SDWebImage總體結(jié)構(gòu)
在平常的項目中司抱，對于加載網(wǎng)絡(luò)圖片蓖康，我們少不了用到SDWebImage铐炫。
僅需要調(diào)用UIImage分類的一行代碼(其余的加載圖片分類方法實際最終也會調(diào)到該方法)：
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock
SDWebImage會自動為我們完成加載cache，更新cache以及下載圖片的功能蒜焊，而且僅需要調(diào)用這一個接口倒信。
SDWebImage是如何做到這些功能的呢？
源碼面前無秘密泳梆，那就讓我們一起來研究學習SDWebImage鳖悠。
首先榜掌，上一張SDWebImage的總體結(jié)構(gòu)圖：
我們在使用SDWebImage時，會調(diào)用UIImageView的分類方法乘综，而在分類方法里面憎账，實際上會調(diào)用單例實例
SDWebImageManager，而對應(yīng)每一個加載圖片操作（這里所謂的加載卡辰，指讀取緩存或下載圖片胞皱，并更新緩存），都有一個SDWebImageCombinedOperation對象九妈，注意這個對象類的名字中含有的‘Combined’反砌，表明這個Operation，其實是讀寫緩存萌朱，下載文件多種操作的一個Combine宴树。
在SDWebImageManager中，又包含兩個只讀屬性
@property (strong, nonatomic, readonly) SDImageCache *imageCache;@property (strong, nonatomic, readonly) SDWebImageDownloader *imageDownloader;看名字就知道晶疼，imageCache負責Cache的讀寫及更新森渐，而imageDownloader則負責網(wǎng)絡(luò)下載圖片。
而在SDWebImageDownloader中冒晰，每一個下載操作，又對應(yīng)于一個SDWebImageDownloaderOperation對象竟块。
從上的結(jié)構(gòu)分析可以知道壶运，對于每個操作，SDWebImage實際會抽象出一個operation對象浪秘，這樣蒋情，就可以實現(xiàn)對于同時進行多個操作的一個管理，也就是對operation對象的管理耸携。
一張圖片是如何加載到UI的
看完上面SDWebImage的總體架構(gòu)圖棵癣，我們再結(jié)合實際代碼，來看看一個網(wǎng)絡(luò)圖片夺衍，是如何下載到本地的狈谊。
下面是UIImageView (WebCache)加載圖片的方法實現(xiàn)：
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
// step1. 先關(guān)掉之前的下載
[self sd_cancelCurrentImageLoad];
// step2. 利用runtime, 存儲當前下載的URL
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
if (!(options & SDWebImageDelayPlaceholder)) { // step3. 設(shè)置placeholder
dispatch_main_async_safe(^{ // dispatch_main_async_safe 宏 用于確保在mainqueue 上執(zhí)行block
self.image = placeholder;
});
}
// step4. 進入圖片 下載/讀緩存 處理
if (url) {
// check if activityView is enabled or not
if ([self showActivityIndicatorView]) {
[self addActivityIndicator];
}
__weak __typeof(self)wself = self;
// step4.1 調(diào)用 SDWebImageManager 單例下載
id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
[wself removeActivityIndicator];
if (!wself) return; // 由于是block回調(diào) 這時weakself 可能已經(jīng)釋放，若釋放沟沙，則return河劝，不做任何回調(diào)處理
dispatch_main_sync_safe(^{
if (!wself) return;
if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock)
{
completedBlock(image, error, cacheType, url);
return;
}
else if (image) {
wself.image = image;
[wself setNeedsLayout];
} else {
if ((options & SDWebImageDelayPlaceholder)) {
wself.image = placeholder;
[wself setNeedsLayout];
}
}
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
// step4.2 保存 當前下載所對應(yīng)的operation(目前operation僅支持cancel 接口)
[self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
} else { // 如果圖片url 為nil，則直接返回錯誤
dispatch_main_async_safe(^{
[self removeActivityIndicator];
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
completedBlock(nil, error, SDImageCacheTypeNone, url);
}
});
}
}
代碼通過注釋矛紫，也不難理解赎瞎。SDWebImage加載圖片，分為如下幾個步驟：
- 先關(guān)閉之前的下載
- 將當前下載圖片的url作為UIImageView的屬性（利用runtime實現(xiàn)）
- 如果有placeholder颊咬，則直接設(shè)置當前image為placeholder
- 調(diào)用SDWebImageManager單實例的downloadImageWithURL方法务甥，加載圖片（這里說加載而不是下載牡辽，是因為最終圖片來源可能是緩存），并返回一個id<SDWebImageOperation>對象（其實是一個SDWebImageCombinedOperation對象）敞临，代表當前的加載圖片操作态辛。
- 將id<SDWebImageOperation>對象保存，當需要取消加載時哟绊，僅需要調(diào)用id<SDWebImageOperation>協(xié)議方法cancel因妙。
