Kill NavigationReducers

Summary:
For navigation actions at high level, reducers from NavigationReducers does not
know anything about the app-specific state thus people won't use these reducers.
Instead, people should build their own reducers.

There are a lot of good libraries available that help people to reducing things if that's
what they really need.

At the low level, for navigation state changes that don't involve app-specific state,
`NavigationStateUtils` should server that kind of need.

`NavigationReducers` serves little benefit cause it does not know the app state, it does
not know how to traverse the navigation states which can be a tree, a list or a map.

That said, we hold no interest in owning in the core navigation library.

Reviewed By: ericvicenti

Differential Revision: D3372910

fbshipit-source-id: 797382b46e7d64b7ad578b51dd37e2b941faa83d

Examples/UIExplorer/NavigationExperimental/NavigationCardStack-example.js

@@ -80,7 +80,7 @@ class YourApplication extends React.Component {
     switch (type) {
       case 'push':
         // push a new route.
-        const route = {key: Date.now()};
+        const route = {key: 'route-' + Date.now()};
         navigationState = NavigationStateUtils.push(navigationState, route);
         break;
 

Examples/UIExplorer/UIExplorerNavigationReducer.js

@@ -22,27 +22,72 @@
  */
 'use strict';
 
-const React = require('react');
 const ReactNative = require('react-native');
 // $FlowFixMe : This is a platform-forked component, and flow seems to only run on iOS?
 const UIExplorerList = require('./UIExplorerList');
+
 const {
   NavigationExperimental,
 } = ReactNative;
+
+
 const {
-  Reducer: NavigationReducer,
+  StateUtils: NavigationStateUtils,
 } = NavigationExperimental;
-const StackReducer = NavigationReducer.StackReducer;
 
 import type {NavigationState} from 'NavigationTypeDefinition';
 
-import type {UIExplorerAction} from './UIExplorerActions';
-
 export type UIExplorerNavigationState = {
   externalExample: ?string;
   stack: NavigationState;
 };
 
+const defaultGetReducerForState = (initialState) => (state) => state || initialState;
+
+function StackReducer({initialState, getReducerForState, getPushedReducerForAction}: any): Function {
+  const getReducerForStateWithDefault = getReducerForState || defaultGetReducerForState;
+  return function (lastState: ?NavigationState, action: any): NavigationState {
+    if (!lastState) {
+      return initialState;
+    }
+    const lastParentState = NavigationStateUtils.getParent(lastState);
+    if (!lastParentState) {
+      return lastState;
+    }
+
+    const activeSubState = lastParentState.routes[lastParentState.index];
+    const activeSubReducer = getReducerForStateWithDefault(activeSubState);
+    const nextActiveState = activeSubReducer(activeSubState, action);
+    if (nextActiveState !== activeSubState) {
+      const nextChildren = [...lastParentState.routes];
+      nextChildren[lastParentState.index] = nextActiveState;
+      return {
+        ...lastParentState,
+        routes: nextChildren,
+      };
+    }
+
+    const subReducerToPush = getPushedReducerForAction(action, lastParentState);
+    if (subReducerToPush) {
+      return NavigationStateUtils.push(
+        lastParentState,
+        subReducerToPush(null, action)
+      );
+    }
+
+    switch (action.type) {
+      case 'back':
+      case 'BackAction':
+        if (lastParentState.index === 0 || lastParentState.routes.length === 1) {
+          return lastParentState;
+        }
+        return NavigationStateUtils.pop(lastParentState);
+    }
+
+    return lastParentState;
+  };
+}
+
 const UIExplorerStackReducer = StackReducer({
   getPushedReducerForAction: (action, lastState) => {
     if (action.type === 'UIExplorerExampleAction' && UIExplorerList.Modules[action.openExample]) {
@@ -106,7 +151,7 @@ function UIExplorerNavigationReducer(lastState: ?UIExplorerNavigationState, acti
     return {
       externalExample: null,
       stack: newStack,
-    }
+    };
   }
   return lastState;
 }