Added `ioQueueAttributes` to use concurrent or control QoS for image cache internal IO Queue

SDWebImage/Core/SDImageCache.m

@@ -109,14 +109,16 @@ static NSString * _defaultDiskCacheDirectory;
     if ((self = [super init])) {
         NSAssert(ns, @"Cache namespace should not be nil");
         
-        // Create IO serial queue
-        _ioQueue = dispatch_queue_create("com.hackemist.SDImageCache", DISPATCH_QUEUE_SERIAL);
-        
         if (!config) {
             config = SDImageCacheConfig.defaultCacheConfig;
         }
         _config = [config copy];
         
+        // Create IO queue
+        dispatch_queue_attr_t ioQueueAttributes = _config.ioQueueAttributes;
+        _ioQueue = dispatch_queue_create("com.hackemist.SDImageCache", ioQueueAttributes);
+        NSAssert(_ioQueue, @"The IO queue should not be nil. Your configured `ioQueueAttributes` may be wrong");
+        
         // Init the memory cache
         NSAssert([config.memoryCacheClass conformsToProtocol:@protocol(SDMemoryCache)], @"Custom memory cache class must conform to `SDMemoryCache` protocol");
         _memoryCache = [[config.memoryCacheClass alloc] initWithConfig:_config];

SDWebImage/Core/SDImageCacheConfig.h

@@ -127,6 +127,15 @@ typedef NS_ENUM(NSUInteger, SDImageCacheConfigExpireType) {
  */
 @property (strong, nonatomic, nullable) NSFileManager *fileManager;
 
+/**
+ * The dispatch queue attr for ioQueue. You can config the QoS and concurrent/serial to internal IO queue. The ioQueue is used by SDImageCache to access read/write for disk data.
+ * Defaults we use `DISPATCH_QUEUE_SERIAL`(NULL), to use serial dispatch queue to ensure single access for disk data. It's safe but may be slow.
+ * @note You can override this to use `DISPATCH_QUEUE_CONCURRENT`, use concurrent queue.
+ * @warning **MAKE SURE** to keep `diskCacheWritingOptions` to use `NSDataWritingAtomic`, or concurrent queue may cause corrupted disk data (because multiple threads read/write same file without atomic is not IO-safe).
+ * @note This value does not support dynamic changes. Which means further modification on this value after cache initialized has no effect.
+ */
+@property (strong, nonatomic, nullable) dispatch_queue_attr_t ioQueueAttributes;
+
 /**
  * The custom memory cache class. Provided class instance must conform to `SDMemoryCache` protocol to allow usage.
  * Defaults to built-in `SDMemoryCache` class.

SDWebImage/Core/SDImageCacheConfig.m

@@ -35,6 +35,8 @@ static const NSInteger kDefaultCacheMaxDiskAge = 60 * 60 * 24 * 7; // 1 week
         _maxDiskAge = kDefaultCacheMaxDiskAge;
         _maxDiskSize = 0;
         _diskCacheExpireType = SDImageCacheConfigExpireTypeModificationDate;
+        _fileManager = nil;
+        _ioQueueAttributes = DISPATCH_QUEUE_SERIAL; // NULL
         _memoryCacheClass = [SDMemoryCache class];
         _diskCacheClass = [SDDiskCache class];
     }
@@ -56,6 +58,7 @@ static const NSInteger kDefaultCacheMaxDiskAge = 60 * 60 * 24 * 7; // 1 week
     config.maxMemoryCount = self.maxMemoryCount;
     config.diskCacheExpireType = self.diskCacheExpireType;
     config.fileManager = self.fileManager; // NSFileManager does not conform to NSCopying, just pass the reference
+    config.ioQueueAttributes = self.ioQueueAttributes; // Pass the reference
     config.memoryCacheClass = self.memoryCacheClass;
     config.diskCacheClass = self.diskCacheClass;