Android自定义View实现水波纹扩散效果

public class RippleView extends View {
 
    public RippleView(Context context) {
        this(context, null);
    }
 
    public RippleView(Context context, @Nullable AttributeSet attrs) {
        //this(context, null, 0);//如果第二个参数写null，则自定义属性将不可用
        this(context, attrs, 0);
    }
　　
    public RippleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        ........
    }    <br>　　
    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        ........
    }
 
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
 
        ........
    }
}       

private Paint centerPaint; //中心圆paint
private int radius = 100; //中心圆半径
private Paint spreadPaint; //扩散圆paint
private float centerX;//圆心x
private float centerY;//圆心y
private int distance = 5; //每次圆递增间距
private int maxRadius = 80; //最大圆半径
private int delayMilliseconds = 30;//扩散延迟间隔，越大扩散越慢
private List<Integer> spreadRadius = new ArrayList<>();//扩散圆层级数，元素为扩散的距离
private List<Integer> alphas = new ArrayList<>();//对应每层圆的透明度　

<resources>
 
  <declare-styleable name="SpreadView">
    <!--中心圆颜色-->
    <attr name="spread_center_color" format="color" />
    <!--中心圆半径-->
    <attr name="spread_radius" format="integer" />
    <!--扩散圆颜色-->
    <attr name="spread_spread_color" format="color" />
    <!--扩散间距-->
    <attr name="spread_distance" format="integer" />
    <!--扩散最大半径-->
    <attr name="spread_max_radius" format="integer" />
    <!--扩散延迟间隔-->
    <attr name="spread_delay_milliseconds" format="integer" />
  </declare-styleable>
 
</resources>

public RippleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
       super(context, attrs, defStyleAttr);
       TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.SpreadView, defStyleAttr, 0);
       radius = typedArray.getInt(R.styleable.SpreadView_spread_radius, radius);
       maxRadius = typedArray.getInt(R.styleable.SpreadView_spread_max_radius, maxRadius);
       int centerColor = typedArray.getColor(R.styleable.SpreadView_spread_center_color,
               ContextCompat.getColor(context, android.R.color.holo_red_light));
       int spreadColor = typedArray.getColor(R.styleable.SpreadView_spread_spread_color,
               ContextCompat.getColor(context, R.color.color_F71816));
       distance = typedArray.getInt(R.styleable.SpreadView_spread_distance, distance);
       typedArray.recycle();
 
   }

public RippleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        
        .....
 
        //中心圆
        centerPaint = new Paint();
        centerPaint.setColor(centerColor);<br>　　　　　//消除锯齿
        centerPaint.setAntiAlias(true);
 
        //水波纹扩散
        spreadPaint = new Paint();
        spreadPaint.setColor(spreadColor);
        spreadPaint.setAntiAlias(true);<br>　　　　　//填充和描边，上面2张图片效果不同取决于该属性
        spreadPaint.setStyle(Paint.Style.STROKE);
        spreadPaint.setAlpha(255);
        //初始化第一个水波纹，扩散半径为0，透明度为255（不透明）
        spreadRadius.add(0);
        alphas.add(255);
    } /**在控件大小发生改变时调用。所以这里初始化会被调用一次*/<br>@Override<br>protected void onSizeChanged(int w, int h, int oldw, int oldh) {<br>    　　super.onSizeChanged(w, h, oldw, oldh);<br>    　　//圆心位置<br>    　　centerX = w / 2;<br>   　　 centerY = h / 2;<br>　　}

@Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
 
        for (int i = 0; i < spreadRadius.size(); i++) {
            //透明度
            int alpha = alphas.get(i);
            //半径
            int width = spreadRadius.get(i);
            spreadPaint.setAlpha(alpha);
            //绘制扩散的圆
            canvas.drawCircle(centerX, centerY, radius + width, spreadPaint);
            if (alpha > 0 && width < 255) {
                //递减
                alpha = (alpha - distance) > 0 ? (alpha - distance) : 1;
                alphas.set(i, alpha);
                //递增
                spreadRadius.set(i, width + distance);
            }
        }
        if (spreadRadius.get(spreadRadius.size() - 1) > maxRadius) {
            spreadRadius.add(0);
            alphas.add(255);
        }
        if (spreadRadius.size() > 8) {
            spreadRadius.remove(0);
            alphas.remove(0);
        }
        //中间的圆
        canvas.drawCircle(centerX, centerY, radius, centerPaint);
        //延迟更新，达到扩散视觉差效果
        postInvalidateDelayed(delayMilliseconds);
    }