45.实现自定义View拖拽效果的5种方法

public class DragView extends View {

private int mLastX;
private int mLastY;

public DragView(Context context) {
super(context);
init();
}

public DragView(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}

public DragView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}

private void init() {
mPaint = new Paint();
mPaint.setStyle(Paint.Style.FILL);
mPaint.setColor(Color.RED);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//计算宽高
setMeasuredDimension(measureDimension(widthMeasureSpec), measureDimension(heightMeasureSpec));
}

private int measureDimension(int measureSpec) {
int result = 0;
//获得测量模式和大小
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
if (specMode == MeasureSpec.EXACTLY) {
result = specSize;
} else {
result = 200;//给一个默认值
if (specMode == MeasureSpec.AT_MOST) {
result = Math.min(result, specSize);
}
}
return result;
}

@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
//画一个圆
canvas.drawCircle(getWidth() / 2, getHeight() / 2, Math.min(getWidth() / 2, getHeight() / 2), mPaint);

@Override
public boolean onTouchEvent(MotionEvent event) {
int x = (int) event.getX();
int y = (int) event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
//记录手指按下时的坐标
mLastX = x;
mLastY = y;
break;
case MotionEvent.ACTION_MOVE:
//计算偏移量
int offsetX = x - mLastX;
int offsetY = y - mLastY;
//重新计算、绘制布局位置
//方法1.
layout(getLeft() + offsetX, getTop() + offsetY, getRight() + offsetX, getBottom() + offsetY);
//以下方法和layout()方法效果一样
//方法2.
//                offsetLeftAndRight(offsetX);
//                offsetTopAndBottom(offsetY);
//方法3.
//                LinearLayout.LayoutParams params = (LinearLayout.LayoutParams) getLayoutParams();
//                params.leftMargin = getLeft() + offsetX;
//                params.topMargin = getTop() + offsetY;
//                setLayoutParams(params);
//方法4.
//                ViewGroup.MarginLayoutParams params = (ViewGroup.MarginLayoutParams) getLayoutParams();
//                params.leftMargin = getLeft() + offsetX;
//                params.topMargin = getTop() + offsetY;
//                setLayoutParams(params);
//方法5.
//                ((View) getParent()).scrollBy(-offsetX, -offsetY);
break;
}
return true;
}
}