cgimage可以添加到cashapelayer的contents属性中吗?

我有一个带有layerClass覆盖的自定义视图,比如:

+ (Class) layerClass
{
    return [CAShapeLayer class];
}

我希望能够向Contents属性添加CGImage,如下所示:

- (void) animateView
{
    UIImage *layerImage = ...;
    CAShapeLayer *layer = (CAShapeLayer *)self.layer;
    layer.contents = layerImage.CGImage;

    //....Do other fun animations here
}

我是否需要设置其他属性,或其他方法来获取形状层以呈现动画图像的内容?