- 在SDWebImageManger的downloadImageWithURL方法的回調(diào)block中，如果加載image圖片成功票髓，則設(shè)置UIImageIView的image屬性攀涵，并調(diào)用completedBlock，若失敗洽沟，則在completedBlock中返回失敗error以故。
以上就是通過SDWebImage加載圖片的整個流程，到目前為止裆操，流程很清楚怒详。
我們可以發(fā)現(xiàn)，這里的核心邏輯踪区，就在于調(diào)用了SDWebImageManger的downloadImageWithURL方法昆烁。那么，我們接下來看看SDWebImageManger的代碼缎岗。
SDWebImageManager
先看SDWebImageManager的頭文件：
/**
* The SDWebImageManager is the class behind the UIImageView+WebCache category and likes.
* It ties the asynchronous downloader (SDWebImageDownloader) with the image cache store (SDImageCache).
* You can use this class directly to benefit from web image downloading with caching in another context than
* a UIView.
*
* Here is a simple example of how to use SDWebImageManager:
*
* @code
SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager downloadImageWithURL:imageURL
options:0
progress:nil
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
if (image) {
// do something with image
}
}];
* @endcode
*/
@interface SDWebImageManager : NSObject
@property (weak, nonatomic) id <SDWebImageManagerDelegate> delegate;
@property (strong, nonatomic, readonly) SDImageCache *imageCache;
@property (strong, nonatomic, readonly) SDWebImageDownloader *imageDownloader;
/**
* The cache filter is a block used each time SDWebImageManager need to convert an URL into a cache key. This can
* be used to remove dynamic part of an image URL.
*
* The following example sets a filter in the application delegate that will remove any query-string from the
* URL before to use it as a cache key:
*
* @code
[[SDWebImageManager sharedManager] setCacheKeyFilter:^(NSURL *url) {
url = [[NSURL alloc] initWithScheme:url.scheme host:url.host path:url.path];
return [url absoluteString];
}];
* @endcode
*/
@property (nonatomic, copy) SDWebImageCacheKeyFilterBlock cacheKeyFilter;
/**
* Returns global SDWebImageManager instance.
*
* @return SDWebImageManager shared instance
*/
+ (SDWebImageManager *)sharedManager;
/**
* Allows to specify instance of cache and image downloader used with image manager.
* @return new instance of `SDWebImageManager` with specified cache and downloader.
*/
- (instancetype)initWithCache:(SDImageCache *)cache downloader:(SDWebImageDownloader *)downloader;
/**
* Downloads the image at the given URL if not present in cache or return the cached version otherwise.
*
* @param url The URL to the image
* @param options A mask to specify options to use for this request
* @param progressBlock A block called while image is downloading
* @param completedBlock A block called when operation has been completed.
*
* This parameter is required.
*
* This block has no return value and takes the requested UIImage as first parameter.
* In case of error the image parameter is nil and the second parameter may contain an NSError.
*
* The third parameter is an `SDImageCacheType` enum indicating if the image was retrieved from the local cache
* or from the memory cache or from the network.
*
* The last parameter is set to NO when the SDWebImageProgressiveDownload option is used and the image is
* downloading. This block is thus called repeatedly with a partial image. When image is fully downloaded, the
* block is called a last time with the full image and the last parameter set to YES.
