Adopt SDWebImage's memory cache cost function, fix the nullable issue

SDWebImageFLPlugin/Classes/FLAnimatedImageBridge/SDFLAnimatedImage.h

@@ -30,9 +30,9 @@ FOUNDATION_EXPORT SDWebImageContextOption _Nonnull const SDWebImageContextPredra
 @interface SDFLAnimatedImage : UIImage <SDAnimatedImage>
 
 /**
- The `FLAnimatedImage` instance for GIF representation
+ The `FLAnimatedImage` instance for GIF representation. This property should be nonnull.
  */
-@property (nonatomic, strong, nullable, readonly) FLAnimatedImage *animatedImage;
+@property (nonatomic, strong, nonnull, readonly) FLAnimatedImage *animatedImage;
 
 /**
  Create the wrapper with specify `FLAnimatedImage` instance. The instance should be nonnull.

SDWebImageFLPlugin/Classes/FLAnimatedImageBridge/SDFLAnimatedImage.m

@@ -7,6 +7,7 @@
  */
 
 #import "SDFLAnimatedImage.h"
+#import <objc/runtime.h>
 
 SDWebImageContextOption _Nonnull const SDWebImageContextOptimalFrameCacheSize = @"optimalFrameCacheSize";
 SDWebImageContextOption _Nonnull const SDWebImageContextPredrawingEnabled = @"predrawingEnabled";
@@ -125,3 +126,22 @@ - (NSUInteger)animatedImageLoopCount {
 }
 
 @end
+
+@implementation SDFLAnimatedImage (MemoryCacheCost)
+
+- (NSUInteger)sd_memoryCost {
+    NSNumber *value = objc_getAssociatedObject(self, @selector(sd_memoryCost));
+    if (value != nil) {
+        return value.unsignedIntegerValue;
+    }
+
+    FLAnimatedImage *animatedImage = self.animatedImage;
+    CGImageRef imageRef = animatedImage.posterImage.CGImage; // / Guaranteed to be loaded, and guaranteed to be CGImage based
+    NSUInteger bytesPerFrame = CGImageGetBytesPerRow(imageRef) * CGImageGetHeight(imageRef);
+    NSUInteger frameCacheSizeCurrent = animatedImage.frameCacheSizeCurrent; // [1...frame count], more suitable than raw frame count because FLAnimatedImage internal actually store a buffer size but not full frames (they called `window`)
+    NSUInteger animatedImageCost = frameCacheSizeCurrent * bytesPerFrame;
+
+    return animatedImageCost;
+}
+
+@end