版本记录
| 版本号 | 时间 |
|---|---|
| V1.0 | 2017.07.15 |
前言
我们做APP，文字和图片是绝对不可缺少的元素，特别是图片一般存储在图床里面，一般公司可以委托第三方保存，NB的公司也可以自己存储图片，ios有很多图片加载的第三方框架，其中最优秀的莫过于SDWebImage，它几乎可以满足你所有的需求，用了好几年这个框架，今天想总结一下。感兴趣的可以看其他几篇。
1. SDWebImage探究（一）
2. SDWebImage探究（二）
详析SDWebImage
下面我们就详析SDWebImage的原理。
- 先看一下入口。
UIImageView+WebCache.m
//这个是入口，下载图片的结果方法最终都是走的这个方法
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
[self sd_internalSetImageWithURL:url
placeholderImage:placeholder
options:options
operationKey:nil
setImageBlock:nil
progress:progressBlock
completed:completedBlock];
}
- 下面进入的是UIView的一个分类
UIView+WebCache.m
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
operationKey:(nullable NSString *)operationKey
setImageBlock:(nullable SDSetImageBlock)setImageBlock
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
[self sd_cancelImageLoadOperationWithKey:validOperationKey];
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
if (!(options & SDWebImageDelayPlaceholder)) {
dispatch_main_async_safe(^{
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
});
}
//如果URL不为空，调用管理器SDWebImageManager
if (url) {
//加载指示器
if ([self sd_showActivityIndicatorView]) {
[self sd_addActivityIndicator];
}
__weak __typeof(self)wself = self;
//调用管理器SDWebImageManager
id operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
//下面的是调用管理器完成的回调
__strong __typeof (wself) sself = wself;
[sself sd_removeActivityIndicator];
if (!sself) {
return;
}
dispatch_main_async_safe(^{
if (!sself) {
return;
}
if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
completedBlock(image, error, cacheType, url);
return;
}
else if (image) {
[sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
}
else {
if ((options & SDWebImageDelayPlaceholder)) {
[sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
}
}
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
//取消下载操作
[self sd_setImageLoadOperation:operation forKey:validOperationKey];
}
//如果url为空就抛出异常
else {
dispatch_main_async_safe(^{
//移除指示器
[self sd_removeActivityIndicator];
if (completedBlock) {
//完成以后的回调
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
completedBlock(nil, error, SDImageCacheTypeNone, url);
}
});
}
}
- 调用管理类SDWebImageManager
SDWebImageManager.m
- (id )loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
//断言，判断completedBlock是否为空
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
//这里是一个容错机制，及时我们给url传递的NSString而不是NSURL，框架也会给我们自动转化
if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
//防崩溃处理，如果不是NSString和NSURL类型，就赋值为nil，防止app崩溃
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;
BOOL isFailedUrl = NO;
if (url) {
//这里的是failedURLs是NSMutableSet集合
@synchronized (self.failedURLs) {
//判断集合中是否包含这个url
isFailedUrl = [self.failedURLs containsObject:url];
}
}
if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
//调用操作完成模块
[self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
return operation;
}
@synchronized (self.runningOperations) {
//如果操作operation不为空，就讲该操作放在可变数组runningOperations里面
[self.runningOperations addObject:operation];
}
//获取urlKey作为操作的key
NSString *key = [self cacheKeyForURL:url];
//查询该操作是否在缓存操作中
operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
//如果缓存中有这个操作，就移除这个operation
if (operation.isCancelled) {
[self safelyRemoveOperationFromRunning:operation];
return;
}
//如果缓存中没有图片，就调用先判断是否可以下载
if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
if (cachedImage && options & SDWebImageRefreshCached) {
//如果缓存中有图片，设置了SDWebImageRefreshCached，就会重新下载，NSURLCache从服务器重新下载
[self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
}
//如果没有图片，利用代理进行下载，下面是下载选项的枚举
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 (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;
if (cachedImage && options & SDWebImageRefreshCached) {
downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
}
//下载图片并给出回调结果
SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (!strongOperation || strongOperation.isCancelled) {
//如果操作不存在或者操作被取消，就什么都不做，如果我们调用完成block，对于同一个对象，在这个block和其他的完成block之间就会产生竞争状态，因此如果这个被二次调用，我们就会覆盖新数据。
}
else if (error) {
[self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];
//错误码处理
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 {
//如果下载失败的话，就将这个url从集合set中移除
if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
if (options & SDWebImageRefreshCached && cachedImage && !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:)]) {
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];
// pass nil if the image was transformed, so we can recalculate the data from the image
[self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
}
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
});
}
else {
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
}
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
}
}
if (finished) {
//完成后才操作集合中移除该操作
[self safelyRemoveOperationFromRunning:strongOperation];
}
}];
//如果操作取消了，同样从操作中移除该操作
operation.cancelBlock = ^{
[self.imageDownloader cancel:subOperationToken];
