SDWebImage是我们最常见的一个第三方库图片缓存库，它提供了UIImageView的一个分类，以支持从远程服务器下载并缓存图片的功能。它具有以下功能：

• 支持网络图片的异步加载与缓存管理 并具有自动缓存过期处理功能
• 支持GIF图片
• 后台图片解压缩处理
• 确保同一个URL的图片不被下载多次
• 确保虚假的URL不会被反复加载
• 确保下载及缓存时，主线程不被阻塞
• 使用GCD和ARC
• 支持ARM64

本文将对SDImageCache进行解析，如有分析不到位之处，还请广大读者指出！

SDImageCache维护一个内存缓存和一个可选的磁盘缓存

SDImageCache的实例

 + (SDImageCache *)sharedImageCache
{
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{instance = self.new;});
    return instance;
}

初始化方法

• 默认初始化方法创建一个default名字的缓存文件夹
  - (id)init
  {
  return [self initWithNamespace:@"default"];
  }

• 用此方法自定义缓存文件夹名字
  - (id)initWithNamespace:(NSString *)ns
  {
  if ((self = [super init]))
  {
  //命名空间
  NSString *fullNamespace = [@"com.hackemist.SDWebImageCache." stringByAppendingString:ns];

      // 创建IO队列
      _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);
  
      // Init default values  初始化缓存时间（默认最大 一周）
      _maxCacheAge = kDefaultCacheMaxCacheAge;
  
      // Init the memory cache  初始化cache
      _memCache = [[NSCache alloc] init];
      _memCache.name = fullNamespace;
  
      // Init the disk cache  设置本地缓存路径
      NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
      _diskCachePath = [paths[0] stringByAppendingPathComponent:fullNamespace];
  
      dispatch_sync(_ioQueue, ^
      {
          _fileManager = NSFileManager.new;
      });
    
    //添加观察者对象 处理内存警告时 清空缓存
      #if TARGET_OS_IPHONE
      // Subscribe to app events  清空内存缓存
      [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(clearMemory)
                                                 name:UIApplicationDidReceiveMemoryWarningNotification
                                               object:nil];
      // 清空disk缓存
      [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(cleanDisk)
                                                 name:UIApplicationWillTerminateNotification
                                               object:nil];
    //后台清空disk缓存
      [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(backgroundCleanDisk)
                                                 name:UIApplicationDidEnterBackgroundNotification
                                               object:nil];
      #endif
    }
  return self;
  }
  

• 清空内存缓存
  - (void)clearMemory
  {
  [self.memCache removeAllObjects];
  }

• 清空磁盘缓存
  - (void)clearDisk
  {
  dispatch_async(self.ioQueue, ^
  {
  [[NSFileManager defaultManager] removeItemAtPath:self.diskCachePath error:nil];
  [[NSFileManager defaultManager] createDirectoryAtPath:self.diskCachePath
  withIntermediateDirectories:YES
  attributes:nil
  error:NULL];
  });
  }

• 清空所有过期的磁盘缓存
  - (void)cleanDisk
  {
  dispatch_async(self.ioQueue, ^
  {
  NSFileManager *fileManager = [NSFileManager defaultManager];
  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];
    unsigned long long 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.
          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])
              {
                  [fileManager removeItemAtURL:fileURL error:nil];
                  continue;
              }
  
              // Store a reference to this file and account for its total size.
              NSNumber *totalAllocatedSize = reso  urceValues[NSURLTotalFileAllocatedSizeKey];
              currentCacheSize += [totalAllocatedSize unsignedLongLongValue];
              [cacheFiles setObject:resourceValues forKey:fileURL];
          }
  
    // 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 unsigned long long 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 unsignedLongLongValue];
  
                      if (currentCacheSize < desiredCacheSize)
                      {
                          break;
                      }
                  }
              }
          }
      });
  }
  

• 后台运行时清空磁盘缓存
  - (void)backgroundCleanDisk
  {
  UIApplication *application = [UIApplication 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;
  }];

      // Start the long-running task and return immediately.
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^
      {
          // Do the work associated with the task, preferably in chunks.
           [self cleanDisk];
    
          [application endBackgroundTask:bgTask];
          bgTask = UIBackgroundTaskInvalid;
      });
  } 
  

• 磁盘高速缓存使用的大小
  - (unsigned long long)getSize;

• 磁盘缓存的图片数量
  - (int)getDiskCount;

• 异步计算磁盘高速缓存的大小。
  - (void)calculateSizeWithCompletionBlock:(void (^)(NSUInteger fileCount, unsigned long long totalSize))completionBlock;

• 检查图像是否存在于缓存了
  - (BOOL)diskImageExistsWithKey:(NSString *)key;

• 通过key 将image 缓存到内存中
  - (void)storeImage:(UIImage *)image forKey:(NSString *)key

• 通过key 将image 缓存 toDisk==TRUE 标示缓存到磁盘中 toDisk==FALSE缓存到内存中
  - (void)storeImage:(UIImage *)image forKey:(NSString *)key toDisk:(BOOL)toDisk;

• 通过key 将image 缓存 image图片 如果data不为nil 则直接缓存data数据 如果data为nil 则将image转为NSData再存储 toDisk==TRUE 标示缓存到磁盘中 toDisk==FALSE缓存到内存中
  - (void)storeImage:(UIImage *)image imageData:(NSData *)data forKey:(NSString *)key toDisk:(BOOL)toDisk;

• 通过key异步查询磁盘高速缓存
  - (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(void (^)(UIImage *image, SDImageCacheType cacheType))doneBlock;

• 通过key异步查询内存缓存
  - (UIImage *)imageFromDiskCacheForKey:(NSString *)key;

• 通过key从磁盘和内存中移除缓存
  - (void)removeImageForKey:(NSString *)key;

• 通过key从磁盘或内存中移除缓存 fromDisk==TRUE 从磁盘中移除反之从内存中移除
  - (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk;