iOS网络——SDWebImage SDImageDownloader源码解析你要知道的NSURLSession都在这里

//下载选项设置的一系列枚举
typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
    SDWebImageDownloaderLowPriority = 1 << 0,
    SDWebImageDownloaderProgressiveDownload = 1 << 1,
    SDWebImageDownloaderUseNSURLCache = 1 << 2,
    SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
    SDWebImageDownloaderContinueInBackground = 1 << 4,
    SDWebImageDownloaderHandleCookies = 1 << 5,
    SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,
    SDWebImageDownloaderHighPriority = 1 << 7,  
    SDWebImageDownloaderScaleDownLargeImages = 1 << 8,
};

//下载图片时的顺序，FIFO或者LIFO
typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
    SDWebImageDownloaderFIFOExecutionOrder,
    SDWebImageDownloaderLIFOExecutionOrder
};

//声明通知的全局变量名
FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageDownloadStartNotification;
FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageDownloadStopNotification;

//进度回调块
typedef void(^SDWebImageDownloaderProgressBlock)(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL);

//下载完成的回调块
typedef void(^SDWebImageDownloaderCompletedBlock)(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, BOOL finished);

//http首部数据字典
typedef NSDictionary<NSString *, NSString *> SDHTTPHeadersDictionary;
//http首部可变数据字典
typedef NSMutableDictionary<NSString *, NSString *> SDHTTPHeadersMutableDictionary;

//http首部过滤的一个回调块
typedef SDHTTPHeadersDictionary * _Nullable (^SDWebImageDownloaderHeadersFilterBlock)(NSURL * _Nullable url, SDHTTPHeadersDictionary * _Nullable headers);

/*
自定义token类，用于取消下载任务
这个token第二个属性其实就是SDWebImageDownloaderOperation中使用的token即回调块的字典
目的相同，都是为了取消特定的下载任务
*/
@interface SDWebImageDownloadToken : NSObject

@property (nonatomic, strong, nullable) NSURL *url;
@property (nonatomic, strong, nullable) id downloadOperationCancelToken;

@end

//异步下载图片
@interface SDWebImageDownloader : NSObject

//是否压缩图片
@property (assign, nonatomic) BOOL shouldDecompressImages;

//支持的最大同时下载图片的数量，其实就是NSOperationQueue支持的最大并发数
@property (assign, nonatomic) NSInteger maxConcurrentDownloads;

//当前正在下载图片的数量，其实就是NSOperationQueue的operationCount即正在执行下载任务的operation的数量
@property (readonly, nonatomic) NSUInteger currentDownloadCount;

//下载时连接服务器的超时时间，默认15s
@property (assign, nonatomic) NSTimeInterval downloadTimeout;

//session运行模式，默认使用默认模式，即 defaultSessionConfiguration
@property (readonly, nonatomic, nonnull) NSURLSessionConfiguration *sessionConfiguration;

//执行下载任务的顺序，FIFO或LIFO
@property (assign, nonatomic) SDWebImageDownloaderExecutionOrder executionOrder;

//类方法，获取全局共享的单例对象
+ (nonnull instancetype)sharedDownloader;

//默认的URL credential
@property (strong, nonatomic, nullable) NSURLCredential *urlCredential;

//用户名，有些图片下载的地址需要做用户认证
@property (strong, nonatomic, nullable) NSString *username;

//密码
@property (strong, nonatomic, nullable) NSString *password;

//过滤http首部的回调块
@property (nonatomic, copy, nullable) SDWebImageDownloaderHeadersFilterBlock headersFilter;

//初始化方法，不使用全局共享的downloader时创建
- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration NS_DESIGNATED_INITIALIZER;

//为http首部设置值
- (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field;

//返回http首部的值
- (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field;

//设置下载类的Class，默认使用SDWebImageDownloaderOperation，开发者可以自定义只需实现相关协议
- (void)setOperationClass:(nullable Class)operationClass;

/*
下载url对应的图片
设置下载配置选项、进度回调块、下载完成回调块
返回一个token，用于取消对应的下载任务
*/
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;

//取消下载任务，需要传入上面创建下载任务时返回的token
- (void)cancel:(nullable SDWebImageDownloadToken *)token;

//设置下载队列NSOperationQueue挂起
- (void)setSuspended:(BOOL)suspended;

