NSCache 删除最老的还是最不常用的？

NSCache的主要方法.png

@interface AutoPurgeCache : NSCache
@end
//  自动清除缓存的Cache类
@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

@interface NSCache  : NSObject {
@private
    id _delegate;
    void *_private[5];
    void *_reserved;
}

@property (copy) NSString *name;

@property (nullable, assign) id delegate;

- (nullable ObjectType)objectForKey:(KeyType)key;
- (void)setObject:(ObjectType)obj forKey:(KeyType)key; // 0 cost
- (void)setObject:(ObjectType)obj forKey:(KeyType)key cost:(NSUInteger)g;
- (void)removeObjectForKey:(KeyType)key;

- (void)removeAllObjects;

@property NSUInteger totalCostLimit;    // limits are imprecise/not strict

// eg:  countLimit = 10; 队列先进先出移除缓存 add 10 remove 0， add 11 remove 1 ......
@property NSUInteger countLimit;    // limits are imprecise/not strict
@property BOOL evictsObjectsWithDiscardedContent;

@end

@protocol NSCacheDelegate 
@optional
- (void)cache:(NSCache *)cache willEvictObject:(id)obj;
@end