在开发项目的过程中会用到很多第三方库，比如AFNetWorking,SDWebImage,FMDB等，但一直都没去好好的研究一下，最近刚好项目不是太紧，闲下来可以给自己充充电，先研究一下SDWebImage的底层实现，源码地址：SDWebImage
先介绍一下SDWebImage，我们使用较多的是它提供的UIImageView分类，支持从远程服务器下载并缓存图片。自从iOS5.0开始，NSURLCache也可以处理磁盘缓存，那么SDWebImage的优势在哪？首先NSURLCache是缓存原始数据(raw data)到磁盘或内存，因此每次使用的时候需要将原始数据转换成具体的对象，如UIImage等，这会导致额外的数据解析以及内存占用等，而SDWebImage则是缓存UIImage对象在内存，缓存在NSCache中，同时直接保存压缩过的图片到磁盘中；还有一个问题是当你第一次在UIImageView中使用image对象的时候，图片的解码是在主线程中运行的！而SDWebImage会强制将解码操作放到子线程中。下图是SDWebImage简单的类图关系：
下面从UIImageView的图片加载开始看起，Let's go! 首先我们在给UIImageView设置图片的时候会调用方法： -
(void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;
其中url为远程图片的地址，而placeholder为预显示的图片。 其实还可以添加一些额外的参数，比如图片选项
SDWebImageOptions typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) { SDWebImageRetryFailed = 1 一般使用的是SDWebImageRetryFailed | SDWebImageLowPriority，
下面看看具体的函数调用：
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock { [self sd_cancelCurrentImageLoad];//取消正在下载的操作 objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);//关联该view对应的图片URL /*...*/ if (url) { __weak UIImageView *wself = self;//防止retain cricle //由SDWebImageManager负责图片的获取 id operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) { /*获取图片到主线层显示*/ }]; [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"]; } }
可以看出图片是从服务端、内存或者硬盘获取是由SDWebImageManager管理的，这个类有几个重要的属性：
@property (strong, nonatomic, readwrite) SDImageCache *imageCache;//负责管理cache，涉及内存缓存和硬盘保存 @property (strong, nonatomic, readwrite) SDWebImageDownloader *imageDownloader;//负责从网络下载图片 @property (strong, nonatomic) NSMutableArray *runningOperations;//包含所有当前正在下载的操作对象
manager会根据URL先去imageCache中查找对应的图片，如果没有在使用downloader去下载，并在下载完成缓存图片到imageCache，接着看实现：
- (id )downloadImageWithURL:(NSURL *)url options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionWithFinishedBlock)completedBlock { /*...*/ //根据URL生成对应的key，没有特殊处理为[url absoluteString]; NSString *key = [self cacheKeyForURL:url]; //去imageCache中寻找图片 operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) { /*...*/ //如果图片没有找到，或者采用的SDWebImageRefreshCached选项，则从网络下载 if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) { dispatch_main_sync_safe(^{ //如果图片找到了，但是采用的SDWebImageRefreshCached选项，通知获取到了图片，并再次从网络下载，使NSURLCache重新刷新 completedBlock(image, nil, cacheType, YES, url); }); } /*下载选项设置*/ //使用imageDownloader开启网络下载 id subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) { /*...*/ if (downloadedImage && finished) { //下载完成后，先将图片保存到imageCache中，然后主线程返回 [self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk]; } dispatch_main_sync_safe(^{ if (!weakOperation.isCancelled) { completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url); } }); } } /*...*/ } else if (image) { //在cache中找到图片了，直接返回 dispatch_main_sync_safe(^{ if (!weakOperation.isCancelled) { completedBlock(image, nil, cacheType, YES, url); } }); } }]; return operation; }
下面先看downloader从网络下载的过程，下载是放在NSOperationQueue中进行的，默认maxConcurrentOperationCount为6，timeout时间为15s：
- (id)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock { __block SDWebImageDownloaderOperation *operation; __weak SDWebImageDownloader *wself = self; /*...*/ //防止NSURLCache和SDImageCache重复缓存 NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy :NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval]; request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies); request.HTTPShouldUsePipelining = YES; request.allHTTPHeaderFields = wself.HTTPHeaders;//设置http头部 //SDWebImageDownloaderOperation派生自NSOperation，负责图片下载工作 operation = [[SDWebImageDownloaderOperation alloc] initWithRequest:request options:options progress:^(NSInteger receivedSize, NSInteger expectedSize) {} completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {} cancelled:^{}]; operation.shouldDecompressImages = wself.shouldDecompressImages;//是否需要解码 if (wself.username && wself.password) { operation.credential = [NSURLCredential credentialWithUser:wself.username password:wself.password persistence:NSURLCredentialPersistenceForSession]; } if (options & SDWebImageDownloaderHighPriority) { operation.queuePriority = NSOperationQueuePriorityHigh; } else if (options & SDWebImageDownloaderLowPriority) { operation.queuePriority = NSOperationQueuePriorityLow; } [wself.downloadQueue addOperation:operation]; if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) { // 如果下载顺序是后面添加的先运行 [wself.lastAddedOperation addDependency:operation]; wself.lastAddedOperation = operation; } }]; return operation; }
SDWebImageDownloaderOperation派生自NSOperation，通过NSURLConnection进行图片的下载，为了确保能够处理下载的数据，需要在后台运行runloop：
