前言
SDWebImage是一个图片加载库，目前在Github上已有将近14k的star了，这篇文章记录了阅读SDWebImage源码的笔记(Version 3.8)，简单的研究了它的实现细节。
流程
SDWebImage加载图片的大致流程，以UIImageView+WebCache为例。
1.调用基础入口方法
sd_setImageWithURL:,sd_setImageWithURL:placeholderImage:,sd_setImageWithURL:placeholderImage:options:等等
2.进入sd_setImageWithURL:placeholderImage:options:progress:completed:
3.进入downloadImageWithURL:options:progress:completed:
4.调用SDWebImageDownloader下载图片downloadImageWithURL:options:progress:completed:
5.调用addProgressCallback:completedBlock:forURL:createCallback:
6.初始化SDWebImageDownloaderOperation
initWithRequest:inSession:options:progress:completed:cancelled:
7.缓存图片调用storeImage:recalculateFromImage:imageData:forKey:toDisk:
以上方法是调用链上的主要方法，将在下文逐一解释。
Utils
Utils有三个类：
-
SDWebImageManager
SDWebImage核心类，用来管理图片的下载和缓存 -
SDWebImageDecoder
解压缩图片 -
SDWebImagePrefetcher
可以预先下载图片
SDWebImageManager
用来管理图片的下载和缓存工具类，提取了SDImageCache和SDWebImageDownloader的相关方法，在流程3中使用的方法具体为：
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock {
// 没有completedBlock调用这个方法是毫无意义的
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
// 判断url合法性，并帮将NSString当NSURL的新手擦屁股
if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
// 判断url合法性
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}
// 创建SDWebImageCombinedOperation对象
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;
BOOL isFailedUrl = NO;
// 给failedURLs属性加锁，确保同一时间只有一个线程操作
@synchronized (self.failedURLs) {
// 检查url是否是下载失败的
isFailedUrl = [self.failedURLs containsObject:url];
}
// 检查url的合法性
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];
}
// 默认使用url作key，也可以自定义cacheKeyFilter过滤
NSString *key = [self cacheKeyForURL:url];
// 查找内存或硬盘中是否有缓存的图片
operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) {
if (operation.isCancelled) {
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
return;
}
if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
if (image && options & SDWebImageRefreshCached) {
dispatch_main_sync_safe(^{
// 如果图片存在，但采用SDWebImageRefreshCached标记，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);
});
}
// 下载图片
// SDWebImageOptions和SDWebImageDownloaderOptions对应关系
// a |= b ------> a = a | b
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) {
// force progressive off if image already cached but forced refreshing
// ~ 按位取反运算符
downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
// ignore image read from NSURLCache if image if cached but force refreshing
downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
}
// 调用SDWebImageDownloader下载图片
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) {
// 下载失败，将url添加到失败urls里
@synchronized (self.failedURLs) {
[self.failedURLs addObject:url];
}
}
}
else {
// 下载失败，但选择了标记SDWebImageRetryFailed，移除失败url
if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
if (options & SDWebImageRefreshCached && image && !downloadedImage) {
// 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:)]) {
// 在全局队列中调用，以不阻塞主线程
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
// 在下载完后，缓存到内存或硬盘之前，做转换图片操作
UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
// 将转换完的图片缓存
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 {
// 将转换完的图片缓存
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);
}
});
}
}
if (finished) {
@synchronized (self.runningOperations) {
if (strongOperation) {
[self.runningOperations removeObject:strongOperation];
}
}
}
}];
operation.cancelBlock = ^{
[subOperation cancel];
@synchronized (self.runningOperations) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation) {
[self.runningOperations removeObject:strongOperation];
}
}
};
}
else if (image) { // 图片存在，即内存或硬盘中有图片
dispatch_main_sync_safe(^{
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation && !strongOperation.isCancelled) {
// 返回image和cacheType
completedBlock(image, nil, cacheType, YES, url);
}
});
// 删除当前operation
@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;
}
此为核心方法，完成了查找缓存、下载图片、缓存图片等操作。值得注意的是SDWebImageManager还有一个delegate属性，并提供了两个可选方法:
@protocol SDWebImageManagerDelegate <NSObject>
@optional
- (BOOL)imageManager:(SDWebImageManager *)imageManager shouldDownloadImageForURL:(NSURL *)imageURL;
// 在下载完后，缓存到内存或硬盘之前，做转换图片操作
- (UIImage *)imageManager:(SDWebImageManager *)imageManager transformDownloadedImage:(UIImage *)image withURL:(NSURL *)imageURL;
