手指在屏幕上滑動，紅色的小球始終跟隨手指移動(自定義控件)

1.自定義控件
BallView類

public class BallView extends View {

    private Paint mpaint;
    private   int  x=100;
    private  int  y=100;
    private  int  r=100;

    public BallView(Context context) {
        this(context,null);
    }

    public BallView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs,0);
    }

    public BallView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);

        //關聯我們所聲明的自定義屬性

        TypedArray ta = context.getTheme().obtainStyledAttributes(attrs, R.styleable.BallView,defStyleAttr,0);

        //獲取屬性
        //獲取內圓半斤
        r = ta.getInt(R.styleable.BallView_ball_size,r);


    }



    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        //畫筆
        mpaint = new Paint();
        //設置畫筆顏色
        mpaint.setColor(Color.parseColor("#FF0000"));
        mpaint.setAntiAlias(true);
        //設置畫布顏色
        canvas.drawColor(Color.parseColor("#FFFFFF"));
//畫圓
        canvas.drawCircle(x,y,r,mpaint);
    }


    @Override
    public boolean onTouchEvent(MotionEvent event) {



        switch(event.getAction())
        {

            case MotionEvent.ACTION_DOWN:
            {
                x=(int) event.getX();
                y= (int) event.getY();
                //System.out.println("按下時：  " + "x座標：" + event.getRawX() + "     " + "y座標：" + event.getRawY());
            }
         break;

            case MotionEvent.ACTION_MOVE:
            {
                x=(int) event.getX();
                y= (int) event.getY();
               // System.out.println("移動時：  " + "x座標：" + event.getRawX() + "     " + "y座標：" + event.getRawY());
            }
                break;
            case MotionEvent.ACTION_UP:
            {
                x=(int) event.getX();
                y= (int) event.getY();
                //System.out.println("擡起時：  " + "x座標：" + event.getRawX() + "     " + "y座標：" + event.getRawY());
            }
                break;




        }

        /*this.x= x;
        this.y=y;*/

        WindowManager manager = (WindowManager) getContext().getSystemService(Context.WINDOW_SERVICE);
        int width = manager.getDefaultDisplay().getWidth();
        int height = manager.getDefaultDisplay().getHeight();
          postInvalidate();

return  true;

    }
}

2.在values中創建attrs.xml
自定義屬性

 <declare-styleable name="BallView">
       <attr name="ball_size" format="integer"></attr>
    </declare-styleable>

3.在佈局中寫控件及屬性

 xmlns:xxx="http://schemas.android.com/apk/res-auto"



 <com.example.ballviewdemo.BallView
        android:id="@+id/ball"
        xxx:ball_size="30"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />