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 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 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

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