在上一篇[SDWebImage源码阅读(五)]（jianshu.com/p/fec6c0b8ed1b）中介绍了SDImageCahe.h文件，包括SDImageCacheConfig文件的阅读，这次开始阅读SDImageCache.m文件中的代码，了解实现思路。
在最初的定义中，定义了开锁和解锁，这个代码值得学习以下
#define LOCK(lock) dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
#define UNLOCK(lock) dispatch_semaphore_signal(lock);
之后定义了一个类SDMemoryCache,继承自NSCache，
在初始化方法中，在缓存中，可能存在内存不足的情况，所以在init中注册了通知
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(didReceiveMemoryWarning:)
name:UIApplicationDidReceiveMemoryWarningNotification
object:nil];
- (void)didReceiveMemoryWarning:(NSNotification *)notification {
//只删除缓存，但是保持弱缓存
[super removeAllObjects];
}
另外，在objectForKey:(id)key中有获取内存的花费的代码
- (id)objectForKey:(id)key {
id obj = [super objectForKey:key];
if (!self.config.shouldUseWeakMemoryCache) {
return obj;
}
if (key && !obj) {
// Check weak cache
LOCK(self.weakCacheLock);
obj = [self.weakCache objectForKey:key];
UNLOCK(self.weakCacheLock);
if (obj) {
// Sync cache
NSUInteger cost = 0;
if ([obj isKindOfClass:[UIImage class]]) {
//获取内存的花费
cost = [(UIImage *)obj sd_memoryCost];
}
[super setObject:obj forKey:key cost:cost];
}
}
return obj;
}
点击跳转后，发现具体的计算内存花费的代码为：
FOUNDATION_STATIC_INLINE NSUInteger SDMemoryCacheCostForImage(UIImage *image) {
#if SD_MAC
return image.size.height * image.size.width;
#elif SD_UIKIT || SD_WATCH
NSUInteger imageSize = image.size.height * image.size.width * image.scale * image.scale;
return image.images ? (imageSize * image.images.count) : imageSize;
#endif
}
接下来看SDImageCache.m中的代码
获取磁盘存储路径的代码
- (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace {
NSArray<NSString *> *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
return [paths[0] stringByAppendingPathComponent:fullNamespace];
}
之后看到初始化的方法
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
diskCacheDirectory:(nonnull NSString *)directory {
//创建io串行队列（名字由自己命名）
_ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);
...
//初始化磁盘缓存
if (directory != nil) {
_diskCachePath = [directory stringByAppendingPathComponent:fullNamespace];
} else {
NSString *path = [self makeDiskCachePath:ns];
_diskCachePath = path;
}
dispatch_sync(_ioQueue, ^{
self.fileManager = [NSFileManager new];
});
//app被杀死时发出通知
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(deleteOldFiles)
name:UIApplicationWillTerminateNotification
object:nil];
//app进入后台时发出通知
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(backgroundDeleteOldFiles)
name:UIApplicationDidEnterBackgroundNotification
object:nil];
}
当app进入后台或者被杀死时，需要相应的执行方法@selector(backgroundDeleteOldFiles)和@selectors(deleteOldFiles)这两个方法。
最终这两个方法都是调用的下面的这个方法
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
}
只不过在backgroundDeleteOldFiles中需要处理任务进入后台请求额外时间的处理，相应的处理代码如下：
- (void)backgroundDeleteOldFiles {
Class UIApplicationClass = NSClassFromString(@"UIApplication");
if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
return;
}
UIApplication *application = [UIApplication performSelector:@selector(sharedApplication)];
__block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
// Clean up any unfinished task business by marking where you
// stopped or ending the task outright.
