fix(ios): fix keyboard displacement bug in iOS 12 WKWebView (#201)

* fix(ios): fix keyboard displacement bug in iOS 12 WKWebView

* add debounce support for cancels on keyboardWillShow to fix bounce during programmatic focus toggling

* simplify by applying to webView.scrollView instead of iterating subviews

src/ios/CDVWKWebViewEngine.m

@@ -126,6 +126,8 @@ @implementation CDVWKWebViewEngine
 
 @synthesize engineWebView = _engineWebView;
 
+NSTimer *timer;
+
 - (instancetype)initWithFrame:(CGRect)frame
 {
     self = [super init];
@@ -363,6 +365,17 @@ - (void)pluginInitialize
      selector:@selector(onSocketError:)
      name:@"socketInUseError" object:nil];
 
+    [[NSNotificationCenter defaultCenter]
+     addObserver:self
+     selector:@selector(keyboardWillHide)
+     name:UIKeyboardWillHideNotification object:nil];
+
+    [[NSNotificationCenter defaultCenter]
+     addObserver:self
+     selector:@selector(keyboardWillShow)
+     name:UIKeyboardWillShowNotification object:nil];
+
+
     NSLog(@"Using Ionic WKWebView");
 
     [self addURLObserver];
@@ -448,6 +461,31 @@ - (void)onAppWillEnterForeground:(NSNotification *)notification {
     }
 }
 
+
+-(void)keyboardWillHide
+{
+    if (@available(iOS 12.0, *)) {
+        timer = [NSTimer scheduledTimerWithTimeInterval:0 target:self selector:@selector(keyboardDisplacementFix) userInfo:nil repeats:false];
+        [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
+    }
+}
+
+-(void)keyboardWillShow
+{
+    if (timer != nil) {
+        [timer invalidate];
+    }
+}
+
+-(void)keyboardDisplacementFix
+{
+    // https://stackoverflow.com/a/9637807/824966
+    [UIView animateWithDuration:.25 animations:^{
+        self.webView.scrollView.contentOffset = CGPointMake(0, 0);
+    }];
+
+}
+
 - (void)onSocketError:(NSNotification *)notification {
     [self loadErrorPage:nil];
 }