看完这篇文章从零开始打造一个iOS图片加载框架（一）,详细的介绍了搭建一个iOS图片加载框架基本思路,再回看YYWebImage,SDWebImage和PINRemoteImage
框架的底层，发现实现的图片加载的思路基本相同，只是细节上不同而已。
推荐文章：
从零开始打造一个iOS图片加载框架（一）
从零开始打造一个iOS图片加载框架（二）
从零开始打造一个iOS图片加载框架（三）
*SDWebImage底层实现简析
- 关键代码分析
- (void)callCacheProcessForOperation:(nonnull SDWebImageCombinedOperation *)operation
url:(nonnull NSURL *)url
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
// Check whether we should query cache
BOOL shouldQueryCache = (options & SDWebImageFromLoaderOnly) == 0;
if (shouldQueryCache) {
id<SDWebImageCacheKeyFilter> cacheKeyFilter = context[SDWebImageContextCacheKeyFilter];
NSString *key = [self cacheKeyForURL:url cacheKeyFilter:cacheKeyFilter];
@weakify(operation);
//开启一个线程，执行通过key从缓存和磁盘中获取图片的任务
operation.cacheOperation = [self.imageCache queryImageForKey:key options:options context:context completion:^(UIImage * _Nullable cachedImage, NSData * _Nullable cachedData, SDImageCacheType cacheType) {
@strongify(operation);
//如果线程已经取消或operation==nil，说明已经从缓存和磁盘中获取到图片，return
if (!operation || operation.isCancelled) {
[self safelyRemoveOperationFromRunning:operation];
return;
}
// Continue download process
//如果没有从缓存和磁盘中获取到图片，则直接网络中下载
[self callDownloadProcessForOperation:operation url:url options:options context:context cachedImage:cachedImage cachedData:cachedData cacheType:cacheType progress:progressBlock completed:completedBlock];
}];
} else {
// Continue download process
[self callDownloadProcessForOperation:operation url:url options:options context:context cachedImage:nil cachedData:nil cacheType:SDImageCacheTypeNone progress:progressBlock completed:completedBlock];
}
}
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options context:(nullable SDWebImageContext *)context done:(nullable SDImageCacheQueryCompletionBlock)doneBlock {
if (!key) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}
id<SDImageTransformer> transformer = context[SDWebImageContextImageTransformer];
if (transformer) {
// grab the transformed disk image if transformer provided
NSString *transformerKey = [transformer transformerKey];
key = SDTransformedKeyForKey(key, transformerKey);
}
// First check the in-memory cache... //从内存中获取缓存
UIImage *image = [self imageFromMemoryCacheForKey:key];
BOOL shouldQueryMemoryOnly = (image && !(options & SDImageCacheQueryMemoryData));
if (shouldQueryMemoryOnly) {
if (doneBlock) {
doneBlock(image, nil, SDImageCacheTypeMemory);
}
return nil;
}
// Second check the disk cache...
NSOperation *operation = [NSOperation new];
// Check whether we need to synchronously query disk
// 1. in-memory cache hit & memoryDataSync
// 2. in-memory cache miss & diskDataSync
BOOL shouldQueryDiskSync = ((image && options & SDImageCacheQueryMemoryDataSync) ||
(!image && options & SDImageCacheQueryDiskDataSync));
void(^queryDiskBlock)(void) = ^{
if (operation.isCancelled) {
// do not call the completion if cancelled
return;
}
//线程池
@autoreleasepool {
// 从磁盘中获取缓存
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage;
SDImageCacheType cacheType = SDImageCacheTypeNone;
NSLog(@"当前的Key:%@",key);
if (image) {
// the image is from in-memory cache, but need image data
diskImage = image;
cacheType = SDImageCacheTypeMemory;
NSLog(@"从内存中获取缓存:%@",image.description);
} else if (diskData) {
cacheType = SDImageCacheTypeDisk;
// decode image data only if in-memory cache missed
NSLog(@"从磁盘中获取缓存:%@",diskData.description);
diskImage = [self diskImageForKey:key data:diskData options:options context:context];
if (diskImage && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = diskImage.sd_memoryCost;
[self.memCache setObject:diskImage forKey:key cost:cost];
}
}
if (doneBlock) {
if (shouldQueryDiskSync) {
doneBlock(diskImage, diskData, cacheType);
} else {
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, diskData, cacheType);
});
}
}
}
};
// Query in ioQueue to keep IO-safe
if (shouldQueryDiskSync) {
dispatch_sync(self.ioQueue, queryDiskBlock);
} else {
dispatch_async(self.ioQueue, queryDiskBlock);
}
return operation;
}
// Download process
- (void)callDownloadProcessForOperation:(nonnull SDWebImageCombinedOperation *)operation
url:(nonnull NSURL *)url
options:(SDWebImageOptions)options
context:(SDWebImageContext *)context
cachedImage:(nullable UIImage *)cachedImage
cachedData:(nullable NSData *)cachedData
cacheType:(SDImageCacheType)cacheType
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
// Check whether we should download image from network
BOOL shouldDownload = (options & SDWebImageFromCacheOnly) == 0;
shouldDownload &= (!cachedImage || options & SDWebImageRefreshCached);
shouldDownload &= (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url]);
shouldDownload &= [self.imageLoader canRequestImageForURL:url];
if (shouldDownload) {
if (cachedImage && options & SDWebImageRefreshCached) {
// 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.
