Scaffold.of会找到遥远的窗口小部件吗？

Question

I am trying to learn more about BuildContext and Scaffold.of() .

I understand that when you are displaying a SnackBar you often need to add a Builder to get the context 'under' the Scaffold you have added.

As such:

class MyScreen extends StatelessWidget {
   build(BuildContext context) {
    return Scaffold(

        body: Builder(
          builder: (context) =>
           RaisedButton(
                child: Text('push me'),
                onPressed: () => Scaffold.of(context).showSnackBar(
                    SnackBar(content: Text('this will work'))),
              ),
        ));
  }
}

My question is this: If Scaffold.of() gets the closest ancestor, shouldn't it usually eventually find something higher up the widget tree, eg the SplashScreen scaffold? When we take away the builder and create this incorrect code:

class MyScreen extends StatelessWidget {
   build(BuildContext context) {
    return Scaffold(

        body: RaisedButton(
                child: Text('push me'),
                onPressed: () => Scaffold.of(context).showSnackBar(
                    SnackBar(content: Text('broken'))),
              ),
        ));
  }
}

then we get the standard error Scaffold.of() called with a context that does not contain a Scaffold. I/flutter ( 7750) Scaffold.of() called with a context that does not contain a Scaffold. I/flutter ( 7750) No Scaffold ancestor could be found starting from the context that was passed to Scaffold.of().

Even though there is no direct parent Scaffold in the BuildContext , how can no Scaffold ancestor be found at all (for example a Grandparent ancestor)? All the previous pages in my application had a scaffold, are they no longer in the widget tree?

I understand that if it found a Scaffold and rendered the SnackBar that we wouldn't be able to see it, but I still don't understand how it doesn't find one. Perhaps I've misunderstood what 'ancestor' means.

Thank you

Answer 1

BuildContext represents the location of one specific widget in the widget tree.

As such, depending on what BuildContext you use, the call Scaffold.of(context) may behave differently.

In your following example:

class MyScreen extends StatelessWidget {
   build(BuildContext context) {
     return Scaffold(
       body: RaisedButton(
         child: Text('push me'),
         onPressed: () => Scaffold.of(context).showSnackBar(
           SnackBar(content: Text('this will work'))),
       ),
     ),
   );
  }
}

Here, you are using the BuildContext of MyScreen . The issue is MyScreen is the widget that creates Scaffold . As such, MyScreen is an ancestor of Scaffold and cannot access it through Scaffold.of(context) .