UIWebView内容缓存

URL Loading System
在说 NSURLProtocol 之前需要对 URL Loading System 进行说明，引用苹果官方文档对 URL Loading System 的一个解释：

The URL loading system is a set of classes and protocols that allow your app to access content referenced by a URL. At the heart of this technology is the NSURL class, which lets your app manipulate URLs and the resources they refer to.

大致的意思就是 URL Loading System 是由一系列的 class 和 Protocol 组成，而我们可以通过这些 class 和 Protocol 来操作相关的 url ，其中处于核心的 class 就是 NSURL 。
其中相关的 class 和 Protocol 可以使用官方的一张图来说明：

URL Loading System

当然 URL Loading System 是由很多个方面组成的详细的情况可以直接查询苹果的官方文档 URL Loading System

URL loading system 原生已经支持了http,https,file,ftp,data这些常见协议，当然也允许我们定义自己的protocol去扩展，或者定义自己的协议。当URL loading system通过NSURLRequest对象进行请求时，将会自动创建NSURLProtocol的实例（可以是自定义的）。这样我们就有机会对该请求进行处理。官方文档里面介绍得比较少，下面我们直接看如何自定义NSURLProtocol，解读一下 RNCachingURLProtocol这个开源库的使用和原理。

首先看一下源码解析

//初始化
+ (void)initialize
{
  if (self == [RNCachingURLProtocol class])
  {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
      RNCachingSupportedSchemesMonitor = [NSObject new];
    });
    
    //设置支持的协议类型
    [self setSupportedSchemes:[NSSet setWithObject:@"http"]];
  }
}

//是否可以处理此次的网络请求 yes 可以  no 丢弃
+ (BOOL)canInitWithRequest:(NSURLRequest *)request
{
  // 判断是否支持协议类型 和 request是否被处理过（防止递归调用）
  if ([[self supportedSchemes] containsObject:[[request URL] scheme]] &&
      ([request valueForHTTPHeaderField:RNCachingURLHeader] == nil))
  {
    return YES;
  }
  return NO;
}

//这边可用干你想干的事情。。更改地址，或者设置里面的请求头。。
+ (NSURLRequest *)canonicalRequestForRequest:(NSURLRequest *)request
{
  return request;
}

//设置请求内容的缓存地址
- (NSString *)cachePathForRequest:(NSURLRequest *)aRequest
{
  // This stores in the Caches directory, which can be deleted when space is low, but we only use it for offline access
  NSString *cachesPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
  NSString *fileName = [[[aRequest URL] absoluteString] sha1];//这里只是一个用SHA1算法将字符串加密的category

  return [cachesPath stringByAppendingPathComponent:fileName];
}

- (void)startLoading
{
    //判断是否已经缓存过
  if (![self useCache]) {
    NSMutableURLRequest *connectionRequest = 
#if WORKAROUND_MUTABLE_COPY_LEAK
      [[self request] mutableCopyWorkaround];
#else
      [[self request] mutableCopy];
#endif
    // 打一下标记
    [connectionRequest setValue:@"" forHTTPHeaderField:RNCachingURLHeader];
    NSURLConnection *connection = [NSURLConnection connectionWithRequest:connectionRequest
                                                                delegate:self];
    [self setConnection:connection];
  }
  else {
    //创建缓存对象
    RNCachedData *cache = [NSKeyedUnarchiver unarchiveObjectWithFile:[self cachePathForRequest:[self request]]];
    if (cache) {
      NSData *data = [cache data];
      NSURLResponse *response = [cache response];
      NSURLRequest *redirectRequest = [cache redirectRequest];
      if (redirectRequest) {
          //重复访问了同一个请求
        [[self client] URLProtocol:self wasRedirectedToRequest:redirectRequest redirectResponse:response];
      } else {
          //处理请求
        [[self client] URLProtocol:self didReceiveResponse:response cacheStoragePolicy:NSURLCacheStorageNotAllowed]; // we handle caching ourselves.
        [[self client] URLProtocol:self didLoadData:data];
        [[self client] URLProtocolDidFinishLoading:self];
      }
    }
    else {
      [[self client] URLProtocol:self didFailWithError:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorCannotConnectToHost userInfo:nil]];
    }
  }
}

//缓存请求的内容 
- (NSURLRequest *)connection:(NSURLConnection *)connection willSendRequest:(NSURLRequest *)request redirectResponse:(NSURLResponse *)response
{
// Thanks to Nick Dowell https://gist.github.com/1885821
  if (response != nil) {
      NSMutableURLRequest *redirectableRequest =
#if WORKAROUND_MUTABLE_COPY_LEAK
      [request mutableCopyWorkaround];
#else
      [request mutableCopy];
#endif
      //标记为nil 防止递归调用 canonicalRequestForRequest
    [redirectableRequest setValue:nil forHTTPHeaderField:RNCachingURLHeader];

    NSString *cachePath = [self cachePathForRequest:[self request]];
    RNCachedData *cache = [RNCachedData new];
    [cache setResponse:response];
    [cache setData:[self data]];
    [cache setRedirectRequest:redirectableRequest];
    [NSKeyedArchiver archiveRootObject:cache toFile:cachePath];
    [[self client] URLProtocol:self wasRedirectedToRequest:redirectableRequest redirectResponse:response];
    return redirectableRequest;
  } else {
    return request;
  }
}

#pragma mark - NSURLConnectionDelegate
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
  [[self client] URLProtocol:self didLoadData:data];
  [self appendData:data];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
  [[self client] URLProtocol:self didFailWithError:error];
  [self setConnection:nil];
  [self setData:nil];
  [self setResponse:nil];
}

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
  [self setResponse:response];
  [[self client] URLProtocol:self didReceiveResponse:response cacheStoragePolicy:NSURLCacheStorageNotAllowed];  // We cache ourselves.
}

注意点：
每次只能只有一个protocol进行处理，如果有多个自定义protocol，系统将采取你registerClass的倒序进行调用，一旦你需要对这个请求进行处理，那么接下来的所有相关操作都需要这个protocol进行管理。
一定要注意标记请求，不然你会无限的循环下去。。。因为一旦你需要处理这个请求，那么系统会创建你这个protocol的实例，然后你自己又开启了connection进行请求的话，又会触发URL Loading system的回调。系统给我们提供了+ (void)setProperty:(id)value forKey:(NSString *)key inRequest:(NSMutableURLRequest *)request;和+ (id)propertyForKey:(NSString *)key inRequest:(NSURLRequest *)request;这两个方法进行标记和区分。
大家在使用的时候只需要在Appdelegate注册一下,然后缓存路径自己处理一下就可以了。

[NSURLProtocol registerClass:[RNCachingURLProtocol class]];

欢迎讨论!