如何使用 NSURLSession 来判断资源是否发生了变化? [英] How to use NSURLSession to determine if resource has changed?

问题描述

我正在使用 NSURLSession 从 HTTP 服务器请求 JSON 资源.服务器使用 Cache-Control 来限制资源在客户端缓存的时间.

I'm using NSURLSession to request a JSON resource from an HTTP server. The server uses Cache-Control to limit the time the resource is cached on clients.

这很好用，但我还想在内存中缓存一个反序列化的 JSON 对象，因为它经常被访问，同时继续利用 NSURLSession 中内置的 HTTP 缓存机制.

This works great, but I'd also like to cache a deserialized JSON object in memory as it is accessed quite often, while continuing to leverage the HTTP caching mechanisms built into NSURLSession.

我想我可以保存一些 HTTP 响应标头:Content-MD5、Etag 和 Last-Modified 以及反序列化的 JSON 对象(我正在使用这 3 个字段，因为我注意到并非所有 HTTP 服务器都返回 Content-MD5，否则这本身就足够了).下次我收到 JSON 对象的响应时，如果这 3 个字段相同，那么我可以重用之前反序列化的 JSON 对象.

I'm thinking I can save a few HTTP response headers: Content-MD5, Etag, and Last-Modified along with the deserialized JSON object (I'm using those 3 fields since I've noticed not all HTTP servers return Content-MD5, otherwise that'd be sufficient by itself). The next time I receive a response for the JSON object, if those 3 fields are the same then I can reuse the previously deserialized JSON object.

这是确定反序列化 JSON 是否仍然有效的可靠方法.如果不是，如何确定反序列化的对象是否是最新的?

Is this a robust way to determine the deserizlied JSON is still valid. If not, how do I determine if the deserialized object is up to date?

推荐答案

我创建了一个 HTTPEntityFingerprint 结构来存储一些实体标头:Content-MD5、Etag 和 Last-Modified.

I created a HTTPEntityFingerprint structure which stores some of the entity headers: Content-MD5, Etag, and Last-Modified.

import Foundation

struct HTTPEntityFingerprint {
    let contentMD5 : String?
    let etag : String?
    let lastModified : String?
}

extension HTTPEntityFingerprint {
    init?(response : NSURLResponse) {
        if let httpResponse = response as? NSHTTPURLResponse {
            let h = httpResponse.allHeaderFields
            contentMD5 = h["Content-MD5"] as? String
            etag = h["Etag"] as? String
            lastModified = h["Last-Modified"] as? String

            if contentMD5 == nil && etag == nil && lastModified == nil {
                return nil
            }
        } else {
            return nil
        }
    }

    static func match(first : HTTPEntityFingerprint?, second : HTTPEntityFingerprint?) -> Bool {
        if let a = first, b = second {
            if let md5A = a.contentMD5, md5B = b.contentMD5 {
                return md5A == md5B
            }
            if let etagA = a.etag, etagB = b.etag {
                return etagA == etagB
            }
            if let lastA = a.lastModified, lastB = b.lastModified {
                return lastA == lastB
            }
        }

        return false
    }
}

当我从 NSURLSession 获得 NSHTTPURLResponse 时，我会从中创建一个 HTTPEntityFingerprint 并使用 将其与之前存储的指纹进行比较>HTTPEntityFingerprint.match.如果指纹匹配，那么 HTTP 资源没有改变，因此我不需要再次反序列化 JSON 响应；但是，如果指纹不匹配，我会反序列化 JSON 响应并保存新指纹.

When I get an NSHTTPURLResponse from an NSURLSession, I create an HTTPEntityFingerprint from it and compare it against a previously stored fingerprint using HTTPEntityFingerprint.match. If the fingerprints match, then the HTTP resource hasn't changed and thus I do not need to deserialized the JSON response again; however, if the fingerprints do not match, then I deserialize the JSON response and save the new fingerprint.

此机制仅在您的服务器至少返回以下 3 个实体标头之一时才有效:Content-MD5、Etag 或 Last-Modified.

This mechanism only works if your server returns at least one of the 3 entity headers: Content-MD5, Etag, or Last-Modified.

NSURLSession 通过 NSURLCache 提供的缓存是透明的，这意味着当您请求以前缓存的资源时，NSURLSession 将调用完成处理程序/委托好像发生了 200 响应.

The caching provided by NSURLSession via NSURLCache is transparent, meaning when you request a previously cached resource NSURLSession will call the completion handlers/delegates as if a 200 response occurred.

如果缓存的响应已经过期，那么 NSURLSession 会向源服务器发送一个新的请求，但是会包含 If-Modified-Since 和 If-None-Match在缓存(尽管已过期)结果中使用 Last-Modified 和 Etag 实体标头的标头；此行为是内置的，除了启用缓存之外，您无需执行任何操作.如果源服务器返回 304(未修改)，则 NSURLSession 会将其转换为应用程序的 200 响应(使其看起来像您获取了资源的新副本，即使它仍然被提供从缓存中).

If the cached response has expired then NSURLSession will send a new request to the origin server, but will include the If-Modified-Since and If-None-Match headers using the Last-Modified and Etag entity headers in the cached (though expired) result; this behavior is built in, you don't have to do anything besides enable caching. If the origin server returns a 304 (Not Modified), then NSURLSession will transform this to a 200 response the application (making it look like you fetched a new copy of the resource, even though it was still served from the cache).

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