SDWebImage源代码阅读(二)

SDWebImage源代码阅读(二)

我们在SDWebImage源代码阅读(一)中讲了下SDWebImage的使用和UIImageView+WebCache

SDWebImageManager：

+ (id)sharedManager {
static dispatch_once_t once;
static id instance;
dispatch_once(&once, ^{
instance = [self new];
});
return instance;
}

SDWebImage

是一个单例，OC中的单例就是这么简单

- (instancetype)init {
SDImageCache *cache = [SDImageCache sharedImageCache];
SDWebImageDownloader *downloader = [SDWebImageDownloader sharedDownloader];
return [self initWithCache:cache downloader:downloader];
}

SDwebImageManager

每次初始化都会有

SDImage

和

SDWebImageDownloader

的单例的赋值

- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock {}

这是SDWebImageManager最关键的方法，实现了，图片的加载保存等功能；progressBlock会在图片下载的时候去调用，completedBlock会在操作完成得时候调用

if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}

首先，我们会对url进行一个判断，因为我们将NSString当成NSURL的这种错误的概率是非常高的，所以这里对是NSString类型的字符串进行了转换，保证之后代码的执行。当然，如果url既不是NSURL又不是NSString，这里就会对它进行nil的设置

__block SDWebImageCombinedOperation *operation =[SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;

接下来这里初始化了一个新的

SDWebImageCombinedOperation

类，并对其进行引用

BOOL isFailedUrl = NO;
@synchronized (self.failedURLs) {
isFailedUrl = [self.failedURLs containsObject:url];
}

这里

@synchronized

的作用是创建一个互斥锁，保证没有其他线程对

self.failedURLs

进行访问修改，起到保护作用。然后查询是否我们正在加载的这个图片的url是曾经加载失败过的url，并对

isFailedUrl

赋值

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;
}

当url长度为0，或者是当self.failedURLs中包含了这个url（url代表的图片曾经加载失败），并且options设置不是SDWebImageRetryFailed(失败后重试)，

dispatch_main_sync_safe

主线程同步队列，初始化NSError并且给completedBlock赋值，并且返回

SDWebImageOperation

变量

@synchronized (self.runningOperations) {
[self.runningOperations addObject:operation];
}

如果url正常，或者错误重试，我们会在

self.runningOperations

中添加这个操作

NSString *key = [self cacheKeyForURL:url];

根据url返回一个cacheKey，之后可以通过

SDWebImageCacheKeyFilterBlock

去删除图像网址动态部分

operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) {
}

用

SDImageCahe

的方法通过关键词去查询是否有

key

的图片存在于高速缓存中或者磁盘中，并且给

SDWebImageCombinedOperation

的属性

cacheOperation

赋值。

if (operation.isCancelled) {
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
return;

当操作已经被取消

operation.isCancelled

，我们将操作从

self.runningOperations

中移除,并且退出该函数

if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
···
}

当图片不存在或者要刷新图片缓存并且状态不属于阻止图片加载

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);
});
}

当图片存在，而且需要刷新图片缓存的时候:赋值completedBlock

SDWebImageDownloaderOpti
4000
ons 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) {
// 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;
}

根据options设置downloaderOptions(按位运算符)

id <SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
···
}

通过url和options下载图片

__strong __typeof(weakOperation) strongOperation = weakOperation;
if (!strongOperation || strongOperation.isCancelled)

__strong __typeof(weakOperation)

因为之前是弱引用，我们需要保证在执行过程中weakOperation不变成nil，所以再加上一个强引用。

当strongOperation不为空或者说操作被取消了

我们什么也不做

当error是nil的时候

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];
}
}

主线程同步队列：如果strongOperation已经变为nil同时操作被取消了，我们给completedBlock赋值。同时error错误原因是我们列出来的，我们将会把url加入到

self.failedURLs

中

当没有error，同时strongOperation为nil且操作没有被取消

if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}

当options是失败重试的时候，我们将对

self.failedURLs

使用互斥锁，并且将url从

self.failedURLs

移出

if (options & SDWebImageRefreshCached && image && !downloadedImage) {
// Image refresh hit the NSURLCache cache, do not call the completion block
}

如果options是刷新图片缓存同时image不是nil同时没有下载的图片，什么都不用执行，（图像刷新NSURLCache缓存，不调用完成闭包）

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);
}
});
});
}

当已经下载了图片了，且图片是动态图，启用了

SDWebImageTransformAnimatedImage

，同时,委托有

imageManager:transformDownloadedImage:withURL:

方法：高优先级异步执行图片转码，当转码完成后图片不为nil，通过self.imageCache保存图片缓存到磁盘上,并且同步执行comletedBlock的赋值

else {
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
}
dispatch_main_sync_safe(^{
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
}

当图片不是动态图，同时不用刷新图片缓存的时候：如果下载图片完毕，调用

self.imageCache

将图片缓存保存在磁盘上，同时同步执行comletedBlock的赋值

if (finished) {
@synchronized (self.runningOperations) {
if (strongOperation) {
[self.runningOperations removeObject:strongOperation];
}
}
}

在图片完成保存，或者不需要保存之后，将operation从

self.runningOperations

移出

operation.cancelBlock = ^{
[subOperation cancel];
@synchronized (self.runningOperations) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation) {
[self.runningOperations removeObject:strongOperation];
}
}
};

设置操作取消的闭包，在操作取消的时候，我们会将操作从将operation从

self.runningOperations

移出

当图片存在（同时不刷新图片缓存······就是上面那个条件不成立）

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];
}

主线程同步队列：

__strong __typeof(weakOperation)

因为之前是弱引用，我们需要保证在执行过程中weakOperation不变成nil，所以再加上一个强引用。如果strongOperation已经变为nil或者说操作被取消了，这时，我们需要给completeBlock赋值，最后从

self.runningOperations

移除这个操作

当图片不存在，同时委托不允许加载图片

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];

主线程同步队列：

__strong __typeof(weakOperation)

因为之前是弱引用，我们需要保证在执行过程中weakOperation不变成nil，所以再加上一个强引用。如果strongOperation已经变为nil或者说操作被取消了，这时，我们需要给completeBlock赋值，最后从

self.runningOperations

移除这个操作，和上面不同的时completeBlock中cacheType是None

简而言之:

SDWebImageManager

是一个单例管理类，负责协调图片缓存和图片下载，是对 SDImageCache和SDWebImageDownloader的封装，一般我们不用调用SDImageCache、SDWebImageDownloader，直接使用SDWebImageManager即可

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