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
//启动长时间运行的任务并立即返回
[self deleteOldFilesWithCompletionBlock:^{
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
}
之后，开始看- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock这个方法中的相应的实现
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
//对于io操作队列，开启一个异步线程，来实现其他的一些东西
dispatch_async(self.ioQueue, ^{
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;
default:
break;
}
NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, cacheContentDateKey, NSURLTotalFileAllocatedSizeKey];
//这个枚举器为缓存文件预取有用的属性。
NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtURL:diskCacheURL
includingPropertiesForKeys:resourceKeys
options:NSDirectoryEnumerationSkipsHiddenFiles
errorHandler:NULL];
//计算国过期的日期，假设self.config.maxCacheAge为7（今天是7月7号），那么得出来的expirationDate的大小为7月2号,删除过期文件只需要删除存储时间是在7月2号之前的就行
NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.config.maxCacheAge];
NSMutableDictionary<NSURL *, NSDictionary<NSString *, id> *> *cacheFiles = [NSMutableDictionary dictionary];
NSUInteger currentCacheSize = 0;
//1. 删除超过过期日期的文件。
// 2.为基于大小的清除传递存储文件属性
NSMutableArray<NSURL *> *urlsToDelete = [[NSMutableArray alloc] init];
for (NSURL *fileURL in fileEnumerator) {
NSError *error;
NSDictionary<NSString *, id> *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:&error];
// 如果当前扫描的是目录，就跳过
if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
continue;
}
// 移除在过期日期以前的文件
NSDate *modifiedDate = resourceValues[cacheContentDateKey];
if ([[modifiedDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
[urlsToDelete addObject:fileURL];
continue;
}
//计算当前已经使用的cache大小， - 并将对应file的属性存到cacheFiles中
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
cacheFiles[fileURL] = resourceValues;
}
//根据需要移除的文件的url删除文件
for (NSURL *fileURL in urlsToDelete) {
[self.fileManager removeItemAtURL:fileURL error:nil];
}
//如果我们当前cache的大小已经超过了允许配置的缓存大小，那就删除已经缓存的文件。
// 删除策略就是，首先删除修改时间更早的缓存文件
if (self.config.maxCacheSize > 0 && currentCacheSize > self.config.maxCacheSize) {
//直接将当前cache大小降到允许最大的cache大小的一半
const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2;
// 根据最后修改时间或最后访问时间(最老的先访问)对剩余的缓存文件进行排序。
NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
usingComparator:^NSComparisonResult(id obj1, id obj2) {
return [obj1[cacheContentDateKey] compare:obj2[cacheContentDateKey]];
}];
//每次删除file后，就计算此时的cache的大小，直到达到我们预期的缓存
for (NSURL *fileURL in sortedFiles) {
if ([self.fileManager removeItemAtURL:fileURL error:nil]) {
NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;
if (currentCacheSize < desiredCacheSize) {
break;
}
}
}
}
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
}
上面介绍的是当app被杀死或者app进入后台时的处理逻辑，下面继续原本的代码看，看到有一个方法
//根据key值获取缓存的fileName，使用了MD5转换
- (nullable NSString *)cachedFileNameForKey:(nullable NSString *)key {
const char *str = key.UTF8String;
if (str == NULL) {
str = "";
}
unsigned char r[CC_MD5_DIGEST_LENGTH];
CC_MD5(str, (CC_LONG)strlen(str), r);
NSURL *keyURL = [NSURL URLWithString:key];
NSString *ext = keyURL ? keyURL.pathExtension : key.pathExtension;
// File system has file name length limit, we need to check if ext is too long, we don't add it to the filename
if (ext.length > SD_MAX_FILE_EXTENSION_LENGTH) {
ext = nil;
}
NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%@",
r[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10],
r[11], r[12], r[13], r[14], r[15], ext.length == 0 ? @"" : [NSString stringWithFormat:@".%@", ext]];
return filename;
}
之后，获取缓存路径的方法为:
- (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace {
