SDWebImage 5.0以后，项目中的代码文件非常多，这边对文件进行了一下简单区分，代码结构如下。
结构图.png
主要分为一下几块
1、Render渲染
2、Coder编码解码
3、AnimatedImage动图加载
4、Utils工具类
5、Cache缓存
6、Downloader下载
7、Category相关类使用接口
今天主要讲一下Cache这部分
- 磁盘存储放在一个串行队列中
- 创建内存存储和磁盘存储的Cache
- 老版本升级图片转移
- 退到后台或者杀掉APP，删除老的缓存
///单利
@property (nonatomic, class, readonly, nonnull) SDImageCache *sharedImageCache
///实现
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
diskCacheDirectory:(nullable NSString *)directory
config:(nullable SDImageCacheConfig *)config {
if ((self = [super init])) {
NSAssert(ns, @"Cache namespace should not be nil");
// 创建串行队列
_ioQueue = dispatch_queue_create("com.hackemist.SDImageCache", DISPATCH_QUEUE_SERIAL);
if (!config) {
config = SDImageCacheConfig.defaultCacheConfig;
}
_config = [config copy];
// Init the memory cache
NSAssert([config.memoryCacheClass conformsToProtocol:@protocol(SDMemoryCache)], @"Custom memory cache class must conform to `SDMemoryCache` protocol");
// 内存缓存
_memoryCache = [[config.memoryCacheClass alloc] initWithConfig:_config];
// Init the disk cache
if (!directory) {
// 默认存储路径
directory = [self.class defaultDiskCacheDirectory];
}
_diskCachePath = [directory stringByAppendingPathComponent:ns];
NSAssert([config.diskCacheClass conformsToProtocol:@protocol(SDDiskCache)], @"Custom disk cache class must conform to `SDDiskCache` protocol");
// 磁盘缓存
_diskCache = [[config.diskCacheClass alloc] initWithCachePath:_diskCachePath config:_config];
// 版本升级图片转移
// 新 ~/Library/Caches/com.hackemist.SDImageCache/default/
// 老 ~/Library/Caches/default/com.hackemist.SDWebImageCache.default/
[self migrateDiskCacheDirectory];
#if SD_UIKIT
// 退到后台或者杀掉APP，删除老的缓存
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationWillTerminate:)
name:UIApplicationWillTerminateNotification
object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationDidEnterBackground:)
name:UIApplicationDidEnterBackgroundNotification
object:nil];
#endif
#if SD_MAC
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(applicationWillTerminate:)
name:NSApplicationWillTerminateNotification
object:nil];
#endif
}
return self;
}
图片存在到内存和磁盘
- 内存存储
- 磁盘存储，异步存储（串行队列）
- 数据编码
- 通过NSKeyedArchiver存储扩展信息，key和图片相同
/// 图片存在到内存和磁盘
- (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;
}
// 内存存储
if (toMemory && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = image.sd_memoryCost;
[self.memoryCache setObject:image forKey:key cost:cost];
}
if (toDisk) {
// 磁盘存储
dispatch_async(self.ioQueue, ^{
// 异步存储（串行队列）
@autoreleasepool {
NSData *data = imageData;
if (!data && [image conformsToProtocol:@protocol(SDAnimatedImage)]) {
// If image is custom animated image class, prefer its original animated data
data = [((id)image) animatedImageData];
}
if (!data && image) {
// Check image's associated image format, may return .undefined
SDImageFormat format = image.sd_imageFormat;
if (format == SDImageFormatUndefined) {
// If image is animated, use GIF (APNG may be better, but has bugs before macOS 10.14)
if (image.sd_isAnimated) {
format = SDImageFormatGIF;
} else {
// If we do not have any data to detect image format, check whether it contains alpha channel to use PNG or JPEG format
if ([SDImageCoderHelper CGImageContainsAlpha:image.CGImage]) {
format = SDImageFormatPNG;
} else {
format = SDImageFormatJPEG;
}
}
}
// 数据编码
data = [[SDImageCodersManager sharedManager] encodedDataWithImage:image format:format options:nil];
}
[self _storeImageDataToDisk:data forKey:key];
if (image) {
// 通过NSKeyedArchiver存储扩展信息，key和图片相同
// Check extended data
id extendedObject = image.sd_extendedObject;
if ([extendedObject conformsToProtocol:@protocol(NSCoding)]) {
NSData *extendedData;
if (@available(iOS 11, tvOS 11, macOS 10.13, watchOS 4, *)) {
NSError *error;
extendedData = [NSKeyedArchiver archivedDataWithRootObject:extendedObject requiringSecureCoding:NO error:&error];
if (error) {
NSLog(@"NSKeyedArchiver archive failed with error: %@", error);
