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=403 "(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 = 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