//设置取消NSOperationQueue队列中的所有下载任务
- (void)cancelAllDownloads;

//要求downloader使用特定运行模式创建一个NSURLSession对象
- (void)createNewSessionWithConfiguration:(nonnull NSURLSessionConfiguration *)sessionConfiguration;

@end

@implementation SDWebImageDownloadToken
@end

//遵守NSURLSessionTaskDelegate和NSURLSessionDataDelegate协议
@interface SDWebImageDownloader () <NSURLSessionTaskDelegate, NSURLSessionDataDelegate>

//定义一个NSOperationQueue的下载队列
@property (strong, nonatomic, nonnull) NSOperationQueue *downloadQueue;
//最近一次添加进队列的operation主要用于LIFO时设置依赖
@property (weak, nonatomic, nullable) NSOperation *lastAddedOperation;
//operationClass默认是SDWebImageDownloaderOperation
@property (assign, nonatomic, nullable) Class operationClass;
//可变字典，key是图片的URL，value是对应的下载任务Operation
@property (strong, nonatomic, nonnull) NSMutableDictionary<NSURL *, SDWebImageDownloaderOperation *> *URLOperations;
//<NSString*, NSString*>类型的字典，存储http首部
@property (strong, nonatomic, nullable) SDHTTPHeadersMutableDictionary *HTTPHeaders;
//一个GCD的队列
@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t barrierQueue;

//NSURLSession对象
@property (strong, nonatomic) NSURLSession *session;

@end

@implementation SDWebImageDownloader

//类方法，类加载的时候执行
+ (void)initialize {
    
    //如果导入了SDNetworkActivityIndicator文件，就会展示一个小菊花
    if (NSClassFromString(@"SDNetworkActivityIndicator")) {

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        id activityIndicator = [NSClassFromString(@"SDNetworkActivityIndicator") performSelector:NSSelectorFromString(@"sharedActivityIndicator")];
#pragma clang diagnostic pop

        //删除通知后重新添加通知，防止重复添加出现异常
        [[NSNotificationCenter defaultCenter] removeObserver:activityIndicator name:SDWebImageDownloadStartNotification object:nil];
        [[NSNotificationCenter defaultCenter] removeObserver:activityIndicator name:SDWebImageDownloadStopNotification object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:activityIndicator
                                                 selector:NSSelectorFromString(@"startActivity")
                                                     name:SDWebImageDownloadStartNotification object:nil];
        [[NSNotificationCenter defaultCenter] addObserver:activityIndicator
                                                 selector:NSSelectorFromString(@"stopActivity")
                                                     name:SDWebImageDownloadStopNotification object:nil];
    }
}

//类方法，返回单例对象
+ (nonnull instancetype)sharedDownloader {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

//初始化方法调用下面的初始化方法，NSURLSession运行在默认模式下
- (nonnull instancetype)init {
    return [self initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
}

//初始化方法
- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration {
    if ((self = [super init])) {
        //默认使用SDWebImageDownloaderOperation作为下载任务Operation
        _operationClass = [SDWebImageDownloaderOperation class];
        _shouldDecompressImages = YES;
        //默认下载顺序FIFO
        _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
        //创建NSOperationQueue并设置最大并发数为6，即同时最多可以下载6张图片
        _downloadQueue = [NSOperationQueue new];
        _downloadQueue.maxConcurrentOperationCount = 6;
        //设置名称
        _downloadQueue.name = @"com.hackemist.SDWebImageDownloader";
        //设置下载webp格式图片的http首部
        _URLOperations = [NSMutableDictionary new];
#ifdef SD_WEBP
        _HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy];
#else
        _HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy];
#endif
        //创建一个GCD并发队列
        _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
        //默认超时时间15s
        _downloadTimeout = 15.0;
        //创建一个sessionConfiguration运行默认的NSURLSession对象
        [self createNewSessionWithConfiguration:sessionConfiguration];
    }
    return self;
}

//创建指定运行模式的NSURLSession对象，可能已经创建过了
- (void)createNewSessionWithConfiguration:(NSURLSessionConfiguration *)sessionConfiguration {
    //可能已经创建过，所以需要取消前一个session的下载任务并打破引用循环
    [self cancelAllDownloads];

    if (self.session) {
        [self.session invalidateAndCancel];
    }

    sessionConfiguration.timeoutIntervalForRequest = self.downloadTimeout;

    self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration
                                                 delegate:self
                                            delegateQueue:nil];
}
//在析构函数中调用session的invalidateAndCancel方法取消下载任务并打破引用循环
- (void)dealloc {
    [self.session invalidateAndCancel];
    self.session = nil;
    //NSOperationQueue取消所有的下载操作
    [self.downloadQueue cancelAllOperations];
    //释放GCD队列
    SDDispatchQueueRelease(_barrierQueue);
}

