iOS中图片缓存策略

在iOS开发中，经常遇到一个问题，就是如何缓存从网上下载好的图片。首先想到的办法是将图片保存在字典中。但是使用NSCache来保存的话，会更好。

NSCache于字典的不同之处在于，当系统资源耗尽时，NSCache可以自行删除缓存，而采用字典要自己编写挂钩。此外NSCache并不会拷贝键，而是采用保留，NSCache对象不拷贝键的原因在于，键都是由不支持拷贝的操作对象来充当的。最后NSCache是线程安全的。

新建一个图片缓存的类

#import <Foundation/Foundation.h>

@class FNSImageCache;

@protocol FNSImageCacheDelegate <NSObject>

@optional

- (void)successFetchData:(NSData *)data;

@end

@interface FNSImageCache :
NSObject

@property (nonatomic,weak)
id<FNSImageCacheDelegate> delegate;

-(void)fetchDataWithURL:(NSURL *)url;

@end

在.m文件中

#import "FNSImageCache.h"

@implementation FNSImageCache

{

    NSCache *_cache;

    

}

- (instancetype)init

{

    if (self = [super
init]) {

        _cache = [NSCache
new];

        

        _cache.countLimit =
100;

        

        _cache.totalCostLimit =
5 * 1024 *
1024;

    }

    return
self;

}

- (void)fetchDataWithURL:(NSURL *)url

{

    NSData *cacheData = [_cache
objectForKey:url];

    if (cacheData) {

        [self useData:cacheData];

    }

    else{

        [self
downloadDataFromInternet:url];

    }

}

- (void)downloadDataFromInternet:(NSURL *)url{

    NSURLRequest *request = [NSURLRequest
reques
c252
tWithURL:url];

    NSURLSession *session = [NSURLSession
sharedSession];

    NSURLSessionDataTask *task = [session
dataTaskWithRequest:request completionHandler:^(NSData *
_Nullable data, NSURLResponse *
_Nullable response,
NSError * _Nullable error) {

        

        [_cache setObject:data
forKey:url cost:data.length];

        [self useData:data];

        

    }];

    [task resume];

    

}

- (void)useData:(NSData *)data{

    if ([self.delegate
respondsToSelector:@selector(successFetchData:)]) {

        [self.delegate
successFetchData:data];

    }

  

}

@end

控制器中

#import "ViewController.h"

#import "FNSImageCache.h"

@interface
ViewController ()<FNSImageCacheDelegate>

@property (weak,
nonatomic) IBOutlet
UIImageView *imageView;

- (IBAction)downloadImageClick:(id)sender;

@property (nonatomic,strong)
FNSImageCache *cache;

@end

@implementation ViewController

- (void)viewDidLoad {

    [super
viewDidLoad];

    

    

}

- (void)didReceiveMemoryWarning {

    [super
didReceiveMemoryWarning];

    // Dispose of any resources that can be recreated.

}

- (IBAction)downloadImageClick:(id)sender {

    FNSImageCache *cache = [[FNSImageCache
alloc]
init];

    cache.delegate =
self;

    NSURL *url = [NSURL
URLWithString:@"https://gd2.alicdn.com/bao/uploaded/i2/TB1cXuXHFXXXXaxXVXXXXXXXXXX_!!0-item_pic.jpg"];

    [cache fetchDataWithURL:url];

    

}

//FNSImageCache的代理方法

- (void)successFetchData:(NSData *)data

{

    UIImage *image = [UIImage
imageWithData:data];

    [self.imageView
setImage:image];

    

}

@end