SDWebImage是一个非常好用的图片下载框架，这里记录一下，一是加深印象方，二是方便自己回顾
下面两张图是SDWebImage官网上的图，第一张是类图，第二张是时序图
SDWebImageClassDiagram
通过类图，我们来看下主要模块
- UIKit的扩展 - 方便我们调用SDWebImage的功能类
- SDWebImageManager - 该功能模块是用来分配任务
- SDImageCache - 缓存模块
- SDWebImageDownloader - 下载模块
SDWebImageSequenceDiagram
通过时序图，我们来看下图片的加载流程
- 拿到图片URL，先去内存查找，如果找到返回图片，否则进行下一步
- 去磁盘查找图片，如果找到返回图片，并写入内存方便下次查找，否则进行下一步
- 进行网络请求，请求得到图片，写入磁盘存入内存方便下次查找，并返回图片
下面我们主要分析下缓存模块与下载模块
缓存模块
内存缓存-SDMemoryCache：该类继承自NSCache
设置内存
- (void)setObject:(id)obj forKey:(id)key cost:(NSUInteger)g {
[super setObject:obj forKey:key cost:g];
if (!self.config.shouldUseWeakMemoryCache) {
return;
}
if (key && obj) {
// Store weak cache
SD_LOCK(self.weakCacheLock);
[self.weakCache setObject:obj forKey:key];
SD_UNLOCK(self.weakCacheLock);
}
}
shouldUseWeakMemoryCache这个值可以控制是否将obj存入weakCache对象，如果设置为否，则只存入NSCache中，这将影响我们下面的查找流程
- (id)objectForKey:(id)key {
id obj = [super objectForKey:key];
if (!self.config.shouldUseWeakMemoryCache) {
return obj;
}
if (key && !obj) {
// Check weak cache
SD_LOCK(self.weakCacheLock);
obj = [self.weakCache objectForKey:key];
SD_UNLOCK(self.weakCacheLock);
if (obj) {
// Sync cache
NSUInteger cost = 0;
if ([obj isKindOfClass:[UIImage class]]) {
cost = [(UIImage *)obj sd_memoryCost];
}
[super setObject:obj forKey:key cost:cost];
}
}
return obj;
}
因为NSCache的清理我们不可控，所以我们存入的值可能会被清理掉，这个时候如果我们shouldUseWeakMemoryCache这个值为YES，那我们在查找存入NSCache的值时，如果查找为空，就会去weakCache里进行一次查找。
注意：
NSMapTable：可以为我们提供更多的内存语义
self.weakCache = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory capacity:0];
当前的value是弱引用的，不会对之外的对象产生影响，当value被释放时，NSMapTable会删除这个key-value
内存缓存的总结，首先内存缓存类SDMemoryCache继承自NSCache，并定义了一个NSMapTable的属性weakCache，然后通过config里的shouldUseWeakMemoryCache这个属性去控制，我们存入的内容是否存入到我们自己的weakCache中，如果存入，那系统清理NSCache取不到存入NSCache中值时，就会去weakCache中去取。
磁盘缓存
创建一个目录，
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns {
return [self initWithNamespace:ns diskCacheDirectory:nil];
}
- (nullable NSString *)userCacheDirectory {
NSArray<NSString *> *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
return paths.firstObject;
}
并以图片路径的MD5为图片名称
static inline NSString * _Nonnull SDDiskCacheFileNameForKey(NSString * _Nullable key) {
const char *str = key.UTF8String;
if (str == NULL) {
str = "";
}
unsigned char r[CC_MD5_DIGEST_LENGTH];
CC_MD5(str, (CC_LONG)strlen(str), r);
NSURL *keyURL = [NSURL URLWithString:key];
NSString *ext = keyURL ? keyURL.pathExtension : key.pathExtension;
// File system has file name length limit, we need to check if ext is too long, we don't add it to the filename
if (ext.length > SD_MAX_FILE_EXTENSION_LENGTH) {
ext = nil;
}
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], ext.length == 0 ? @"" : [NSString stringWithFormat:@".%@", ext]];
return filename;
}
在了解了内存与磁盘模块后，我们来看下这个双缓存是如何一个查找流程：
- SDWebImageManager中loadImageWithURL ->
callCacheProcessForOperation
- (void)callCacheProcessForOperation:(nonnull SDWebImageCombinedOperation *)operation
url:(nonnull NSURL *)url
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
// Check whether we should query cache
BOOL shouldQueryCache = !SD_OPTIONS_CONTAINS(options, SDWebImageFromLoaderOnly);
if (shouldQueryCache) {
NSString *key = [self cacheKeyForURL:url context:context];
@weakify(operation);
operation.cacheOperation = [self.imageCache queryImageForKey:key options:options context:context completion:^(UIImage * _Nullable cachedImage, NSData * _Nullable cachedData, SDImageCacheType cacheType) {
@strongify(operation);
if (!operation || operation.isCancelled) {
// Image combined operation cancelled by user
[self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorCancelled userInfo:@{NSLocalizedDescriptionKey : @"Operation cancelled by user during querying the cache"}] url:url];
[self safelyRemoveOperationFromRunning:operation];
return;
}
// Continue download process
[self callDownloadProcessForOperation:operation url:url options:options context:context cachedImage:cachedImage cachedData:cachedData cacheType:cacheType progress:progressBlock completed:completedBlock];
}];
} else {
// Continue download process
[self callDownloadProcessForOperation:operation url:url options:options context:context cachedImage:nil cachedData:nil cacheType:SDImageCacheTypeNone progress:progressBlock completed:completedBlock];
}
}
在上述方法中执行了SDImageCache的实例imageCache的queryImageForKey方法，再调用queryCacheOperationForKey方法
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options context:(nullable SDWebImageContext *)context done:(nullable SDImageCacheQueryCompletionBlock)doneBlock {
...
