dart - The best way to passing data between widgets in Flutter

Question

I develop my own custom widget which is used in some other view. In this custom widget, I have one class property which stores information, let's say that this is a list which gains new items inside the widget. Now I want to get items from this list from the level of my main widget.

How to do that? I don't want to create a variable like this: var customWidget = MyCustomWidget() and then, get the inside variable like customWidget.createState().myList - I think this is a terrible solution (and I'm not sure if it will work). Also passing a list to the constructor of my custom widget looks very ugly.

Is there any other way to get other widget's state?

Answer 1

First, bear in mind that in Flutter data can only be passed downward. It is by design not possible to access data of children, only parents (although there are some hacks around it).

From this point, there are 2 main solutions to pass data:

1. Add the wanted data to your widgets constructors.

I don't think there is much to say here. Easy to use. But boring when you want to pass one field to all your widget tree. Use this method only when the scope of a value is limited. If it's something like configurations or user details, go for the second solution.

class Bar extends StatelessWidget {
  final String data;

  Bar({this.data});

  @override
  Widget build(BuildContext context) {
    return Text(data);
  }
}

2. Using Flutter's BuildContext

Each widget has access to a BuildContext. This class allows one widget to fetch information from any of their ancestors using one of the following methods:

• inheritFromWidgetOfExactType(Type)

• ancestorStateOfType(TypeMatcher)

• ancestorWidgetOfExactType(Type)

• ...

As a matter of facts, if there's a data that needs to be accessed many times, prefer inheritFromWidgetOfExactType.

This uses InheritedWidget; which are specifics kind of widgets that are extremely fast to access to.

See Flutter: How to correctly use an Inherited Widget? for more details on their usage

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As a side note, there a third solution. Which is GlobalKey

I won't go into too many details, as this is more a hack than a proper solution. (see Builder versus GlobalKey)

But basically, it allows getting the state/context of any widgets outside of the build call. Be it parents or children.