FutureBuilder<T> class

A widget that builds itself based on the latest snapshot of interaction with a Future.

Managing the future

The future must have been obtained earlier, e.g. during State.initState, State.didUpdateWidget, or State.didChangeDependencies. It must not be created during the State.build or StatelessWidget.build method call when constructing the FutureBuilder. If the future is created at the same time as the FutureBuilder, then every time the FutureBuilder's parent is rebuilt, the asynchronous task will be restarted.

A general guideline is to assume that every build method could get called every frame, and to treat omitted calls as an optimization.

Timing

Widget rebuilding is scheduled by the completion of the future, using State.setState, but is otherwise decoupled from the timing of the future. The builder callback is called at the discretion of the Flutter pipeline, and will thus receive a timing-dependent sub-sequence of the snapshots that represent the interaction with the future.

A side-effect of this is that providing a new but already-completed future to a FutureBuilder will result in a single frame in the ConnectionState.waiting state. This is because there is no way to synchronously determine that a Future has already completed.

Builder contract

For a future that completes successfully with data, assuming initialData is null, the builder will be called with either both or only the latter of the following snapshots:

• AsyncSnapshot<String>.withData(ConnectionState.waiting, null)
• AsyncSnapshot<String>.withData(ConnectionState.done, 'some data')

If that same future instead completed with an error, the builder would be called with either both or only the latter of:

• AsyncSnapshot<String>.withData(ConnectionState.waiting, null)
• AsyncSnapshot<String>.withError(ConnectionState.done, 'some error', someStackTrace)

The initial snapshot data can be controlled by specifying initialData. You would use this facility to ensure that if the builder is invoked before the future completes, the snapshot carries data of your choice rather than the default null value.

The data and error fields of the snapshot change only as the connection state field transitions from waiting to done, and they will be retained when changing the FutureBuilder configuration to another future. If the old future has already completed successfully with data as above, changing configuration to a new future results in snapshot pairs of the form:

• AsyncSnapshot<String>.withData(ConnectionState.none, 'data of first future')
• AsyncSnapshot<String>.withData(ConnectionState.waiting, 'data of second future')

In general, the latter will be produced only when the new future is non-null, and the former only when the old future is non-null.

A FutureBuilder behaves identically to a StreamBuilder configured with future?.asStream(), except that snapshots with ConnectionState.active may appear for the latter, depending on how the stream is implemented.