ios url缓存策略——NSURLCache、 NSURLRequest

一：url 缓存策略

NSURLRequest

requestWithURL:cachePolicy:timeoutInterval:

1：NSURLRequestUseProtocolCachePolicy

This flag will use the underlying protocol’s caching mechanism if the protocol

supports it.

基础策略

2：NSURLRequestReloadIgnoringLocalCacheData

This flag specifies that the local cached copy of the resource that is about to be

downloaded must be disregarded and the remote cache policy must be effective.

If there is a local copy of the resource, managed by the framework itself, it will be

ignored.

忽略本地缓存

3：NSURLRequestReturnCacheDataElseLoad

This flag specifies that the cached data must be used before attempting to load the

data from the original source. The cached data could be protocol-based cached or

locally cached. If there is no cached data, the data will be downloaded from the

original source.

首先使用缓存，如果没有本地缓存，才从原地址下载

4：NSURLRequestReturnCacheDataDontLoad

This flag specifies that only the local cached data must be used. If the data has not

been cached, the request will fail. This is a great flag to use whenever your application

wants to perform operations in offline mode (such as the Offline Mode in

web browsers).

使用本地缓存，从不下载，如果本地没有缓存，则请求失败。此策略多用于离线操作

5：NSURLRequestReloadIgnoringLocalAndRemoteCacheData

This flag disregards any type of caching involved in the process, local and remote,

and always attempts to download the data from the original source.

无视任何的缓存策略，无论是本地的还是远程的，总是从原地址重新下载

6：NSURLRequestReloadRevalidatingCacheData

This flag specifies that the original source of data must validate the local cache (if

any) before an attempt is made to download the data from the original source. If

there is a copy of the original source cached locally and the remote source specifies

that the cached data is valid, the data won’t be downloaded again. In any other

case, the data will be downloaded from the original source.

如果本地缓存是有效的则不下载。其他任何情况都从原地址重新下载

二：NSURLCache

NSURLCache *urlCache = [NSURLCache sharedURLCache];

In iOS, NSURLCache supports caching data only in memory and not

on disk.

（NSURLCache 仅支持内存缓存，不支持硬盘缓存）

- (void) downloadURL:(NSString *)paramURLAsString{

if ([paramURLAsString length] == 0){

NSLog(@”Nil or empty URL is given”);

return;

}

/* Get the shared URL Cache object. No need to create a new one */

NSURLCache *urlCache = [NSURLCache sharedURLCache];

/* We will store up to 1 Megabyte of data into the cache */

[urlCache setMemoryCapacity:1*1024*1024];

/* For our request, we need an instance of NSURL so let’s retrieve

that from the string that we created before */

NSURL *url = [NSURL URLWithString:paramURLAsString];

/* And this is our request */

NSMutableURLRequest *request =[NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0f];

/* Try to get a cached response to our request.

This might come back as nil */

NSCachedURLResponse *response =[urlCache cachedResponseForRequest:request];

/* Do we have a cached response? */

if (response != nil){

NSLog(@”Cached response exists. Loading data from cache…”);

[request setCachePolicy:NSURLRequestReturnCacheDataDontLoad];

}

self.connection = nil;

/* Start the connection with the request */

NSURLConnection *newConnection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];

self.connection = newConnection;

[newConnection release];

}