[Android]笔记3-跟随手指的小球

为了实现一个跟随的小球，自定义UI组件，这个UI组件将会在指定位置绘制一个小球，这个位置可以动态改变。当用户通过手指在屏幕上拖动时，程序监听这个手指动作，把手指的位置传入自定义UI组件，并通知该组件重绘制即可。

DrawView UI组件

package com.dezai.customerviewactivity;

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;


/**
 * Created by A292 on 2017/5/8.
 */

public class DrawView  extends View {
    public float currentX=40;
    public float currentY=50;
    Paint p=new Paint();
    public DrawView(Context context) {
        super(context);
    }
    public DrawView(Context context,AttributeSet set){
        super(context,set);
    }
    @Override
    public void onDraw(Canvas canvas){
        super.onDraw(canvas);
        //设置画笔颜色
        p.setColor(Color.RED);
        //绘制一个小圆(作为一个小球)
        canvas.drawCircle(currentX,currentY,25,p);
    }
    @Override
    public boolean onTouchEvent(MotionEvent event){
        //修改currentX currentY 两个属性
        currentX =event.getX();
        currentY=event.getY();
        //通知当前组件重绘制自己
        invalidate();
        //返回true表明该处理方法已处理该事件
        return true;
    }
}

XML:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.dezai.customerviewactivity.MainActivity">


    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:id="@+id/root">
        <com.dezai.customerviewactivity.DrawView
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </LinearLayout>
</android.support.constraint.ConstraintLayout>

MainActivity

package com.dezai.customerviewactivity;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}