// First check the in-memory cache...
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
if (options & SDImageCacheDecodeFirstFrameOnly) {
// Ensure static image
Class animatedImageClass = image.class;
if (image.sd_isAnimated || ([animatedImageClass isSubclassOfClass:[UIImage class]] && [animatedImageClass conformsToProtocol:@protocol(SDAnimatedImage)])) {
#if SD_MAC
image = [[NSImage alloc] initWithCGImage:image.CGImage scale:image.scale orientation:kCGImagePropertyOrientationUp];
#else
image = [[UIImage alloc] initWithCGImage:image.CGImage scale:image.scale orientation:image.imageOrientation];
#endif
}
} else if (options & SDImageCacheMatchAnimatedImageClass) {
// Check image class matching
Class animatedImageClass = image.class;
Class desiredImageClass = context[SDWebImageContextAnimatedImageClass];
if (desiredImageClass && ![animatedImageClass isSubclassOfClass:desiredImageClass]) {
image = nil;
}
}
}
BOOL shouldQueryMemoryOnly = (image && !(options & SDImageCacheQueryMemoryData));
if (shouldQueryMemoryOnly) {
if (doneBlock) {
doneBlock(image, nil, SDImageCacheTypeMemory);
}
return nil;
}
// Second check the disk cache...
NSOperation *operation = [NSOperation new];
// Check whether we need to synchronously query disk
// 1. in-memory cache hit & memoryDataSync
// 2. in-memory cache miss & diskDataSync
BOOL shouldQueryDiskSync = ((image && options & SDImageCacheQueryMemoryDataSync) ||
(!image && options & SDImageCacheQueryDiskDataSync));
void(^queryDiskBlock)(void) = ^{
if (operation.isCancelled) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return;
}
@autoreleasepool {
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage;
SDImageCacheType cacheType = SDImageCacheTypeNone;
if (image) {
// the image is from in-memory cache, but need image data
diskImage = image;
cacheType = SDImageCacheTypeMemory;
} else if (diskData) {
cacheType = SDImageCacheTypeDisk;
// decode image data only if in-memory cache missed
diskImage = [self diskImageForKey:key data:diskData options:options context:context];
if (diskImage && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = diskImage.sd_memoryCost;
[self.memoryCache setObject:diskImage forKey:key cost:cost];
}
}
if (doneBlock) {
if (shouldQueryDiskSync) {
doneBlock(diskImage, diskData, cacheType);
} else {
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, diskData, cacheType);
});
}
}
}
};
// Query in ioQueue to keep IO-safe
if (shouldQueryDiskSync) {
dispatch_sync(self.ioQueue, queryDiskBlock);
} else {
dispatch_async(self.ioQueue, queryDiskBlock);
}
return operation;
}
至此，完成图片缓存查找的流程。
- (void)storeImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
forKey:(nullable NSString *)key
toMemory:(BOOL)toMemory
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock {
...
// if memory cache is enabled
if (toMemory && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = image.sd_memoryCost;
[self.memoryCache setObject:image forKey:key cost:cost];
}
if (toDisk) {
...
[self _storeImageDataToDisk:data forKey:key];
...
} else {
if (completionBlock) {
completionBlock();
}
}
}
通过上面的源码，我们知道了图片在内存中存储的是UIImage对象，在磁盘中存储的则是NSData数据流
下载流程
两个重要的类：
SDWebImageDownloader - 配置下载相关的参数、控制下载队列的先后顺序及最大任务数
SDWebImageDownloaderOperation - 具体的下载任务
下载流程代码大体执行顺序
调用SDWebImageManager的loadImageWithURL
-callCacheProcessForOperation
-缓存查找失败后
-callDownloadProcessForOperation
-调用imageLoader的requestImageWithURL
-downloadImageWithURL
到此downloadImageWithURL会创建任务，并将任务加入OperationQueue，当任务执行时，调用start方法，开始发起请求
注：
downloadImageWithURL中调用的createDownloaderOperationWithUrl方法创建一个Operation对象时，可以控制任务执行的优先级
- (nullable NSOperation<SDWebImageDownloaderOperation> *)createDownloaderOperationWithUrl:(nonnull NSURL *)url
options:(SDWebImageDownloaderOptions)options
context:(nullable SDWebImageContext *)context {
...
if (self.config.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
// Emulate LIFO execution order by systematically, each previous adding operation can dependency the new operation
// This can gurantee the new operation to be execulated firstly, even if when some operations finished, meanwhile you appending new operations
// Just make last added operation dependents new operation can not solve this problem. See test case #test15DownloaderLIFOExecutionOrder
for (NSOperation *pendingOperation in self.downloadQueue.operations) {
[pendingOperation addDependency:operation];
}
}
return operation;
}
这个方法最后，当任务优先级选择LIFO，也就是后进先出时，会将所有任务执行条件依赖于当前任务执行结束后
- (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
NSOperation<SDWebImageDownloaderOperation> *dataOperation = [self operationWithTask:dataTask];
if ([dataOperation respondsToSelector:@selector(URLSession:dataTask:didReceiveResponse:completionHandler:)]) {
[dataOperation URLSession:session dataTask:dataTask didReceiveResponse:response completionHandler:completionHandler];
} else {
if (completionHandler) {
completionHandler(NSURLSessionResponseAllow);
}
}
}
这个地方将SDWebImageDownloader中请求代理的响应统一放到了SDWebImageDownloaderOperation类中
生活如此美好，今天就点到为止。。。
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