Multithreaded programming in Android (ii) The principle of handler (attached source)

The principle of handler in Android

I. Principle of handler:

The relationship between 1.Handler, Looper and MessageQueue.

(1). Handler class: sends a message to the MessageQueue message queue, receives the message returned by the Looper, and processes it.

(2). Looper class: a container that stores message queues. Responsible for receiving handler sent messages, and directly to the message back to handler himself.

(3). MessageQueue class: stores messages.

2. Relationship:

(1). When you create a handler object, it is bound to the default thread (the UI thread), and there is a message queue in the thread.

privatenew Handler() {    publicvoidhandleMessage(android.os.Message msg) {        "主线程 Thread UI:" + Thread.currentThread().getId());        mView.setText("111");    

(2). If you are opening a new child thread, when you manipulate handler in that thread, create a Looper object yourself to find the appropriate message queue. In other words, handle and looper are interrelated.

 PackageCom.chengdong.su.handlerdemo;Importandroid.app.Activity;ImportAndroid.app.ActionBar;ImportAndroid.app.Fragment;ImportAndroid.os.Bundle;ImportAndroid.os.Handler;ImportAndroid.os.Looper;ImportAndroid.os.Message;ImportAndroid.util.Log;ImportAndroid.util.SparseArray;ImportAndroid.view.LayoutInflater;ImportAndroid.view.Menu;ImportAndroid.view.MenuItem;ImportAndroid.view.View;ImportAndroid.view.ViewGroup;ImportAndroid.widget.ImageView;ImportAndroid.widget.TextView;ImportAndroid.widget.Toast;ImportAndroid.os.Build;/** * * @author SCD * */ Public  class thridactivity extends Activity {    PrivateString TAG = GetClass (). Getsimplename ();PrivateTextView MView;PrivateMyThread Mthread;/** * Create a default handler * /    PrivateHandler Mhandler =NewHandler () { Public void Handlemessage(Android.os.Message msg) {LOG.D (TAG,"-------UI Thread:"+ Thread.CurrentThread (). GetId ()); Mview.settext ("Hello");    }; };@Override    protected void onCreate(Bundle savedinstancestate) {Super. OnCreate (Savedinstancestate);        Setcontentview (R.layout.activity_main);        Init (); Mthread =NewMyThread (); Mthread.start ();Try{Mthread.sleep ( -); }Catch(Interruptedexception e)        {E.printstacktrace (); }//Sub-threadsMThread.myHandler.sendEmptyMessage (0);//Main threadMhandler.sendemptymessage (1); }Private void Init() {MView = (TextView) Findviewbyid (R.ID.TEXTVIEW1); }/** * Custom Thread * * @author SCD * */     Public  class MyThread extends Thread {         PublicHandler MyHandler;@Override         Public void Run() {looper.prepare (); MyHandler =NewHandler () {@Override                 Public void Handlemessage(Message msg) {Super. Handlemessage (msg); LOG.D (TAG,"------>thread:"+ Thread.CurrentThread (). GetId ()); System.out.println ("---child thread----"); }            };//Loop processing messagesLooper.loop (); }    }}

However, using the above method will lead to multi-threaded concurrency caused by the null pointer problem, in order to avoid this problem, Google has encapsulated a very useful class: Handlerthread class.

3.HandlerThread class:

(1). Resolves null pointer problems caused by multithreading concurrency.
The use code for Handlerthread is as follows:

 PackageCom.chengdong.su.handlerdemo;Importandroid.app.Activity;ImportAndroid.os.Bundle;ImportAndroid.os.Handler;ImportAndroid.os.HandlerThread;ImportAndroid.os.Message;ImportAndroid.util.Log;/** * * @author SCD * */ Public  class fouractivity extends Activity {    PrivateString TAG = GetClass (). Getsimplename ();PrivateString Mname ="Handlerthread";PrivateHandlerthread Mthread;PrivateHandler Mhandler;@Override    protected void onCreate(Bundle savedinstancestate) {Super. OnCreate (Savedinstancestate);//Package classMthread =NewHandlerthread (Mname);//Open threadMthread.start ();//Way One: handler is on a child threadMhandler =NewHandler (Mthread.getlooper ()) {@Override             Public void Handlemessage(Message msg) {Super. Handlemessage (msg); LOG.D (TAG,"---child thread----handlerthread:"+ Thread.CurrentThread (). GetId ()); }        };//Mode two: handler is in the main threadMhandler =NewHandler () {@Override             Public void Handlemessage(Message msg) {Super. Handlemessage (msg); LOG.D (TAG,"---main thread UI----handlerthread:"+ Thread.CurrentThread (). GetId ());        }        }; Mhandler.sendemptymessage (0); LOG.D (TAG,"---main thread----uithread:"+ Thread.CurrentThread (). GetId ()); }}

Note: The Mhandler in mode one is run in a sub-thread, which can handle time-consuming asynchronous tasks, send messages, process messages, and so on in the handler.

(2). Communication between the main thread and child threads in Android:

A: Strand Cheng sent to the main thread message: Let the main thread update the UI interface
B: The main line Cheng to the child thread message: Let the child thread do the corresponding logical processing.
The code is as follows:

 PackageCom.chengdong.su.handlerdemo;Importandroid.app.Activity;ImportAndroid.os.Bundle;ImportAndroid.os.Handler;ImportAndroid.os.HandlerThread;ImportAndroid.os.Message;ImportAndroid.util.Log;ImportAndroid.view.View;ImportAndroid.view.View.OnClickListener;ImportAndroid.widget.Button;/** * Send messages to each other between the main thread and the child threads for communication * * @author SCD * */ Public  class fiveactivity extends Activity  implements  Onclicklistener {    PrivateString TAG = GetClass (). Getsimplename ();PrivateString Mname ="Handlerthread";//Initialize the handler of the main thread    PrivateHandler Mhandler =NewHandler () { Public void Handlemessage(Message msg) {Message message =NewMessage ();//Mainline Cheng to child thread messagesmthreadhandler.sendmessagedelayed (Message, +); LOG.D (TAG,"--------> Mainline Cheng to Child thread messages");    }; };//Sub-threading Object    PrivateHandlerthread Mthread;//Sub-thread handler    PrivateHandler Mthreadhandler;PrivateButton MButton1;PrivateButton MButton2;@Override    protected void onCreate(Bundle savedinstancestate) {Super. OnCreate (Savedinstancestate);        Setcontentview (R.layout.activity_main);    Init (); }/** * Init the View * *    Private void Init() {MButton1 = (Button) Findviewbyid (R.id.button1);        MButton2 = (Button) Findviewbyid (R.id.button2); Mbutton1.setonclicklistener ( This); Mbutton2.setonclicklistener ( This);//Initialize the handler of the child threadMthread =NewHandlerthread (Mname); Mthread.start ();//Initialize the handler in the child threadMthreadhandler =NewHandler (Mthread.getlooper ()) {@Override             Public void Handlemessage(Message msg) {Message message =NewMessage (); mhandler.sendmessagedelayed (Message, +); LOG.D (TAG,"--------> Strand Cheng to the main thread message");        }        }; LOG.D (TAG,"------->init main thread"); }@Override     Public void OnClick(View v) {Switch(V.getid ()) { CaseR.id.button1: {//main thread send messageMhandler.sendemptymessage (0); Break; } CaseR.id.button2: {//Cancel messageMhandler.removemessages (0); Break; }default: Break; }    }}

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Multithreaded programming in Android (ii) The principle of handler (attached source)