NSArray<NSString *> *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
return [paths[0] stringByAppendingPathComponent:fullNamespace];
}
接下来就是一连串的存储图片数据的方法
- (void)storeImage:(nullable UIImage *)image
forKey:(nullable NSString *)key
completion:(nullable SDWebImageNoParamsBlock)completionBlock {
[self storeImage:image imageData:nil forKey:key toDisk:YES completion:completionBlock];
}
- (void)storeImage:(nullable UIImage *)image
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock {
[self storeImage:image imageData:nil forKey:key toDisk:toDisk completion:completionBlock];
}
- (void)storeImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock {
}
最终都是调用的第三个方法，
- (void)storeImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
forKey:(nullable NSString *)key
toDisk:(BOOL)toDisk
completion:(nullable SDWebImageNoParamsBlock)completionBlock {
if (!image || !key) {
if (completionBlock) {
completionBlock();
}
return;
}
// 如果允许存储到内存中
if (self.config.shouldCacheImagesInMemory) {
NSUInteger cost = image.sd_memoryCost;
//将图片以及对应的花费存储到内存NSCache中
[self.memCache setObject:image forKey:key cost:cost];
}
//如果允许存储到磁盘中
if (toDisk) {
dispatch_async(self.ioQueue, ^{
@autoreleasepool {
NSData *data = imageData;
if (!data && image) {
//如果我们没有任何数据来检测图像格式，检查它是否包含alpha通道来使用PNG或JPEG格式
SDImageFormat format;
if (SDCGImageRefContainsAlpha(image.CGImage)) {
format = SDImageFormatPNG;
} else {
format = SDImageFormatJPEG;
}
//判断图片的格式，用于编码成数据
data = [[SDWebImageCodersManager sharedInstance] encodedDataWithImage:image format:format];
}
//调用storeImageDataToDisk，将图像数据存储到磁盘
[self _storeImageDataToDisk:data forKey:key];
}
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
} else {
if (completionBlock) {
completionBlock();
}
}
}
- (void)_storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
if (!imageData || !key) {
return;
}
if (![self.fileManager fileExistsAtPath:_diskCachePath]) {
[self.fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
}
// 根据key获取缓存的路径
NSString *cachePathForKey = [self defaultCachePathForKey:key];
// 转换成URL
NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
[imageData writeToURL:fileURL options:self.config.diskCacheWritingOptions error:nil];
//不允许iClound备份
if (self.config.shouldDisableiCloud) {
[fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];
}
}
上面讲得是存储方面的操作，下面讲的是查询和检索的操作
//查询磁盘缓存中图片是否存在
- (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
dispatch_async(self.ioQueue, ^{
BOOL exists = [self _diskImageDataExistsWithKey:key];
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock(exists);
});
}
});
}
下面有一个从缓存中获取图片对象的方法，
//首先检查内存中是否有对应缓存，如果有，则将其取出使用，如果没有，则去检查磁盘中是否有对应的缓存，有的话就使用磁盘中的缓存
- (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key {
// First check the in-memory cache...
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
return image;
}
// Second check the disk cache...
image = [self imageFromDiskCacheForKey:key];
return image;
}
还有一个比较复杂的方法
- (nullable NSData *)diskImageDataForKey:(nullable NSString *)key {
if (!key) {
return nil;
}
__block NSData *imageData = nil;
//这个地方为什么要开启一个同步的队列
dispatch_sync(self.ioQueue, ^{
//搜索所有的path以获取data
imageData = [self diskImageDataBySearchingAllPathsForKey:key];
});
return imageData;
}
//搜索的策略
- (nullable NSData *)diskImageDataBySearchingAllPathsForKey:(nullable NSString *)key {
//首先根据默认的缓存路径尝试获取data，如果获取到，则直接返回
NSString *defaultPath = [self defaultCachePathForKey:key];
NSData *data = [NSData dataWithContentsOfFile:defaultPath options:self.config.diskCacheReadingOptions error:nil];
