react-native-screens/README.md

# react-native-screens

This project aims to expose native navigation container components to React Native. It is not designed to be used as a standalone library but rather as a dependency of a [full-featured navigation library](https://github.com/react-navigation/react-navigation).

## How can I take advantage of that?

Screens are already integrated with the React Native's most popular navigation library [react-navigation](https://github.com/react-navigation/react-navigation) and [Expo](https://expo.io).
Read usage guide depending on if you are [using Expo](#usage-in-expo-with-react-navigation) or [not](#usage-with-react-navigation-without-expo).

## Supported react-native version

Since version 2.0.0 react-native-screens requires RN v0.60+. Check 1.0.0-alpha for Expo support or older versions of React Native.

## Usage with [react-navigation](https://github.com/react-navigation/react-navigation) (without Expo)

Screens support is built into [react-navigation](https://github.com/react-navigation/react-navigation) starting from version [2.14.0](https://github.com/react-navigation/react-navigation/releases/tag/2.14.0) for all the different navigator types (stack, tab, drawer, etc). We plan on adding it to other navigators in near future.

To configure react-navigation to use screens instead of plain RN Views for rendering screen views, follow the steps below:

1. Add this library as a dependency to your project:

```bash
yarn add react-native-screens
```

2.Link native modules this library ships with into your app:

```bash
react-native link react-native-screens
```

> If you are not familiar with the concept of linking libraries [read on here](https://facebook.github.io/react-native/docs/linking-libraries-ios).

3.Enable screens support before any of your navigation screen renders. Add the following code to your main application file (e.g. App.js):

```js
import { enableScreens } from 'react-native-screens';

enableScreens();
```

Note that the above code need to execute before first render of a navigation screen. You can check Example's app [App.js](https://github.com/kmagiera/react-native-screens/blob/master/Example/App.js#L16) file as a reference.

4. Make sure that the version of [react-navigation](https://github.com/react-navigation/react-navigation) you are using is 2.14.0 or higher

5. You are all set 🎉 – when screens are enabled in your application code react-navigation will automatically use them instead of relying on plain React Native Views.

## Usage in Expo with [react-navigation](https://github.com/react-navigation/react-navigation) v1.0.0

Screens support is built into Expo [SDK 30](https://blog.expo.io/expo-sdk-30-0-0-is-now-available-e64d8b1db2a7) and react-navigation starting from [2.14.0](https://github.com/react-navigation/react-navigation/releases/tag/2.14.0). Make sure your app use these versions before you start.

1. Add screens library as dependency to your project – you can skip this step when using snack as the dependency will be imported when you import it in one of the JS files

```bash
yarn add react-native-screens
```

2. Open your App.js file and add the following snippet somewhere near the top of the file (e.g. right after import statements):

```js
import { enableScreens } from 'react-native-screens';

enableScreens();
```

3. That's all 🎉 – enjoy faster navigation in your Expo app. Keep in mind screens are in pretty early phase so please report if you discover some issues.

## Interop with [react-native-navigation](https://github.com/wix/react-native-navigation)

React-native-navigation library already uses native containers for rendering navigation scenes so wrapping these scenes with `<ScreenContainer>` or `<Screen>` component does not provide any benefits. Yet if you would like to build a component that uses screens primitives under the hood (for example a view pager component) it is safe to use `<ScreenContainer>` and `<Screen>` components for that as these work out of the box when rendered on react-native-navigation scenes.

## Using native stack navigator

In order to take advantage of the native stack navigator primitive introduced in version 2.0 you need to use navigator creator function exported by react-native-screens package:

```js
import createNativeStackNavigator from 'react-native-screens/createNativeStackNavigator';
```

Then replace places when you use `createStackNavigator` with `createNativeStackNavigator`. Note that not all the screen customization options are supported. There are some technical limitations for implementing some of the stack header options. Documenting the supported parameters is on an immediate roadmap and will be available soon.

## Interop with other libraries

This library should work out of the box with all existing react-native libraries. If you experience problems with interoperability please [report an issue](https://github.com/kmagiera/react-native-screens/issues).

