fix: drop isFirstRouteInParent method (#145)

The `isFirstRouteInParent` method was added to determine whether the back button should be shown in the header for stack navigator or not.
This was mainly due to the API of the old version of stack whose public API of header didn't have all required info to determine whether it should be shown.
It was probably a mistake to add it, because this method doesn't look at history and so pretty much useless for other navigators which aren't stack.

In the new stack, the header receives a `previous` prop and the `headerLeft` option receives a `canGoBack` prop which can be used for this purpose.
It's also possible to check if a route is the first by doing `navigation.dangerouslyGetState().routes[0].key === route.key`.

So it's time to drop this method.

packages/compat/src/createCompatNavigationProp.tsx

@@ -173,6 +173,12 @@ export default function createCompatNavigationProp<
 
       return defaultValue;
     },
+    isFirstRouteInParent(): boolean {
+      const { routes } = navigation.dangerouslyGetState();
+
+      // @ts-ignore
+      return routes[0].key === state.key;
+    },
     dangerouslyGetParent() {
       const parent = navigation.dangerouslyGetParent();
 

packages/compat/src/types.tsx

@@ -25,6 +25,7 @@ export type CompatNavigationProp<
       paramName: T,
       defaultValue?: ParamList[RouteName][T]
     ): ParamList[RouteName][T];
+    isFirstRouteInParent(): boolean;
     dangerouslyGetParent<
       T = NavigationProp<ParamListBase> | undefined
     >(): T extends NavigationProp<ParamListBase>

packages/core/src/types.tsx

@@ -394,13 +394,6 @@ export type NavigationProp<
    */
   setOptions(options: Partial<ScreenOptions>): void;
 
-  /**
-   * Check if the screen is the first route in the navigator.
-   * This method returns `true` if the index of the route is `0`, `false` otherwise.
-   * It can be useful to decide whether to display a back button in a stack.
-   */
-  isFirstRouteInParent(): boolean;
-
   /**
    * Returns the parent navigator, if any. Reason why the function is called
    * dangerouslyGetParent is to warn developers against overusing it to eg. get parent

packages/core/src/useNavigationCache.tsx

@@ -66,11 +66,9 @@ export default function useNavigationCache<
   cache.current = state.routes.reduce<NavigationCache<State, ScreenOptions>>(
     (acc, route, index) => {
       const previous = cache.current[route.key];
-      const isFirst = route.key === state.routes[0].key;
 
-      if (previous && previous.isFirstRouteInParent() === isFirst) {
-        // If a cached navigation object already exists and has same `isFirstRouteInParent`, reuse it
-        // This method could return different result if the index of the route changes somehow
+      if (previous) {
+        // If a cached navigation object already exists, reuse it
         acc[route.key] = previous;
       } else {
         // eslint-disable-next-line @typescript-eslint/no-unused-vars
@@ -121,7 +119,6 @@ export default function useNavigationCache<
             // This makes sure that we return the focus state in the whole tree, not just this navigator
             return navigation ? navigation.isFocused() : true;
           },
-          isFirstRouteInParent: () => isFirst,
         };
       }