/**
 * 根据fraction值来计算当前的颜色。
 */
private int getCurrentColor(float fraction, int startColor, int endColor) {
  int redCurrent;
  int blueCurrent;
  int greenCurrent;
  int alphaCurrent;

  int redStart = Color.red(startColor);
  int blueStart = Color.blue(startColor);
  int greenStart = Color.green(startColor);
  int alphaStart = Color.alpha(startColor);

  int redEnd = Color.red(endColor);
  int blueEnd = Color.blue(endColor);
  int greenEnd = Color.green(endColor);
  int alphaEnd = Color.alpha(endColor);

  int redDifference = redEnd - redStart;
  int blueDifference = blueEnd - blueStart;
  int greenDifference = greenEnd - greenStart;
  int alphaDifference = alphaEnd - alphaStart;

  redCurrent = (int) (redStart + fraction * redDifference);
  blueCurrent = (int) (blueStart + fraction * blueDifference);
  greenCurrent = (int) (greenStart + fraction * greenDifference);
  alphaCurrent = (int) (alphaStart + fraction * alphaDifference);

  return Color.argb(alphaCurrent, redCurrent, greenCurrent, blueCurrent);
}