SDWebImage之缓存策略
目录
- 取值 - 内存缓存
- 取值 - 磁盘缓存
- 存值 - 内存/磁盘缓存
简介
本章节只是对SDWebImage中内存策略一块的理解。
取值 - 内存缓存
- 1、判读类型不等与SDImageCacheTypeDisk，从内存中取值
- 2、从self.memoryCache中取值，key是url
- 3、self.memoryCache是SDMemoryCache类型，SDMemoryCache是NSCache类型
1、从内存缓存中取值首先会判读类型不等与SDImageCacheTypeDisk磁盘缓存
//SDImageCache
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options context:(nullable SDWebImageContext *)context cacheType:(SDImageCacheType)queryCacheType done:(nullable SDImageCacheQueryCompletionBlock)doneBlock {
...
//memory内存缓存取值
UIImage *image;
if (queryCacheType != SDImageCacheTypeDisk) {
image = [self imageFromMemoryCacheForKey:key];
}
...
}
2、从self.memoryCache中取值，key是url
//SDImageCache
- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key {
return [self.memoryCache objectForKey:key];
}
3、self.memoryCache是SDMemoryCache类型，SDMemoryCache是NSCache类型。
//SDImageCache
@property (nonatomic, strong, readonly, nonnull) id<SDMemoryCache> memoryCache;
@interface SDMemoryCache <KeyType, ObjectType> : NSCache <KeyType, ObjectType> <SDMemoryCache>
//SDMemoryCache
@property (nonatomic, strong, nonnull, readonly) SDImageCacheConfig *config;
@end
取值-磁盘取值
- 1、从self.diskCache中取diskData
- 2、self.diskCache类型为SDDiskCache
- 3、SDDiskCache磁盘缓存是用NSFileManager实现的
//SDImageCache
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options context:(nullable SDWebImageContext *)context cacheType:(SDImageCacheType)queryCacheType done:(nullable SDImageCacheQueryCompletionBlock)doneBlock {
...
//disk磁盘取值
1、从self.diskCache中取diskData
@autoreleasepool {
//从self.diskCache中取diskData
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage;
SDImageCacheType cacheType = SDImageCacheTypeNone;
if (image) {
// the image is from in-memory cache, but need image data
diskImage = image;
cacheType = SDImageCacheTypeMemory;
} else if (diskData) {
cacheType = SDImageCacheTypeDisk;
// decode image data only if in-memory cache missed
diskImage = [self diskImageForKey:key data:diskData options:options context:context];
if (diskImage && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = diskImage.sd_memoryCost;
[self.memoryCache 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);
});
}
}
}
...
}
1、从self.diskCache中取diskData
//SDImageCache
- (nullable NSData *)diskImageDataBySearchingAllPathsForKey:(nullable NSString *)key {
...
NSData *data = [self.diskCache dataForKey:key];
...
}
2、self.diskCache类型为SDDiskCache
//SDImageCache
@property (nonatomic, strong, readonly, nonnull) id<SDDiskCache> diskCache;
3、SDDiskCache磁盘缓存是用NSFileManager实现的
@property (nonatomic, strong, nonnull) NSFileManager *fileManager;
- (NSData *)dataForKey:(NSString *)key {
NSParameterAssert(key);
NSString *filePath = [self cachePathForKey:key];
NSData *data = [NSData dataWithContentsOfFile:filePath options:self.config.diskCacheReadingOptions error:nil];
if (data) {
return data;
}
// fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
// checking the key with and without the extension
data = [NSData dataWithContentsOfFile:filePath.stringByDeletingPathExtension options:self.config.diskCacheReadingOptions error:nil];
if (data) {
return data;
}
return nil;
}
- (void)setData:(NSData *)data forKey:(NSString *)key {
NSParameterAssert(data);
NSParameterAssert(key);
if (![self.fileManager fileExistsAtPath:self.diskCachePath]) {
[self.fileManager createDirectoryAtPath:self.diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
}
// get cache Path for image key
NSString *cachePathForKey = [self cachePathForKey:key];
// transform to NSUrl
NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
[data writeToURL:fileURL options:self.config.diskCacheWritingOptions error:nil];
// disable iCloud backup
if (self.config.shouldDisableiCloud) {
// ignore iCloud backup resource value error
[fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];
}
}
存值-内存缓存
- 1、检测缓存，如果缓存中没有，则请求获取
- 2、创建loaderOperation <SDWebImageOperation>下载任务
- 3、下载之后，进行缓存
//SDWebImageManager
- (void)callCacheProcessForOperation:(nonnull SDWebImageCombinedOperation *)operation
url:(nonnull NSURL *)url
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
...
operation.cacheOperation = [imageCache queryImageForKey:key options:options context:context cacheType:queryCacheType completion:^(UIImage * _Nullable cachedImage, NSData * _Nullable cachedData, SDImageCacheType cacheType) {
//优先从缓存中获取，如果没有获取到再请求获取
...
