Merge pull request #2863 from dreampiggy/bugfix_revert_apng_constants

Revert the change for APNG constants fix for iOS 8 devices which cause crash

SDWebImage/Core/SDImageAPNGCoder.m

@@ -14,20 +14,15 @@
 #endif
 
 // iOS 8 Image/IO framework binary does not contains these APNG contants, so we define them. Thanks Apple :)
-static NSString * kSDCGImagePropertyAPNGLoopCount = @"LoopCount";
-static NSString * kSDCGImagePropertyAPNGDelayTime = @"DelayTime";
-static NSString * kSDCGImagePropertyAPNGUnclampedDelayTime = @"UnclampedDelayTime";
+// We can not use runtime @available check for this issue, because it's a global symbol and should be loaded during launch time by dyld. So hack if the min deployment target version < iOS 9.0, whatever it running on iOS 9+ or not.
+#if (__IPHONE_OS_VERSION_MIN_REQUIRED && __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_9_0)
+const CFStringRef kCGImagePropertyAPNGLoopCount = (__bridge CFStringRef)@"LoopCount";
+const CFStringRef kCGImagePropertyAPNGDelayTime = (__bridge CFStringRef)@"DelayTime";
+const CFStringRef kCGImagePropertyAPNGUnclampedDelayTime = (__bridge CFStringRef)@"UnclampedDelayTime";
+#endif
 
 @implementation SDImageAPNGCoder
 
-+ (void)initialize {
-    if (@available(iOS 9, *)) {
-        kSDCGImagePropertyAPNGLoopCount = (__bridge NSString *)kCGImagePropertyAPNGLoopCount;
-        kSDCGImagePropertyAPNGDelayTime = (__bridge NSString *)kCGImagePropertyAPNGDelayTime;
-        kSDCGImagePropertyAPNGUnclampedDelayTime = (__bridge NSString *)kCGImagePropertyAPNGUnclampedDelayTime;
-    }
-}
-
 + (instancetype)sharedCoder {
     static SDImageAPNGCoder *coder;
     static dispatch_once_t onceToken;
@@ -52,15 +47,15 @@ static NSString * kSDCGImagePropertyAPNGUnclampedDelayTime = @"UnclampedDelayTim
 }
 
 + (NSString *)unclampedDelayTimeProperty {
-    return kSDCGImagePropertyAPNGUnclampedDelayTime;
+    return (__bridge NSString *)kCGImagePropertyAPNGUnclampedDelayTime;
 }
 
 + (NSString *)delayTimeProperty {
-    return kSDCGImagePropertyAPNGDelayTime;
+    return (__bridge NSString *)kCGImagePropertyAPNGDelayTime;
 }
 
 + (NSString *)loopCountProperty {
-    return kSDCGImagePropertyAPNGLoopCount;
+    return (__bridge NSString *)kCGImagePropertyAPNGLoopCount;
 }
 
 + (NSUInteger)defaultLoopCount {