Adds 'hiddenUntilFinishLoad' param

This param is true by default, so it does not change the current behaviour: a hidden webview is automatically revealed once stateChanged.finishLoad is received.

It can be set to false so that a hidden webview is not automatically revealed when the page has finished loading.

This way, the user can keep the webview hidden as long as required (i.e. to wait for custom javascript evaluation after the page finishes loading).

lib/src/base.dart

@@ -117,6 +117,7 @@ class FlutterWebviewPlugin {
   /// - [clearCache] clear the cache of the Webview
   /// - [clearCookies] clear all cookies of the Webview
   /// - [hidden] not show
+  /// - [hiddenUntilFinishLoad] stay hidden until finishLoad event
   /// - [rect]: show in rect, fullscreen if null
   /// - [enableAppScheme]: false will enable all schemes, true only for httt/https/about
   ///     android: Not implemented yet
@@ -150,6 +151,7 @@ class FlutterWebviewPlugin {
     bool clearCookies,
     bool mediaPlaybackRequiresUserGesture,
     bool hidden,
+    bool hiddenUntilFinishLoad,
     bool enableAppScheme,
     Rect rect,
     String userAgent,
@@ -174,6 +176,7 @@ class FlutterWebviewPlugin {
       'withJavascript': withJavascript ?? true,
       'clearCache': clearCache ?? false,
       'hidden': hidden ?? false,
+      'hiddenUntilFinishLoad': hiddenUntilFinishLoad ?? true,
       'clearCookies': clearCookies ?? false,
       'mediaPlaybackRequiresUserGesture': mediaPlaybackRequiresUserGesture ?? true,
       'enableAppScheme': enableAppScheme ?? true,

lib/src/webview_scaffold.dart

@@ -34,6 +34,7 @@ class WebviewScaffold extends StatefulWidget {
     this.supportMultipleWindows,
     this.appCacheEnabled,
     this.hidden = false,
+    this.hiddenUntilFinishLoad = true,
     this.initialChild,
     this.allowFileURLs,
     this.resizeToAvoidBottomInset = false,
@@ -65,6 +66,7 @@ class WebviewScaffold extends StatefulWidget {
   final bool supportMultipleWindows;
   final bool appCacheEnabled;
   final bool hidden;
+  final bool hiddenUntilFinishLoad;
   final Widget initialChild;
   final bool allowFileURLs;
   final bool resizeToAvoidBottomInset;
@@ -110,7 +112,7 @@ class _WebviewScaffoldState extends State<WebviewScaffold> {
       }
     });
 
-    if (widget.hidden) {
+    if (widget.hidden && widget.hiddenUntilFinishLoad) {
       _onStateChanged =
           webviewReference.onStateChanged.listen((WebViewStateChanged state) {
         if (state.type == WebViewState.finishLoad) {
@@ -136,9 +138,7 @@ class _WebviewScaffoldState extends State<WebviewScaffold> {
     _onBack?.cancel();
     _resizeTimer?.cancel();
     webviewReference.close();
-    if (widget.hidden) {
-      _onStateChanged.cancel();
-    }
+    _onStateChanged?.cancel();
     webviewReference.dispose();
   }