# build method

1. @override

Widget build (BuildContext context)

Describes the part of the user interface represented by this widget.

The framework calls this method in a number of different situations:

- After calling initState.
- After calling didUpdateWidget.
- After receiving a call to setState.
- After a dependency of this `State` object changes (e.g., an `InheritedWidget` referenced by the previous build changes).
- After calling deactivate and then reinserting the `State` object into the tree at another location.

The framework replaces the subtree below this widget with the widget returned by this method, either by updating the existing subtree or by removing the subtree and inflating a new subtree, depending on whether the widget returned by this method can update the root of the existing subtree, as determined by calling `Widget.canUpdate`.

Typically implementations return a newly created constellation of widgets that are configured with information from this widget's constructor, the given `BuildContext`, and the internal state of this `State` object.

The given `BuildContext` contains information about the location in the tree at which this widget is being built. For example, the context provides the set of inherited widgets for this location in the tree. The `BuildContext` argument is always the same as the `context` property of this `State` object and will remain the same for the lifetime of this object. The `BuildContext` argument is provided redundantly here so that this method matches the signature for a `WidgetBuilder`.

## Design discussion

### Why is the build method on `State`, and not `StatefulWidget`?

Putting a `Widget build(BuildContext context)` method on `State` rather putting a `Widget build(BuildContext context, State state)` method on `StatefulWidget` gives developers more flexibility when subclassing `StatefulWidget`.

For example, `AnimatedWidget` is a subclass of `StatefulWidget` that introduces an abstract `Widget build(BuildContext context)` method for its subclasses to implement. If `StatefulWidget` already had a build method that took a `State` argument, `AnimatedWidget` would be forced to provide its `State` object to subclasses even though its `State` object is an internal implementation detail of `AnimatedWidget`.

Conceptually, `StatelessWidget` could also be implemented as a subclass of `StatefulWidget` in a similar manner. If the build method were on `StatefulWidget` rather than `State`, that would not be possible anymore.

Putting the build function on `State` rather than `StatefulWidget` also helps avoid a category of bugs related to closures implicitly capturing `this`. If you defined a closure in a build function on a `StatefulWidget`, that closure would implicitly capture `this`, which is the current widget instance, and would have the (immutable) fields of that instance in scope:

    class MyButton extends StatefulWidget {
      ...
      final Color color;

      @override
      Widget build(BuildContext context, MyButtonState state) {
        ... () { print("color: $color"); } ...
      }
    }

For example, suppose the parent builds `MyButton` with `color` being blue, the `$color` in the print function refers to blue, as expected. Now, suppose the parent rebuilds `MyButton` with green. The closure created by the first build still implicitly refers to the original widget and the `$color` still prints blue even through the widget has been updated to green.

In contrast, with the build function on the `State` object, closures created during build implicitly capture the `State` instance instead of the widget instance:

    class MyButtonState extends State<MyButton> {
      ...
      @override
      Widget build(BuildContext context) {
        ... () { print("color: ${widget.color}"); } ...
      }
    }

Now when the parent rebuilds `MyButton` with green, the closure created by the first build still refers to `State` object, which is preserved across rebuilds, but the framework has updated that `State` object's widget property to refer to the new `MyButton` instance and `${widget.color}` prints green, as expected.

See also:

- The discussion on performance considerations at `StatefulWidget`.

## Implementation

    @override
    Widget build(BuildContext context) {

      return GoogleMap(
        onMapCreated: _onMapCreated,
      );
    }