Android Handler 原理分析及實例代碼

Android Handler 原理分析

Handler一個讓無數android開發者頭疼的東西，希望我今天這邊文章能為您徹底根治這個問題

今天就為大家詳細剖析下Handler的原理

Handler使用的原因

1.多線程更新Ui會導致UI界面錯亂
2.如果加鎖會導致性能下降
3.只在主線程去更新UI,輪詢處理

Handler使用簡介

其實關鍵方法就2個一個sendMessage，用來接收消息

另一個是handleMessage，用來處理接收到的消息

下面是我參考瘋狂android講義，寫的一個子線程和主線程之間相互通信的demo

對原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發送消息處理消息（一般都是將消息發送給自己），因為hanler在不同線程是可使用的

2.Looper管理MessageQueue

Looper.loop死循環，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(); 
    } 
  } 

這個是Looper.loop的源碼，實質就是一個死循環，不斷讀取自己的MessQueue的消息

3.MessQueue一個消息隊列，Handler發送的消息會添加到與自己內聯的Looper的MessQueue中，受Looper管理

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

這個是Looper構造器，其中做了2個工作，

1.生成與自己關聯的Message

2.綁定到當前線程

主線程在初始化的時候已經生成Looper，

其他線程如果想使用handler需要通過Looper.prepare()生成一個自己線程綁定的looper

這就是Looper.prepare()源碼，其實質也是使用構造器生成一個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發送消息會將消息保存在自己相關聯的Looper的MessageQueue中,那它是如何找到這個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; 
  } 

這個是Handler的構造方法，它會找到一個自己關聯的一個Looper

public static Looper myLooper() { 
    return sThreadLocal.get(); 
  } 

沒錯，他們之間也是通過線程關聯的，得到Looper之後自然就可以獲得它的MessageQueue了

5.我們再看下handler如發送消息，又是如何在發送完消息後，回調HandlerMessage的

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { 
    msg.target = this; 
    if (mAsynchronous) { 
      msg.setAsynchronous(true); 
    } 
    return queue.enqueueMessage(msg, uptimeMillis); 
  } 

這個就是Handler發送消息的最終源碼，可見就是將一個message添加到MessageQueue中，那為什麼發送完消息又能及時回調handleMessage方法呢

大家請看上邊那個loop方法，其中的for循環裡面有一句話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，看到了吧裡面會調用hanlerMessage,一切都聯系起來了吧

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