AndroidShimmerLayout实现微光效果解析

private Bitmap getSourceMaskBitmap() {
  if (sourceMaskBitmap != null) {
   return sourceMaskBitmap;
  }

  int width = maskRect.width();
  int height = getHeight();

  /* 通过LinearGradient在遮罩Bitmap上绘制渐变效果 */
  final int edgeColor = reduceColorAlphaValueToZero(shimmerColor);
  LinearGradient gradient = new LinearGradient(
    -maskRect.left, 0,
    width + maskRect.left, 0,
    /* 透明色 - 微光颜色 - 微光颜色 - 透明色 */
    new int[]{edgeColor, shimmerColor, shimmerColor, edgeColor},
    new float[]{0.25F, 0.47F, 0.53F, 0.75F},
    Shader.TileMode.CLAMP);

  Paint paint = new Paint();
  paint.setShader(gradient);

  sourceMaskBitmap = createBitmap(width, height);
  /* 对微光效果的bitmap做一些旋转效果 */
  Canvas canvas = new Canvas(sourceMaskBitmap);
  canvas.rotate(shimmerAngle, width / 2, height / 2);
  canvas.drawRect(-maskRect.left, maskRect.top, width + maskRect.left, maskRect.bottom, paint);

  return sourceMaskBitmap;
 }

/* 获取微光效果Bitmap */
localMaskBitmap = getSourceMaskBitmap();
canvas.save();
/* 先绘制GroupView本身的界面 */
super.dispatchDraw(canvas);
/* 再绘制微光效果Bitmap */
canvas.drawBitmap(localMaskBitmap, 0, 0, maskPaint);
canvas.restore();

/* 获取微光效果Bitmap */
localMaskBitmap = getSourceMaskBitmap();
canvas.save();
/* canvas 裁剪显示 */
canvas.clipRect(maskOffsetX, 0,
        maskOffsetX + localMaskBitmap.getWidth(),
        getHeight());
/* 先绘制GroupView本身的界面 */
super.dispatchDraw(canvas);
/* 再绘制微光效果Bitmap */
canvas.drawBitmap(localMaskBitmap, maskOffsetX, 0, maskPaint);
canvas.restore();