//为http首部设置键值对
- (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field {
    if (value) {
        self.HTTPHeaders[field] = value;
    } else {
        [self.HTTPHeaders removeObjectForKey:field];
    }
}

//获取http首部的值
- (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field {
    return self.HTTPHeaders[field];
}

//设置最大同时下载图片的数量，即NSOperationQueue最大并发数
- (void)setMaxConcurrentDownloads:(NSInteger)maxConcurrentDownloads {
    _downloadQueue.maxConcurrentOperationCount = maxConcurrentDownloads;
}

//当前正在下载图片数量，即NSOperationQueue中正在执行的operation数量
- (NSUInteger)currentDownloadCount {
    return _downloadQueue.operationCount;
}

//获取最大同时下载图片的数量
- (NSInteger)maxConcurrentDownloads {
    return _downloadQueue.maxConcurrentOperationCount;
}

//获取NSURLSession的运行模式配置
- (NSURLSessionConfiguration *)sessionConfiguration {
    return self.session.configuration;
}

//设置operation的Class类对象
- (void)setOperationClass:(nullable Class)operationClass {
    if (operationClass && [operationClass isSubclassOfClass:[NSOperation class]] && [operationClass conformsToProtocol:@protocol(SDWebImageDownloaderOperationInterface)]) {
        _operationClass = operationClass;
    } else {
        _operationClass = [SDWebImageDownloaderOperation class];
    }
}

//下载图片的方法
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    __weak SDWebImageDownloader *wself = self;
    //直接调用另一个方法，后面大片的block代码目的就是为了创建一个SDWebImageDownloaderOperation类的对象
    return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{
        //block中为了防止引用循环和空指针，先weak后strong
        __strong __typeof (wself) sself = wself;
        //设置超时时间
        NSTimeInterval timeoutInterval = sself.downloadTimeout;
        if (timeoutInterval == 0.0) {
            timeoutInterval = 15.0;
        }

        //设置缓存策略
        NSURLRequestCachePolicy cachePolicy = options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData;
        //创建一个可变的request镀锡
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url
                                                                    cachePolicy:cachePolicy
                                                                timeoutInterval:timeoutInterval];
        //设置cookie的处理策略
        request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
        request.HTTPShouldUsePipelining = YES;
        //过滤http首部
        if (sself.headersFilter) {
            request.allHTTPHeaderFields = sself.headersFilter(url, [sself.HTTPHeaders copy]);
        }
        else {
            request.allHTTPHeaderFields = sself.HTTPHeaders;
        }
        //创建一个SDWebImageDownloaderOperation类的对象
        //传入request、session和下载选项配置options
        SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];
        //设置是否压缩图片
        operation.shouldDecompressImages = sself.shouldDecompressImages;
        //设置认证凭证和https相关
        if (sself.urlCredential) {
            operation.credential = sself.urlCredential;
        } else if (sself.username && sself.password) {
            operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];
        }
        //设置下载优先级
        if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
        }
        //向队列中添加创建的下载任务，之后这个operation就会被线程调度来执行其start方法
        [sself.downloadQueue addOperation:operation];
        //如果是LIFO就设置一个依赖
        if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
            // Emulate LIFO execution order by systematically adding new operations as last operation's dependency
            [sself.lastAddedOperation addDependency:operation];
            sself.lastAddedOperation = operation;
        }
        //返回operation
        return operation;
    }];
}
//取消一个下载任务，需要传入上一个方法返回的token，其实具体的token是由下一个方法创建的
- (void)cancel:(nullable SDWebImageDownloadToken *)token {
    //异步执行，阻塞队列从而串行的删除任务，可以避免竞争条件的产生
    dispatch_barrier_async(self.barrierQueue, ^{
        //通过token的url获取到这个Operation
        SDWebImageDownloaderOperation *operation = self.URLOperations[token.url];
        //调用Operation自定义的cancel方法来取消任务，传入一个回调块字典的token
        BOOL canceled = [operation cancel:token.downloadOperationCancelToken];
        //如果取消了就从字典中移除掉这个键值对
        if (canceled) {
            [self.URLOperations removeObjectForKey:token.url];
        }
    });
}

