Android控件的拖动

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;
}
});
}