程關(guān)聯(lián)的，得到Looper之后自然就可以獲得它的MessageQueue了
5.我們?cè)倏聪耯andler如發(fā)送消息，又是如何在發(fā)送完消息后，回調(diào)HandlerMessage的
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); } 這個(gè)就是Handler發(fā)送消息的最終源碼，可見就是將一個(gè)message添加到MessageQueue中，那為什么發(fā)送完消息又能及時(shí)回調(diào)handleMessage方法呢
大家請(qǐng)看上邊那個(gè)loop方法，其中的for循環(huán)里面有一句話msg.target.dispatchMessage(msg);
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } 這就是這句話，看到了吧里面會(huì)調(diào)用hanlerMessage,一切都聯(lián)系起來了吧
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