feat(onScroll): Add `onScroll` callback for iOS & Android (#516)

* Add `onScroll` callback for iOS & Android

This code was mostly extracted from https://github.com/react-native-community/react-native-webview/pull/202

I tried and tried to make it work with `Animated.event`'s `useNativeDriver`, but I was unsuccessful 😢 that'll have to be done later once I understand better how Animated's native stuff is hooked up.

* fix crash for missing onScroll

android/src/main/java/com/reactnativecommunity/webview/RNCWebViewManager.java

 import android.webkit.WebViewClient;
 import android.widget.FrameLayout;
 
+import com.facebook.react.views.scroll.ScrollEvent;
+import com.facebook.react.views.scroll.ScrollEventType;
+import com.facebook.react.views.scroll.OnScrollDispatchHelper;
 import com.facebook.react.bridge.Arguments;
 import com.facebook.react.bridge.LifecycleEventListener;
 import com.facebook.react.bridge.ReactContext;
     }
     export.put(TopLoadingProgressEvent.EVENT_NAME, MapBuilder.of("registrationName", "onLoadingProgress"));
     export.put(TopShouldStartLoadWithRequestEvent.EVENT_NAME, MapBuilder.of("registrationName", "onShouldStartLoadWithRequest"));
+    export.put(ScrollEventType.SCROLL.getJSEventName(), MapBuilder.of("registrationName", "onScroll"));
     return export;
   }
 
     protected @Nullable
     RNCWebViewClient mRNCWebViewClient;
     protected boolean sendContentSizeChangeEvents = false;
+    private final OnScrollDispatchHelper mOnScrollDispatchHelper = new OnScrollDispatchHelper();
 
     /**
      * WebView must be created with an context of the current activity
       dispatchEvent(this, new TopMessageEvent(this.getId(), message));
     }
 
+    protected void onScrollChanged(int x, int y, int oldX, int oldY) {
+      super.onScrollChanged(x, y, oldX, oldY);
+
+      if (mOnScrollDispatchHelper.onScrollChanged(x, y)) {
+        ScrollEvent event = ScrollEvent.obtain(
+                this.getId(),
+                ScrollEventType.SCROLL,
+                x,
+                y,
+                mOnScrollDispatchHelper.getXFlingVelocity(),
+                mOnScrollDispatchHelper.getYFlingVelocity(),
+                this.computeHorizontalScrollRange(),
+                this.computeVerticalScrollRange(),
+                this.getWidth(),
+                this.getHeight());
+
+        dispatchEvent(this, event);
+      }
+    }
+
     protected void cleanupCallbacksAndDestroy() {
       setWebViewClient(null);
       destroy();

ios/RNCWKWebView.m

 @property (nonatomic, copy) RCTDirectEventBlock onLoadingProgress;
 @property (nonatomic, copy) RCTDirectEventBlock onShouldStartLoadWithRequest;
 @property (nonatomic, copy) RCTDirectEventBlock onMessage;
+@property (nonatomic, copy) RCTDirectEventBlock onScroll;
 @property (nonatomic, copy) WKWebView *webView;
 @end
 
   if (!_scrollEnabled) {
     scrollView.bounds = _webView.bounds;
   }
+  else if (_onScroll != nil) {
+    NSDictionary *event = @{
+      @"contentOffset": @{
+          @"x": @(scrollView.contentOffset.x),
+          @"y": @(scrollView.contentOffset.y)
+          },
+      @"contentInset": @{
+          @"top": @(scrollView.contentInset.top),
+          @"left": @(scrollView.contentInset.left),
+          @"bottom": @(scrollView.contentInset.bottom),
+          @"right": @(scrollView.contentInset.right)
+          },
+      @"contentSize": @{
+          @"width": @(scrollView.contentSize.width),
+          @"height": @(scrollView.contentSize.height)
+          },
+      @"layoutMeasurement": @{
+          @"width": @(scrollView.frame.size.width),
+          @"height": @(scrollView.frame.size.height)
+          },
+      @"zoomScale": @(scrollView.zoomScale ?: 1),
+      };
+    _onScroll(event);
+  }
 }
 
 - (void)setDirectionalLockEnabled:(BOOL)directionalLockEnabled

ios/RNCWKWebViewManager.m

  */
 RCT_EXPORT_VIEW_PROPERTY(messagingEnabled, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(onMessage, RCTDirectEventBlock)
+RCT_EXPORT_VIEW_PROPERTY(onScroll, RCTDirectEventBlock)
 
 RCT_EXPORT_METHOD(postMessage:(nonnull NSNumber *)reactTag message:(NSString *)message)
 {

src/WebView.ios.tsx

         onLoadingProgress={this.onLoadingProgress}
         onLoadingStart={this.onLoadingStart}
         onMessage={this.onMessage}
+        onScroll={this.props.onScroll}
         onShouldStartLoadWithRequest={onShouldStartLoadWithRequest}
         ref={this.webViewRef}
         scalesPageToFit={scalesPageToFit}

src/WebViewTypes.ts

   NativeMethodsMixin,
   Constructor,
   UIManagerStatic,
+  NativeScrollEvent,
 } from 'react-native';
 
 export interface WebViewCommands {
   injectedJavaScript?: string;
   mediaPlaybackRequiresUserAction?: boolean;
   messagingEnabled: boolean;
+  onScroll?: (event: NativeScrollEvent) => void;
   onLoadingError: (event: WebViewErrorEvent) => void;
   onLoadingFinish: (event: WebViewNavigationEvent) => void;
   onLoadingProgress: (event: WebViewProgressEvent) => void;
    */
   renderLoading?: () => ReactElement;
 
+  /**
+   * Function that is invoked when the `WebView` scrolls.
+   */
+  onScroll?: (event: NativeScrollEvent) => void;
+
   /**
    * Function that is invoked when the `WebView` has finished loading.
    */