Fix CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF warning

@@ -26,13 +26,12 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 @property (strong, nonatomic, nonnull) NSString *diskCachePath;
 @property (strong, nonatomic, nullable) NSMutableArray<NSString *> *customPaths;
 @property (strong, nonatomic, nullable) dispatch_queue_t ioQueue;
+@property (strong, nonatomic, nonnull) NSFileManager *fileManager;
 
 @end
 
 
-@implementation SDImageCache {
-    NSFileManager *_fileManager;
-}
+@implementation SDImageCache
 
 #pragma mark - Singleton, init, dealloc
 
@@ -224,8 +223,8 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
         return;
     }
     
-    if (![_fileManager fileExistsAtPath:_diskCachePath]) {
-        [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
+    if (![self.fileManager fileExistsAtPath:_diskCachePath]) {
+        [self.fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
     }
     
     // get cache Path for image key
@@ -271,12 +270,12 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
     if (!key) {
         return NO;
     }
-    BOOL exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key]];
+    BOOL exists = [self.fileManager fileExistsAtPath:[self defaultCachePathForKey:key]];
     
     // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
     // checking the key with and without the extension
     if (!exists) {
-        exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key].stringByDeletingPathExtension];
+        exists = [self.fileManager fileExistsAtPath:[self defaultCachePathForKey:key].stringByDeletingPathExtension];
     }
     
     return exists;
@@ -447,7 +446,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 
     if (fromDisk) {
         dispatch_async(self.ioQueue, ^{
-            [_fileManager removeItemAtPath:[self defaultCachePathForKey:key] error:nil];
+            [self.fileManager removeItemAtPath:[self defaultCachePathForKey:key] error:nil];
             
             if (completion) {
                 dispatch_async(dispatch_get_main_queue(), ^{
@@ -487,8 +486,8 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 
 - (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion {
     dispatch_async(self.ioQueue, ^{
-        [_fileManager removeItemAtPath:self.diskCachePath error:nil];
-        [_fileManager createDirectoryAtPath:self.diskCachePath
+        [self.fileManager removeItemAtPath:self.diskCachePath error:nil];
+        [self.fileManager createDirectoryAtPath:self.diskCachePath
                 withIntermediateDirectories:YES
                                  attributes:nil
                                       error:NULL];
@@ -511,7 +510,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
         NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];
 
         // This enumerator prefetches useful properties for our cache files.
-        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
+        NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtURL:diskCacheURL
                                                    includingPropertiesForKeys:resourceKeys
                                                                       options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                  errorHandler:NULL];
@@ -548,7 +547,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
         }
         
         for (NSURL *fileURL in urlsToDelete) {
-            [_fileManager removeItemAtURL:fileURL error:nil];
+            [self.fileManager removeItemAtURL:fileURL error:nil];
         }
 
         // If our remaining disk cache exceeds a configured maximum size, perform a second
@@ -565,7 +564,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 
             // Delete files until we fall below our desired cache size.
             for (NSURL *fileURL in sortedFiles) {
-                if ([_fileManager removeItemAtURL:fileURL error:nil]) {
+                if ([self.fileManager removeItemAtURL:fileURL error:nil]) {
                     NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
                     NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
                     currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;
@@ -611,10 +610,10 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 - (NSUInteger)getSize {
     __block NSUInteger size = 0;
     dispatch_sync(self.ioQueue, ^{
-        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
+        NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtPath:self.diskCachePath];
         for (NSString *fileName in fileEnumerator) {
             NSString *filePath = [self.diskCachePath stringByAppendingPathComponent:fileName];
-            NSDictionary<NSString *, id> *attrs = [_fileManager attributesOfItemAtPath:filePath error:nil];
+            NSDictionary<NSString *, id> *attrs = [self.fileManager attributesOfItemAtPath:filePath error:nil];
             size += [attrs fileSize];
         }
     });
@@ -624,7 +623,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 - (NSUInteger)getDiskCount {
     __block NSUInteger count = 0;
     dispatch_sync(self.ioQueue, ^{
-        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
+        NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtPath:self.diskCachePath];
         count = fileEnumerator.allObjects.count;
     });
     return count;
@@ -637,7 +636,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
         NSUInteger fileCount = 0;
         NSUInteger totalSize = 0;
 
-        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
+        NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtURL:diskCacheURL
                                                    includingPropertiesForKeys:@[NSFileSize]
                                                                       options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                  errorHandler:NULL];