Implement message passing!

Summary:
@public

This diff implements message passing between the `WKWebView` and React Native. As with `<WebView/>`, we can only send/receive strings.

**Usage:**
1. Set `messagingEnabled` to `true`.
1. To send data from the web view to React Native, call `postMessage(data)` within the web view. This forces React Native to execute the `onMessage` prop on the `WKWebView` component. `onMessage` will be called with an event `e`, where `e.nativeEvent.data` will be the data you passed into `postMessage`.
1. To send data from React Native to the web view, call `UIManager.dispatchViewManagerCommand` to dispatch the `UIManager.RCTWKWebView.Commands.postMessage` command. Look at [[ https://fburl.com/u1wusf2f | this part of the existing `<WebView/>` ]] component for more details. After you make the call, React Native will dispatch a `'message'` event to the `document` object within the webview. You can listen to the event by doing `document.addEventListener('message', callback)`. Let the event dispatched be `e`. Then, `e.data` is the data you sent over from React Native.

[[ P58627181 | This Playground.js ]] illustrates the usage.

Reviewed By: shergin

Differential Revision: D6304850

fbshipit-source-id: 29075ef753296e9fb5a9cddeb1ad0f4ff7e28650

React/Views/RCTWKWebView.h

@@ -18,6 +18,9 @@
 
 @property (nonatomic, weak) id<RCTWKWebViewDelegate> delegate;
 @property (nonatomic, copy) NSDictionary *source;
+@property (nonatomic, assign) BOOL messagingEnabled;
 @property (nonatomic, copy) NSString *injectedJavaScript;
 
+- (void)postMessage:(NSString *)message;
+
 @end

React/Views/RCTWKWebView.m

@@ -1,15 +1,15 @@
 #import "RCTWKWebView.h"
-
 #import <WebKit/WebKit.h>
-
 #import <React/RCTConvert.h>
-
 #import "RCTAutoInsetsProtocol.h"
 
-@interface RCTWKWebView () <WKUIDelegate, WKNavigationDelegate>
+static NSString *const MessageHanderName = @"ReactNative";
+
+@interface RCTWKWebView () <WKUIDelegate, WKNavigationDelegate, WKScriptMessageHandler>
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingStart;
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingFinish;
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingError;
+@property (nonatomic, copy) RCTDirectEventBlock onMessage;
 @property (nonatomic, copy) WKWebView *webView;
 @end
 
@@ -26,14 +26,32 @@ - (instancetype)initWithFrame:(CGRect)frame
 {
   if ((self = [super initWithFrame:frame])) {
     super.backgroundColor = [UIColor clearColor];
-    _webView = [[WKWebView alloc] initWithFrame:self.bounds];
+    WKWebViewConfiguration *wkWebViewConfig = [WKWebViewConfiguration new];
+    wkWebViewConfig.userContentController = [WKUserContentController new];
+    [wkWebViewConfig.userContentController addScriptMessageHandler: self name: MessageHanderName];
+
+    _webView = [[WKWebView alloc] initWithFrame:self.bounds configuration: wkWebViewConfig];
     _webView.UIDelegate = self;
     _webView.navigationDelegate = self;
     [self addSubview:_webView];
   }
   return self;
 }
 
+/**
+ * This method is called whenever JavaScript running within the web view calls:
+ *   - window.webkit.messageHandlers.[MessageHanderName].postMessage
+ */
+- (void)userContentController:(WKUserContentController *)userContentController
+       didReceiveScriptMessage:(WKScriptMessage *)message
+{
+  if (_onMessage != nil) {
+    NSMutableDictionary<NSString *, id> *event = [self baseEvent];
+    [event addEntriesFromDictionary: @{@"data": message.body}];
+    _onMessage(event);
+  }
+}
+
 - (void)setSource:(NSDictionary *)source
 {
   if (![_source isEqualToDictionary:source]) {
@@ -67,9 +85,21 @@ - (void)setSource:(NSDictionary *)source
   }
 }
 
+- (void)postMessage:(NSString *)message
+{
+  NSDictionary *eventInitDict = @{@"data": message};
+  NSString *source = [NSString
+    stringWithFormat:@"document.dispatchEvent(new MessageEvent('message', %@));",
+    RCTJSONStringify(eventInitDict, NULL)
+  ];
+  [self evaluateJS: source thenCall: nil];
+}
+
 - (void)layoutSubviews
 {
   [super layoutSubviews];
+
+  // Ensure webview takes the position and dimensions of RCTWKWebView
   _webView.frame = self.bounds;
 }
 
@@ -161,7 +191,7 @@ - (void)evaluateJS:(NSString *)js
           thenCall: (void (^)(NSString*)) callback
 {
   [self.webView evaluateJavaScript: js completionHandler: ^(id result, NSError *error) {
-    if (error == nil) {
+    if (error == nil && callback != nil) {
       callback([NSString stringWithFormat:@"%@", result]);
     }
   }];
@@ -175,6 +205,31 @@ - (void)evaluateJS:(NSString *)js
 - (void)      webView:(WKWebView *)webView
   didFinishNavigation:(WKNavigation *)navigation
 {
+  if (_messagingEnabled) {
+    #if RCT_DEV
+
+    // Implementation inspired by Lodash.isNative.
+    NSString *isPostMessageNative = @"String(String(window.postMessage) === String(Object.hasOwnProperty).replace('hasOwnProperty', 'postMessage'))";
+    [self evaluateJS: isPostMessageNative thenCall: ^(NSString *result) {
+      if (! [result isEqualToString:@"true"]) {
+        RCTLogError(@"Setting onMessage on a WebView overrides existing values of window.postMessage, but a previous value was defined");
+      }
+    }];
+    #endif
+
+    NSString *source = [NSString stringWithFormat:
+      @"(function() {"
+        "window.originalPostMessage = window.postMessage;"
+
+        "window.postMessage = function(data) {"
+          "window.webkit.messageHandlers.%@.postMessage(String(data));"
+        "};"
+      "})();",
+      MessageHanderName
+    ];
+    [self evaluateJS: source thenCall: nil];
+  }
+
   if (_injectedJavaScript) {
     [self evaluateJS: _injectedJavaScript thenCall: ^(NSString *jsEvaluationValue) {
       NSMutableDictionary *event = [self baseEvent];

React/Views/RCTWKWebViewManager.h

@@ -0,0 +1,6 @@
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+#import <React/RCTViewManager.h>
+
+@interface RCTWKWebViewManager : RCTViewManager
+@end

React/Views/RCTWKWebViewManager.m

@@ -1,8 +1,7 @@
-#import "RCTViewManager.h"
-#import "RCTWKWebView.h"
+#import "RCTWKWebViewManager.h"
 
-@interface RCTWKWebViewManager : RCTViewManager
-@end
+#import "RCTUIManager.h"
+#import "RCTWKWebView.h"
 
 @implementation RCTWKWebViewManager
 
@@ -19,4 +18,22 @@ - (UIView *)view
 RCT_EXPORT_VIEW_PROPERTY(onLoadingError, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(injectedJavaScript, NSString)
 
+/**
+ * Expose methods to enable messaging the webview.
+ */
+RCT_EXPORT_VIEW_PROPERTY(messagingEnabled, BOOL)
+RCT_EXPORT_VIEW_PROPERTY(onMessage, RCTDirectEventBlock)
+
+RCT_EXPORT_METHOD(postMessage:(nonnull NSNumber *)reactTag message:(NSString *)message)
+{
+  [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 postMessage:message];
+    }
+  }];
+}
+
 @end