NSURLRequestCachePolicy 缓存策略

代码摘自:SDWebImage

- (void)start{    // In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests    NSURLRequest *request = [[NSURLRequest alloc] initWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalCacheData timeoutInterval:15];    self.connection = [[[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:NO] autorelease];    // Ensure we aren't blocked by UI manipulations (default runloop mode for NSURLConnection is NSEventTrackingRunLoopMode)    [connection scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];    [connection start];    [request release];    if (connection)    {        self.imageData = [NSMutableData data];        [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:nil];    }    else    {        if ([delegate respondsToSelector:@selector(imageDownloader:didFailWithError:)])        {            [delegate performSelector:@selector(imageDownloader:didFailWithError:) withObject:self withObject:nil];        }    }}

1

NSURLRequest *request = [[NSURLRequest alloc] initWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalCacheData timeoutInterval:15];

1

1

1

1

1

 NSURLRequestCachePolicy 缓存策略

原文地址：点击打开链接

1> NSURLRequestUseProtocolCachePolicy = 0, 默认的缓存策略，如果缓存不存在，直接从服务端获取。如果缓存存在，会根据response中的Cache-Control字段判断下一步操作，如:Cache-Control字段为must-revalidata,则询问服务端该数据是否有更新，无更新的话直接返回给用户缓存数据，若已更新，则请求服务端.

2> NSURLRequestReloadIgnoringLocalCacheData = 1,忽略本地缓存数据，直接请求服务端.

3> NSURLRequestIgnoringLocalAndRemoteCacheData = 4,忽略本地缓存，代理服务器以及其他中介，直接请求源服务端.

4> NSURLRequestReloadIgnoringCacheData =NSURLRequestReloadIgnoringLocalCacheData

5> NSURLRequestReturnCacheDataElseLoad= 2, 有缓存就使用，不管其有效性(即忽略Cache-Control字段), 无则请求服务端.

6> NSURLRequ