Android控件拖动实例详解

Android控件拖动

Android控件的拖动，主要是通过设置控件的setOnTouchListener()方法，重写它的onTouch()方法。然后通过MotionEvent的不同事件，进行判断，主要是在MotionEvent.ACTION_MOVE中不断重绘控件在画布中的位置，而实现拖动的效果。

public class DragViewActivity extends AppCompatActivity {
  private Button btn_drag; 

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

    // 获取屏幕的宽高
    DisplayMetrics dm = getResources().getDisplayMetrics();
    final int screenWith = dm.widthPixels;
    final int screenHeight = dm.heightPixels; 

    btn_drag = (Button) findViewById(R.id.btn_drag);
    btn_drag.setOnTouchListener(new View.OnTouchListener() {
        int lastX;
        int lastY;
        boolean isDraging = false; 

      @Override
      public boolean onTouch(View v, MotionEvent event) {
        int ee = event.getAction();
        switch (ee){
          case MotionEvent.ACTION_DOWN:
            isDraging = true;
            lastX = (int) event.getRawX();
            lastY = (int) event.getRawY();
            break;
          case MotionEvent.ACTION_MOVE:
            if (isDraging){
              // x轴方向的位移差
              int dx = (int) event.getRawX() - lastX;
              // y轴方向的位移差
              int dy = (int) event.getRawY() - lastY; 

              int l = v.getLeft() + dx;
              int t = v.getTop() + dy;
              int r = v.getRight() + dx;
              int b = v.getBottom() + dy; 

              // 判断超出屏幕
              if (l < 0){
                l = 0;
                r = l + v.getWidth();
              }
              if (t < 0){
                t = 0;
                b = t + v.getHeight();
              }
              if (r > screenWith){
                r = screenWith;
                l = r - v.getWidth();
              }
              if (b > screenHeight){
                b = screenHeight;
                t = b - v.getHeight();
              } 

              // 重绘view
              v.layout(l, t, r, b);
              v.postInvalidate(); 

              lastX = (int) event.getRawX();
              lastY = (int) event.getRawY();
            }
            break; 

          case MotionEvent.ACTION_UP:
            isDraging = false;
            break;
          default:
            break;
        }
        return false;
      }
    });
  }

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