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Catalog

One 、 brief introduction

1、Handler How to implement ？

2、 Send and process messages

Two 、 Practical cases

1、Handler Object sends a message to the newly started child thread

2、 Create in main thread Handler、 And get the processing message

3、demo Demonstration effect

3、 ... and 、 To sum up

1、Handler Basic usage

2、Handler Common problems that can be solved ？

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Series articles

Handler Asynchronous messaging mechanism （ One ）Handler Common basic usage

Handler Asynchronous messaging mechanism （ Two ） Create... In a child thread Handler

Handler Asynchronous messaging mechanism （ 3、 ... and ） In the main thread 、 Create... In a child thread Handler, Source code （Android 9.0） analysis

Handler Asynchronous messaging mechanism （ Four ）Handler Send message flow , Source code （Android 9.0） analysis

One 、 brief introduction

Android The message passing mechanism of is mainly to solve Android Multithreading of applications , and Android The platform does not allow Activity The newly started child thread accesses this Activity The interface components inside , This will make the newly started thread unable to dynamically change the property value of the interface component .

Such as ：Android In actual development , Especially when it comes to game development, new threads are often required , Periodically access and change the attribute values of interface components . So how to solve it ？ We can use Handler、AsyncTask Relevant mechanisms to solve .

This article is mainly about Handler, of AsyncTask For reference ：Android Special topic AsyncTask（ One ） Introduction to basic concepts

1、Handler How to implement ？

  （1） Create in main thread Handler, Get through callback 、 Process the message

  （2） Send a message in a newly started thread    

 

2、 Send and process messages

In order to make the main thread better handle the messages sent by the newly started thread , Obviously, only through The way of callback To achieve .

Developers just need to rewrite Handler Class to process messages , When a newly started child thread sends a message ,Handler Class is automatically called back .

Handler Class contains the following methods to send 、 Process the message ：

Two 、 Practical cases

Here I'll take the previous project “app Download updates and install them automatically ” The code has been changed , Because it's mainly a demonstration Handler Usage of , So the other steps are presented in the form of simulation or output ！

1、Handler Object sends a message to the newly started child thread

Simulation download app, And send a message to the newly started child thread

public class DownLoadAppFile {
    public void download(String urlPath, Handler handler, ProgressBar pb) {

        try {
            // download apk Code for , Here, thread sleep is used to simulate 
            Thread.currentThread().sleep(3*1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        Message msg = Message.obtain();
        msg.what =1;// success 
        //msg.what =2;// Failure 
        handler.sendMessage(msg);//( Sub thread ) Send a message 

    }
}

2、 Create in main thread Handler、 And get the processing message

public class MainActivity extends AppCompatActivity {
    private  TextView show_text;
    private String strMsg;

    // Get... In the main thread 、 Process the message 
    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case 1:// Download successful 
                    strMsg = strMsg +"\n"+ "2、apk Download successful ... Start automatic installation of downloaded apk！";
                    show_text.setText(strMsg);
                    Toast.makeText(MainActivity.this,"apk Download successful ！",Toast.LENGTH_SHORT).show();
                    break;
                case 2:// Download failed 
                    strMsg = strMsg +"\n"+ "apk Download failed ！";
                    show_text.setText(strMsg);
                    Toast.makeText(MainActivity.this,"apk Download failed ！",Toast.LENGTH_SHORT).show();
                    break;

                default:
                    break;
            }
        }
    };


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        show_text = (TextView) findViewById(R.id.show_text);

        downloadApkFile();

    }


    /**
     *   download apk file 
     *   notes ：Android 6.0 The above need to apply for read-write permission 
     */
    private void downloadApkFile() {

        strMsg = "1、 Start the download apk...";
        show_text.setText(strMsg);
        Toast.makeText(MainActivity.this," Start the download apk...",Toast.LENGTH_SHORT).show();

        new Thread() {// In the newly started child thread , Call Download app Code for , And send messages 、 Feedback results 
            public void run() {

                DownLoadAppFile downLoadFile = new DownLoadAppFile();
                downLoadFile.download(null, handler, null);
            };
        }.start();
    }
}

3、demo Demonstration effect

demo Address ：https://download.csdn.net/download/yudbl/10894028

The effect is as follows ：

3、 ... and 、 To sum up

1、Handler Basic usage

（1） Create in main thread Handler, Get through callback 、 Process the message

（2） Send a message in a newly started thread

2、Handler Common problems that can be solved ？

problem ：

The newly started thread cannot dynamically change the attribute value of the interface component

reason ：

Android The message passing mechanism of is mainly to solve Android Multithreading of applications ,

and Android The platform does not allow Activity The newly started child thread accesses this Activity The interface components inside .

Examples of common usage scenarios ：

（1）app Upgrade download installation , Update the progress bar in real time

（2） Game development often requires newly started threads , Periodically access and change the attribute values of interface components