if (data) {
return data;
}
//在默认路径去除最后的扩展尝试获取data，如果获取到，则直接返回
data = [NSData dataWithContentsOfFile:defaultPath.stringByDeletingPathExtension options:self.config.diskCacheReadingOptions error:nil];
if (data) {
return data;
}
NSArray<NSString *> *customPaths = [self.customPaths copy];
for (NSString *path in customPaths) {
//尝试从自定义的路径那里获取data，如果获取到，则直接返回
NSString *filePath = [self cachePathForKey:key inPath:path];
NSData *imageData = [NSData dataWithContentsOfFile:filePath options:self.config.diskCacheReadingOptions error:nil];
if (imageData) {
return imageData;
}
// 对自定义路径去除后面的扩展，从这个路径尝试获取data，如果获取到则直接返回
imageData = [NSData dataWithContentsOfFile:filePath.stringByDeletingPathExtension options:self.config.diskCacheReadingOptions error:nil];
if (imageData) {
return imageData;
}
}
return nil;
}
上面获取到的都是NSData，需要将NSData转换成UIImage对象，需要解码，解码的相关的方法为：
- (nullable UIImage *)diskImageForKey:(nullable NSString *)key data:(nullable NSData *)data options:(SDImageCacheOptions)options {
if (data) {
UIImage *image = [[SDWebImageCodersManager sharedInstance] decodedImageWithData:data];
image = [self scaledImageForKey:key image:image];
if (self.config.shouldDecompressImages) {
BOOL shouldScaleDown = options & SDImageCacheScaleDownLargeImages;
image = [[SDWebImageCodersManager sharedInstance] decompressedImageWithImage:image data:&data options:@{SDWebImageCoderScaleDownLargeImagesKey: @(shouldScaleDown)}];
}
return image;
} else {
return nil;
}
}
之后，获取相关的NSOperation实例
- (NSOperation *)queryCacheOperationForKey:(NSString *)key done:(SDCacheQueryCompletedBlock)doneBlock {
return [self queryCacheOperationForKey:key options:0 done:doneBlock];
}
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options done:(nullable SDCacheQueryCompletedBlock)doneBlock {
if (!key) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}
// 从内存中获取cache 的image
UIImage *image = [self imageFromMemoryCacheForKey:key];
BOOL shouldQueryMemoryOnly = (image && !(options & SDImageCacheQueryDataWhenInMemory));
//只需要去查询内存，不需要查询磁盘
if (shouldQueryMemoryOnly) {
if (doneBlock) {
doneBlock(image, nil, SDImageCacheTypeMemory);
}
return nil;
}
NSOperation *operation = [NSOperation new];
void(^queryDiskBlock)(void) = ^{
if (operation.isCancelled) {
// do not call the completion if cancelled
return;
}
@autoreleasepool {
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage;
SDImageCacheType cacheType = SDImageCacheTypeNone;
if (image) {
// the image is from in-memory cache
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];
if (diskImage && self.config.shouldCacheImagesInMemory) {
NSUInteger cost = diskImage.sd_memoryCost;
[self.memCache setObject:diskImage forKey:key cost:cost];
}
}
if (doneBlock) {
//当需要同步查询磁盘的数据时，直接调用doneBlock
if (options & SDImageCacheQueryDiskSync) {
doneBlock(diskImage, diskData, cacheType);
} else {
//当需要异步查询磁盘数据，同步查询的内存数据，使用dispatch_async
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, diskData, cacheType);
});
}
}
}
};
//当需要同步执行这个block时，不需要开辟新的线程，使用原本的线程就好
if (options & SDImageCacheQueryDiskSync) {
queryDiskBlock();
} else {
//需要异步查询时，用ioQueue执行queryDiskBlock
dispatch_async(self.ioQueue, queryDiskBlock);
}
return operation;
}
上面的代码目前就先看到这里，到这里留有几个疑问：一是SDWebImageCompat.h的实现，里面主要是判断图片的格式；二是SDWebImageCoderManager中的实现，这个类中的实现是将图片编码和解码以及压缩（NSData和UIImage对象的转换），三是各个线程的开辟和使用，特别是最后两个方法涉及的，有点摸不清头脑，需要自己理顺一下。
SDWebImage源码阅读(一)
SDWebImage源码阅读(二)
SDWebImage源码阅读(三)
SDWebImage源码阅读(四)
SDWebImage源码阅读(五)
SDWebImage源码阅读(六)
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