SDWebImage源碼剖析（一）
在開發項目的過程當中會用到不少第三方庫，好比AFNetWorking,SDWebImage,FMDB等，但一直都沒去好好的研究一下，最近恰好項目不是太緊，閒下來能夠給本身充充電，先研究一下SDWebImage的底層實現，源碼地址：SDWebImage
先介紹一下SDWebImage，咱們使用較多的是它提供的UIImageView分類，支持從遠程服務器下載並緩存圖片。自從iOS5.0開始，NSURLCache也能夠處理磁盤緩存，那麼SDWebImage的優點在哪？首先NSURLCache是緩存原始數據(raw data)到磁盤或內存，所以每次使用的時候須要將原始數據轉換成具體的對象，如UIImage等，這會致使額外的數據解析以及內存佔用等，而SDWebImage則是緩存UIImage對象在內存，緩存在NSCache中，同時直接保存壓縮過的圖片到磁盤中；還有一個問題是當你第一次在UIImageView中使用image對象的時候，圖片的解碼是在主線程中運行的！而SDWebImage會強制將解碼操做放到子線程中。下圖是SDWebImage簡單的類圖關係：git
SDWebImage.png
下面從UIImageView的圖片加載開始看起，Let's go!github
首先咱們在給UIImageView設置圖片的時候會調用方法：緩存
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;
其中url爲遠程圖片的地址，而placeholder爲預顯示的圖片。
其實還能夠添加一些額外的參數，好比圖片選項SDWebImageOptions服務器
typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
SDWebImageRetryFailed = 1
通常使用的是SDWebImageRetryFailed | SDWebImageLowPriority，下面看看具體的函數調用：網絡
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock
{
[self sd_cancelCurrentImageLoad];//取消正在下載的操做
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);//關聯該view對應的圖片URL
/*...*/
if (url) {
__weak UIImageView *wself = self;//防止retain cricle
//由SDWebImageManager負責圖片的獲取
id operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
/*獲取圖片到主線層顯示*/
}];
[self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
}
}
能夠看出圖片是從服務端、內存或者硬盤獲取是由SDWebImageManager管理的，這個類有幾個重要的屬性：app
@property (strong, nonatomic, readwrite) SDImageCache *imageCache;//負責管理cache，涉及內存緩存和硬盤保存 @property (strong, nonatomic, readwrite) SDWebImageDownloader *imageDownloader;//負責從網絡下載圖片 @property (strong, nonatomic) NSMutableArray *runningOperations;//包含全部當前正在下載的操做對象
manager會根據URL先去imageCache中查找對應的圖片，若是沒有在使用downloader去下載，並在下載完成緩存圖片到imageCache，接着看實現：async
- (id )downloadImageWithURL:(NSURL *)url options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock
{
/*...*/
//根據URL生成對應的key，沒有特殊處理爲[url absoluteString];
NSString *key = [self cacheKeyForURL:url];
//去imageCache中尋找圖片
operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType)
{
/*...*/
//若是圖片沒有找到，或者採用的SDWebImageRefreshCached選項，則從網絡下載
if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
dispatch_main_sync_safe(^{
//若是圖片找到了，可是採用的SDWebImageRefreshCached選項，通知獲取到了圖片，並再次從網絡下載，使NSURLCache從新刷新
completedBlock(image, nil, cacheType, YES, url);
});
}
/*下載選項設置*/
//使用imageDownloader開啓網絡下載
id subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
/*...*/
if (downloadedImage && finished) {
//下載完成後，先將圖片保存到imageCache中，而後主線程返回
[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中找到圖片了，直接返回
dispatch_main_sync_safe(^{
if (!weakOperation.isCancelled) {
completedBlock(image, nil, cacheType, YES, url);
}
});
}
}];
return operation;
}
下面先看downloader從網絡下載的過程，下載是放在NSOperationQueue中進行的，默認maxConcurrentOperationCount爲6，timeout時間爲15s：函數
- (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重複緩存
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy :NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
request.HTTPShouldUsePipelining = YES;
request.allHTTPHeaderFields = wself.HTTPHeaders;//設置http頭部
//SDWebImageDownloaderOperation派生自NSOperation，負責圖片下載工做
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;//是否須要解碼
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) {
// 若是下載順序是後面添加的先運行
[wself.lastAddedOperation addDependency:operation];
wself.lastAddedOperation = operation;
}
}];
return operation;
}
SDWebImageDownloaderOperation派生自NSOperation，經過NSURLConnection進行圖片的下載，爲了確保可以處理下載的數據，須要在後臺運行runloop：oop
- (void)start {
/*...*/
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
//開啓後臺下載
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);
}
//註冊和發送通知須要在一個線程
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
});
CFRunLoopRun();//在默認模式下運行當前runlooprun，直到調用CFRunLoopStop中止運行
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
}
下載過程當中，在代理 － (void)connection:(NSURLConnection )connection didReceiveData:(NSData )data中將接收到的數據保存到NSMutableData中，[self.imageData appendData:data]，下載完成後在該線程完成圖片的解碼，並在完成的completionBlock中進行imageCache的緩存：post
- (void)connectionDidFinishLoading:(NSURLConnection *)aConnection {
SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
@synchronized(self) {
CFRunLoopStop(CFRunLoopGetCurrent());//中止當前對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];//圖片解碼
}
}
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];
}
後續的圖片緩存能夠參考：SDWebImage源碼剖析（二）
相關文章
- 1. SDWebImage源碼解析(一)
- 2. SDWebImage 源碼分析
- 3. SDWebImage源碼分析
- 4. SDWebImage 源碼解析
- 5. SDWebImage源碼解析
- 6. SDWebImage 源碼一覽
- 7. SDWebImage源代碼分析（下）
- 8. SDWebImage源碼解析(三)
- 9. SDWebImage源碼解析(四)
- 10. 源碼淺析 SDWebImage 5.6
- 更多相關文章...
- • TCP滑動窗口機制深度剖析 - TCP/IP教程
- • Docker 資源彙總 - Docker教程
- • 互聯網組織的未來：剖析GitHub員工的任性之源
- • Java Agent入門實戰（二）-Instrumentation源碼概述
相關標籤/搜索
每日一句
-
每一个你不满意的现在，都有一个你没有努力的曾经。
最新文章
- 1. 在windows下的虛擬機中,安裝華爲電腦的deepin操作系統
- 2. 強烈推薦款下載不限速解析神器
- 3. 【區塊鏈技術】孫宇晨：區塊鏈技術帶來金融服務的信任變革
- 4. 搜索引起的鏈接分析-計算網頁的重要性
- 5. TiDB x 微衆銀行 | 耗時降低 58%，分佈式架構助力實現普惠金融
- 6. 《數字孿生體技術白皮書》重磅發佈（附完整版下載）
- 7. 雙十一「避坑」指南：區塊鏈電子合同爲電商交易保駕護航！
- 8. 區塊鏈產業，怎樣「鏈」住未來？
- 9. OpenglRipper使用教程
- 10. springcloud請求一次好用一次不好用zuul Name or service not known
歡迎關注本站公眾號,獲取更多信息
相關文章