*
* @return Returns an NSObject conforming to SDWebImageOperation. Should be an instance of SDWebImageDownloaderOperation
*/
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock;
/**
* Saves image to cache for given URL
*
* @param image The image to cache
* @param url The URL to the image
*
*/
- (void)saveImageToCache:(UIImage *)image forURL:(NSURL *)url;
/**
* Cancel all current operations
*/
- (void)cancelAll;
/**
* Check one or more operations running
*/
- (BOOL)isRunning;
/**
* Check if image has already been cached
*
* @param url image url
*
* @return if the image was already cached
*/
- (BOOL)cachedImageExistsForURL:(NSURL *)url;
/**
* Check if image has already been cached on disk only
*
* @param url image url
*
* @return if the image was already cached (disk only)
*/
- (BOOL)diskImageExistsForURL:(NSURL *)url;
/**
* Async check if image has already been cached
*
* @param url image url
* @param completionBlock the block to be executed when the check is finished
*
* @note the completion block is always executed on the main queue
*/
- (void)cachedImageExistsForURL:(NSURL *)url
completion:(SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
* Async check if image has already been cached on disk only
*
* @param url image url
* @param completionBlock the block to be executed when the check is finished
*
* @note the completion block is always executed on the main queue
*/
- (void)diskImageExistsForURL:(NSURL *)url
completion:(SDWebImageCheckCacheCompletionBlock)completionBlock;
/**
*Return the cache key for a given URL
*/
- (NSString *)cacheKeyForURL:(NSURL *)url;
@end
注釋很多静尼，其實真正的代碼并不多。注意到開頭的一段注釋（結(jié)合自己理解簡單翻譯一下）：
SDWebImageManager為UIImageView+WebCache加載圖片提供了核心實現(xiàn)传泊。它其實是管理了圖片異步的下載以及緩存處理鼠渺，同時，我們可以直接使用SDWebImageManager來加載網(wǎng)絡(luò)圖片眷细，而不僅僅是將其用于UIImageView中拦盹。所以，以后我們想下載網(wǎng)絡(luò)圖片溪椎，而不僅僅是為了做UI展示時普舆，可以直接使用SDWebImageManager來下載圖片以及緩存管理。
在這里池磁，我們還是先關(guān)注圖片是如何加載的奔害，除掉其他無關(guān)的代碼，SDWebImageManager可以簡化為：
@interface SDWebImageManager : NSObject
@property (weak, nonatomic) id <SDWebImageManagerDelegate> delegate;
@property (strong, nonatomic, readonly) SDImageCache *imageCache; // 處理緩存相關(guān)
@property (strong, nonatomic, readonly) SDWebImageDownloader *imageDownloader; // 處理圖片下載
@property (nonatomic, copy) SDWebImageCacheKeyFilterBlock cacheKeyFilter; // 用于過濾處理圖片的url（這里可先不關(guān)注）
// 表明我是一個單例
+ (SDWebImageManager *)sharedManager;
- (instancetype)initWithCache:(SDImageCache *)cache downloader:(SDWebImageDownloader *)downloader;
// 核心加載圖片方法 downloadImageWithURL
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock;
SDWebImageManager的利用downloadImageWithURL方法加載圖片地熄。我們就來分析它的源代碼：
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock {
// Invoking this method without a completedBlock is pointless
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
// Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, XCode won't
// throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.