## Guide for navigation library authors

If you are building navigation library you may want to use react-native-screens to have a control which parts of the react component tree are attached to the native view hierarchy.
To do that react-native-screens provides you with two components documented below:

### `<ScreenContainer/>`

This component is a container for one or more `Screen` components.
It does not accept other component types as direct children.
The role of the container is to control which of its children screens should be attached to the view hierarchy.
It does that by monitoring the `active` property of each of its children.
It is possible to have as many `active` children as you'd like but in order for the component to be the most efficient we should keep the number of active screens to a minimum.
In a case of a stack navigator or tabs navigator we only want to have one active screen (the top most view on a stack or the selected tab).
While transitioning between views we may want to activate a second screen for the duration of the transition, and then go back to just one active screen.

### `<Screen/>`

This component is a container for views we want to display on a navigation screen.
It is designed to only be rendered as a direct child of `ScreenContainer`.
In addition to plain React Native [View props](http://facebook.github.io/react-native/docs/view#props) this component only accepts a single additional property called `active`.
When `active` is set to `0`, the parent container will detach its views from the native view hierarchy.
Otherwise the views will be attached as long as the parent container is attached too.

#### Example

```js
<ScreenContainer>
  <Screen>{tab1}</Screen>
  <Screen active={1}>{tab2}</Screen>
  <Screen>{tab3}</Screen>
</ScreenContainer>
```

### `<ScreenStack>`

Screen stack component expects one or more `Screen` components as direct children and renders them in a platform native stack container (for iOS it is `UINavigationController` and for Android inside `Fragment` container). For `Screen` components placed as children of `ScreenStack` the `active` property is ignored and instead the screen that corresponds to the last child is rendered as active. All type of updates done to the list of children are acceptable, when the top element is exchanged the container will use platform default (unless customized) animation to transition between screens.

`StackScreen` extends the capabilities of `Screen` component to allow additional customizations and to make it possible to handle events such as using hardware back or back gesture to dismiss the top screen. Below is the list of additional properties that can be used for `Screen` component:

#### `onDismissed`

A callback that gets called when the current screen is dismissed by hardware back (on Android) or dismiss gesture (swipe back or down). The callback takes no arguments.

#### `stackAnimation`

Allows for the customization of how the given screen should appear/dissapear when pushed or popped at the top of the stack. The followin values are currently supported:
 - `"default"` – uses a platform default animation
 - `"fade"` – fades screen in or out
 - `"flip"` – flips the screen, requires `stackPresentation: "modal"` (iOS only)
 - `"none"` – the screen appears/dissapears without an animation

 #### `stackPresentation`

Defines how the method that should be used to present the given screen. It is a separate property from `stackAnimation` as the presentation mode can carry additional semantic. The allowed values are:
 - `"push"` – the new screen will be pushed onto a stack which on iOS means that the default animation will be slide from the side, the animation on Android may vary depending on the OS version and theme.
 - `"modal"` – the new screen will be presented modally. In addition this allow for a nested stack to be rendered inside such screens
 - `"transparentModal"` – the new screen will be presented modally but in addition the second to last screen will remain attached to the stack container such that if the top screen is non opaque the content below can still be seen. If `"modal"` is used instead the below screen will get unmounted as soon as the transition ends.

### `<ScreenStackHeaderConfig>`

The config component is expected to be rendered as a direct children of `<Screen>`. It provides an ability to configure native navigation header that gets rendered as a part of native screen stack. The component acts as a "virtual" element that is not directly rendered under `Screen`. You can use its properties to customize platform native header for the parent screen and also render react-native components that you'd like to be displayed inside the header (e.g. in the title are or on the side).

Along with this component properties that can be used to customize header behavior one can also use one or the below component containers to render custom react-native content in different areas of the native header:
 - `ScreenStackHeaderCenterView` – the childern will render in the center of the native navigation bar.
 - `ScreenStackHeaderRightView` – the children will render on the right hand side of the navigation bar (or on the left hand side in case LTR locales are set on the user's device).
 - `ScreenStackHeaderLeftView` – the children will render on the left hand side of the navigation bar (or on the right hand side in case LTR locales are set on the user's device).

Below is a list of properties that can be set with `ScreenStackHeaderConfig` component:


#### `hidden`

When set to `true` the header will be hidden while the parent `Screen` is on the top of the stack. The default value is `false`.

#### `color`

Controls the color of items rendered on the header. This includes back icon, back text (iOS only) and title text. If you want the title to have different color use `titleColor` property.

