自定义组件，通过回调实现跟随手指的小球

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class DrawView extends View {
  public float currentX = 40;
  public float currentY = 50;
  Paint p = new Paint();
  
  public DrawView(Context context, AttributeSet attrs) {
    super(context, attrs);
    // TODO Auto-generated constructor stub
  }

  @Override
  public void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    p.setColor(Color.RED);
    canvas.drawCircle(currentX, currentY, 15, p);
  }
  
  @Override
  public boolean onTouchEvent(MotionEvent event) {
    this.currentX = event.getX();
    this.currentY = event.getY();
    this.invalidate();
    return true;
  }
  
}