UBNTDiscardableThumbnail.m

// public interface
@interface UBNTDiscardableThumbnail : NSObject<NSDiscardableContent>

+ (instancetype)discardableThumbnailWithImage:(UIImage *)image;
- (instancetype)initWithImage:(UIImage *)thumbnail;

@property (nonatomic, readonly) UIImage *thumbnail;

@end

// private interface
@interface UBNTDiscardableThumbnail()

@property (nonatomic) UIImage *thumbnail;
@property (nonatomic, assign) NSUInteger accessCounter;

@end

@implementation UBNTDiscardableThumbnail

+ (instancetype)discardableThumbnailWithImage:(UIImage *)image
{
    return [[UBNTDiscardableThumbnail alloc] initWithImage:image];
}

- (instancetype)initWithImage:(UIImage *)image;
{
    self = [super init];
    if (self)
    {
        _thumbnail = image;
    }
    return self;
}

- (BOOL)beginContentAccess
{
    if (!self.thumbnail)
        return NO;
    
    self.accessCounter++;
    
    return YES;
}

- (void)endContentAccess
{
    if (self.accessCounter > 0)
        self.accessCounter--;
}

- (void)discardContentIfPossible
{
    if (self.accessCounter == 0)
    {
        self.thumbnail = nil;
    }
}

- (BOOL)isContentDiscarded
{
    return self.thumbnail == nil;
}

@end