@end
SDWebImageDecoder
用来解压缩图片，会在取图片时做一次decode操作。
因为通过 imageNamed 创建 UIImage 时，系统实际上只是在 Bundle 内查找到文件名，然后把这个文件名放到 UIImage 里返回，并没有进行实际的文件读取和解码。当 UIImage 第一次显示到屏幕上时，其内部的解码方法才会被调用，同时解码结果会保存到一个全局缓存去。在图片解码后，App 第一次退到后台和收到内存警告时，该图片的缓存才会被清空，其他情况下缓存会一直存在。具体的说就是一个UIImage加载了jpeg或者png，当UIImageView将要显示这个UIImage的时候会先把png和jpeg解码成未压缩格式，所以SDWebImage有一个decodeImage方法，就是把这一步放在了异步线程做，防止tableViewCell中的imageView加载图片的时候在主线程解码图片，导致滑动卡顿。这样效率很低，但是只有瞬时的内存需求。为了提高效率通过SDWebImageDecoder将包装在Data下的资源解压，然后画在另外一张图片上，这样这张新图片就不再需要重复解压了,这种做法是典型的空间换时间的做法，如下从硬盘中去图片时，分别对图片进行了缩放和解压缩操作。
关于处理图片可以看看YY这篇博文
- (UIImage *)diskImageForKey:(NSString *)key {
NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
if (data) {
UIImage *image = [UIImage sd_imageWithData:data];
// 缩放图片
image = [self scaledImageForKey:key image:image];
if (self.shouldDecompressImages) {
// 解压缩
image = [UIImage decodedImageWithImage:image];
}
return image;
}
else {
return nil;
}
}
DownLoader
DownLoader有两个类：
-
SDWebImageDownloader
下载核心类，用来管理图片的下载 -
SDWebImageDownloaderOperation
每一个图片下载的具体操作
SDWebImageDownloader
SDWebImageDownloader一个单例类,负责图片的下载操作和管理,其在流程4和流程5中的主要方法为：
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
__block SDWebImageDownloaderOperation *operation;
__weak __typeof(self)wself = self;
[self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^{
// 过期时间
NSTimeInterval timeoutInterval = wself.downloadTimeout;
if (timeoutInterval == 0.0) {
timeoutInterval = 15.0;
}
// 创建请求对象，并根据options参数设置其属性
// 为了避免潜在的重复缓存(NSURLCache + SDImageCache)，如果没有明确告知需要缓存，则禁用图片请求的缓存操作
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
request.HTTPShouldUsePipelining = YES;
// header过滤
if (wself.headersFilter) {
request.allHTTPHeaderFields = wself.headersFilter(url, [wself.HTTPHeaders copy]);
}
else {
request.allHTTPHeaderFields = wself.HTTPHeaders;
}
// 初始化SDWebImageDownloaderOperation
operation = [[wself.operationClass alloc] initWithRequest:request
inSession:self.session
options:options
progress:^(NSInteger receivedSize, NSInteger expectedSize) {
SDWebImageDownloader *sself = wself;
if (!sself) return;
__block NSArray *callbacksForURL;
dispatch_sync(sself.barrierQueue, ^{
callbacksForURL = [sself.URLCallbacks[url] copy];
});
// 从URLCallbacks取出progress回调
for (NSDictionary *callbacks in callbacksForURL) {
dispatch_async(dispatch_get_main_queue(), ^{
SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
if (callback) callback(receivedSize, expectedSize);
});
}
}
completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {
SDWebImageDownloader *sself = wself;
if (!sself) return;
__block NSArray *callbacksForURL;
dispatch_barrier_sync(sself.barrierQueue, ^{
callbacksForURL = [sself.URLCallbacks[url] copy];
if (finished) {
[sself.URLCallbacks removeObjectForKey:url];
}
});
// 从URLCallbacks取出complete回调
for (NSDictionary *callbacks in callbacksForURL) {
SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
if (callback) callback(image, data, error, finished);
}
}
cancelled:^{
SDWebImageDownloader *sself = wself;
if (!sself) return;
dispatch_barrier_async(sself.barrierQueue, ^{
[sself.URLCallbacks removeObjectForKey:url];
});
}];
// 对图片进行解压缩，默认为YES
operation.shouldDecompressImages = wself.shouldDecompressImages;
if (wself.urlCredential) {
operation.credential = wself.urlCredential;
} else if (wself.username && wself.password) {
operation.credential = [NSURLCredential credentialWithUser:wself.username password:wself.password persistence:NSURLCredentialPersistenceForSession];
}
// 设置优先级
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
// 将操作加入到操作队列downloadQueue中
[wself.downloadQueue addOperation:operation];
// 如果是LIFO顺序，则将新的操作作为原队列中最后一个操作的依赖，然后将新操作设置为最后一个操作
if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
// Emulate LIFO execution order by systematically adding new operations as last operation's dependency
[wself.lastAddedOperation addDependency:operation];
wself.lastAddedOperation = operation;
}
}];
return operation;
}
- (void)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock forURL:(NSURL *)url createCallback:(SDWebImageNoParamsBlock)createCallback {
// The URL will be used as the key to the callbacks dictionary so it cannot be nil. If it is nil immediately call the completed block with no image or data.
