iOS下载图片失败

一、具体问题　　

　　开发的过程中，发现某个界面部分图片的显示出现了问题只显示占位图片，取出图片的url在浏览器却是能打开的，各种尝试甚至找同行的朋友帮忙在他们项目里展示都会存在问题，最终发现通过第三方框架SDWebImage或者YYWebImage下载带有逗号的url图片链接都会下载失败，在下载方法完成的回调block里面打印信息如下：

Error Domain=NSURLErrorDomain Code= "(null)"

　　现列举两个不能正常展示的图片url：

http://img1.imgtn.bdimg.com/it/u=3044191397,2911599132&fm=27&gp=0.jpg

http://img2.imgtn.bdimg.com/it/u=3509004173,840437551&fm=27&gp=0.jpg

　　有兴趣的小伙伴可以拿到自己的项目里试试

二、问题原因

　　网上有小伙伴提出是因为缺少User-Agent用户代理导致的。只有设置了用户代理，才能访问到这张带有逗号的url图片。至于这个用户代理的格式，只要有值或者约定的特定格式字符串都可以。

三、具体解决

1.第三方框架YYWebImage

　　找到YYWebImageManager.m文件，定位到设置http请求头的属性即_headers的地方，加入一个User-Agent的键值对，具体改动可以看下面的方法

- (instancetype)initWithCache:(YYImageCache *)cache queue:(NSOperationQueue *)queue{
    self = [super init];
    if (!self) return nil;
    _cache = cache;
    _queue = queue;
    _timeout = 15.0;

    NSString *userAgent = @"";
    userAgent = [NSString stringWithFormat:@"%@/%@ (%@; iOS %@; Scale/%0.2f)", [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleExecutableKey] ?: [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleIdentifierKey], [[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleShortVersionString"] ?: [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleVersionKey], [[UIDevice currentDevice] model], [[UIDevice currentDevice] systemVersion], [[UIScreen mainScreen] scale]];

    if (userAgent) {
        if (![userAgent canBeConvertedToEncoding:NSASCIIStringEncoding]) {
            NSMutableString *mutableUserAgent = [userAgent mutableCopy];
            if (CFStringTransform((__bridge CFMutableStringRef)(mutableUserAgent), NULL, (__bridge CFStringRef)@"Any-Latin; Latin-ASCII; [:^ASCII:] Remove", false)) {
                userAgent = mutableUserAgent;
            }
        }
    }

    //带有逗号的图片url不显示的问题，重要的是设置代理才能解决
    if (YYImageWebPAvailable()) {
        _headers = @{ @"Accept" : @"image/webp,image/*;q=0.8", @"User-Agent" : userAgent};
    } else {
        _headers = @{ @"Accept" : @"image/*;q=0.8", @"User-Agent" : userAgent};
    }
    return self;
}

2.第三方框架SDWebImage

　　找到SDWebImageDownloader.m文件，也是定位到设置http请求头的属性即_HTTPHeaders的地方，加入一个User-Agent的键值对，具体改动可以看下面的方法

- (id)init {
    if ((self = [super init])) {
        _operationClass = [SDWebImageDownloaderOperation class];
        _shouldDecompressImages = YES;
        _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
        _downloadQueue = [NSOperationQueue new];
        _downloadQueue.maxConcurrentOperationCount = ;
        _URLCallbacks = [NSMutableDictionary new];
        /***********************/
        NSString *userAgent = @"";
        userAgent = [NSString stringWithFormat:@"%@/%@ (%@; iOS %@; Scale/%0.2f)", [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleExecutableKey] ?: [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleIdentifierKey], [[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleShortVersionString"] ?: [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleVersionKey], [[UIDevice currentDevice] model], [[UIDevice currentDevice] systemVersion], [[UIScreen mainScreen] scale]];

        if (userAgent) {
            if (![userAgent canBeConvertedToEncoding:NSASCIIStringEncoding]) {
                NSMutableString *mutableUserAgent = [userAgent mutableCopy];
                if (CFStringTransform((__bridge CFMutableStringRef)(mutableUserAgent), NULL, (__bridge CFStringRef)@"Any-Latin; Latin-ASCII; [:^ASCII:] Remove", false)) {
                    userAgent = mutableUserAgent;
                }
            }
        }
        /***********************/
#ifdef SD_WEBP
        _HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8", userAgent : @"User-Agent"} mutableCopy];
#else
        _HTTPHeaders = [@{@"Accept": @"image/*;q=0.8", userAgent : @"User-Agent"} mutableCopy];
#endif
        _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
        _downloadTimeout = 15.0;
    }
    return self;
}

或者是直接在UIImageView+WebCache.m文件中，在统一下载图片入口最前面添加如下代码

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
    /***********************/
    NSString *userAgent = @"";
    userAgent = [NSString stringWithFormat:@"%@/%@ (%@; iOS %@; Scale/%0.2f)", [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleExecutableKey] ?: [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleIdentifierKey], [[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleShortVersionString"] ?: [[[NSBundle mainBundle] infoDictionary] objectForKey:(__bridge NSString *)kCFBundleVersionKey], [[UIDevice currentDevice] model], [[UIDevice currentDevice] systemVersion], [[UIScreen mainScreen] scale]];

    if (userAgent) {
        if (![userAgent canBeConvertedToEncoding:NSASCIIStringEncoding]) {
            NSMutableString *mutableUserAgent = [userAgent mutableCopy];
            if (CFStringTransform((__bridge CFMutableStringRef)(mutableUserAgent), NULL, (__bridge CFStringRef)@"Any-Latin; Latin-ASCII; [:^ASCII:] Remove", false)) {
                userAgent = mutableUserAgent;
            }
        }

        [[SDWebImageDownloader sharedDownloader] setValue:userAgent forHTTPHeaderField:@"User-Agent"];
    }
    /***********************/
    [self sd_cancelCurrentImageLoad];

    ........省略原源码
}

3.其他第三方下载图片的框架

　　直接全局搜索字符串"Accept"，因为虽然缺少设置User-Agent用户代理，但是http请求头一般都会有设置"Accept"，所以定位后，直接再加一个User-Agent的键值对就可以了