介绍：水波纹散开效果的控件在 App 里面还是比较常见的，例如 网易云音乐歌曲识别，附近搜索场景。

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看下实现的效果：

实现思路： 先将大圆半径与最小圆半径间距分成几等份，从内到外，Paint 透明度依次递减，绘制出同心圆，然后不断的改变这些同心圆的半径大小，延迟一定时间重绘，便达到了想外散开的动画效果了。

public class WaveView extends View {

 private static final String TAG = "WaveView";

 private int waveColor;

 private int waveCount;

 private Bitmap waveCenterIcon;

 private Paint paint;

 private int mWidth;

 private int mHeight;

 private int centerX;

 private int centerY;

 private float radius; // 最外圆半径，即大半径

 private float innerRadius; // 最内圆的半径，即最小半径

 private int centerIconWidth;

 private int centerIconHeight;

 private float[] waveDegreeArr;

 private boolean isRunning = true;

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

 public WaveView(Context context, AttributeSet attrs) {
  super(context, attrs);
  readAttrs(context, attrs);
  init();
 }

 private void init() {
  paint = new Paint(Paint.ANTI_ALIAS_FLAG);
  paint.setColor(waveColor);
  paint.setStyle(Paint.Style.FILL);
  waveDegreeArr = new float[waveCount];

  // 设置中间 drawable 点击事件


 }

 private void readAttrs(Context context, AttributeSet attrs) {
  TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.WaveView);
  try {
   waveColor = typedArray.getColor(R.styleable.WaveView_waveColor, 0xffff0000);
   waveCount = typedArray.getInt(R.styleable.WaveView_waveCount, 4);
   Drawable centerDrawable = typedArray.getDrawable(R.styleable.WaveView_waveCenterIcon);
   waveCenterIcon = ((BitmapDrawable) centerDrawable).getBitmap();
  } catch (Exception e) {

  } finally {
   typedArray.recycle();
  }
 }

 @Override
 protected void onSizeChanged(int w, int h, int oldw, int oldh) {
  super.onSizeChanged(w, h, oldw, oldh);
  mWidth = w;
  mHeight = h;
  centerX = mWidth / 2;
  centerY = mHeight / 2;
  radius = Math.min(mWidth, mHeight) / 2f;
  centerIconWidth = waveCenterIcon.getWidth();
  centerIconHeight = waveCenterIcon.getHeight();
  innerRadius = Math.max(centerIconWidth, centerIconHeight) * 1.2f;

  for (int i = 0; i < waveCount; i++) {
   waveDegreeArr[i] = innerRadius + (radius - innerRadius) / waveCount * i;
  }

 }

 @Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  int widthMode = MeasureSpec.getMode(widthMeasureSpec);
  int heightMode = MeasureSpec.getMode(heightMeasureSpec);
  if (widthMode == MeasureSpec.UNSPECIFIED || widthMode == MeasureSpec.AT_MOST) {
   widthMeasureSpec = MeasureSpec.makeMeasureSpec(dp2Px(120), MeasureSpec.EXACTLY);
  }
  if (heightMode == MeasureSpec.UNSPECIFIED || heightMode == MeasureSpec.AT_MOST) {
   heightMeasureSpec = MeasureSpec.makeMeasureSpec(dp2Px(120), MeasureSpec.EXACTLY);
  }
  super.onMeasure(widthMeasureSpec, heightMeasureSpec);
 }


 @Override
 protected void onDraw(Canvas canvas) {
  drawWave(canvas);
  drawCenterCircle(canvas);
  drawCenterIcon(canvas);

 }

 private void drawCenterCircle(Canvas canvas) {
  canvas.drawCircle(centerX, centerY, innerRadius, paint);
 }

 private void drawWave(Canvas canvas) {
  for (int i = 0; i < waveCount; i++) {
   paint.setAlpha((int) (255 - 255 * waveDegreeArr[i] / radius));
   canvas.drawCircle(centerX, centerY, waveDegreeArr[i], paint);
  }
  for (int i = 0; i < waveDegreeArr.length; i++) {
   if ((waveDegreeArr[i] += 4) > radius) {
    waveDegreeArr[i] = innerRadius;
   }
  }
  if (isRunning) {
   postInvalidateDelayed(50);
  }
 }

 private void drawCenterIcon(Canvas canvas) {
  paint.setAlpha(255);
  int left = centerX - centerIconWidth / 2;
  int top = centerY - centerIconHeight / 2;
  canvas.drawBitmap(waveCenterIcon, left, top, paint);
 }

 @Override
 public boolean onTouchEvent(MotionEvent event) {
  switch (event.getAction()) {
   case MotionEvent.ACTION_UP:
    // 处理事件逻辑
    handleEvent(event);
    return true;
  }
  return true;
 }

 private void handleEvent(MotionEvent event) {
  float touchX = event.getX();
  float touchY = event.getY();
  Log.i(TAG, "handleEvent: " + "(" + touchX + "," + touchY + ")");
  float distanceX = Math.abs(touchX - centerX);
  float distanceY = Math.abs(touchY - centerY);
  // 计算触摸点距离中心点的距离
  float distance = (float) Math.sqrt(distanceX * distanceX + distanceY * distanceY);
  // 当点击的点距离中心点距离小于最内圆半径时，认为是点击有效，否则无效
  if (distance < innerRadius) {
   if (listener != null) {
    listener.onCenterWaveClick();
   }
  }
 }

 OnCenterWaveClickListener listener;

 public interface OnCenterWaveClickListener {
  void onCenterWaveClick();
 }

 public void setOnCenterWaveClickListener(OnCenterWaveClickListener listener) {
  this.listener = listener;
 }

 public void toggle() {
  isRunning = !isRunning;
  invalidate();
 }

 public boolean isWaveRunning() {
  return isRunning;
 }


 private int dp2Px(int dpValue) {
  return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dpValue, getResources().getDisplayMetrics());
 }

}

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