if (url == nil) {
if (completedBlock != nil) {
completedBlock(nil, nil, nil, NO);
}
return;
}
// 以dispatch_barrier_sync操作来保证同一时间只有一个线程能对URLCallbacks进行操作
// 为了保证URLCallbacks操作(添加、删除)的线程安全性，SDWebImageDownloader将这些操作作为一个个任务放到barrierQueue队列中，并设置屏障来确保同一时间只有一个线程操作URLCallbacks属性
dispatch_barrier_sync(self.barrierQueue, ^{
BOOL first = NO;
if (!self.URLCallbacks[url]) {
self.URLCallbacks[url] = [NSMutableArray new];
first = YES;
}
// Handle single download of simultaneous download request for the same URL
// 处理同一URL的同步下载请求的单个下载
NSMutableArray *callbacksForURL = self.URLCallbacks[url];
NSMutableDictionary *callbacks = [NSMutableDictionary new];
if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];
if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];
[callbacksForURL addObject:callbacks];
self.URLCallbacks[url] = callbacksForURL;
if (first) {
createCallback();
}
});
}
需要注意的是SDWebImageDownloader管理的下载是放在一个NSOperationQueue队列中来完成的，其中每个下载操作都是一个单独的SDWebImageDownloaderOperation，将这些操作放到一个操作队列中，这样可以实现图片的并发下载。
SDWebImageDownloaderOperation
SDWebImageDownloaderOperation继承自NSOperation，实际中每个图片的下载都交给一个SDWebImageDownloaderOperation，需要注意的是在SDWebImage Version3.8中，下载已经有原先的NSURLConnection切换到了NSURLSession了，所以我们要关注的是NSURLSessionDataDelegate中的URLSession:dataTask:didReceiveResponse:completionHandler:方法，此方法主要用来接收数据:
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
// 每次附加新数据
[self.imageData appendData:data];
if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0 && self.completedBlock) {
// 记录当前总大小
const NSInteger totalSize = self.imageData.length;
// 每次接收到数据时，用现有的数据创建一个CGImageSourceRef对象
CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)self.imageData, NULL);
if (width + height == 0) {
// 从这些包含图像信息的数据中取出图像的长、宽、方向等信息以备使用
CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
if (properties) {
NSInteger orientationValue = -1;
CFTypeRef val = CFDictionaryGetValue(properties, kCGImagePropertyPixelHeight);
if (val) CFNumberGetValue(val, kCFNumberLongType, &height);
val = CFDictionaryGetValue(properties, kCGImagePropertyPixelWidth);
if (val) CFNumberGetValue(val, kCFNumberLongType, &width);
val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);
if (val) CFNumberGetValue(val, kCFNumberNSIntegerType, &orientationValue);
CFRelease(properties);
// 绘制到Core Graphics时，会丢失方向信息，这意味着有时候由initWithCGIImage创建的图片
// 的方向会不对，所以在这边先保存这个信息并在后面使用
orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)];
}
}
// 下载未完成
if (width + height > 0 && totalSize < self.expectedSize) {
// Create the image
// 使用现有的数据创建图片对象，如果数据中存有多张图片，则取第一张
CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);
#ifdef TARGET_OS_IPHONE
// Workaround for iOS anamorphic image
if (partialImageRef) {
const size_t partialHeight = CGImageGetHeight(partialImageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef bmContext = CGBitmapContextCreate(NULL, width, height, 8, width * 4, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
CGColorSpaceRelease(colorSpace);
if (bmContext) {
CGContextDrawImage(bmContext, (CGRect){.origin.x = 0.0f, .origin.y = 0.0f, .size.width = width, .size.height = partialHeight}, partialImageRef);
CGImageRelease(partialImageRef);
