typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
    /**
     * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
     * This flag disable this blacklisting.
     */
    SDWebImageRetryFailed = 1 << 0,

    /**
     * By default, image downloads are started during UI interactions, this flags disable this feature,
     * leading to delayed download on UIScrollView deceleration for instance.
     */
    SDWebImageLowPriority = 1 << 1,

    /**
     * This flag disables on-disk caching
     */
    SDWebImageCacheMemoryOnly = 1 << 2,

    /**
     * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
     * By default, the image is only displayed once completely downloaded.
     */
    SDWebImageProgressiveDownload = 1 << 3,

    /**
     * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
     * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
     * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
     * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
     *
     * Use this flag only if you can't make your URLs static with embedded cache busting parameter.
     */
    SDWebImageRefreshCached = 1 << 4,

    /**
     * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
     * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
     */
    SDWebImageContinueInBackground = 1 << 5,

    /**
     * Handles cookies stored in NSHTTPCookieStore by setting
     * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
     */
    SDWebImageHandleCookies = 1 << 6,

    /**
     * Enable to allow untrusted SSL certificates.
     * Useful for testing purposes. Use with caution in production.
     */
    SDWebImageAllowInvalidSSLCertificates = 1 << 7,

    /**
     * By default, images are loaded in the order in which they were queued. This flag moves them to
     * the front of the queue.
     */
    SDWebImageHighPriority = 1 << 8,
    
    /**
     * By default, placeholder images are loaded while the image is loading. This flag will delay the loading
     * of the placeholder image until after the image has finished loading.
     */
    SDWebImageDelayPlaceholder = 1 << 9,

    /**
     * We usually don't call transformDownloadedImage delegate method on animated images,
     * as most transformation code would mangle it.
     * Use this flag to transform them anyway.
     */
    SDWebImageTransformAnimatedImage = 1 << 10,
    
    /**
     * By default, image is added to the imageView after download. But in some cases, we want to
     * have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)
     * Use this flag if you want to manually set the image in the completion when success
     */
    SDWebImageAvoidAutoSetImage = 1 << 11
};

/**
 * Set the imageView `image` with an `url`, placeholder and custom options.
 *
 * The download is asynchronous and cached.
 *
 * @param url            The url for the image.
 * @param placeholder    The image to be set initially, until the image request finishes.
 * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
 * @param completedBlock A block called when operation has been completed. This block has no return value
 *                       and takes the requested UIImage as first parameter. In case of error the image parameter
 *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
 *                       indicating if the image was retrieved from the local cache or from the network.
 *                       The fourth parameter is the original image url.
 */
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options completed:(SDWebImageCompletionBlock)completedBlock;

// UIImageView+IETransition.h

#import <UIKit/UIKit.h>

@interface UIImageView (IETransition)<CAAnimationDelegate>

/** 渐变动画加载 URL 图片
 *
 * @param imageURL					图片 URL
 * @param placeholderImage	占位图
 *
 */
- (void)ie_transitionFadeLoadImage:(NSURL *)imageURL placeholderImage:(UIImage * _Nullable)placeholderImage;

@end

//  UIImageView+IETransition.m

#import "UIImageView+IETransition.h"

static NSString *const ANIMATIONKEY_TRANSITION_FADE = @"IETransitionFade";

@implementation UIImageView (IETransition)

#pragma mark - 渐变动画加载 URL 图片

- (void)ie_transitionFadeLoadImage:(NSURL *)imageURL placeholderImage:(UIImage * _Nullable)placeholderImage {
// - Note: 如果需要纯色占位图，可以将下面注释打开替换成具体颜色
//    if (!placeholderImage) {
//        self.backgroundColor = MainDefaultImageBgColor;
//    }
    @weakify(self)
    [self sd_setImageWithURL:imageURL placeholderImage:placeholderImage options:SDWebImageLowPriority completed:^(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL) {
        @strongify(self)
        if (image && cacheType == SDImageCacheTypeNone) { // 只有当缓存中没有图片，也就是首次加载时才实现CATransition动画
            CATransition *transition = [CATransition animation];
            transition.delegate = self;
            transition.type = kCATransitionFade; // 褪色效果，渐进效果的基础
            transition.duration = 0.85f;
            transition.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut]; // 先慢后快再慢
            [self.layer addAnimation:transition forKey:ANIMATIONKEY_TRANSITION_FADE];
        }
    }];
}

#pragma mark - CAAnimationDelegate

- (void)animationDidStop:(CAAnimation *)anim finished:(BOOL)flag {
    if (flag) {
        [self.layer removeAnimationForKey:ANIMATIONKEY_TRANSITION_FADE];
    }
}

@end