feat(WKWebview): Add fix for keyboard dismiss leaving viewport shifted in iOS 12 (#134)

ios/RNCWKWebView.m

 
 #import "objc/runtime.h"
 
+static NSTimer *keyboardTimer;
 static NSString *const MessageHanderName = @"ReactNative";
 
 // runtime trick to remove WKWebView keyboard default toolbar
     _automaticallyAdjustContentInsets = YES;
     _contentInset = UIEdgeInsetsZero;
   }
+    
+  // Workaround for a keyboard dismissal bug present in iOS 12
+  // https://openradar.appspot.com/radar?id=5018321736957952
+  if (@available(iOS 12.0, *)) {
+    [[NSNotificationCenter defaultCenter]
+      addObserver:self
+      selector:@selector(keyboardWillHide)
+      name:UIKeyboardWillHideNotification object:nil];
+    [[NSNotificationCenter defaultCenter]
+      addObserver:self
+      selector:@selector(keyboardWillShow)
+      name:UIKeyboardWillShowNotification object:nil];
+  }
   return self;
 }
 
   }
 }
 
+-(void)keyboardWillHide
+{
+    keyboardTimer = [NSTimer scheduledTimerWithTimeInterval:0 target:self selector:@selector(keyboardDisplacementFix) userInfo:nil repeats:false];
+    [[NSRunLoop mainRunLoop] addTimer:keyboardTimer forMode:NSRunLoopCommonModes];
+}
+-(void)keyboardWillShow
+{
+    if (keyboardTimer != nil) {
+        [keyboardTimer invalidate];
+    }
+}
+-(void)keyboardDisplacementFix
+{
+    // https://stackoverflow.com/a/9637807/824966
+    [UIView animateWithDuration:.25 animations:^{
+        self.webView.scrollView.contentOffset = CGPointMake(0, 0);
+    }];
+}
+
 - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
     if ([keyPath isEqual:@"estimatedProgress"] && object == self.webView) {
         if(_onLoadingProgress){