[self callDownloadProcessForOperation:operation url:url options:options context:context cachedImage:cachedImage cachedData:cachedData cacheType:cacheType progress:progressBlock completed:completedBlock];
...
}
...
}
2、创建loaderOperation <SDWebImageOperation>下载任务
//SDWebImageDownloader
- (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 {
...
//2、创建loaderOperation <SDWebImageOperation>下载任务
operation.loaderOperation = [imageLoader requestImageWithURL:url options:options context:context progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
...
// Continue store cache process
[self callStoreCacheProcessForOperation:operation url:url options:options context:context downloadedImage:downloadedImage downloadedData:downloadedData finished:finished progress:progressBlock completed:completedBlock];
...
}
...
}
3、下载之后，进行缓存
- 3.1：如果满足内存缓存，则进行内存缓存
//SDImageCache
- (void)storeImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
forKey:(nullable NSString *)key
toMemory:(BOOL)toMemory
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock {
if (!image || !key) {
if (completionBlock) {
completionBlock();
}
return;
}
// if memory cache is enabled
//3.1：如果满足内存缓存条件，则进行内存缓存
if (toMemory && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = image.sd_memoryCost;
[self.memoryCache setObject:image forKey:key cost:cost];
}
//3.2：如果满足磁盘缓存条件，则进行缓存缓存
if (toDisk) {
dispatch_async(self.ioQueue, ^{
......
})
}
}
image.png
缓存策略整体流程：
1、每次加载图片优先取缓存中取
2、如果memory有，则直接返回
3、如果memory没有，则到disk中取
4、如果disk有，则对内存添加一份
5、如果disk没有，则进行网络请求加载
6、加载完成之后，对memory和disk各存一份。
面试题
1、SDWebImage 的最大并发数是多少，默认超时时间是多少
//SDWebImageDownloaderConfig
- (instancetype)init {
self = [super init];
if (self) {
_maxConcurrentDownloads = 6;
_downloadTimeout = 15.0;
_executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
}
return self;
}
最大并发数6，默认超时时间15.0秒，外部可以修改
2、SDWebImage 缓存技术是什么
//SDMemoryCache
@interface SDMemoryCache <KeyType, ObjectType> : NSCache <KeyType, ObjectType> <SDMemoryCache>
@property (nonatomic, strong, nonnull, readonly) SDImageCacheConfig *config;
@end
//self.diskCache类型为SDDiskCache
@property (nonatomic, strong, nonnull) NSFileManager *fileManager;
- (NSData *)dataForKey:(NSString *)key {}
- (void)setData:(NSData *)data forKey:(NSString *)key {}
内存缓存使用NSCache
磁盘缓存使用NSFileManager
内存管理使用NSCache
- NSCache是一个类似NSDictionary一个可变的集合。
- 提供了可设置缓存的数目与内存大小限制的方式。
- 保证了处理的数据的线程安全性。
- 缓存使用的key不需要是实现NSCopying的类。
- 当内存警告时内部自动清理部分缓存数据。
https://www.jianshu.com/p/986e4560f10f
https://blog.csdn.net/perry0528/article/details/91044144
3、SDWebImage的Disk删除时机
3.1：SDImageCache类还监听了UIApplicationDidReceiveMemoryWarningNotification通知，当收到内存警告时候清除Memory缓存
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(didReceiveMemoryWarning:)
name:UIApplicationDidReceiveMemoryWarningNotification
object:nil];
#endif
}
// Current this seems no use on macOS (macOS use virtual memory and do not clear cache when memory warning). So we only override on iOS/tvOS platform.
#if SD_UIKIT
- (void)didReceiveMemoryWarning:(NSNotification *)notification {
// Only remove cache, but keep weak cache
[super removeAllObjects];
}
3.2：图片默认缓存时间是一周
//SDImageCacheConfig
static const NSInteger kDefaultCacheMaxDiskAge = 60 * 60 * 24 * 7; // 1 week
【参考】
作者:小篆风
原文链接:https://www.jianshu.com/p/32410bbf4c9a