Revert "Revert "Add getParam navigation helper (#3510)" (#3532)"

This reverts commit 5febb81a1c.

packages/react-navigation/src/__tests__/addNavigationHelpers-test.js

@@ -76,4 +76,43 @@ describe('addNavigationHelpers', () => {
     });
     expect(mockedDispatch.mock.calls.length).toBe(1);
   });
+
+  it('handles GetParams action', () => {
+    const mockedDispatch = jest
+      .fn(() => false)
+      .mockImplementationOnce(() => true);
+    expect(
+      addNavigationHelpers({
+        state: { key: 'B', routeName: 'Settings', params: { name: 'Peter' } },
+        dispatch: mockedDispatch,
+        addListener: dummyEventSubscriber,
+      }).getParam('name', 'Brent')
+    ).toEqual('Peter');
+  });
+
+  it('handles GetParams action with default param', () => {
+    const mockedDispatch = jest
+      .fn(() => false)
+      .mockImplementationOnce(() => true);
+    expect(
+      addNavigationHelpers({
+        state: { key: 'B', routeName: 'Settings' },
+        dispatch: mockedDispatch,
+        addListener: dummyEventSubscriber,
+      }).getParam('name', 'Brent')
+    ).toEqual('Brent');
+  });
+
+  it('handles GetParams action with param value as null', () => {
+    const mockedDispatch = jest
+      .fn(() => false)
+      .mockImplementationOnce(() => true);
+    expect(
+      addNavigationHelpers({
+        state: { key: 'B', routeName: 'Settings', params: { name: null } },
+        dispatch: mockedDispatch,
+        addListener: dummyEventSubscriber,
+      }).getParam('name')
+    ).toEqual(null);
+  });
 });

packages/react-navigation/src/addNavigationHelpers.js

@@ -61,6 +61,16 @@ export default function(navigation) {
       return navigation.dispatch(NavigationActions.setParams({ params, key }));
     },
 
+    getParam: (paramName, defaultValue) => {
+      const params = navigation.state.params;
+
+      if (params && paramName in params) {
+        return params[paramName];
+      }
+
+      return defaultValue;
+    },
+
     push: (routeName, params, action) =>
       navigation.dispatch(
         NavigationActions.push({ routeName, params, action })

packages/react-navigation/src/navigators/__tests__/__snapshots__/StackNavigator-test.js.snap

@@ -98,6 +98,7 @@ exports[`StackNavigator applies correct values when headerRight is present 1`] =
         Object {
           "addListener": [Function],
           "dispatch": [Function],
+          "getParam": [Function],
           "goBack": [Function],
           "navigate": [Function],
           "pop": [Function],
@@ -337,6 +338,7 @@ exports[`StackNavigator renders successfully 1`] = `
         Object {
           "addListener": [Function],
           "dispatch": [Function],
+          "getParam": [Function],
           "goBack": [Function],
           "navigate": [Function],
           "pop": [Function],