Implement 'onShouldStartLoadWithRequest' prop

Summary:
@public

This diff introduces the native backend for a new WKWebView prop: `onShouldStartLoadWithRequest`.

In the final component, the behaviour will be as follows: Whenever the user navigates around in the web view, we call `onShouldStartLoadWithRequest` with the navigation event. If `onShouldStartLoadWithRequest` returns `true`, we continue the navigation. Otherwise, we abort it.

Reviewed By: shergin

Differential Revision: D6370317

fbshipit-source-id: e3cdd7e2a755125aebdb6df67e7b39116228bdfb

Author: RSNara

React/Views/RCTWKWebView.h

@@ -12,6 +12,11 @@
 @class RCTWKWebView;
 
 @protocol RCTWKWebViewDelegate <NSObject>
+
+- (BOOL)webView:(RCTWKWebView *)webView
+shouldStartLoadForRequest:(NSMutableDictionary<NSString *, id> *)request
+   withCallback:(RCTDirectEventBlock)callback;
+
 @end
 
 @interface RCTWKWebView : RCTView

React/Views/RCTWKWebView.m

@@ -9,6 +9,7 @@ @interface RCTWKWebView () <WKUIDelegate, WKNavigationDelegate, WKScriptMessageH
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingStart;
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingFinish;
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingError;
+@property (nonatomic, copy) RCTDirectEventBlock onShouldStartLoadWithRequest;
 @property (nonatomic, copy) RCTDirectEventBlock onMessage;
 @property (nonatomic, copy) WKWebView *webView;
 @end
@@ -154,6 +155,20 @@ - (void)                  webView:(WKWebView *)webView
   WKNavigationType navigationType = navigationAction.navigationType;
   NSURLRequest *request = navigationAction.request;
 
+  if (_onShouldStartLoadWithRequest) {
+    NSMutableDictionary<NSString *, id> *event = [self baseEvent];
+    [event addEntriesFromDictionary: @{
+      @"url": (request.URL).absoluteString,
+      @"navigationType": navigationTypes[@(navigationType)]
+    }];
+    if (![self.delegate webView:self
+      shouldStartLoadForRequest:event
+                   withCallback:_onShouldStartLoadWithRequest]) {
+      decisionHandler(WKNavigationResponsePolicyCancel);
+      return;
+    }
+  }
+
   if (_onLoadingStart) {
     // We have this check to filter out iframe requests and whatnot
     BOOL isTopFrame = [request.URL isEqual:request.mainDocumentURL];

React/Views/RCTWKWebViewManager.m

@@ -3,19 +3,29 @@
 #import "RCTUIManager.h"
 #import "RCTWKWebView.h"
 
+@interface RCTWKWebViewManager () <RCTWKWebViewDelegate>
+@end
+
 @implementation RCTWKWebViewManager
+{
+  NSConditionLock *_shouldStartLoadLock;
+  BOOL _shouldStartLoad;
+}
 
 RCT_EXPORT_MODULE()
 
 - (UIView *)view
 {
-  return [RCTWKWebView new];
+  RCTWKWebView *webView = [RCTWKWebView new];
+  webView.delegate = self;
+  return webView;
 }
 
 RCT_EXPORT_VIEW_PROPERTY(source, NSDictionary)
 RCT_EXPORT_VIEW_PROPERTY(onLoadingStart, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onLoadingFinish, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onLoadingError, RCTDirectEventBlock)
+RCT_EXPORT_VIEW_PROPERTY(onShouldStartLoadWithRequest, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(injectedJavaScript, NSString)
 
 /**
@@ -105,4 +115,38 @@ - (UIView *)view
   }];
 }
 
+#pragma mark - Exported synchronous methods
+
+- (BOOL)          webView:(RCTWKWebView *)webView
+shouldStartLoadForRequest:(NSMutableDictionary<NSString *, id> *)request
+             withCallback:(RCTDirectEventBlock)callback
+{
+  _shouldStartLoadLock = [[NSConditionLock alloc] initWithCondition:arc4random()];
+  _shouldStartLoad = YES;
+  request[@"lockIdentifier"] = @(_shouldStartLoadLock.condition);
+  callback(request);
+
+  // Block the main thread for a maximum of 250ms until the JS thread returns
+  if ([_shouldStartLoadLock lockWhenCondition:0 beforeDate:[NSDate dateWithTimeIntervalSinceNow:.25]]) {
+    BOOL returnValue = _shouldStartLoad;
+    [_shouldStartLoadLock unlock];
+    _shouldStartLoadLock = nil;
+    return returnValue;
+  } else {
+    RCTLogWarn(@"Did not receive response to shouldStartLoad in time, defaulting to YES");
+    return YES;
+  }
+}
+
+RCT_EXPORT_METHOD(startLoadWithResult:(BOOL)result lockIdentifier:(NSInteger)lockIdentifier)
+{
+  if ([_shouldStartLoadLock tryLockWhenCondition:lockIdentifier]) {
+    _shouldStartLoad = result;
+    [_shouldStartLoadLock unlockWithCondition:0];
+  } else {
+    RCTLogWarn(@"startLoadWithResult invoked with invalid lockIdentifier: "
+               "got %lld, expected %lld", (long long)lockIdentifier, (long long)_shouldStartLoadLock.condition);
+  }
+}
+
 @end