如何使用NSCache

Question

谁能给出一个如何使用NSCache缓存字符串的例子？或者任何人有链接到一个很好的解释？我好像找不到任何..

Answer 1

使用它的方式与使用NSMutableDictionary的方式相同。不同的是，当NSCache检测到过多的内存压力(即它缓存了太多的值)时，它会释放其中的一些值来腾出空间。

如果您可以在运行时重新创建这些值(通过从互联网下载，进行计算，等等)，那么NSCache可能会满足您的需求。如果数据不能被重新创建(例如，它是用户输入，它是时间敏感的，等等)那么您不应该将其存储在NSCache中，因为它将在那里被销毁。

例如，不考虑线程安全：

// Your cache should have a lifetime beyond the method or handful of methods
// that use it. For example, you could make it a field of your application
// delegate, or of your view controller, or something like that. Up to you.
NSCache *myCache = ...;
NSAssert(myCache != nil, @"cache object is missing");

// Try to get the existing object out of the cache, if it's there.
Widget *myWidget = [myCache objectForKey: @"Important Widget"];
if (!myWidget) {
    // It's not in the cache yet, or has been removed. We have to
    // create it. Presumably, creation is an expensive operation,
    // which is why we cache the results. If creation is cheap, we
    // probably don't need to bother caching it. That's a design
    // decision you'll have to make yourself.
    myWidget = [[[Widget alloc] initExpensively] autorelease];

    // Put it in the cache. It will stay there as long as the OS
    // has room for it. It may be removed at any time, however,
    // at which point we'll have to create it again on next use.
    [myCache setObject: myWidget forKey: @"Important Widget"];
}

// myWidget should exist now either way. Use it here.
if (myWidget) {
    [myWidget runOrWhatever];
}

Answer 2

@implementation ViewController
{    
    NSCache *imagesCache;    
}


- (void)viewDidLoad
{    
    imagesCache = [[NSCache alloc] init];
}


// How to save and retrieve NSData into NSCache
NSData *imageData = [imagesCache objectForKey:@"KEY"];
[imagesCache setObject:imageData forKey:@"KEY"];

Answer 3

在Swift中使用NSCache缓存字符串的示例代码：

var cache = NSCache()
cache.setObject("String for key 1", forKey: "Key1")
var result = cache.objectForKey("Key1") as String
println(result) // Prints "String for key 1"

要创建单个应用程序范围的NSCache实例(单例)，您可以轻松地扩展NSCache以添加sharedInstance属性。只需将以下代码放在一个名为NSCache+Singleton.swift的文件中：

import Foundation

extension NSCache {
    class var sharedInstance : NSCache {
        struct Static {
            static let instance : NSCache = NSCache()
        }
        return Static.instance
    }
}

然后，您可以在应用程序中的任何位置使用缓存：

NSCache.sharedInstance.setObject("String for key 2", forKey: "Key2")
var result2 = NSCache.sharedInstance.objectForKey("Key2") as String
println(result2) // Prints "String for key 2"