SDWebImage源码解析

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 <SDWebImageOperation>)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