partialImageRef = CGBitmapContextCreateImage(bmContext);
CGContextRelease(bmContext);
}
else {
CGImageRelease(partialImageRef);
partialImageRef = nil;
}
}
#endif
if (partialImageRef) {
UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
// 对图片进行缩放
UIImage *scaledImage = [self scaledImageForKey:key image:image];
if (self.shouldDecompressImages) {
// 对图片解压缩
image = [UIImage decodedImageWithImage:scaledImage];
}
else {
image = scaledImage;
}
CGImageRelease(partialImageRef);
dispatch_main_sync_safe(^{
if (self.completedBlock) {
self.completedBlock(image, nil, nil, NO);
}
});
}
}
CFRelease(imageSource);
}
if (self.progressBlock) {
self.progressBlock(self.imageData.length, self.expectedSize);
}
}
当然SDWebImageDownloaderOperation本质是个NSOperation，有start方法启动，并在start方法里创建NSURLSession下载图片，更具体的可以看我注解的源码，这里不再赘述。
Cache
主要就一个类SDImageCache,这个类管理了对图片的缓存操作，包括内存缓存和磁盘缓存。
内存缓存是用NSCache实现的，以Key-Value的形式存储图片，当内存不够的时候会清除所有缓存图片。
磁盘缓存则是缓存到沙盒中，需要注意的文件名是key做MD5后的字符串，文件替换方式是以时间为单位，剔除时间大于一周的图片文件。
流程7中调用的主要代码为：
- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk {
if (!image || !key) {
return;
}
// 内存缓存 存入NSCache
if (self.shouldCacheImagesInMemory) {
// 图片cost值
NSUInteger cost = SDCacheCostForImage(image);
[self.memCache setObject:image forKey:key cost:cost];
}
// 磁盘缓存
if (toDisk) {
// 磁盘缓存，则将缓存操作作为一个任务放入ioQueue(串行队列)中
dispatch_async(self.ioQueue, ^{
NSData *data = imageData;
if (image && (recalculate || !data)) {
#if TARGET_OS_IPHONE
// 需要确定图片是PNG还是JPEG。PNG图片容易检测，因为有一个唯一签名。PNG图像的前8个字节总是包含以下值：137 80 78 71 13 10 26 10
// 在imageData为nil的情况下假定图像为PNG。我们将其当作PNG以避免丢失透明度。而当有图片数据时，我们检测其前缀，确定图片的类型
int alphaInfo = CGImageGetAlphaInfo(image.CGImage);
BOOL hasAlpha = !(alphaInfo == kCGImageAlphaNone ||
alphaInfo == kCGImageAlphaNoneSkipFirst ||
alphaInfo == kCGImageAlphaNoneSkipLast);
BOOL imageIsPng = hasAlpha;
// 当有图片数据时，我们检测其前缀，确定图片的类型
if ([imageData length] >= [kPNGSignatureData length]) {
imageIsPng = ImageDataHasPNGPreffix(imageData);
}
if (imageIsPng) {
data = UIImagePNGRepresentation(image);
}
else {
data = UIImageJPEGRepresentation(image, (CGFloat)1.0);
}
#else
data = [NSBitmapImageRep representationOfImageRepsInArray:image.representations usingType: NSJPEGFileType properties:nil];
#endif
}
// 创建缓存文件并存储图片
[self storeImageDataToDisk:data forKey:key];
});
}
}
磁盘缓存的自动清理机制为：
- (void)cleanDiskWithCompletionBlock:(SDWebImageNoParamsBlock)completionBlock {
dispatch_async(self.ioQueue, ^{
NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
NSArray *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];
// 该枚举器预先获取缓存文件的有用的属性
NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
includingPropertiesForKeys:resourceKeys
options:NSDirectoryEnumerationSkipsHiddenFiles
errorHandler:NULL];
NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.maxCacheAge];
NSMutableDictionary *cacheFiles = [NSMutableDictionary dictionary];
NSUInteger currentCacheSize = 0;
// 枚举缓存文件夹中所有文件，该迭代有两个目的：移除比过期日期更老的文件；存储文件属性以备后面执行基于缓存大小的清理操作
NSMutableArray *urlsToDelete = [[NSMutableArray alloc] init];
for (NSURL *fileURL in fileEnumerator) {
NSDictionary *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:NULL];
// Skip directories.
