refactor: simpler way to check canGoBack (#54)

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

@@ -96,10 +96,6 @@ export default function MockRouter(options: DefaultRouterOptions) {
     shouldActionChangeFocus() {
       return false;
     },
-
-    canGoBack() {
-      return false;
-    },
   };
 
   return router;

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

@@ -102,14 +102,68 @@ it("returns correct value for canGoBack when it's not overridden", () => {
   expect(result).toEqual(false);
 });
 
-it("returns correct value for canGoBack when it's overridden", () => {
+it(`returns false for canGoBack when current router doesn't handle GO_BACK`, () => {
+  function TestRouter(options: DefaultRouterOptions) {
+    const CurrentMockRouter = MockRouter(options);
+    const ChildRouter: Router<NavigationState, MockActions> = {
+      ...CurrentMockRouter,
+
+      getStateForAction(state, action) {
+        if (action.type === 'GO_BACK') {
+          return null;
+        }
+
+        return CurrentMockRouter.getStateForAction(state, action);
+      },
+    };
+    return ChildRouter;
+  }
+
+  const TestNavigator = (props: any) => {
+    const { state, descriptors } = useNavigationBuilder<
+      NavigationState,
+      any,
+      any
+    >(TestRouter, 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>
+      <TestNavigator>
+        <Screen name="baz" component={TestScreen} />
+      </TestNavigator>
+    </NavigationContainer>
+  );
+
+  render(root).update(root);
+
+  expect(result).toBe(false);
+});
+
+it('returns true for canGoBack when current router handles GO_BACK', () => {
   function ParentRouter(options: DefaultRouterOptions) {
     const CurrentMockRouter = MockRouter(options);
     const ChildRouter: Router<NavigationState, MockActions> = {
       ...CurrentMockRouter,
 
-      canGoBack() {
-        return true;
+      getStateForAction(state, action) {
+        if (action.type === 'GO_BACK') {
+          return state;
+        }
+
+        return CurrentMockRouter.getStateForAction(state, action);
       },
     };
     return ChildRouter;
@@ -160,17 +214,21 @@ it("returns correct value for canGoBack when it's overridden", () => {
 
   render(root).update(root);
 
-  expect(result).toEqual(true);
+  expect(result).toBe(true);
 });
 
-it('returns correct value for canGoBack when parent router overrides it', () => {
+it('returns true for canGoBack when parent router handles GO_BACK', () => {
   function OverrodeRouter(options: DefaultRouterOptions) {
     const CurrentMockRouter = MockRouter(options);
     const ChildRouter: Router<NavigationState, MockActions> = {
       ...CurrentMockRouter,
 
-      canGoBack() {
-        return true;
+      getStateForAction(state, action) {
+        if (action.type === 'GO_BACK') {
+          return state;
+        }
+
+        return CurrentMockRouter.getStateForAction(state, action);
       },
     };
     return ChildRouter;
@@ -228,7 +286,7 @@ it('returns correct value for canGoBack when parent router overrides it', () =>
 
   render(root).update(root);
 
-  expect(result).toEqual(false);
+  expect(result).toBe(false);
 });
 
 it('sets options with options prop as a fuction', () => {

packages/core/src/types.tsx

@@ -169,13 +169,6 @@ 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.
    */

packages/core/src/useNavigationHelpers.tsx

@@ -68,7 +68,8 @@ export default function useNavigationHelpers<
         ? parentNavigationHelpers.isFocused
         : () => true,
       canGoBack: () =>
-        router.canGoBack(getState()) ||
+        router.getStateForAction(getState(), BaseActions.goBack() as Action) !==
+          null ||
         (parentNavigationHelpers && parentNavigationHelpers.canGoBack()) ||
         false,
     } as NavigationHelpers<ParamListBase> &

packages/routers/src/StackRouter.tsx

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

packages/routers/src/TabRouter.tsx

@@ -211,10 +211,6 @@ export default function TabRouter({
       return action.type === 'NAVIGATE';
     },
 
-    canGoBack(state) {
-      return router.getStateForAction(state, { type: 'GO_BACK' }) !== null;
-    },
-
     actionCreators: TabActions,
   };