Purge nscache on system memory notifications

@@ -11,6 +11,29 @@
 #import "UIImage+MultiFormat.h"
 #import <CommonCrypto/CommonDigest.h>
 
+// See https://github.com/rs/SDWebImage/pull/1141 for discussion
+@interface AutoPurgeCache : NSCache
+@end
+
+@implementation AutoPurgeCache
+
+- (id)init
+{
+    self = [super init];
+    if (self) {
+        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(removeAllObjects) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
+    }
+    return self;
+}
+
+- (void)dealloc
+{
+    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
+
+}
+
+@end
+
 static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
 // PNG signature bytes and data (below)
 static unsigned char kPNGSignatureBytes[8] = {0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A};