iOS下載圖片失敗

一、具體問題　　

　　開發的過程中，發現某個界面部分圖片的顯示出現了問題只顯示占位圖片，取出圖片的url在瀏覽器卻是能打開的，各種嘗試甚至找同行的朋友幫忙在他們項目里展示都會存在問題，最終發現通過第三方框架SDWebImage或者YYWebImage下載帶有逗號的url圖片鏈接都會下載失敗，在下載方法完成的回調block里面打印信息如下：

Error Domain=NSURLErrorDomain Code=403 "(null)"

　　現列舉兩個不能正常展示的圖片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

　　有興趣的小伙伴可以拿到自己的項目里試試

二、問題原因

　　網上有小伙伴提出是因為缺少User-Agent用戶代理導致的。只有設置了用戶代理，才能訪問到這張帶有逗號的url圖片。至於這個用戶代理的格式，只要有值或者約定的特定格式字符串都可以。

三、具體解決

1.第三方框架YYWebImage

　　找到YYWebImageManager.m文件，定位到設置http請求頭的屬性即_headers的地方，加入一個User-Agent的鍵值對，具體改動可以看下面的方法

- (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;
            }
        }
    }

    //帶有逗號的圖片url不顯示的問題，重要的是設置代理才能解決
    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，也是定位到設置http請求頭的屬性即_HTTPHeaders的地方，加入一個User-Agent的鍵值對，具體改動可以看下面的方法

- (id)init {
    if ((self = [super init])) {
        _operationClass = [SDWebImageDownloaderOperation class];
        _shouldDecompressImages = YES;
        _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
        _downloadQueue = [NSOperationQueue new];
        _downloadQueue.maxConcurrentOperationCount = 6;
        _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，在統一下載圖片入口最前面添加如下代碼

- (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];
    
    ........省略原源碼
}

3.其他第三方下載圖片的框架

　　直接全局搜索字符串"Accept"，因為雖然缺少設置User-Agent用戶代理，但是http請求頭一般都會有設置"Accept"，所以定位后，直接再加一個User-Agent的鍵值對就可以了