}
} else {
@try {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
extendedData = [NSKeyedArchiver archivedDataWithRootObject:extendedObject];
#pragma clang diagnostic pop
} @catch (NSException *exception) {
NSLog(@"NSKeyedArchiver archive failed with exception: %@", exception);
}
}
if (extendedData) {
[self.diskCache setExtendedData:extendedData forKey:key];
}
}
}
}
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
} else {
if (completionBlock) {
completionBlock();
}
}
}
查询图片
- 首先从内存中查询
- 对需要解码和动图处理
- 从磁盘中查询
- 内存中不存在，从磁盘解码出图片数据并存入磁盘中
// 查询图片
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options context:(nullable SDWebImageContext *)context cacheType:(SDImageCacheType)queryCacheType done:(nullable SDImageCacheQueryCompletionBlock)doneBlock {
if (!key) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}
// Invalid cache type
if (queryCacheType == SDImageCacheTypeNone) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}
// 首先从内存中查询
UIImage *image;
if (queryCacheType != SDImageCacheTypeDisk) {
image = [self imageFromMemoryCacheForKey:key];
}
if (image) {
// 对需要解码和动图处理
if (options & SDImageCacheDecodeFirstFrameOnly) {
// Ensure static image
Class animatedImageClass = image.class;
if (image.sd_isAnimated || ([animatedImageClass isSubclassOfClass:[UIImage class]] && [animatedImageClass conformsToProtocol:@protocol(SDAnimatedImage)])) {
#if SD_MAC
image = [[NSImage alloc] initWithCGImage:image.CGImage scale:image.scale orientation:kCGImagePropertyOrientationUp];
#else
image = [[UIImage alloc] initWithCGImage:image.CGImage scale:image.scale orientation:image.imageOrientation];
#endif
}
} else if (options & SDImageCacheMatchAnimatedImageClass) {
// Check image class matching
Class animatedImageClass = image.class;
Class desiredImageClass = context[SDWebImageContextAnimatedImageClass];
if (desiredImageClass && ![animatedImageClass isSubclassOfClass:desiredImageClass]) {
image = nil;
}
}
}
BOOL shouldQueryMemoryOnly = (queryCacheType == SDImageCacheTypeMemory) || (image && !(options & SDImageCacheQueryMemoryData));
if (shouldQueryMemoryOnly) {
if (doneBlock) {
doneBlock(image, nil, SDImageCacheTypeMemory);
}
return nil;
}
// 从磁盘中查询
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) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return;
}
@autoreleasepool {
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage;
if (image) {
// the image is from in-memory cache, but need image data
diskImage = image;
} else if (diskData) {
// 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, SDImageCacheTypeDisk);
} else {
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
});
}
}
}
};
// Query in ioQueue to keep IO-safe
if (shouldQueryDiskSync) {
dispatch_sync(self.ioQueue, queryDiskBlock);
} else {
dispatch_async(self.ioQueue, queryDiskBlock);
}
return operation;
}
SDMemoryCache 内存缓存
这个对象是继承自NSCache的。同时这个支持弱存储。
NSCache是通过下面2个方法实现的
- (void)setObject:(nullable id)object forKey:(nonnull id)key;
- (void)removeObjectForKey:(nonnull id)key;
弱存储主要是通过NSMapTable，因为图片存储是多线程的，所以这边对weakCache的操作都进行了加锁处理（SD_LOCK）。
@property (nonatomic, strong, nonnull) NSMapTable
// `setObject:forKey:` just call this with 0 cost. Override this is enough
- (void)setObject:(id)obj forKey:(id)key cost:(NSUInteger)g {
[super setObject:obj forKey:key cost:g];
if (!self.config.shouldUseWeakMemoryCache) {
return;
}
if (key && obj) {
// Store weak cache
SD_LOCK(self.weakCacheLock);
[self.weakCache setObject:obj forKey:key];
SD_UNLOCK(self.weakCacheLock);
}
}
- (id)objectForKey:(id)key {
id obj = [super objectForKey:key];
if (!self.config.shouldUseWeakMemoryCache) {
return obj;
}
if (key && !obj) {
// Check weak cache
SD_LOCK(self.weakCacheLock);
obj = [self.weakCache objectForKey:key];
SD_UNLOCK(self.weakCacheLock);
if (obj) {
// Sync cache
NSUInteger cost = 0;
if ([obj isKindOfClass:[UIImage class]]) {
cost = [(UIImage *)obj sd_memoryCost];
}
