Enable to change clear cache option

SDWebImage/SDImageCache.m

@@ -606,7 +606,23 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 - (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
     dispatch_async(self.ioQueue, ^{
         NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
-        NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];
+
+        // Compute content date key to be used for tests
+        NSURLResourceKey cacheContentDateKey = NSURLContentModificationDateKey;
+        switch (self.config.cacheClearBy) {
+            case SDImageCacheConfigExpireTypeAccessDate:
+                cacheContentDateKey = NSURLContentAccessDateKey;
+                break;
+
+            case SDImageCacheConfigExpireTypeModificationDate:
+                cacheContentDateKey = NSURLContentModificationDateKey;
+                break;
+
+            default:
+                break;
+        }
+        
+        NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, cacheContentDateKey, NSURLTotalFileAllocatedSizeKey];
 
         // This enumerator prefetches useful properties for our cache files.
         NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtURL:diskCacheURL
@@ -633,8 +649,8 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
             }
 
             // Remove files that are older than the expiration date;
-            NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
+            NSDate *modifiedDate = resourceValues[cacheContentDateKey];
-            if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
+            if ([[modifiedDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
                 [urlsToDelete addObject:fileURL];
                 continue;
             }

SDWebImage/SDImageCacheConfig.h

@@ -9,6 +9,17 @@
 #import <Foundation/Foundation.h>
 #import "SDWebImageCompat.h"
 
+typedef NS_ENUM(NSUInteger, SDImageCacheConfigExpireType) {
+    /**
+     * When the image is accessed it will update this value
+     */
+    SDImageCacheConfigExpireTypeAccessDate,
+    /**
+     * The image was obtained from the disk cache (Default)
+     */
+    SDImageCacheConfigExpireTypeModificationDate
+};
+
 @interface SDImageCacheConfig : NSObject
 
 /**
@@ -49,4 +60,10 @@
  */
 @property (assign, nonatomic) NSUInteger maxCacheSize;
 
+/**
+ * The attribute which the clear cache will be checked against when clearing the disk cache
+ * Default is Modified Date
+ */
+@property (assign, nonatomic) SDImageCacheConfigExpireType cacheClearBy;
+
 @end

SDWebImage/SDImageCacheConfig.m

@@ -21,6 +21,7 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
         _diskCacheWritingOptions = NSDataWritingAtomic;
         _maxCacheAge = kDefaultCacheMaxCacheAge;
         _maxCacheSize = 0;
+        _cacheClearBy = SDImageCacheConfigExpireTypeModificationDate;
     }
     return self;
 }