if ([resourceValues[NSURLIsDirectoryKey] boolValue]) {
continue;
}
// 移除旧的过期文件
NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
[urlsToDelete addObject:fileURL];
continue;
}
// 存储文件的引用，计算文件的总大小
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize += [totalAllocatedSize unsignedIntegerValue];
[cacheFiles setObject:resourceValues forKey:fileURL];
}
for (NSURL *fileURL in urlsToDelete) {
[_fileManager removeItemAtURL:fileURL error:nil];
}
// 如果磁盘缓存的大小大于我们配置的最大大小，则执行基于文件大小的清理，我们首先删除最老的文件
if (self.maxCacheSize > 0 && currentCacheSize > self.maxCacheSize) {
// 以设置的最大缓存大小的一半作为清理目标
const NSUInteger desiredCacheSize = self.maxCacheSize / 2;
// 按照最后修改时间来排序剩下的缓存文件
NSArray *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
usingComparator:^NSComparisonResult(id obj1, id obj2) {
return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
}];
// 删除文件，直到缓存总大小降到我们期望的大小
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();
});
}
});
}
SDWebImageManager下载图片前会先搜索内存层面的数据，如果有直接返回，没有的话去访问磁盘，将图片从磁盘读取出来，然后做Decoder，再将图片对象放到内存做备份。
更为具体关于图片的查询、删除可以看我注解的源码，这里不再赘述。
Categories
这里主要看UIImageView+WebCache这个类，在流程2中核心方法为：
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
// 取消上一次操作
[self sd_cancelCurrentImageLoad];
// 拿到url，在这里setAssociatedObject
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
if (!(options & SDWebImageDelayPlaceholder)) {
// 确保在主线程中执行操作
dispatch_main_async_safe(^{
self.image = placeholder;
});
}
if (url) {
// check if activityView is enabled or not
if ([self showActivityIndicatorView]) {
[self addActivityIndicator];
}
__weak __typeof(self)wself = self;
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;
dispatch_main_sync_safe(^{
if (!wself) return;
// 选择了SDWebImageAvoidAutoSetImage标记，会先返回image不加载图片，以便添加动画效果或过滤器
if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock)
{
completedBlock(image, error, cacheType, url);
return;
}
// 只有image，下载成功后加载
else if (image) {
wself.image = image;
[wself setNeedsLayout];
} else {
// 图片不存在或下载失败或SDWebImageDelayPlaceholder，加载placeholder
if ((options & SDWebImageDelayPlaceholder)) {
wself.image = placeholder;
[wself setNeedsLayout];
}
}
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
// 保存本次操作,保存前会先cancel上次操作
[self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
} else {
dispatch_main_async_safe(^{
[self removeActivityIndicator];
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
// url不存在，SDImageCacheTypeNone
completedBlock(nil, error, SDImageCacheTypeNone, url);
}
});
}
}
总结
- 1.主动为开发者考虑
// 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];
}
代码里在判断url的合法性上，主动帮将NSString当NSURL的新手擦屁股，连这个问题都考虑到了，是不是有很多在这里报错的人给他提issue呢？（笑~
-
2.位移枚举
又回顾了一遍位移枚举。 -
3.dispatch_main_async_safe
#define dispatch_main_async_safe(block)\
if ([NSThread isMainThread]) {\
block();\
} else {\
dispatch_async(dispatch_get_main_queue(), block);\
}
目的是想在主线程中执行操作，为什么要判断当前线程是否就是主线程中呢？
如果在主线程中执行dispatch_sync(dispatch_get_main_queue(), block) 同步操作时，会出现死锁问题，因为主线程正在执行当前代码，根本无法将block添加到主队列中。
-
4.@synchronized
相当于加锁，在SDWebImage里表现为@synchronized(syncObject) { },具体的说，是在某个对象实例内，可以防止多个线程同时访问这个对象的synchronized方法（如果一个对象有多个synchronized方法，只要一个线程访问了其中的一个synchronized方法，其它线程不能同时访问这个对象中任何一个synchronized方法）。这时，不同的对象实例的 synchronized方法是不相干扰的。也就是说，其它线程照样可以同时访问相同类的另一个对象实例中的synchronized方法；
这里引申一个，若同时重写某属性的setter和getter方法后，需加上@synthesize propertyName = _propertyName;，这是因为同时重写后，系统就不会帮你自动生成这个成员变量。 -
5.开启后台任务
-(void)backgroundCleanDisk {
Class UIApplicationClass = NSClassFromString(@"UIApplication");
if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
return;
}
UIApplication *application = [UIApplication performSelector:@selector(sharedApplication)];
__block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
// 清理任何未完成的任务作业，标记完全停止或结束任务。
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
// 开始长时间后台运行的任务并且立即return。
[self cleanDiskWithCompletionBlock:^{
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
}
想要提高还得看源码，总的来说阅读源码获益匪浅。
相关的注解在这里。
Reference
https://github.com/rs/SDWebImage
http://blog.csdn.net/uxyheaven/article/details/7909373
http://southpeak.github.io/blog/2015/02/07/sourcecode-sdwebimage/
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