[super setObject:obj forKey:key cost:cost];
}
}
return obj;
}
SDDiskCache 磁盘存储
磁盘存储主要是通过NSFileManager对数据进行存储和删除
移除过期的磁盘数据
/// NSFileManager的方法，可以获取到含有时间信息的URL的枚举数据
- (nullable NSDirectoryEnumerator *)enumeratorAtURL:(NSURL *)url includingPropertiesForKeys:(nullable NSArray *)keys options:(NSDirectoryEnumerationOptions)mask errorHandler:(nullable BOOL (^)(NSURL *url, NSError *error))handler API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0));
/// 字典进行排序
- (NSArray *)keysSortedByValueWithOptions:(NSSortOptions)opts usingComparator:(NSComparator NS_NOESCAPE)cmptr API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0));
- (void)removeExpiredData {
NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
// Compute content date key to be used for tests
NSURLResourceKey cacheContentDateKey = NSURLContentModificationDateKey;
switch (self.config.diskCacheExpireType) {
case SDImageCacheConfigExpireTypeAccessDate:
cacheContentDateKey = NSURLContentAccessDateKey;
break;
case SDImageCacheConfigExpireTypeModificationDate:
cacheContentDateKey = NSURLContentModificationDateKey;
break;
case SDImageCacheConfigExpireTypeCreationDate:
cacheContentDateKey = NSURLCreationDateKey;
break;
case SDImageCacheConfigExpireTypeChangeDate:
cacheContentDateKey = NSURLAttributeModificationDateKey;
break;
default:
break;
}
NSArray *resourceKeys = @[NSURLIsDirectoryKey, cacheContentDateKey, NSURLTotalFileAllocatedSizeKey];
// This enumerator prefetches useful properties for our cache files.
NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtURL:diskCacheURL
includingPropertiesForKeys:resourceKeys
options:NSDirectoryEnumerationSkipsHiddenFiles
errorHandler:NULL];
NSDate *expirationDate = (self.config.maxDiskAge < 0) ? nil: [NSDate dateWithTimeIntervalSinceNow:-self.config.maxDiskAge];
NSMutableDictionary *> *cacheFiles = [NSMutableDictionary dictionary];
NSUInteger currentCacheSize = 0;
// Enumerate all of the files in the cache directory. This loop has two purposes:
//
// 1. Removing files that are older than the expiration date.
// 2. Storing file attributes for the size-based cleanup pass.
NSMutableArray *urlsToDelete = [[NSMutableArray alloc] init];
for (NSURL *fileURL in fileEnumerator) {
NSError *error;
NSDictionary *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:&error];
// Skip directories and errors.
if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
continue;
}
// Remove files that are older than the expiration date;
NSDate *modifiedDate = resourceValues[cacheContentDateKey];
if (expirationDate && [[modifiedDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
[urlsToDelete addObject:fileURL];
continue;
}
// Store a reference to this file and account for its total size.
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
cacheFiles[fileURL] = resourceValues;
}
for (NSURL *fileURL in urlsToDelete) {
[self.fileManager removeItemAtURL:fileURL error:nil];
}
// If our remaining disk cache exceeds a configured maximum size, perform a second
// size-based cleanup pass. We delete the oldest files first.
NSUInteger maxDiskSize = self.config.maxDiskSize;
if (maxDiskSize > 0 && currentCacheSize > maxDiskSize) {
// Target half of our maximum cache size for this cleanup pass.
const NSUInteger desiredCacheSize = maxDiskSize / 2;
// Sort the remaining cache files by their last modification time or last access time (oldest first).
NSArray *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
usingComparator:^NSComparisonResult(id obj1, id obj2) {
return [obj1[cacheContentDateKey] compare:obj2[cacheContentDateKey]];
}];
// Delete files until we fall below our desired cache size.
for (NSURL *fileURL in sortedFiles) {
if ([self.fileManager removeItemAtURL:fileURL error:nil]) {
NSDictionary *resourceValues = cacheFiles[fileURL];
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;
if (currentCacheSize < desiredCacheSize) {
break;
}
}
}
}
}