安卓Handler使用執行個體（含源代碼

第一個是主線程處理兩子線程發送的消息

第二個是點選按鈕下載下傳csdn頭像并用imageView控件顯示

安卓端：

控制台：

第一個的源代碼

public class MainActivity extends AppCompatActivity {

    private Handler handler = new Handler(){
        @Override
        public void handleMessage(@NonNull Message msg) {
            switch(msg.what){
                case 1:
                    System.out.println("處理thread1:的消息"+msg.arg1);
                    break;
                case 2:
                    System.out.println("處理thread2:的消息"+msg.arg1);
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        thread1.start();
        thread2.start();
    }
    Thread thread1 = new Thread(new Runnable(){

        @Override
        public void run() {
            Message msg = handler.obtainMessage();
            msg.what = 1;//線程标記
            msg.arg1 = 6001;//線程參數
            handler.sendMessage(msg);//發送消息
        }
    });
    Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            Message msg = handler.obtainMessage();
            msg.what = 2;
            msg.arg1 = 6002;
            handler.sendMessage(msg);
        }
    });
}
           

第一個的源代碼

AndroidManifest.xml

activity_main.xml

<Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="點選下載下傳"
        tools:ignore="MissingConstraints"
        tools:layout_editor_absoluteX="84dp"
        tools:layout_editor_absoluteY="128dp" />

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="156dp"
        android:layout_marginTop="275dp"
        android:padding="2dp"
        android:scaleType="centerInside"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:srcCompat="@tools:sample/avatars" />
           

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private Button button;
    private ImageView imageView;
    private static final String imageUrl = "https://avatar.csdnimg.cn/8/4/A/2_qq_41083450_1604883740.jpg";

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

        System.out.println("正在下載下傳...");
        imageView = (ImageView) this.findViewById(R.id.imageView);
        button = (Button) this.findViewById(R.id.button);
        //1、給按鈕控件添加事件監聽器
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //2、點選按鈕後建立子線程并執行
                new Thread(){
                    @Override
                    public void run() {
                        Bitmap bitmap = loadImgFromUrl(imageUrl);
                        if (bitmap != null){
                            //子線程發送消息給主線程
                            Message msg = mHandler.obtainMessage(0, bitmap);
                            mHandler.sendMessage(msg);
                        }else {
                            Message msg = mHandler.obtainMessage(1, null);
                            mHandler.sendMessage(msg);
                        }
                    }
                }.start();
            }
        });
    }
    private Handler mHandler = new Handler(){
        //主線程從消息隊列取消息來處理
        @Override
        public void handleMessage(@NonNull Message msg) {
            System.out.println("異步下載下傳圖檔");
            switch (msg.what){
                case 0:
                    //拿到消息中的bitmap用控件顯示出來
                    Bitmap bitmap = (Bitmap) msg.obj;
                    imageView.setImageBitmap(bitmap);
                    System.out.println("已下載下傳完成");
                    break;
                case 1:
                    System.out.println("下載下傳失敗");
                    break;
            }
            super.handleMessage(msg);
        }
    };

    static Bitmap loadImgFromUrl (String imageUrl){
        Bitmap bitmap = null;
        try {
            InputStream in = new java.net.URL(imageUrl).openStream();
            bitmap = BitmapFactory.decodeStream(in);
            in.close();
        }catch (Exception e){
            e.printStackTrace();
        }
        return bitmap;
    }
}