Cache policy control for image source

Summary:
In the context of an app an image exists in three resolutions on the server: `thumb` (30px) `feed` (300px) `full` (900px). When looking at an individual item a user can come either from the feed, via a permalink or from other parts of the app. This allows a situation where the `feed` image might or might not already be loaded somewhere in the app. In the detail view I want to render `thumb` with a blur (to quickly display something), then the `feed` image if it exists to have something decent to display until `full` loads. However it is quite a waste to load the `feed` image if it isn't already in cache, and will slow down the time until `full` is loaded. It is possible to track the navigation from feed->detail and that the `feed` image has actually completed loading by the feed component however as component hierarchies grow this turns into quite a lot of prop passing and bad separation of concerns.

NSURLRequests accepts a [Cache Policy](https://developer.apple.com/reference/fo
Closes https://github.com/facebook/react-native/pull/10844

Differential Revision: D4425959

Pulled By: lacker

fbshipit-source-id: 679835439c761a2fc894f56eb6d744c036cf0b49

React/Base/RCTConvert.h

@@ -47,6 +47,7 @@
 + (NSData *)NSData:(id)json;
 + (NSIndexSet *)NSIndexSet:(id)json;
 
++ (NSURLRequestCachePolicy)NSURLRequestCachePolicy:(id)json;
 + (NSURL *)NSURL:(id)json;
 + (NSURLRequest *)NSURLRequest:(id)json;
 

React/Base/RCTConvert.m

@@ -118,6 +118,14 @@ RCT_CUSTOM_CONVERTER(NSData *, NSData, [json dataUsingEncoding:NSUTF8StringEncod
   }
 }
 
+RCT_ENUM_CONVERTER(NSURLRequestCachePolicy, (@{
+                                               @"default": @(NSURLRequestUseProtocolCachePolicy),
+                                               @"reload": @(NSURLRequestReloadIgnoringLocalCacheData),
+                                               @"force-cache": @(NSURLRequestReturnCacheDataElseLoad),
+                                               @"only-if-cached": @(NSURLRequestReturnCacheDataDontLoad),
+                                               }), NSURLRequestUseProtocolCachePolicy, integerValue)
+
+
 + (NSURLRequest *)NSURLRequest:(id)json
 {
   if ([json isKindOfClass:[NSString class]]) {
@@ -140,8 +148,9 @@ RCT_CUSTOM_CONVERTER(NSData *, NSData, [json dataUsingEncoding:NSUTF8StringEncod
 
     NSData *body = [self NSData:json[@"body"]];
     NSString *method = [self NSString:json[@"method"]].uppercaseString ?: @"GET";
+    NSURLRequestCachePolicy cachePolicy = [self NSURLRequestCachePolicy:json[@"cache"]];
     NSDictionary *headers = [self NSDictionary:json[@"headers"]];
-    if ([method isEqualToString:@"GET"] && headers == nil && body == nil) {
+    if ([method isEqualToString:@"GET"] && headers == nil && body == nil && cachePolicy == NSURLRequestUseProtocolCachePolicy) {
       return [NSURLRequest requestWithURL:URL];
     }
 
@@ -164,6 +173,7 @@ RCT_CUSTOM_CONVERTER(NSData *, NSData, [json dataUsingEncoding:NSUTF8StringEncod
     NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
     request.HTTPBody = body;
     request.HTTPMethod = method;
+    request.cachePolicy = cachePolicy;
     request.allHTTPHeaderFields = headers;
     return [request copy];
   }