async_builder - Improved Future and Stream builder for Flutter.

This package provides AsyncBuilder, a widget similar to StreamBuilder / FutureBuilder which is designed to reduce boilerplate and improve error handling.

It also provides InitBuilder, which makes it easier to start async tasks safely.

How to use

1. Add to dependencies

  async_builder: ^1.2.0

2. Import

import 'package:async_builder/async_builder.dart';
import 'package:async_builder/init_builder.dart';

AsyncBuilder Examples


  future: myFuture,
  waiting: (context) => Text('Loading...'),
  builder: (context, value) => Text('$value'),
  error: (context, error, stackTrace) => Text('Error! $error'),


  stream: myStream,
  waiting: (context) => Text('Loading...'),
  builder: (context, value) => Text('$value'),
  error: (context, error, stackTrace) => Text('Error! $error'),
  closed: (context, value) => Text('$value (closed)'),

Note that you cannot provide both a stream and future.

AsyncBuilder Features

Separate builders

Instead of a single builder, AsyncBuilder allows you to specify separate builders depending on the state of the asynchronous operation:

  • waiting(context) - Called when no events have fired yet.
  • builder(context, value) - Required. Called when a value is available.
  • error(context, error, stackTrace) - Called if there was an error.
  • closed(context, value) - Called if the stream was closed.

If any of these are not provided then it defaults to calling builder (potentially with a null value).

Error handling

AsyncBuilder does not silently ignore errors by default.

If an exception occurs and error is provided, the widget will rebuild and call that builder. Otherwise, if error is not provided and silent not true then the exception and stack trace will be printed to console (default behavior).

Initial value

If initial is provided, it is used in place of the value before one is available.

RxDart ValueStream

If stream is a ValueStream (BehaviorSubject) holding an existing value, that value will be available immediately on first build.

Stream pausing

The StreamSubscription for this widget can be paused with the pause parameter, this is useful if you want to notify the upstream StreamController that you don't need updates.


InitBuilder is a widget that initializes a value only when its configuration changes, this is extremely useful because it allows you to safely start async tasks without making a whole new StatefulWidget.

The basic usage of this widget is to make a separate function outside of build that starts the task and then pass it to InitBuilder, for example:

static Future<int> getNumber() async => ...;

build(context) => InitBuilder<int>(
  getter: getNumber,
  builder: (context, future) => AsyncBuilder<int>(
    future: future,
    builder: (context, value) => Text('$value'),

In this case, getNumber is only ever called on the first build.

You may also want to pass arguments to the getter, for example to query shared preferences:

final String prefsKey;

build(context) => InitBuilder.arg<String, String>(
  getter: sharedPrefs.getString,
  arg: prefsKey,
  builder: (context, future) => AsyncBuilder<String>(
    future: future,
    builder: (context, value) => Text('$value'),

The alternate constructors InitBuilder.arg to InitBuilder.arg7 can be used to pass arguments to the getter, these will re-initialize the value if and only if either getter or the arguments change.