Implement 'injectJavaScript' method

Summary:
@public

This diff introduces a method called `injectJavaScript(script)` on the React Native `<WKWebView/>` component. When called with a string, it evaluates that string as JavaScript within the web view.

Reviewed By: shergin

Differential Revision: D6367445

fbshipit-source-id: f68afeff42535dc991747f96a63f3c956faf13d3

Author: RSNara

React/Views/RCTWKWebView.h

@@ -24,5 +24,6 @@
 @property (nonatomic, assign) CGFloat decelerationRate;
 
 - (void)postMessage:(NSString *)message;
+- (void)injectJavaScript:(NSString *)script;
 
 @end

React/Views/RCTWKWebView.m

@@ -255,4 +255,9 @@ - (void)      webView:(WKWebView *)webView
   }
 }
 
+- (void)injectJavaScript:(NSString *)script
+{
+  [self evaluateJS: script thenCall: nil];
+}
+
 @end

React/Views/RCTWKWebViewManager.m

@@ -45,4 +45,16 @@ - (UIView *)view
   view.decelerationRate = json == nil ? UIScrollViewDecelerationRateNormal : [RCTConvert CGFloat: json];
 }
 
+RCT_EXPORT_METHOD(injectJavaScript:(nonnull NSNumber *)reactTag script:(NSString *)script)
+{
+  [self.bridge.uiManager addUIBlock:^(__unused RCTUIManager *uiManager, NSDictionary<NSNumber *, RCTWKWebView *> *viewRegistry) {
+    RCTWKWebView *view = viewRegistry[reactTag];
+    if (![view isKindOfClass:[RCTWKWebView class]]) {
+      RCTLogError(@"Invalid view returned from registry, expecting RCTWebView, got: %@", view);
+    } else {
+      [view injectJavaScript:script];
+    }
+  }];
+}
+
 @end