if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
// Prevents app crashing on argument type error like sending NSNull instead of NSURL
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;
// 如果是曾經(jīng)失敗的url 且當前的options不需重新嘗試失敗的鏈接华临，則直接返回錯誤
BOOL isFailedUrl = NO;
@synchronized (self.failedURLs) {
isFailedUrl = [self.failedURLs containsObject:url];
}
if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
dispatch_main_sync_safe(^{
NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil];
completedBlock(nil, error, SDImageCacheTypeNone, YES, url);
});
return operation;
}
@synchronized (self.runningOperations) {
[self.runningOperations addObject:operation];
}
NSString *key = [self cacheKeyForURL:url];
// step1. 先讀取緩存
operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) {
// 讀取緩存結(jié)束(可能有緩存， 也可能沒有)
/////////////////////////////////////////////
if (operation.isCancelled) {
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
return;
}
//// 緩存沒有或必須刷新緩存等端考， 走network download路線
///////////////////////////////////////
if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
if (image && options & SDWebImageRefreshCached) {
dispatch_main_sync_safe(^{
// If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
// AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
completedBlock(image, nil, cacheType, YES, url);
});
}
// download if no image or requested to refresh anyway, and download allowed by delegate
/////////////////////////////////////////////////////////////////////
// step1. 將外部傳入的SDWebImageOptions 轉(zhuǎn)化為 SDWebImageDownloaderOptions
SDWebImageDownloaderOptions downloaderOptions = 0;
if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
if (image && options & SDWebImageRefreshCached) { // 特殊處理一下 SDWebImageRefreshCached 請求的options
// force progressive off if image already cached but forced refreshing
downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
// ignore image read from NSURLCache if image if cached but force refreshing
downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
}
// step2. 利用SDWebImageDownloader開始下載 并返回一個 id <SDWebImageOperation>subOperation 代表當前下載operation(這里利用operation的cancel block對subOperaton 進行retain)
id <SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (!strongOperation || strongOperation.isCancelled) {
// Do nothing if the operation was cancelled
// See #699 for more details
// if we would call the completedBlock, there could be a race condition between this block and another completedBlock for the same object, so if this one is called second, we will overwrite the new data
}
else if (error) {
dispatch_main_sync_safe(^{
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(nil, error, SDImageCacheTypeNone, finished, url);
}
});
if ( error.code != NSURLErrorNotConnectedToInternet
&& error.code != NSURLErrorCancelled
&& error.code != NSURLErrorTimedOut
&& error.code != NSURLErrorInternationalRoamingOff
&& error.code != NSURLErrorDataNotAllowed
&& error.code != NSURLErrorCannotFindHost
&& error.code != NSURLErrorCannotConnectToHost) {
@synchronized (self.failedURLs) {
[self.failedURLs addObject:url];
}
}
}
else {
if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}
// 下載成功雅潭，做cache
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
if (options & SDWebImageRefreshCached && image && !downloadedImage) { // 如果download是因為refresh cache引起揭厚，而又沒有真正的download image下來，什么也不做
// Image refresh hit the NSURLCache cache, do not call the completion block
}
else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
if (transformedImage && finished) {
BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
[self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:(imageWasTransformed ? nil : data) forKey:key toDisk:cacheOnDisk];
}
dispatch_main_sync_safe(^{
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(transformedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
});
}
else {
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
}
// 回調(diào)
dispatch_main_sync_safe(^{
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
}
}
// 如果結(jié)束扶供，則刪除該SDWebImageCombinedOperation
if (finished) {
@synchronized (self.runningOperations) {
if (strongOperation) {
[self.runningOperations removeObject:strongOperation];
}
}
}
}];
// step3. 設(shè)置operation的cancel block
operation.cancelBlock = ^{
[subOperation cancel];
@synchronized (self.runningOperations) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation) {
[self.runningOperations removeObject:strongOperation];
}
}
};
}
else if (image) {
dispatch_main_sync_safe(^{
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(image, nil, cacheType, YES, url);
}
});
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
else {
// Image not in cache and download disallowed by delegate
dispatch_main_sync_safe(^{
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation && !weakOperation.isCancelled) {
completedBlock(nil, nil, SDImageCacheTypeNone, YES, url);
}
});
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
}];
return operation;
}
代碼的邏輯大致分為三部分：
- 先判斷image的URL是否合法
2.若URL合法筛圆，則先讀取緩存
3.若緩存沒有該文件，則在網(wǎng)絡(luò)上下載
通過注釋椿浓，應(yīng)該也不難理解太援。
略過檢查URL合法性的代碼，我們首先來看SDWebImage是如何讀取Cache的扳碍。
也就是
SDImageCache 類 的
- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock; 方法提岔。
SDImageCache
我們先看queryDiskCacheForKey的方法聲明：
typedef NS_ENUM(NSInteger, SDImageCacheType) {
/**
* The image wasn't available the SDWebImage caches, but was downloaded from the web.