[self callCompletionBlockForOperation:operation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
// Pass the cached image to the image loader. The image loader should check whether the remote image is equal to the cached image.
SDWebImageMutableContext *mutableContext;
if (context) {
mutableContext = [context mutableCopy];
} else {
mutableContext = [NSMutableDictionary dictionary];
}
mutableContext[SDWebImageContextLoaderCachedImage] = cachedImage;
context = [mutableContext copy];
}
// `SDWebImageCombinedOperation` -> `SDWebImageDownloadToken` -> `downloadOperationCancelToken`, which is a `SDCallbacksDictionary` and retain the completed block below, so we need weak-strong again to avoid retain cycle
@weakify(operation);
operation.loaderOperation = [self.imageLoader requestImageWithURL:url options:options context:context progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
@strongify(operation);
if (!operation || operation.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 (cachedImage && options & SDWebImageRefreshCached && [error.domain isEqualToString:SDWebImageErrorDomain] && error.code == SDWebImageErrorCacheNotModified) {
// Image refresh hit the NSURLCache cache, do not call the completion block
} else if (error) {
[self callCompletionBlockForOperation:operation completion:completedBlock error:error url:url];
BOOL shouldBlockFailedURL = [self shouldBlockFailedURLWithURL:url error:error];
if (shouldBlockFailedURL) {
SD_LOCK(self.failedURLsLock);
[self.failedURLs addObject:url];
SD_UNLOCK(self.failedURLsLock);
}
} else {
if ((options & SDWebImageRetryFailed)) {
SD_LOCK(self.failedURLsLock);
[self.failedURLs removeObject:url];
SD_UNLOCK(self.failedURLsLock);
}
[self callStoreCacheProcessForOperation:operation url:url options:options context:context downloadedImage:downloadedImage downloadedData:downloadedData finished:finished progress:progressBlock completed:completedBlock];
}
if (finished) {
[self safelyRemoveOperationFromRunning:operation];
}
}];
} else if (cachedImage) {
[self callCompletionBlockForOperation:operation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
[self safelyRemoveOperationFromRunning:operation];
} else {
// Image not in cache and download disallowed by delegate
[self callCompletionBlockForOperation:operation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
[self safelyRemoveOperationFromRunning:operation];
}
}
-
SDWebImage 流程
SDWebImage 流程.jpg
面试题
1.使用SDWebImage加载图片，有一个图片的url链接第一次加载完了后，后台将url链接对应的图片内容更改（即保持url不变），使用SDWebImage去再次显示时，图片会变吗?如果会，请说明理由，如果不会，如何才能显示服务器里面真正存放的图片？
1.该题考察的是对SDWebImage的底层实现，首先SD会将这个url作为Key，从内存缓存以及磁盘缓存中去获取该图片，如果内存缓存以及磁盘缓存都没有找到该Key对应图片，则才会执行网络下载的步骤。
2.因为url不变,所以key不变，所以第二次加载会从磁盘中得到该图片直接显示，图片不会发生变化。
3.如果确认url内容发生改变,[[SDImageCache sharedImageCache]removeImageForKey:url];可先清除该url上的缓存。