/*
前面download方法调用的方法，返回一个token
createCallback就是download方法写的，用于创建一个SDWebImageDownloaderOperation
*/
- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
                                           completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
                                                   forURL:(nullable NSURL *)url
                                           createCallback:(SDWebImageDownloaderOperation *(^)(void))createCallback {
    //URL为nil就调用下载完成回调块，返回nil
    if (url == nil) {
        if (completedBlock != nil) {
            completedBlock(nil, nil, nil, NO);
        }
        return nil;
    }
    //定义一个token变量
    __block SDWebImageDownloadToken *token = nil;
    //同步方法，阻塞当前线程也阻塞队列，防止产生竞争条件
    dispatch_barrier_sync(self.barrierQueue, ^{
        //通过URL获取Operation
        SDWebImageDownloaderOperation *operation = self.URLOperations[url];
        
        if (!operation) {
            //如果URL对应的Operation不存在就调用，createCallback块创建一个
            operation = createCallback();
            //添加进字典中
            self.URLOperations[url] = operation;

            __weak SDWebImageDownloaderOperation *woperation = operation;
            //设置Operation下载完成的回调块
            operation.completionBlock = ^{
                //同步方法阻塞当前线程阻塞队列
                dispatch_barrier_sync(self.barrierQueue, ^{
                    //先weak后strong
                    SDWebImageDownloaderOperation *soperation = woperation;
                    if (!soperation) return;
                    //下载完成就从字典中删除
                    if (self.URLOperations[url] == soperation) {
                        [self.URLOperations removeObjectForKey:url];
                    };
                });
            };
        }
        //取消下载任务时的token，第一个值就是url，第二个值就是回调块字典
        id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];

        token = [SDWebImageDownloadToken new];
        token.url = url;
        token.downloadOperationCancelToken = downloadOperationCancelToken;
    });

    return token;
}

//设置是否挂起下载队列
- (void)setSuspended:(BOOL)suspended {
    self.downloadQueue.suspended = suspended;
}

//取消所有的下载任务
- (void)cancelAllDownloads {
    [self.downloadQueue cancelAllOperations];
}

#pragma mark Helper methods
//通过NSURLSessionTask找到NSOPerationQueue里的任务
- (SDWebImageDownloaderOperation *)operationWithTask:(NSURLSessionTask *)task {
    SDWebImageDownloaderOperation *returnOperation = nil;
    for (SDWebImageDownloaderOperation *operation in self.downloadQueue.operations) {
        if (operation.dataTask.taskIdentifier == task.taskIdentifier) {
            returnOperation = operation;
            break;
        }
    }
    return returnOperation;
}

#pragma mark NSURLSessionDataDelegate
//下面这些代理方法，都是先通过NSURLSessionTask找到对应的SDWebImageDownloaderOperation
//然后调用Operation的代理方法，
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {

    // Identify the operation that runs this task and pass it the delegate method
    SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask];

    [dataOperation URLSession:session dataTask:dataTask didReceiveResponse:response completionHandler:completionHandler];
}

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {

    // Identify the operation that runs this task and pass it the delegate method
    SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask];

    [dataOperation URLSession:session dataTask:dataTask didReceiveData:data];
}

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
 willCacheResponse:(NSCachedURLResponse *)proposedResponse
 completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler {

    // Identify the operation that runs this task and pass it the delegate method
    SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask];

    [dataOperation URLSession:session dataTask:dataTask willCacheResponse:proposedResponse completionHandler:completionHandler];
}

#pragma mark NSURLSessionTaskDelegate

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    // Identify the operation that runs this task and pass it the delegate method
    SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:task];

    [dataOperation URLSession:session task:task didCompleteWithError:error];
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task willPerformHTTPRedirection:(NSHTTPURLResponse *)response newRequest:(NSURLRequest *)request completionHandler:(void (^)(NSURLRequest * _Nullable))completionHandler {
    
    completionHandler(request);
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {

    // Identify the operation that runs this task and pass it the delegate method
    SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:task];

    [dataOperation URLSession:session task:task didReceiveChallenge:challenge completionHandler:completionHandler];
}

@end