NSCache相关

#import "ViewController.h"

@interface ViewController () <NSCacheDelegate>

@property (nonatomic,strong)NSCache *cache;
@property (nonatomic,strong)NSOperationQueue *queue;

@end

@implementation ViewController

- (NSCache *)cache
{
    if(_cache == nil)
    {
        _cache = [[NSCache alloc]init];

        //_cache.countLimit = 5;//设置最多可以缓存多少个对象
        //_cache.totalCostLimit = 5;//设置缓存最大代价（成本），如果当前缓存达到最大代价（成本），会自动先回收之前的对象
        _cache.delegate = self;
    }
    return _cache;
}

//存入缓存数据
- (IBAction)writeCache:(id)sender {

    NSBlockOperation *download = [NSBlockOperation blockOperationWithBlock:^{
        for(int i = 0; i < 10; i++)
        {
            NSURL *url = [NSURL URLWithString:@"http://p16.qhimg.com/dr/48_48_/t0125e8d438ae9d2fbb.png"];
            NSData *data = [NSData dataWithContentsOfURL:url];

            if(data)
            {
                [[NSOperationQueue mainQueue] addOperationWithBlock:^{

                    [self.cache setObject:data forKey:@(i)];
                    //[self.cache setObject:data forKey:@(i) cost:1];//存入缓存并设置代价（成本）
                    NSLog(@"---存入缓存 %d---",i);
                    NSLog(@"----save cache----%@",[NSThread currentThread]);
                }];
            }
        }
        NSLog(@"----download----%@",[NSThread currentThread]);
    }];

    self.queue = [[NSOperationQueue alloc]init];

    [self.queue addOperation:download];

    NSLog(@"%s",__func__);
 }

//取出缓存数据
- (IBAction)checkCache:(id)sender {

    for(int i = 0; i < 10; i++)
    {
        NSData *data = [self.cache objectForKey:@(i)];
        if(data)
        {
            NSLog(@"---检查到缓存 %d---",i);
        }
        else
        {
            NSLog(@"---没有检查到缓存---");
        }
    }
}

//清除缓存数据
- (IBAction)clearCache:(id)sender {

    [self.cache removeAllObjects];

    NSLog(@"---缓存已清空---");

}

//代理方法，即将要回收对象时会调用
- (void)cache:(NSCache *)cache willEvictObject:(id)obj
{
    NSData *data = (NSData*)obj;
    NSLog(@"回收对象的长度%zd",[data length]);
}

@end

注意：NSCache的Key只是对对象进行strong引用，而不是copy，而NSDictionary的key是copy