swift - NSURLRequestCachePolicy.UserProtocolCachPolicy 要求

Question

我正在使用以下代码进行缓存，从服务器收到的响应具有以下 header 。是否需要从请求端设置任何 header ，以便缓存工作 10 秒。

Connection Received Resopnse Headers= [Date: Sat, 12 Sep 2015 22:51:16 GMT, Transfer-Encoding: Identity, Server: Apache-Coyote/1.1, Content-Type: application/json;charset=UTF-8, Expires: Sat, 12 Sep 2015 22:51:26 GMT, Cache-Control: max-age=10, must-revalidate]

没有缓存的强大代码。

import UIKit

class HTTPJSONDonwload: NSObject , NSURLConnectionDataDelegate , NSURLConnectionDelegate {
    static let httpjsonDownloader:HTTPJSONDonwload = HTTPJSONDonwload()

    func startDownload(){
        let serverRequest = getServerURL()

        NSURLConnection(request: serverRequest, delegate: self, startImmediately: true)
    }

    func getServerURL() -> NSMutableURLRequest{

        let request:NSMutableURLRequest = NSMutableURLRequest(URL:NSURL(string:"http://citiesfav-jcitiesj.rhcloud.com/Cache/getAllCities")! )
        request.cachePolicy = NSURLRequestCachePolicy.UseProtocolCachePolicy
        request.HTTPMethod = "POST"
        return request
    }

    func connection(connection: NSURLConnection, didReceiveData data: NSData) {

        print("Connection Data= \(NSString(data: data, encoding: NSUTF8StringEncoding))")
    }

    func connection(connection: NSURLConnection, didReceiveResponse response: NSURLResponse) {
        print("Connection Received Resopnse Headers= \((response as! NSHTTPURLResponse).allHeaderFields)")
    }

    func connection(connection: NSURLConnection, willCacheResponse cachedResponse: NSCachedURLResponse) -> NSCachedURLResponse? {
        print("Connection will cache Response")       
        return cachedResponse
    }
}

从 header 中删除 must-revalidate 后，它仍在获取请求。

Connection Received Resopnse Headers= [Cache-Control: max-age=10, Transfer-Encoding: Identity, Date: Sun, 13 Sep 2015 18:35:43 GMT, Content-Type: application/json;charset=UTF-8, Server: Apache-Coyote/1.1, Expires: Sun, 13 Sep 2015 18:35:53 GMT]

后来的发现表明 POST 请求确实被缓存了，但不像 GET 那样工作，后者考虑了 max-age。

func startDownload(){
    let serverRequest = getServerURL()
    let cache = NSURLCache.sharedURLCache()
    let response = cache.cachedResponseForRequest(serverRequest)

    if response != nil {
        serverRequest.cachePolicy = NSURLRequestCachePolicy.ReturnCacheDataDontLoad
    }

    NSURLConnection(request: serverRequest, delegate: self, startImmediately: true)
}

Answer 1

tl;dr

您需要使用 GET 而不是 POST。

冗长的解释

问题是您请求的是 POST。

func getServerURL() -> NSMutableURLRequest{
    ...
    request.HTTPMethod = "POST"
    ...
}

通常，POST 请求用于在服务器上创建(或有时也用于更新)资源。为创建或更新请求重用缓存的响应没有多大意义，因为无论如何您都必须将请求发送到服务器(否则不会创建或更新任何内容)。似乎 iOS 会自动绕过 POST 请求的缓存。

但是，在您的特定情况下，您实际上并不需要 POST，因为您只是从服务器读取数据。这意味着您应该改用 GET 请求。

func getServerURL() -> NSMutableURLRequest{
    ...
    request.HTTPMethod = "GET"
    ...
}

我通过以下代码片段验证了 iOS 系统确实重用了缓存。

let d = HTTPJSONDonwload()

// Initial request. Can not reuse cache.
d.startDownload()

// Subsequent request after 5 seconds. Should be able to reuse the cache.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(5 * NSEC_PER_SEC)), dispatch_get_main_queue()) {
    d.startDownload()
}

// Subsequent request after 11 seconds. Cannot reuse the cache because
// the expiration timeout is 10 seconds.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(11 * NSEC_PER_SEC)), dispatch_get_main_queue()) {
    d.startDownload()
}

当我在模拟器中运行它并使用 Charles Proxy 监控网络调用时，我确实只看到两个事件:

第一次调用是初始请求

第二个调用是延迟11秒后发出的第三个请求。

请注意，在 5 秒后发出的第二个请求没有出现，这意味着响应是从缓存中检索的。然而，NSURLConnection 的委托(delegate)方法仍将被调用，就像响应来自网络一样。因此，通过代码中的日志输出，您将在控制台上看到所有三个请求。

Connection Received Resopnse Headers= [Server: Apache-Coyote/1.1, Content-Type: application/json;charset=UTF-8, Keep-Alive: timeout=15, max=100, Proxy-Connection: Keep-alive, Date: Mon, 14 Sep 2015 06:28:05 GMT, Content-Encoding: gzip, Content-Length: 36, Cache-Control: max-age=10, Vary: Accept-Encoding]
Connection Data= Optional({"1":"New York"})
Connection will cache Response
Connection Received Resopnse Headers= [Server: Apache-Coyote/1.1, Content-Type: application/json;charset=UTF-8, Keep-Alive: timeout=15, max=100, Proxy-Connection: Keep-alive, Date: Mon, 14 Sep 2015 06:28:05 GMT, Content-Encoding: gzip, Content-Length: 36, Cache-Control: max-age=10, Vary: Accept-Encoding]
Connection Data= Optional({"1":"New York"})
Connection Received Resopnse Headers= [Server: Apache-Coyote/1.1, Content-Type: application/json;charset=UTF-8, Keep-Alive: timeout=15, max=99, Proxy-Connection: Keep-alive, Date: Mon, 14 Sep 2015 06:28:16 GMT, Content-Encoding: gzip, Content-Length: 36, Cache-Control: max-age=10, Vary: Accept-Encoding]
Connection Data= Optional({"1":"New York"})
Connection will cache Response

请注意，在第二次请求后没有Connection will cache Response，因为响应是从缓存中检索的，再次缓存它没有意义。