__strong __typeof(weakOperation) strongOperation = weakOperation;
[self safelyRemoveOperationFromRunning:strongOperation];
};
}
//如果有缓存图片
else if (cachedImage) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
//调用完成回调，并且将该操作从数组中移除
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
[self safelyRemoveOperationFromRunning:operation];
}
else {
//缓存中没有图片，也不用代理下载，并移除操作
__strong __typeof(weakOperation) strongOperation = weakOperation;
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
[self safelyRemoveOperationFromRunning:operation];
}
}];
return operation;
}
- 在上面的那个步骤下载前，需要先检查缓存SDImageCache。
SDImageCache.m
//查询是否缓存过该操作
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock
{
//不存在这个key，就调用完成block
if (!key) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}
// 先检查内存是否存在该图片
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
NSData *diskData = nil;
if ([image isGIF]) {//gif
diskData = [self diskImageDataBySearchingAllPathsForKey:key];
}
if (doneBlock) {
doneBlock(image, diskData, SDImageCacheTypeMemory);
}
return nil;
}
//实例化一个新操作
NSOperation *operation = [NSOperation new];
dispatch_async(self.ioQueue, ^{
if (operation.isCancelled) {
//取消操作就返回
return;
}
@autoreleasepool {
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage = [self diskImageForKey:key];
//如果盘中存在图片，并且如果可以使用缓存
if (diskImage && self.config.shouldCacheImagesInMemory) {
//图片的尺寸
NSUInteger cost = SDCacheCostForImage(diskImage);
//将硬盘中的图片在缓存中存一份
[self.memCache setObject:diskImage forKey:key cost:cost];
}
if (doneBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
//完成的回调，返回图片，数据以及图片的获取途径
// typedef NS_ENUM(NSInteger, SDImageCacheType) {
// //缓存中不存在图片，要从网上下载
// SDImageCacheTypeNone,
// //图片缓存在磁盘
// SDImageCacheTypeDisk,
// 图片缓存在内存
// SDImageCacheTypeMemory
// };
doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
});
}
}
});
return operation;
}
- 如果上面的缓存中没有，下面就是下载图片了SDWebImageDownloader。
SDWebImageDownloader.m
//下面就是下载图片的方法了
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
__weak SDWebImageDownloader *wself = self;
//进度回调
return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{
__strong __typeof (wself) sself = wself;
//设置超时15s
NSTimeInterval timeoutInterval = sself.downloadTimeout;
if (timeoutInterval == 0.0) {
timeoutInterval = 15.0;
}
// NSURLCache + SDImageCache)防止重复缓存，开始请求图片
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
request.HTTPShouldUsePipelining = YES;
//过滤HTTP请求，该字典用来HTTP请求的header
if (sself.headersFilter) {
request.allHTTPHeaderFields = sself.headersFilter(url, [sself.HTTPHeaders copy]);
}
else {
request.allHTTPHeaderFields = sself.HTTPHeaders;
}
SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];
operation.shouldDecompressImages = sself.shouldDecompressImages;
//设置默认的 URL credential
if (sself.urlCredential) {
operation.credential = sself.urlCredential;
} else if (sself.username && sself.password) {
operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];
}
//设置队列的优先级级别
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
//将操作放在NSOperationQueue中
[sself.downloadQueue addOperation:operation];
//设置操作的执行顺序
//typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
//队列模式 先进先出，这个是默认值
//SDWebImageDownloaderFIFOExecutionOrder,
//堆栈模式，后进先出
//SDWebImageDownloaderLIFOExecutionOrder
//};
if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
//添加操作的依赖关系，这里利用lastAddedOperation记录操作
[sself.lastAddedOperation addDependency:operation];
sself.lastAddedOperation = operation;
}
return operation;
}];
}
- 上面的SDWebImageDownloadToken是从下面这个方法中获取的。
SDWebImageDownloader.m
//加入进度回调，返回SDWebImageDownloadToken对象
- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
forURL:(nullable NSURL *)url
createCallback:(SDWebImageDownloaderOperation *(^)())createCallback {
//如果URL用于回调字典的key，不能为空，如果为空，就会立即调用完成block，不带有图片和数据
if (url == nil) {
if (completedBlock != nil) {
completedBlock(nil, nil, nil, NO);
}
return nil;
}
__block SDWebImageDownloadToken *token = nil;
//加入GCD阻塞
dispatch_barrier_sync(self.barrierQueue, ^{
//根据url作为key返回操作
SDWebImageDownloaderOperation *operation = self.URLOperations[url];
if (!operation) {
operation = createCallback();
self.URLOperations[url] = operation;
__weak SDWebImageDownloaderOperation *woperation = operation;
operation.completionBlock = ^{
SDWebImageDownloaderOperation *soperation = woperation;
if (!soperation)
return;
//如果这次加入的操作已经存在，就移除该操作
if (self.URLOperations[url] == soperation) {
[self.URLOperations removeObjectForKey:url];
};
};
}
id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
//重新初始化SDWebImageDownloadToken对象
token = [SDWebImageDownloadToken new];
token.url = url;
token.downloadOperationCancelToken = downloadOperationCancelToken;
});
return token;
}
- 下载完图片就对图片进行解码。
#import
#import "SDWebImageCompat.h"
@interface UIImage (ForceDecode)
+ (nullable UIImage *)decodedImageWithImage:(nullable UIImage *)image;
+ (nullable UIImage *)decodedAndScaledDownImageWithImage:(nullable UIImage *)image;
@end
主要的这些功能了，剩下的就是下载完成以后，对图片进行进一步的缓存处理。
后记
这个框架写的确实很好，从整个架构的考虑，到细节处理，从编程思想到具体实现，都是我们可以学习的地方，未完，待续~~~
ocean && sky