feat: add canGoBack (facebook#50)

packages/core/src/BaseRouter.tsx

@@ -78,6 +78,10 @@ const BaseRouter = {
   shouldActionChangeFocus(action: CommonAction) {
     return action.type === 'NAVIGATE';
   },
+
+  canGoBack() {
+    return false;
+  },
 };
 
 export default BaseRouter;

packages/core/src/NavigationContainer.tsx

@@ -88,9 +88,7 @@ const Container = React.forwardRef(function NavigationContainer(
 
   const dispatch = (action: NavigationAction) =>
     context.performTransaction(() => {
-      for (let i = 0; i < actionListeners.length; i++) {
-        actionListeners[i](action, undefined, null);
-      }
+      actionListeners[0](action, undefined, null);
     });
 
   React.useImperativeHandle(ref, () => ({

packages/core/src/__tests__/__fixtures__/MockRouter.tsx

@@ -97,7 +97,9 @@ export default function MockRouter(options: DefaultRouterOptions) {
       return false;
     },
 
-    actionCreators: {},
+    canGoBack() {
+      return false;
+    },
   };
 
   return router;

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

@@ -3,8 +3,11 @@ import { render, act } from 'react-native-testing-library';
 import useNavigationBuilder from '../useNavigationBuilder';
 import NavigationContainer from '../NavigationContainer';
 import Screen from '../Screen';
-import MockRouter, { MockRouterKey } from './__fixtures__/MockRouter';
-import { NavigationState } from '../types';
+import MockRouter, {
+  MockActions,
+  MockRouterKey,
+} from './__fixtures__/MockRouter';
+import { DefaultRouterOptions, NavigationState, Router } from '../types';
 
 jest.useFakeTimers();
 
@@ -54,6 +57,180 @@ it('sets options with options prop as an object', () => {
       `);
 });
 
+it("returns correct value for canGoBack when it's not overridden", () => {
+  const TestNavigator = (props: any) => {
+    const { state, descriptors } = useNavigationBuilder<
+      NavigationState,
+      { title?: string },
+      any
+    >(MockRouter, props);
+    const { render, options } = descriptors[state.routes[state.index].key];
+
+    return (
+      <main>
+        <h1>{options.title}</h1>
+        <div>{render()}</div>
+      </main>
+    );
+  };
+
+  let result = true;
+
+  const TestScreen = ({ navigation }: any): any => {
+    React.useEffect(() => {
+      result = navigation.canGoBack();
+    });
+
+    return null;
+  };
+
+  const root = (
+    <NavigationContainer>
+      <TestNavigator>
+        <Screen
+          name="foo"
+          component={TestScreen}
+          options={{ title: 'Hello world' }}
+        />
+        <Screen name="bar" component={jest.fn()} />
+      </TestNavigator>
+    </NavigationContainer>
+  );
+
+  render(root).update(root);
+
+  expect(result).toEqual(false);
+});
+
+it("returns correct value for canGoBack when it's overridden", () => {
+  function ParentRouter(options: DefaultRouterOptions) {
+    const CurrentMockRouter = MockRouter(options);
+    const ChildRouter: Router<NavigationState, MockActions> = {
+      ...CurrentMockRouter,
+
+      canGoBack() {
+        return true;
+      },
+    };
+    return ChildRouter;
+  }
+
+  const ParentNavigator = (props: any) => {
+    const { state, descriptors } = useNavigationBuilder<
+      NavigationState,
+      { title?: string },
+      any
+    >(ParentRouter, props);
+    return descriptors[state.routes[state.index].key].render();
+  };
+
+  const ChildNavigator = (props: any) => {
+    const { state, descriptors } = useNavigationBuilder<
+      NavigationState,
+      { title?: string },
+      any
+    >(MockRouter, props);
+
+    return descriptors[state.routes[state.index].key].render();
+  };
+
+  let result = false;
+
+  const TestScreen = ({ navigation }: any): any => {
+    React.useEffect(() => {
+      result = navigation.canGoBack();
+    });
+
+    return null;
+  };
+
+  const root = (
+    <NavigationContainer>
+      <ParentNavigator>
+        <Screen name="baz">
+          {() => (
+            <ChildNavigator>
+              <Screen name="qux" component={TestScreen} />
+            </ChildNavigator>
+          )}
+        </Screen>
+      </ParentNavigator>
+    </NavigationContainer>
+  );
+
+  render(root).update(root);
+
+  expect(result).toEqual(true);
+});
+
+it('returns correct value for canGoBack when parent router overrides it', () => {
+  function OverrodeRouter(options: DefaultRouterOptions) {
+    const CurrentMockRouter = MockRouter(options);
+    const ChildRouter: Router<NavigationState, MockActions> = {
+      ...CurrentMockRouter,
+
+      canGoBack() {
+        return true;
+      },
+    };
+    return ChildRouter;
+  }
+
+  const OverrodeNavigator = (props: any) => {
+    const { state, descriptors } = useNavigationBuilder<
+      NavigationState,
+      { title?: string },
+      any
+    >(OverrodeRouter, props);
+    return descriptors[state.routes[state.index].key].render();
+  };
+
+  const TestNavigator = (props: any) => {
+    const { state, descriptors } = useNavigationBuilder<
+      NavigationState,
+      { title?: string },
+      any
+    >(MockRouter, props);
+
+    return descriptors[state.routes[state.index].key].render();
+  };
+
+  let result = true;
+
+  const TestScreen = ({ navigation }: any): any => {
+    React.useEffect(() => {
+      result = navigation.canGoBack();
+    });
+
+    return null;
+  };
+
+  const root = (
+    <NavigationContainer>
+      <TestNavigator>
+        <Screen name="baz">
+          {() => (
+            <TestNavigator>
+              <Screen name="qux" component={TestScreen} />
+            </TestNavigator>
+          )}
+        </Screen>
+        <Screen name="qux">
+          {() => (
+            <OverrodeNavigator>
+              <Screen name="qux" component={() => null} />
+            </OverrodeNavigator>
+          )}
+        </Screen>
+      </TestNavigator>
+    </NavigationContainer>
+  );
+
+  render(root).update(root);
+
+  expect(result).toEqual(false);
+});
+
 it('sets options with options prop as a fuction', () => {
   const TestNavigator = (props: any) => {
     const { state, descriptors } = useNavigationBuilder<

