Android Handler 原理分析及实例代码

Android Handler 原理分析

Handler一个让无数android开发者头疼的东西，希望我今天这边文章能为您彻底根治这个问题

今天就为大家详细剖析下Handler的原理

Handler使用的原因

1.多线程更新Ui会导致UI界面错乱
2.如果加锁会导致性能下降
3.只在主线程去更新UI,轮询处理

Handler使用简介

其实关键方法就2个一个sendMessage，用来接收消息

另一个是handleMessage，用来处理接收到的消息

下面是我参考疯狂android讲义，写的一个子线程和主线程之间相互通信的demo

对原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发送消息处理消息（一般都是将消息发送给自己），因为hanler在不同线程是可使用的

2.Looper管理MessageQueue

Looper.loop死循环，不断从MessageQueue取消息，如果有消息就处理消息，没有消息就阻塞

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();
    }
  }

这个是Looper.loop的源码，实质就是一个死循环，不断读取自己的MessQueue的消息

3.MessQueue一个消息队列，Handler发送的消息会添加到与自己内联的Looper的MessQueue中，受Looper管理

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

这个是Looper构造器，其中做了2个工作，

1.生成与自己关联的Message

2.绑定到当前线程

主线程在初始化的时候已经生成Looper，

其他线程如果想使用handler需要通过Looper.prepare()生成一个自己线程绑定的looper

这就是Looper.prepare()源码，其实质也是使用构造器生成一个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));
  }

4.handler发送消息会将消息保存在自己相关联的Looper的MessageQueue中,那它是如何找到这个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;
  }

这个是Handler的构造方法，它会找到一个自己关联的一个Looper

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

没错，他们之间也是通过线程关联的，得到Looper之后自然就可以获得它的MessageQueue了

5.我们再看下handler如发送消息，又是如何在发送完消息后，回调HandlerMessage的

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

这个就是Handler发送消息的最终源码，可见就是将一个message添加到MessageQueue中，那为什么发送完消息又能及时回调handleMessage方法呢

大家请看上边那个loop方法，其中的for循环里面有一句话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);
    }
  }

这就是这句话，看到了吧里面会调用hanlerMessage,一切都联系起来了吧

感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！