SDWebImage源码剖析（二）

SDWebImageCache管理着SDWebImage的缓存，其中内存缓存采用NSCache，同时会创建一个ioQueue负责对硬盘的读写，并且会添加观察者，在收到内存警告、关闭或进入后台时完成对应的处理:

- (id)init {
_memCache = [[NSCache alloc] init];
_ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);
//收到内存警告时，清除NSCache：[self.memCache removeAllObjects];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(clearMemory)
name:UIApplicationDidReceiveMemoryWarningNotification
object:nil];
//程序关闭时，会对硬盘文件做一些处理
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(cleanDisk)
name:UIApplicationWillTerminateNotification
object:nil];
//程序进入后台时，也会进行硬盘文件处理
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(backgroundCleanDisk)
name:UIApplicationDidEnterBackgroundNotification
object:nil];
}

[/code]

查询图片

每次向SDWebImageCache索取图片的时候，会先根据图片URL对应的key值先检查内存中是否有对应的图片，如果有则直接返回；如果没有则在ioQueue中去硬盘中查找，其中文件名是是根据URL生成的MD5值，找到之后先将图片缓存在内存中，然后在把图片返回：

- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock {
/*...*/
// 首先查找内存缓存
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
doneBlock(image, SDImageCacheTypeMemory);
return nil;
}
//硬盘查找
NSOperation *operation = [NSOperation new];
dispatch_async(self.ioQueue, ^{
//创建自动释放池，内存及时释放
@autoreleasepool {
UIImage *diskImage = [self diskImageForKey:key];
if (diskImage) {
CGFloat cost = diskImage.size.height * diskImage.size.width * diskImage.scale * diskImage.scale;
//缓存到NSCache中
[self.memCache setObject:diskImage forKey:key cost:cost];
}
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, SDImageCacheTypeDisk);
});
}
});
return operation;
}

在硬盘查询的时候，会在后台将NSData转成UIImage，并完成相关的解码工作:

- (UIImage *)diskImageForKey:(NSString *)key {
NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
if (data) {
UIImage *image = [UIImage sd_imageWithData:data];
image = [self scaledImageForKey:key image:image];
if (self.shouldDecompressImages) {
image = [UIImage decodedImageWithImage:image];
}
return image;
}
else {
return nil;
}
}

保存图片

当下载完图片后，会先将图片保存到NSCache中，并把图片像素大小作为该对象的cost值，同时如果需要保存到硬盘，会先判断图片的格式，PNG或者JPEG，并保存对应的NSData到缓存路径中，文件名为URL的MD5值：

- (NSString *)cachedFileNameForKey:(NSString *)key {
//根据key生成对应的MD5值作为文件名
const char *str = [key UTF8String];
if (str == NULL) {
str = "";
}
unsigned char r[CC_MD5_DIGEST_LENGTH];
CC_MD5(str, (CC_LONG)strlen(str), r);
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]];

return filename;
}

- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk
{
//保存到NSCache，cost为像素值
[self.memCache setObject:image forKey:key cost:image.size.height * image.size.width * image.scale * image.scale];
if (toDisk) {
dispatch_async(self.ioQueue, ^{
NSData *data = imageData;
if (image && (recalculate || !data)) {
//判断图片格式
BOOL imageIsPng = YES;
// 查看imagedata的前缀是否是PNG的前缀格式
if ([imageData length] >= [kPNGSignatureData length]) {
imageIsPng = ImageDataHasPNGPreffix(imageData);
}
if (imageIsPng) {
data = UIImagePNGRepresentation(image);
}

4000
else {
data = UIImageJPEGRepresentation(image, (CGFloat)1.0);
}
}
if (data) {
if (![_fileManager fileExistsAtPath:_diskCachePath]) {
[_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
}
//保存data到指定的路径中
[_fileManager createFileAtPath:[self defaultCachePathForKey:key] contents:data attributes:nil];
}
});
}
}

[/code]

硬盘文件的管理

在程序退出或者进入后台时，会出图片文件进行管理，具体的策略：
清除过期的文件，默认一星期
如果设置了最大缓存，并且当前缓存的文件超过了这个限制，则删除最旧的文件，直到当前缓存文件的大小为最大缓存大小的一半

- (void)cleanDiskWithCompletionBlock:(SDWebImageNoParamsBlock)completionBlock {
dispatch_async(self.ioQueue, ^{
NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
NSArray *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];

// This enumerator prefetches useful properties for our cache files.
NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
includingPropertiesForKeys:resourceKeys
options:NSDirectoryEnumerationSkipsHiddenFiles
errorHandler:NULL];

NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.maxCacheAge];
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) {
NSDictionary *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:NULL];

// Skip directories.
if ([resourceValues[NSURLIsDirectoryKey] boolValue]) {
continue;
}

// Remove files that are older than the expiration date;
NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
if ([[modificationDate 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 setObject:resourceValues forKey:fileURL];
}

for (NSURL *fileURL in urlsToDelete) {
[_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.
if (self.maxCacheSize > 0 && currentCacheSize > self.maxCacheSize) {
// Target half of our maximum cache size for this cleanup pass.
const NSUInteger desiredCacheSize = self.maxCacheSize / 2;

// Sort the remaining cache files by their last modification time (oldest first).
NSArray *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
usingComparator:^NSComparisonResult(id obj1, id obj2) {
return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
}];

// Delete files until we fall below our desired cache size.
for (NSURL *fileURL in sortedFiles) {
if ([_fileManager removeItemAtURL:fileURL error:nil]) {
NSDictionary *resourceValues = cacheFiles[fileURL];
NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
currentCacheSize -= [totalAllocatedSize unsignedIntegerValue];

if (currentCacheSize < desiredCacheSize) {
break;
}
}
}
}
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
}

[/code]

总结

接口设计简单
通常我们使用较多的UIImageView分类：

[self.imageView sd_setImageWithURL:[NSURL URLWithString:@"url"]
placeholderImage:[UIImage imageNamed:@"placeholder"]];

[/code]一个简单的接口将其中复杂的实现细节全部隐藏：简单就是美。
采用NSCache作为内存缓
耗时较长的请求，都采用异步形式，在回调函数块中处理请求结果
NSOperation和NSOperationQueue：可以取消任务处理队列中的任务，设置最大并发数，设置operation之间的依赖关系。
图片缓存清理的策略
dispatch_barrier_sync：前面的任务执行结束后它才执行，而且它后面的任务要等它执行完成之后才会执行。
使用weak self strong self 防止retain circle
如果子线程进需要不断处理一些事件，那么设置一个Run Loop是最好的处理方式

作者：树下老男孩
链接：http://www.jianshu.com/p/d401ec7626eb
來源：简书
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。