CALayer规模不受框架影响？

Question

我正在添加一个包含CGImage内容的子图层。我创建了它的框架，我认为它包含了它的边界和位置。我还设置了图层contentsGravity，因此图像会在UIImageView中正确调整大小，但imageview只显示部分图像，表明CALayer没有正确缩放图像。这里到底发生了什么？为什么CALayer没有合适的比例？

-(void)setImageInView:(UIImage*)img {
    CALayer *mosaicLayer = [CALayer layer];

    mosaicLayer.frame = CGRectMake(0.0f,0.0f, 720.0f, 960.0f);
    mosaicLayer.contentsGravity = kCAGravityResizeAspect;
    mosaicLayer.contents = (id)[img CGImage];

    [imageView.layer addSublayer:mosaicLayer];

    UIGraphicsBeginImageContext(mosaicLayer.bounds.size);
    [mosaicLayer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *saver = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    UIImageWriteToSavedPhotosAlbum(saver, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);   }