时间：2022-04-10 10:32:06 | 栏目：Android代码 | 点击：次
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！