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

@@ -31,6 +31,9 @@ import android.webkit.WebView;
 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;
@@ -446,6 +449,7 @@ public class RNCWebViewManager extends SimpleViewManager<WebView> {
     }
     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;
   }
 
@@ -780,6 +784,7 @@ public class RNCWebViewManager extends SimpleViewManager<WebView> {
     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
@@ -886,6 +891,26 @@ public class RNCWebViewManager extends SimpleViewManager<WebView> {
       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();