Add `NavigationStateUtils.back()` and `NavigationStateUtils.forward()`.

Summary: This makes it easy to build Pager.

Reviewed By: ericvicenti

Differential Revision: D3479979

fbshipit-source-id: 71c0536eb190e8ad64feea99e4bda5e2d28801d2

Libraries/NavigationExperimental/NavigationStateUtils.js

@@ -111,6 +111,24 @@ function jumpTo(state: NavigationState, key: string): NavigationState {
   return jumpToIndex(state, index);
 }
 
+/**
+ * Sets the focused route to the previous route.
+ */
+function back(state: NavigationState): NavigationState {
+  const index = state.index - 1;
+  const route = state.routes[index];
+  return route ? jumpToIndex(state, index) : state;
+}
+
+/**
+ * Sets the focused route to the next route.
+ */
+function forward(state: NavigationState): NavigationState {
+  const index = state.index + 1;
+  const route = state.routes[index];
+  return route ? jumpToIndex(state, index) : state;
+}
+
 /**
  * Replace a route by a key.
  * Note that this moves the index to the positon to where the new route in the
@@ -190,6 +208,8 @@ function reset(
 }
 
 const NavigationStateUtils = {
+  back,
+  forward,
   get: get,
   has,
   indexOf,

Libraries/NavigationExperimental/__tests__/NavigationStateUtils-test.js

@@ -83,6 +83,20 @@ describe('NavigationStateUtils', () => {
     expect(() => NavigationStateUtils.jumpTo(state, 'c')).toThrow();
   });
 
+  it('move backwards', () => {
+    const state = {index: 1, routes: [{key: 'a'}, {key: 'b'}]};
+    const newState = {index: 0, routes: [{key: 'a'}, {key: 'b'}]};
+    expect(NavigationStateUtils.back(state)).toEqual(newState);
+    expect(NavigationStateUtils.back(newState)).toBe(newState);
+  });
+
+  it('move forwards', () => {
+    const state = {index: 0, routes: [{key: 'a'}, {key: 'b'}]};
+    const newState = {index: 1, routes: [{key: 'a'}, {key: 'b'}]};
+    expect(NavigationStateUtils.forward(state)).toEqual(newState);
+    expect(NavigationStateUtils.forward(newState)).toBe(newState);
+  });
+
   // Replace
   it('Replaces by key', () => {
     const state = {index: 0, routes: [{key: 'a'}, {key: 'b'}]};