- (void)start { /*...*/ #if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0 //开启后台下载 if ([self shouldContinueWhenAppEntersBackground]) { __weak __typeof__ (self) wself = self; self.backgroundTaskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{ __strong __typeof (wself) sself = wself; if (sself) { [sself cancel]; [[UIApplication sharedApplication] endBackgroundTask:sself.backgroundTaskId]; sself.backgroundTaskId = UIBackgroundTaskInvalid; } }]; } #endif self.executing = YES; self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self startImmediately:NO]; } [self.connection start]; if (self.connection) { if (self.progressBlock) { self.progressBlock(0, NSURLResponseUnknownLength); } //在主线程发通知，这样也保证在主线程收到通知 dispatch_async(dispatch_get_main_queue(), ^{ [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self]; }); CFRunLoopRun();//在默认模式下运行当前runlooprun，直到调用CFRunLoopStop停止运行 if (!self.isFinished) { [self.connection cancel]; [self connection:self.connection didFailWithError:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorTimedOut userInfo:@{NSURLErrorFailingURLErrorKey : self.request.URL}]]; } } #if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0 if (self.backgroundTaskId != UIBackgroundTaskInvalid) { [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskId]; self.backgroundTaskId = UIBackgroundTaskInvalid; } #endif } 下载过程中，在代理 － (void)connection:(NSURLConnection )connection didReceiveData:(NSData )data中将接收到的数据保存到NSMutableData中，[self.imageData appendData:data]，下载完成后在该线程完成图片的解码，并在完成的completionBlock中进行imageCache的缓存：
- (void)connectionDidFinishLoading:(NSURLConnection *)aConnection { SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock; @synchronized(self) { CFRunLoopStop(CFRunLoopGetCurrent());//停止当前对runloop /*...*/ if (completionBlock) { /*...*/ UIImage *image = [UIImage sd_imageWithData:self.imageData]; NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL]; image = [self scaledImageForKey:key image:image]; // Do not force decoding animated GIFs if (!image.images) { if (self.shouldDecompressImages) { image = [UIImage decodedImageWithImage:image];//图片解码 } } if (CGSizeEqualToSize(image.size, CGSizeZero)) { completionBlock(nil, nil, [NSError errorWithDomain:@"SDWebImageErrorDomain" code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}], YES); } else { completionBlock(image, self.imageData, nil, YES); } } } self.completionBlock = nil; [self done]; } SDWebImageCache管理着SDWebImage的缓存，其中内存缓存采用NSCache，同时会创建一个ioQueue负责对硬盘的读写，并且会添加观察者，在收到内存警告、关闭或进入后台时完成对应的处理:
- (id)init { _memCache = [[NSCache alloc] init]; _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL); //收到内存警告时，清除NSCache：[self.memCache removeAllObjects]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(clearMemory) name:UIApplicationDidReceiveMemoryWarningNotification object:nil]; //程序关闭时，会对硬盘文件做一些处理 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(cleanDisk) name:UIApplicationWillTerminateNotification object:nil]; //程序进入后台时，也会进行硬盘文件处理 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(backgroundCleanDisk) name:UIApplicationDidEnterBackgroundNotification object:nil]; }
查询图片
每次向SDWebImageCache索取图片的时候，会先根据图片URL对应的key值先检查内存中是否有对应的图片，如果有则直接返回；如果没有则在ioQueue中去硬盘中查找，其中文件名是是根据URL生成的MD5值，找到之后先将图片缓存在内存中，然后在把图片返回：
- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock { /*...*/ // 首先查找内存缓存 UIImage *image = [self imageFromMemoryCacheForKey:key]; if (image) { doneBlock(image, SDImageCacheTypeMemory); return nil; } //硬盘查找 NSOperation *operation = [NSOperation new]; dispatch_async(self.ioQueue, ^{ //创建自动释放池，内存及时释放 @autoreleasepool { UIImage *diskImage = [self diskImageForKey:key]; if (diskImage) { CGFloat cost = diskImage.size.height * diskImage.size.width * diskImage.scale * diskImage.scale; //缓存到NSCache中 [self.memCache setObject:diskImage forKey:key cost:cost]; } dispatch_async(dispatch_get_main_queue(), ^{ doneBlock(diskImage, SDImageCacheTypeDisk); }); } }); return operation; } 在硬盘查询的时候，会在后台将NSData转成UIImage，并完成相关的解码工作:
- (UIImage *)diskImageForKey:(NSString *)key { NSData *data = [self diskImageDataBySearchingAllPathsForKey:key]; if (data) { UIImage *image = [UIImage sd_imageWithData:data]; image = [self scaledImageForKey:key image:image]; if (self.shouldDecompressImages) { image = [UIImage decodedImageWithImage:image]; } return image; } else { return nil; } }
保存图片
当下载完图片后，会先将图片保存到NSCache中，并把图片像素大小作为该对象的cost值，同时如果需要保存到硬盘，会先判断图片的格式，PNG或者JPEG，并保存对应的NSData到缓存路径中，文件名为URL的MD5值：
