Android开发实现跟随手指的小球效果示例

public class DrawView extends View {
  public float currentX = 40;
  public float currentY = 50;
  //定义并创建画笔
  Paint p = new Paint();
  public DrawView(Context context)
  {
    super(context);
  }
  public DrawView(Context context , AttributeSet set)
  {
    super(context,set);
  }
  @Override
  public void onDrawForeground(Canvas canvas) {
    super.onDrawForeground(canvas);
    //设置画笔颜色
    p.setColor(Color.RED);
    //绘制一个小球
    canvas.drawCircle(currentX , currentY , 30 , p);
  }
  //为组建的触碰实践重写处理方法
  @Override
  public boolean onTouchEvent(MotionEvent event) {
    //修改currentX,currentY的两个属性
    currentX = event.getX();
    currentY = event.getY();
    //通知当前组建重绘自己
    invalidate();
    //放回true表明该处理方法已经处理该事件
    return true;
  }
}

public class MainActivity extends AppCompatActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //获取Linearlayout布局容器
    LinearLayout root = (LinearLayout) findViewById(R.id.root);
    //创建DrawView组件
    final DrawView draw = new DrawView(this);
    //设定自定义组件的最小宽度、高度
    draw.setMinimumWidth(300);
    draw.setMinimumHeight(500);
    root.addView(draw);
  }
}

<?xml version="1.0" encoding="utf-8"? 
<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:id="@+id/root"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical" 
  <com.example.a30797.myapplication.DrawView
    android:layout_width="match_parent"
    android:layout_height="match_parent" / 
</LinearLayout 

public class MainActivity extends AppCompatActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
  }
}