Custom NSURLSession configuration (#27701)

Summary:
While it is possible in the React Native implementation for Android to provide a custom configuration for HTTP requests, the iOS implementation does not allow for the same customization. As the NSURLSession used for HTTP requests on iOS is configured internally, one may for instance not supply an ephemeral configuration for HTTP requests. Other concerns related to the given problem have been addressed in the community: react-native-community/discussions-and-proposals#166. I did make a PR with an RFC in the community repo, but after some discussion in the said repo, I figured I might as well make a PR with a suggestion :)

## Changelog

[iOS] [Added] - Allow for configuring the NSURLSessionConfiguration

Implement a C function `RCTSetCustomNSURLSessionConfigurationProvider` which gives the app programmer the ability to provide a block which provides an NSURLSessionConfiguration that will be used for all HTTP requests instead of the default configuration. The provided block will be called when the session configuration is needed.

Pull Request resolved: #27701

Test Plan: Unsure if this can be tested in any other way than uncommenting the example code in `RNTester/RNTester/AppDelegate.mm`.

Reviewed By: yungsters

Differential Revision: D28680384

Pulled By: JoshuaGross

fbshipit-source-id: ae24399955581a1cc9f4202f0f6f497bfe067a5c

Libraries/Network/RCTHTTPRequestHandler.h

@@ -8,6 +8,11 @@
 #import <React/RCTInvalidating.h>
 #import <React/RCTURLRequestHandler.h>
 
+typedef NSURLSessionConfiguration* (^NSURLSessionConfigurationProvider)(void);
+/**
+ *  The block provided via this function will provide the NSURLSessionConfiguration for all HTTP requests made by the app.
+*/
+RCT_EXTERN void RCTSetCustomNSURLSessionConfigurationProvider(NSURLSessionConfigurationProvider);
 /**
  * This is the default RCTURLRequestHandler implementation for HTTP requests.
  */

Libraries/Network/RCTHTTPRequestHandler.mm

@@ -18,6 +18,12 @@ @interface RCTHTTPRequestHandler () <NSURLSessionDataDelegate, RCTTurboModule>
 
 @end
 
+static NSURLSessionConfigurationProvider urlSessionConfigurationProvider;
+
+void RCTSetCustomNSURLSessionConfigurationProvider(NSURLSessionConfigurationProvider provider) {
+  urlSessionConfigurationProvider = provider;
+}
+
 @implementation RCTHTTPRequestHandler
 {
   NSMapTable *_delegates;
@@ -75,14 +81,20 @@ - (NSURLSessionDataTask *)sendRequest:(NSURLRequest *)request
     NSOperationQueue *callbackQueue = [NSOperationQueue new];
     callbackQueue.maxConcurrentOperationCount = 1;
     callbackQueue.underlyingQueue = [[_moduleRegistry moduleForName:"Networking"] methodQueue];
-    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
-    // Set allowsCellularAccess to NO ONLY if key ReactNetworkForceWifiOnly exists AND its value is YES
-    if (useWifiOnly) {
-      configuration.allowsCellularAccess = ![useWifiOnly boolValue];
+    NSURLSessionConfiguration *configuration;
+    if (urlSessionConfigurationProvider) {
+      configuration = urlSessionConfigurationProvider();
+    } else {
+      configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
+      // Set allowsCellularAccess to NO ONLY if key ReactNetworkForceWifiOnly exists AND its value is YES
+      if (useWifiOnly) {
+        configuration.allowsCellularAccess = ![useWifiOnly boolValue];
+      }
+      [configuration setHTTPShouldSetCookies:YES];
+      [configuration setHTTPCookieAcceptPolicy:NSHTTPCookieAcceptPolicyAlways];
+      [configuration setHTTPCookieStorage:[NSHTTPCookieStorage sharedHTTPCookieStorage]];
     }
-    [configuration setHTTPShouldSetCookies:YES];
-    [configuration setHTTPCookieAcceptPolicy:NSHTTPCookieAcceptPolicyAlways];
-    [configuration setHTTPCookieStorage:[NSHTTPCookieStorage sharedHTTPCookieStorage]];
+    assert(configuration != nil);
     _session = [NSURLSession sessionWithConfiguration:configuration
                                              delegate:self
                                         delegateQueue:callbackQueue];