SDWebImage源代码解析（二）Downloader

前言

本文简书地址：http://www.jianshu.com/p/685e9b7ec4b2
本文的中文注释代码demo更新在我的github上。

上篇文章讲解的了SDWebImage的Cache部分，这篇讲讲一下Download部分。

Download

Download部分主要包含如下2个方法
* SDWebImageDownloader
图片下载控制类
* SDWebImageDownloaderOperation
NSOperation子类

SDWebImageDownloader

SDWebImageDownloader主要包含以下内容：
* 初始化信息
* 相关请求信息设置与获取
* 请求图片方法
* NSURLSession相关回调

初始化信息

SDWebImageDownloader.h中的property声明

//下载完成执行顺序
typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
    //先进先出
    SDWebImageDownloaderFIFOExecutionOrder,
    //先进后出
    SDWebImageDownloaderLIFOExecutionOrder
};

@interface SDWebImageDownloader : NSObject
//是否压缩图片
@property (assign, nonatomic) BOOL shouldDecompressImages;
//最大下载数量
@property (assign, nonatomic) NSInteger maxConcurrentDownloads;
//当前下载数量
@property (readonly, nonatomic) NSUInteger currentDownloadCount;
//下载超时时间
@property (assign, nonatomic) NSTimeInterval downloadTimeout;
//下载策略
@property (assign, nonatomic) SDWebImageDownloaderExecutionOrder executionOrder;

SDWebImageDownloader.m中的Extension

@interface SDWebImageDownloader () <NSURLSessionTaskDelegate, NSURLSessionDataDelegate>
//下载的queue
@property (strong, nonatomic) NSOperationQueue *downloadQueue;
//上一个添加的操作
@property (weak, nonatomic) NSOperation *lastAddedOperation;
//操作类
@property (assign, nonatomic) Class operationClass;
//url请求缓存
@property (strong, nonatomic) NSMutableDictionary *URLCallbacks;
//请求头
@property (strong, nonatomic) NSMutableDictionary *HTTPHeaders;
// This queue is used to serialize the handling of the network responses of all the download operation in a single queue
//这个queue为了能否序列化处理所有网络结果的返回
@property (SDDispatchQueueSetterSementics, nonatomic) dispatch_queue_t barrierQueue;

// The session in which data tasks will run
//数据runsession
@property (strong, nonatomic) NSURLSession *session;

@end

相关方法实现如下：

//判断用了SDNetworkActivityIndicator，去替换该方法的内容
+ (void)initialize {
    // Bind SDNetworkActivityIndicator if available (download it here: http://github.com/rs/SDNetworkActivityIndicator )
    // To use it, just add #import "SDNetworkActivityIndicator.h" in addition to the SDWebImage import
    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

        // Remove observer in case it was previously added.
        [[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];
    }
}

//单例
+ (SDWebImageDownloader *)sharedDownloader {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

//初始化方法
- (id)init {
    if ((self = [super init])) {
        _operationClass = [SDWebImageDownloaderOperation class];
        _shouldDecompressImages = YES;
        _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
        _downloadQueue = [NSOperationQueue new];
        _downloadQueue.maxConcurrentOperationCount = 6;
        _downloadQueue.name = @"com.hackemist.SDWebImageDownloader";
        _URLCallbacks = [NSMutableDictionary new];
#ifdef SD_WEBP
        _HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy];
#else
        _HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy];
#endif
        _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
        _downloadTimeout = 15.0;

        NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
        sessionConfig.timeoutIntervalForRequest = _downloadTimeout;

        /**
         *  Create the session for this task
         *  We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate
         *  method calls and completion handler calls.
         */
        //session初始化
        self.session = [NSURLSession sessionWithConfiguration:sessionConfig
                                                     delegate:self
                                                delegateQueue:nil];
    }
    return self;
}

- (void)dealloc {
    //停止session
    [self.session invalidateAndCancel];
    self.session = nil;

    //停止所有downloadqueue
    [self.downloadQueue cancelAllOperations];
    SDDispatchQueueRelease(_barrierQueue);
}

从这边也可以看出，SDWebImageDownloader的下载方法就是用NSOperation。使用NSOperationQueue去控制最大操作数量和取消所有操作，在NSOperation中运行NSUrlSession方法请求参数。

相关请求信息设置与获取

相关方法如下：

//设置hedaer头
- (void)setValue:(NSString *)value forHTTPHeaderField:(NSString *)field {
    if (value) {
        self.HTTPHeaders[field] = value;
    }
    else {
        [self.HTTPHeaders removeObjectForKey:field];
    }
}

- (NSString *)valueForHTTPHeaderField:(NSString *)field {
    return self.HTTPHeaders[field];
}

//设置多大并发数量
- (void)setMaxConcurrentDownloads:(NSInteger)maxConcurrentDownloads {
    _downloadQueue.maxConcurrentOperationCount = maxConcurrentDownloads;
}

- (NSUInteger)currentDownloadCount {
    return _downloadQueue.operationCount;
}

- (NSInteger)maxConcurrentDownloads {
    return _downloadQueue.maxConcurrentOperationCount;
}

//设置operation的类型，可以自行再继承子类去实现
- (void)setOperationClass:(Class)operationClass {
    _operationClass = operationClass ?: [SDWebImageDownloaderOperation class];
}

//暂停下载
- (void)setSuspended:(BOOL)suspended {
    [self.downloadQueue setSuspended:suspended];
}
//取消所有下载
- (void)cancelAllDownloads {
    [self.downloadQueue cancelAllOperations];
}

这一部分唯一提一下的是setOperationClass:方法，是为了可以自己实现NSOperation的子类去完成相关请求的设置。

请求图片方法

相关方法实现如下：

//请求图片方法
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
    __block SDWebImageDownloaderOperation *operation;
    __weak __typeof(self)wself = self;

    //处理新请求的封装
    [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^{
        NSTimeInterval timeoutInterval = wself.downloadTimeout;
        if (timeoutInterval == 0.0) {
            timeoutInterval = 15.0;
        }

        // In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise
        //为了防止潜在的重复cache
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
        //设置是否处理cookies
        request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
        request.HTTPShouldUsePipelining = YES;
        //有header处理block，则调用block，返回headerfieleds
        if (wself.headersFilter) {
            request.allHTTPHeaderFields = wself.headersFilter(url, [wself.HTTPHeaders copy]);
        }
        else {
            request.allHTTPHeaderFields = wself.HTTPHeaders;
        }
        //这边的创建方式允许自己定义SDWebImageDownloaderOperation的子类进行替换
        operation = [[wself.operationClass alloc] initWithRequest:request
                                                        inSession:self.session