feat: add a useIsFocused hook to get focus state (#52)

README.md

@@ -228,7 +228,17 @@ Screens cannot emit events as there is no `emit` method on a screen's `navigatio
 
 If you don't need to get notified of focus change, but just need to check if the screen is currently focused in a callback, you can use the `navigation.isFocused()` method which returns a boolean. Note that it's not safe to use this in `render`. Only use it in callbacks, event listeners etc.
 
-## Side-effects in focused screen
+## Additional utilities
+
+### Access navigation anywhere
+
+Passing the `navigation` prop down can be tedious. The library exports a `useNavigation` hook which can access the `navigation` prop from the parent screen:
+
+```js
+const navigation = useNavigation();
+```
+
+### Side-effects in focused screen
 
 Sometimes we want to run side-effects when a screen is focused. A side effect may involve things like adding an event listener, fetching data, updating document title, etc. While this can be achieved using `focus` and `blur` events, it's not very ergonomic.
 
@@ -257,12 +267,12 @@ The `useFocusEffect` is analogous to React's `useEffect` hook. The only differen
 
 **NOTE:** To avoid the running the effect too often, it's important to wrap the callback in `useCallback` before passing it to `useFocusEffect` as shown in the example.
 
-## Access navigation anywhere
+### Render based on focus state
 
-Passing the `navigation` prop down can be tedious. The library exports a `useNavigation` hook which can access the `navigation` prop from the parent screen.
+We might want to render different content based on the current focus state of the screen. The library exports a `useIsFocused` hook to make this easier:
 
 ```js
-const navigation = useNavigation();
+const isFocused = useIsFocused();
 ```
 
 ## Type-checking

packages/core/package.json

@@ -4,15 +4,16 @@
   "main": "src/index",
   "license": "MIT",
   "dependencies": {
-    "shortid": "^2.2.14"
+    "shortid": "^2.2.14",
+    "use-subscription": "^1.0.0"
   },
   "devDependencies": {
     "@babel/core": "^7.4.5",
+    "@types/react": "^16.8.19",
+    "@types/shortid": "^0.0.29",
     "react": "^16.8.3",
     "react-native-testing-library": "^1.9.1",
-    "react-test-renderer": "^16.8.3",
-    "@types/react": "^16.8.19",
-    "@types/shortid": "^0.0.29"
+    "react-test-renderer": "^16.8.3"
   },
   "peerDependencies": {
     "react": "^16.8.3"

packages/core/src/__tests__/useIsFocused.test.tsx

@@ -0,0 +1,49 @@
+import * as React from 'react';
+import { render, act } from 'react-native-testing-library';
+import useNavigationBuilder from '../useNavigationBuilder';
+import useIsFocused from '../useIsFocused';
+import NavigationContainer from '../NavigationContainer';
+import Screen from '../Screen';
+import MockRouter from './__fixtures__/MockRouter';
+
+it('renders correct focus state', () => {
+  const TestNavigator = (props: any): any => {
+    const { state, descriptors } = useNavigationBuilder(MockRouter, props);
+
+    return state.routes.map(route => descriptors[route.key].render());
+  };
+
+  const Test = () => {
+    const isFocused = useIsFocused();
+
+    return (
+      <React.Fragment>{isFocused ? 'focused' : 'unfocused'}</React.Fragment>
+    );
+  };
+
+  const navigation = React.createRef<any>();
+
+  const root = render(
+    <NavigationContainer ref={navigation}>
+      <TestNavigator>
+        <Screen name="first">{() => null}</Screen>
+        <Screen name="second" component={Test} />
+        <Screen name="third">{() => null}</Screen>
+      </TestNavigator>
+    </NavigationContainer>
+  );
+
+  expect(root).toMatchInlineSnapshot(`"unfocused"`);
+
+  act(() => navigation.current.navigate('second'));
+
+  expect(root).toMatchInlineSnapshot(`"focused"`);
+
+  act(() => navigation.current.navigate('third'));
+
+  expect(root).toMatchInlineSnapshot(`"unfocused"`);
+
+  act(() => navigation.current.navigate('second'));
+
+  expect(root).toMatchInlineSnapshot(`"focused"`);
+});

