我们获取的网络图片, 设置到按钮活tabbar上往往都很模糊,如何使网络下载的图片按照2x或3x显示呢?(比如60*60的原始图片, 显示为30*30的大小,实现2倍率显示)

#pragma mark - 无损缩小图片：按比例
-(UIImage *)scaleImage:(UIImage *)image scale:(CGFloat)scale{
    NSData *imageData = UIImagePNGRepresentation(image);
    UIImage *imageNew = [UIImage imageWithData:imageData scale:scale];
    return imageNew;
}

SDWebImageManager *manager = [SDWebImageManager sharedManager];
NSURL *url = [NSURL URLWithString:iconUrl] 
[manager downloadImageWithURL:url options:SDWebImageRetryFailed progress:nil completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
    if (image) {
        image = [self scaleImage:image scale:2.0];
        shouyeVC.tabBarItem.selectedImage = [image imageWithRenderingMode:UIImageRenderingModeAutomatic];
    }
}];

还有一个比较好用的方法, 就是将得到的网络图片image进行尺寸压缩到指定大小, 而图片不失真.

/*
*return 新生成的图片
* targetSize 新生成图片的尺寸(你想得到的尺寸)
* image原始图片
*/
- (UIImage *)imageByScalingToSize:(CGSize)targetSize ForImage:(UIImage *)image{
    //用作3倍率缩放, 保证图片不失真
    targetSize = CGSizeMake(targetSize.width*3, targetSize.height*3);

    UIImage *sourceImage = image;
    UIImage *newImage = nil;
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = targetSize.width;
    CGFloat targetHeight = targetSize.height;
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    CGPoint thumbnailPoint = CGPointMake(0.0,0.0);
    if (CGSizeEqualToSize(imageSize, targetSize) == NO) {
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
        if (widthFactor < heightFactor)
            scaleFactor = widthFactor;
        else
            scaleFactor = heightFactor;
        scaledWidth  = width * scaleFactor;
        scaledHeight = height * scaleFactor;
        // center the image
        if (widthFactor < heightFactor) {

            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
        } else if (widthFactor > heightFactor) {
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }
    // this is actually the interesting part:
    UIGraphicsBeginImageContext(targetSize);
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width  = scaledWidth;
    thumbnailRect.size.height = scaledHeight;
    [sourceImage drawInRect:thumbnailRect];
    newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    if(newImage == nil)
        NSLog(@"could not scale image");

    UIImage *image3Scale = [[UIImage alloc] initWithData:UIImagePNGRepresentation(newImage) scale:3.0];
    return image3Scale;
}