#### `title`

String that representing screen title that will get rendered in the middle section of the header. On iOS the title is centered on the header while on Android it is aligned to the left and placed next to back button (if one is present).

#### `titleFontFamily`

Customize font family to be used for the title.

#### `titleFontSize`

Customize the size of the font to be used for the title.

#### `titleColor`

Allows for setting text color of the title.

#### `backgroundColor`

Controlls the color of the navigation header.

#### `hideShadow`

Boolean that allows for disabling drop shadow under navigation header. The default value is `true`.

#### `hideBackButton`

If set to `true` the back button will not be rendered as a part of navigation header.

#### `gestureEnabled`

When set to `false` the back swipe gesture will be disabled when the parent `Screen` is on top of the stack. The default value is `true`.

#### `translucent` (iOS only)

When set to `true`, it makes native navigation bar on iOS semi transparent with blur effect. It is a common way of presenting navigation bar introduced in iOS 11. The default value is `false`.

#### `backTitle` (iOS only)

Allows for controlling the string to be rendered next to back button. By default iOS uses the title of the previous screen.

#### `backTitleFontFamily` (iOS only)

Allows for customizing font family to be used for back button title on iOS.

#### `backTitleFontSize` (iOS only)

Allows for customizing font size to be used for back button title on iOS.

#### `largeTitle` (iOS only)

When set to `true` it makes the title display using the large title effect.

#### `largeTitleFontFamily` (iOS only)

Customize font family to be used for the large title.

#### `largeTitleFontSize` (iOS only)

Customize the size of the font to be used for the large title.

## Guide for native component authors

If you are adding a new native component to be used from the React Native app, you may want it to respond to navigation lifecycle events.

Good example is a map component that shows current user location. When component is on the top-most screen it should register for location updates and display users location on the map. But if we navigate away from a screen that has a map, e.g. by pushing new screen on top of it or if it is in one of a tabs and user just switched to the previous app, we may want to stop listening to location updates.

In order to achieve that we need to know at the native component level when our native view goes out of sight. With react-native-screens you can do that in the following way:

### Navigation lifecycle on iOS

In order for your native view on iOS to be notified when its parent navigation container goes into background override `didMoveToWindow` method:

```objective-c
- (void)didMoveToWindow
{
  [super didMoveToWindow];
  BOOL isVisible = self.superview && self.window;
  if (isVisible) {
    // navigation container this view belongs to became visible
  } else {
    // we are in a background
  }
}
```

You can check our example app for a fully functional demo see [RNSSampleLifecycleAwareView.m](https://github.com/kmagiera/react-native-screens/blob/master/Example/ios/ScreensExample/RNSSampleLifecycleAwareView.m) for more details.

### Navigation lifecycle on Android

On Android you can use [LifecycleObserver](https://developer.android.com/reference/android/arch/lifecycle/LifecycleObserver) interface which is a part of Android compat library to make your view handle lifecycle events.
Check [LifecycleAwareView.java](https://github.com/kmagiera/react-native-screens/blob/master/Example/android/app/src/main/java/com/swmansion/rnscreens/example/LifecycleAwareView.java) from our example app for more details on that.

In addition to that you will need to register for receiving these updates. This can be done using [`LifecycleHelper.register`](https://github.com/kmagiera/react-native-screens/blob/master/android/src/main/java/com/swmansion/rnscreens/LifecycleHelper.java#L50).
Remember to call [`LifecycleHelper.unregister`](https://github.com/kmagiera/react-native-screens/blob/master/android/src/main/java/com/swmansion/rnscreens/LifecycleHelper.java#L59) before the view is dropped.
Please refer to [SampleLifecycleAwareViewManager.java](https://github.com/kmagiera/react-native-screens/blob/master/Example/android/app/src/main/java/com/swmansion/rnscreens/example/SampleLifecycleAwareViewManager.java) from our example app to see what are the best ways of using the above methods.

## License

React native screens library is licensed under [The MIT License](LICENSE).

## Credits

This project is supported by amazing people from [Expo.io](https://expo.io) and [Software Mansion](https://swmansion.com)

[![expo](https://avatars2.githubusercontent.com/u/12504344?v=3&s=100 'Expo.io')](https://expo.io)
[![swm](https://avatars1.githubusercontent.com/u/6952717?v=3&s=100 'Software Mansion')](https://swmansion.com)