packages/core/src/index.tsx

@@ -9,5 +9,6 @@ export { default as createNavigator } from './createNavigator';
 export { default as useNavigationBuilder } from './useNavigationBuilder';
 export { default as useNavigation } from './useNavigation';
 export { default as useFocusEffect } from './useFocusEffect';
+export { default as useIsFocused } from './useIsFocused';
 
 export * from './types';

packages/core/src/types.tsx

@@ -317,6 +317,7 @@ type NavigationHelpersCommon<
    * Check if the screen is focused. The method returns `true` if focused, `false` otherwise.
    * Note that this method doesn't re-render screen when the focus changes. So don't use it in `render`.
    * To get notified of focus changes, use `addListener('focus', cb)` and `addListener('blur', cb)`.
+   * To conditionally render content based on focus state, use the `useIsFocused` hook.
    */
   isFocused(): boolean;
 

packages/core/src/useFocusEffect.tsx

@@ -4,7 +4,7 @@ import useNavigation from './useNavigation';
 type EffectCallback = (() => void) | (() => () => void);
 
 /**
- * Hook to an effect in a focused screen, similar to `React.useEffect`.
+ * Hook to run an effect in a focused screen, similar to `React.useEffect`.
  * This can be used to perform side-effects such as fetching data or subscribing to events.
  * The passed callback should be wrapped in `React.useCallback` to avoid running the effect too often.
  *

packages/core/src/useIsFocused.tsx

@@ -0,0 +1,35 @@
+import * as React from 'react';
+import { useSubscription } from 'use-subscription';
+import useNavigation from './useNavigation';
+
+/**
+ * Hook to get the current focus state of the screen. Returns a `true` if screen is focused, otherwise `false`.
+ * This can be used if a component needs to render something based on the focus state.
+ * It uses `use-subscription` under the hood for safer use in concurrent mode.
+ */
+export default function useIsFocused(): boolean {
+  const navigation = useNavigation();
+  const getCurrentValue = React.useCallback(navigation.isFocused, [navigation]);
+  const subscribe = React.useCallback(
+    (callback: (value: boolean) => void) => {
+      const unsubscribeFocus = navigation.addListener('focus', () =>
+        callback(true)
+      );
+
+      const unsubscribeBlur = navigation.addListener('blur', () =>
+        callback(false)
+      );
+
+      return () => {
+        unsubscribeFocus();
+        unsubscribeBlur();
+      };
+    },
+    [navigation]
+  );
+
+  return useSubscription({
+    getCurrentValue,
+    subscribe,
+  });
+}

typings/use-subscription.d.ts

@@ -0,0 +1,6 @@
+declare module 'use-subscription' {
+  export function useSubscription<T>(options: {
+    getCurrentValue: () => T;
+    subscribe: (callback: (value: T) => void) => () => void;
+  }): T;
+}

yarn.lock

@@ -13961,6 +13961,11 @@ urlgrey@^0.4.4:
   resolved "https://registry.yarnpkg.com/urlgrey/-/urlgrey-0.4.4.tgz#892fe95960805e85519f1cd4389f2cb4cbb7652f"
   integrity sha1-iS/pWWCAXoVRnxzUOJ8stMu3ZS8=
 
+use-subscription@^1.0.0:
+  version "1.0.0"
+  resolved "https://registry.yarnpkg.com/use-subscription/-/use-subscription-1.0.0.tgz#25ed2161f75e9f6bd8c5c4acfe6087bfebfbfef4"
+  integrity sha512-PkDL9KSMN01nJYfa5c8O7Qbs7lmpzirfRWhWfIQN053hbIj5s1o5L7hrTzCfCCO2FsN2bKrU7ciRxxRcinmxAA==
+
 use@^3.1.0:
   version "3.1.1"
   resolved "https://registry.yarnpkg.com/use/-/use-3.1.1.tgz#d50c8cac79a19fbc20f2911f56eb973f4e10070f"