- (NSString *)cachedFileNameForKey:(NSString *)key { //根据key生成对应的MD5值作为文件名 const char *str = [key UTF8String]; if (str == NULL) { str = ""; } unsigned char r[CC_MD5_DIGEST_LENGTH]; CC_MD5(str, (CC_LONG)strlen(str), r); NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x", r[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10], r[11], r[12], r[13], r[14], r[15]]; return filename; } - (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk { //保存到NSCache，cost为像素值 [self.memCache setObject:image forKey:key cost:image.size.height * image.size.width * image.scale * image.scale]; if (toDisk) { dispatch_async(self.ioQueue, ^{ NSData *data = imageData; if (image && (recalculate || !data)) { //判断图片格式 BOOL imageIsPng = YES; // 查看imagedata的前缀是否是PNG的前缀格式 if ([imageData length] >= [kPNGSignatureData length]) { imageIsPng = ImageDataHasPNGPreffix(imageData); } if (imageIsPng) { data = UIImagePNGRepresentation(image); } else { data = UIImageJPEGRepresentation(image, (CGFloat)1.0); } } if (data) { if (![_fileManager fileExistsAtPath:_diskCachePath]) { [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL]; } //保存data到指定的路径中 [_fileManager createFileAtPath:[self defaultCachePathForKey:key] contents:data attributes:nil]; } }); } } 硬盘文件的管理
在程序退出或者进入后台时，会出图片文件进行管理，具体的策略：
清除过期的文件，默认一星期
如果设置了最大缓存，并且当前缓存的文件超过了这个限制，则删除最旧的文件，直到当前缓存文件的大小为最大缓存大小的一半
- (void)cleanDiskWithCompletionBlock:(SDWebImageNoParamsBlock)completionBlock { dispatch_async(self.ioQueue, ^{ NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES]; NSArray *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey]; // This enumerator prefetches useful properties for our cache files. NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL includingPropertiesForKeys:resourceKeys options:NSDirectoryEnumerationSkipsHiddenFiles errorHandler:NULL]; NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.maxCacheAge]; NSMutableDictionary *cacheFiles = [NSMutableDictionary dictionary]; NSUInteger currentCacheSize = 0; // Enumerate all of the files in the cache directory. This loop has two purposes: // // 1. Removing files that are older than the expiration date. // 2. Storing file attributes for the size-based cleanup pass. NSMutableArray *urlsToDelete = [[NSMutableArray alloc] init]; for (NSURL *fileURL in fileEnumerator) { NSDictionary *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:NULL]; // Skip directories. if ([resourceValues[NSURLIsDirectoryKey] boolValue]) { continue; } // Remove files that are older than the expiration date; NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey]; if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) { [urlsToDelete addObject:fileURL]; continue; } // Store a reference to this file and account for its total size. NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey]; currentCacheSize += [totalAllocatedSize unsignedIntegerValue]; [cacheFiles setObject:resourceValues forKey:fileURL]; } for (NSURL *fileURL in urlsToDelete) { [_fileManager removeItemAtURL:fileURL error:nil]; } // If our remaining disk cache exceeds a configured maximum size, perform a second // size-based cleanup pass. We delete the oldest files first. if (self.maxCacheSize > 0 && currentCacheSize > self.maxCacheSize) { // Target half of our maximum cache size for this cleanup pass. const NSUInteger desiredCacheSize = self.maxCacheSize / 2; // Sort the remaining cache files by their last modification time (oldest first). NSArray *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent usingComparator:^NSComparisonResult(id obj1, id obj2) { return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]]; }]; // Delete files until we fall below our desired cache size. for (NSURL *fileURL in sortedFiles) { if ([_fileManager removeItemAtURL:fileURL error:nil]) { NSDictionary *resourceValues = cacheFiles[fileURL]; NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey]; currentCacheSize -= [totalAllocatedSize unsignedIntegerValue]; if (currentCacheSize < desiredCacheSize) { break; } } } } if (completionBlock) { dispatch_async(dispatch_get_main_queue(), ^{ completionBlock(); }); } }); } 总结
接口设计简单通常我们使用较多的UIImageView分类：
[self.imageView sd_setImageWithURL:[NSURL URLWithString:@"url"] placeholderImage:[UIImage imageNamed:@"placeholder"]];
一个简单的接口将其中复杂的实现细节全部隐藏：简单就是美。
采用NSCache作为内存缓
耗时较长的请求，都采用异步形式，在回调函数块中处理请求结果
NSOperation和NSOperationQueue：可以取消任务处理队列中的任务，设置最大并发数，设置operation之间的依赖关系。
图片缓存清理的策略
dispatch_barrier_sync：前面的任务执行结束后它才执行，而且它后面的任务要等它执行完成之后才会执行。
使用weak self strong self 防止retain circle
如果子线程进需要不断处理一些事件，那么设置一个Run Loop是最好的处理方式