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！
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