*/
SDImageCacheTypeNone = 1,
/**
* The image was obtained from the disk cache.
*/
SDImageCacheTypeDisk,
/**
* The image was obtained from the memory cache.
*/
SDImageCacheTypeMemory
};
typedef void(^SDWebImageQueryCompletedBlock)(UIImage *image, SDImageCacheType cacheType);
/**
* Query the disk cache asynchronously.
*
* @param key The unique key used to store the wanted image
*/
- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock;
- queryDiskCacheForKey的返回值為一個NSOperation對象，外部可以通過該對象取消cache的查找過程笋敞。
注意碱蒙，這里SDWebImage多次使用了該思路（加載Image，下載Image）夯巷，通過同步的返回一個代表當前操作的對象赛惩，來對異步操作進行管理。 - 參數(shù)接受一個URL MD5值作為Key查找Cache趁餐。
- 在回調(diào)block中喷兼，返回UIImage，并用SDImageCacheType來表明緩存文件的來源：沒有后雷，內(nèi)存褒搔，磁盤。
這里喷面，我們也可以得知SDWebImage的cache策略，先讀內(nèi)存走孽，沒有命中惧辈，才去讀磁盤。
我們再來看他的具體實現(xiàn)
- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock {
if (!doneBlock) {
return nil;
}
if (!key) {
doneBlock(nil, SDImageCacheTypeNone);
return nil;
}
// First check the in-memory cache...
UIImage *image = [self imageFromMemoryCacheForKey:key]; // NSCache is threadsafe, don't need lock
if (image) {
doneBlock(image, SDImageCacheTypeMemory);
return nil;
}
// Second check disk cache...
NSOperation *operation = [NSOperation new]; // 這個NSOperation 對象的意義在于向外提供了cancel讀取disk cache的對象
dispatch_async(self.ioQueue, ^{ // aync call. Note there sync io operation by serial queue
if (operation.isCancelled) {
return;
}
@autoreleasepool {
UIImage *diskImage = [self diskImageForKey:key];
if (diskImage && self.shouldCacheImagesInMemory) {
NSUInteger cost = SDCacheCostForImage(diskImage);
[self.memCache setObject:diskImage forKey:key cost:cost];
}
dispatch_async(dispatch_get_main_queue(), ^{ // return to main queue
doneBlock(diskImage, SDImageCacheTypeDisk);
});
}
});
return operation;
}
- First check in-memory cahce.
這里的memory cahce用了繼承自系統(tǒng)的NSCache的AutoPurgeCache對象磕瓷。該對象在init時會注冊監(jiān)聽系統(tǒng)的
UIApplicationDidReceiveMemoryWarningNotification通知盒齿。
當系統(tǒng)內(nèi)存告警時，會自動清除所有的memory cache：
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(removeAllObjects) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
Second check disk cache困食。
這里注意到边翁，之前都是同步的返回nil，而在讀取disk緩存時硕盹，才返回一個operation對象符匾。這是因為之前的讀取內(nèi)存cache都是同步的，而這里disk讀取Cache時異步的瘩例，為了為異步操作做管理啊胶，才需要想外返回一個operation對象以便隨時可以cancel該異步操作甸各。
在讀取disk cache時，為了防止多線程同時讀寫一個文件焰坪，這里用到了
串行隊列
self.ioQueue
來串行化io操作趣倾。這是我們可以借鑒的地方，利用串行隊列而非lock來保證線程安全某饰。當從disk讀取緩存后儒恋，再將disk緩存存入memory cache。
調(diào)用done block黔漂，將UIImage的cache（如果有的話）诫尽，以及Cache來源返回。
SDWebImage的Cache策略
那么瘟仿，SDWebImage通過SDWebCache讀取Cache的流程如下：
在下一篇文章中箱锐，我會接著和大家一起分析，當Cache未命中時劳较，SDWebImage是如何在網(wǎng)絡(luò)下載圖片驹止，并更新Cahce的。
