// 头像信息类，记录大小、位置等信息
private static class DrawableInfo {
 int mId = View.NO_ID;
 Drawable mDrawable;
 // 中心点位置
 float mCenterX;
 float mCenterY;
 // 头像上缺口弧所在圆上的圆心位置，其实就是下一个相邻头像的中心点
 float mGapCenterX;
 float mGapCenterY;
 boolean mHasGap;
 // 头像边界
 final RectF mBounds = new RectF();
 // 圆形蒙板路径，把头像弄成圆形
 final Path mMaskPath = new Path();
}

private void layoutDrawables() {
 mSteinerCircleRadius = 0;
 mOffsetY = 0;

 int width = getWidth() - getPaddingLeft() - getPaddingRight();
 int height = getHeight() - getPaddingTop() - getPaddingBottom();

 mContentSize = Math.min(width, height);
 final List<DrawableInfo> drawables = mDrawables;
 final int N = drawables.size();
 float center = mContentSize * .5f;
 if (mContentSize > 0 && N > 0) {
 // 图像圆的半径。
 final float r;
 if (N == 1) {
  r = mContentSize * .5f;
 } else if (N == 2) {
  r = (float) (mContentSize / (2 + 2 * Math.sin(Math.PI / 4)));
 } else if (N == 4) {
  r = mContentSize / 4.f;
 } else {
  r = (float) (mContentSize / (2 * (2 * Math.sin(((N - 2) * Math.PI) / (2 * N)) + 1)));
  final double sinN = Math.sin(Math.PI / N);
  // 以所有图像圆为内切圆的圆的半径
  final float R = (float) (r * ((sinN + 1) / sinN));
  mOffsetY = (float) ((mContentSize - R - r * (1 + 1 / Math.tan(Math.PI / N))) / 2f);
 }

 // 初始化第一个头像的中心位置
 final float startX, startY;
 if (N % 2 == 0) {
  startX = startY = r;
 } else {
  startX = center;
  startY = r;
 }

 // 变换矩阵
 final Matrix matrix = mLayoutMatrix;
 // 坐标点临时数组
 final float[] pointsTemp = this.mPointsTemp;

 matrix.reset();

 for (int i = 0; i < drawables.size(); i++) {
  DrawableInfo drawable = drawables.get(i);
  drawable.reset();

  drawable.mHasGap = i > 0;
  // 缺口弧的中心
  if (drawable.mHasGap) {
  drawable.mGapCenterX = pointsTemp[0];
  drawable.mGapCenterY = pointsTemp[1];
  }

  pointsTemp[0] = startX;
  pointsTemp[1] = startY;
  if (i > 0) {
  // 以上一个圆的圆心旋转计算得出当前圆的圆位置
  matrix.postRotate(360.f / N, center, center + mOffsetY);
  matrix.mapPoints(pointsTemp);
  }

  // 取出中心点位置
  drawable.mCenterX = pointsTemp[0];
  drawable.mCenterY = pointsTemp[1];

  // 设置边界
  drawable.mBounds.inset(-r, -r);
  drawable.mBounds.offset(drawable.mCenterX, drawable.mCenterY);

  // 设置“蒙板”路径
  drawable.mMaskPath.addCircle(drawable.mCenterX, drawable.mCenterY, r, Path.Direction.CW);
  drawable.mMaskPath.setFillType(Path.FillType.INVERSE_WINDING);
 }

 // 设置第一个头像的缺口，头像数量少于3个的时候没有
 if (N > 2) {
  DrawableInfo first = drawables.get(0);
  DrawableInfo last = drawables.get(N - 1);
  first.mHasGap = true;
  first.mGapCenterX = last.mCenterX;
  first.mGapCenterY = last.mCenterY;
 }

 mSteinerCircleRadius = r;
 }

 invalidate();
}

drawable.mMaskPath.addCircle(drawable.mCenterX, drawable.mCenterY, r, Path.Direction.CW);
drawable.mMaskPath.setFillType(Path.FillType.INVERSE_WINDING);

@Override
protected void onDraw(Canvas canvas) {
 super.onDraw(canvas);
 ...
 canvas.translate(0, mOffsetY);

 final Paint paint = mPaint;
 final float gapRadius = mSteinerCircleRadius * (mGap + 1f);
 for (int i = 0; i < drawables.size(); i++) {
  DrawableInfo drawable = drawables.get(i);
  RectF bounds = drawable.mBounds;
  final int savedLayer = canvas.saveLayer(0, 0, mContentSize, mContentSize, null, Canvas.ALL_SAVE_FLAG);

  // 设置Drawable的边界
  drawable.mDrawable.setBounds((int) bounds.left, (int) bounds.top,
    Math.round(bounds.right), Math.round(bounds.bottom));
  // 绘制Drawable
  drawable.mDrawable.draw(canvas);

  // 绘制“蒙板”路径，将Drawable绘制的图像“剪”成圆形
  canvas.drawPath(drawable.mMaskPath, paint);
  // “剪”出弧形的缺口
  if (drawable.mHasGap && mGap > 0f) {
   canvas.drawCircle(drawable.mGapCenterX, drawable.mGapCenterY, gapRadius, paint);
  }

  canvas.restoreToCount(savedLayer);
 }
}

public interface Callback {
 /**
  * 当drawable需要重新绘制时调用。此时的view应该使其自身失效(至少drawable展示部分失效)
  * @param who 要求重新绘制的drawable
  */
 void invalidateDrawable(@NonNull Drawable who);

 /**
  * drawable可以通过调用该方法来安排动画的下一帧。
  * @param who 要预定的drawable
  * @param what 要执行的动作
  * @param when 执行的时间（以毫秒为单位），基于android.os.SystemClock.uptimeMillis()
  */
 void scheduleDrawable(@NonNull Drawable who, @NonNull Runnable what, long when);

 /**
  * drawable可以通过调用该方法来取消先前通过scheduleDrawable(Drawable, Runnable, long)调度的动作。
  * @param who 要取消预定的drawable
  * @param what 要取消执行的动作
  */
 void unscheduleDrawable(@NonNull Drawable who, @NonNull Runnable what);
}

protected boolean verifyDrawable(@NonNull Drawable who) {
 // ...
 return who == mBackground || (mForegroundInfo != null && mForegroundInfo.mDrawable == who);
}

private boolean hasSameDrawable(Drawable drawable) {
 for (DrawableInfo d : mDrawables) {
  if (d.mDrawable == drawable) {
   return true;
  }
 }
 return false;
}

@Override
protected boolean verifyDrawable(@NonNull Drawable drawable) {
 return hasSameDrawable(drawable) || super.verifyDrawable(drawable);
}

// 状态改变时被调用
@Override
protected void drawableStateChanged() {
 super.drawableStateChanged();
 boolean invalidate = false;
 for (DrawableInfo drawable : mDrawables) {
  Drawable d = drawable.mDrawable;
  // 判断Drawable是否支持状态并更新状态
  if (d.isStateful() && d.setState(getDrawableState())) {
   invalidate = true;
  }
 }
 if (invalidate) {
  invalidate();
 }
}

// 这个方法主要针对状态改变时有过渡动画的Drawable
@Override
public void jumpDrawablesToCurrentState() {
 super.jumpDrawablesToCurrentState();
 for (DrawableInfo drawable : mDrawables) {
  drawable.mDrawable.jumpToCurrentState();
 }
}