Name UI states, navigate between them, remember where you've been.
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"Name-giving will be the foundation of our science." — Linnaeus

"The winds and waves are always on the side of the ablest navigators." — Gibbon

"Memory is the treasury and guardian of all things." — Cicero

Flow gives names to your Activity's UI states, navigates between them, and remembers where it's been.


Navigate between UI states. Support the back button easily without confusing your users with surprising results.

Remember the UI state, and its history, as you navigate and across configuration changes and process death.

Manage resources with set-up/tear-down hooks invoked for each UI state. UI states can easily share resources, and they'll be disposed when no longer needed.

Manage all types of UIs-- complex master-detail views, multiple layers, and window-based dialogs are all simple to manage.

Using Flow


compile 'com.squareup.flow:flow:1.0.0-alpha3'

Install Flow into your Activity:

public class MainActivity {
  @Override protected void attachBaseContext(Context baseContext) {
    baseContext = Flow.configure(baseContext, this).install();

By default, Flow will take over your Activity's content view. When you start your Activity, you should see a "Hello world" screen. Of course you'll want to change this-- that's covered under Controlling UI below.

Defining UI states with key objects

Your Activity's UI states are represented in Flow by Objects, which Flow refers to as "keys". Keys are typically value objects with just enough information to identify a discrete UI state.

Flow relies on a key's equals and hashCode methods for its identity. Keys should be immutable-- that is, their equals and hashCode methods should always behave the same.

To give an idea of what keys might look like, here are some examples:

public enum TabKey {

public final class HomeKey extends flow.ClassKey {

public final class ArticleKey {
  public final String articleId;

  public ArticleKey(String articleId) {
    this.articleId = articleId;

  public boolean equals(Object o) {
    return o instanceof ArticleKey
        && articleId.equals(((ArticleKey) o).articleId);
  public int hashCode() {
    return articleId.hashCode();

See the Sample Projects below for more example keys.

Navigation and History

Flow offers simple commands for navigating within your app.

Flow#goBack() -- Goes back to the previous key. Think "back button".

Flow#set(key) -- Goes to the requested key. Goes back or forward depending on whether the key is already in the History.

Flow also lets you rewrite history safely and easily.

Flow#setHistory(history, direction) -- Change history to whatever you want.

See the Flow class for other convenient operators.

As you navigate the app, Flow keeps track of where you've been. And Flow makes it easy to save view state (and any other state you wish) so that when your users go back to a place they've been before, it's just as they left it.

Controlling UI

Navigation only counts if it changes UI state. Because every app has different needs, Flow lets you plug in your own logic for responding to navigation and updating your UI.

See the Basic Sample, Tree Sample, and MultiKey Sample below for examples.

Managing resources

Your app requires different resources when it's in different states; sometimes those resources are shared between states. Flow makes it easy to associate resources with keys so they're set up when needed and torn down (only) when they're not anymore.

See the Tree Sample for an example.

Surviving configuration changes and process death

Android is a hostile environment. One of its greatest challenges is that your Activity or even your process can be destroyed and recreated under a variety of circumstances. Flow makes it easy to weather the storm, by automatically remembering your app's state and its history.

You supply the serialization for your keys, and Flow does the rest. Flow automatically saves and restores your History (including any state you've saved), taking care of all of the Android lifecycle events so you don't have to worry about them.

Sample projects


Copyright 2013 Square, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.