packages/core/src/types.tsx

@@ -154,6 +154,13 @@ export type Router<
    */
   shouldActionChangeFocus(action: NavigationAction): boolean;
 
+  /**
+   * Whether the back action will be handled by navigation
+   *
+   * @param state State object to check.
+   */
+  canGoBack(state: State): boolean;
+
   /**
    * Action creators for the router.
    */
@@ -264,6 +271,12 @@ type NavigationHelpersCommon<
    * To get notified of focus changes, use `addListener('focus', cb)` and `addListener('blur', cb)`.
    */
   isFocused(): boolean;
+
+  /**
+   * Check if dispatching back action will be handled by navigation.
+   * Note that this method doesn't re-render screen when the result changes. So don't use it in `render`.
+   */
+  canGoBack(): boolean;
 } & PrivateValueStore<ParamList, keyof ParamList, {}>;
 
 export type NavigationHelpers<

packages/core/src/useNavigationBuilder.tsx

@@ -198,6 +198,7 @@ export default function useNavigationBuilder<
     getState,
     setState,
     emitter,
+    router,
     actionCreators: router.actionCreators,
   });
 

packages/core/src/useNavigationHelpers.tsx

@@ -10,6 +10,7 @@ import {
   NavigationState,
   ActionCreators,
   ParamListBase,
+  Router,
 } from './types';
 
 type Options<Action extends NavigationAction> = {
@@ -21,6 +22,7 @@ type Options<Action extends NavigationAction> = {
   setState: (state: NavigationState) => void;
   actionCreators?: ActionCreators<Action>;
   emitter: NavigationEventEmitter;
+  router: Router<NavigationState, Action>;
 };
 
 export default function useNavigationHelpers<Action extends NavigationAction>({
@@ -29,6 +31,7 @@ export default function useNavigationHelpers<Action extends NavigationAction>({
   setState,
   actionCreators,
   emitter,
+  router,
 }: Options<Action>) {
   const parentNavigationHelpers = React.useContext(NavigationContext);
   const { performTransaction } = React.useContext(NavigationStateContext);
@@ -68,14 +71,19 @@ export default function useNavigationHelpers<Action extends NavigationAction>({
       isFocused: parentNavigationHelpers
         ? parentNavigationHelpers.isFocused
         : () => true,
+      canGoBack: () =>
+        router.canGoBack(getState()) ||
+        (parentNavigationHelpers && parentNavigationHelpers.canGoBack()) ||
+        false,
     };
   }, [
     actionCreators,
+    router,
+    getState,
     parentNavigationHelpers,
     emitter.emit,
     performTransaction,
     setState,
-    getState,
     onAction,
   ]);
 }

packages/routers/src/StackRouter.tsx

@@ -225,6 +225,10 @@ export default function StackRouter(options: StackRouterOptions) {
       }
     },
 
+    canGoBack(state) {
+      return state.routes.length > 1;
+    },
+
     actionCreators: StackActions,
   };