Memoize prop transformation functions and specify their arguments

Meant to be similar to reselect's `createSelector`, but instead of
reaching into redux `state`, it reaches into the component's `this`.

components/AddTodo.js

@@ -4,7 +4,7 @@ export default class AddTodo extends Component {
   render() {
     return (
       <div>
-        <input type='text' ref='input' />
+        Add: <input type='text' ref='input' />
         <button onClick={(e) => this.handleClick(e)}>
           Add
         </button>

containers/App.js

@@ -4,20 +4,51 @@ import { addTodo, completeTodo, setVisibilityFilter, changeTheme, VisibilityFilt
 import AddTodo from '../components/AddTodo'
 import TodoList from '../components/TodoList'
 import Footer from '../components/Footer'
+import { memoize, createMemoizedFunction } from '../memoize'
+
+function selectTodos(todos, filter) {
+  console.log("Recalculating selectTodos");
+  switch (filter) {
+    case VisibilityFilters.SHOW_ALL:
+      return todos
+    case VisibilityFilters.SHOW_COMPLETED:
+      return todos.filter(todo => todo.completed)
+    case VisibilityFilters.SHOW_ACTIVE:
+      return todos.filter(todo => !todo.completed)
+  }
+}
+
+function selectMatchingTodos(todos, search) {
+  console.log("Recalculating matchingTodos");
+  return todos.filter((todo) => { return todo.text.search(search) >= 0; });
+}
 
 class App extends Component {
+  constructor(props, context) {
+    super(props, context);
+    this.state = { search: '' };
+  }
+
+  visibleTodos = createMemoizedFunction(() => [this.props.todos, this.props.visibilityFilter], selectTodos);
+  matchingTodos = createMemoizedFunction(() => [this.visibleTodos(), this.state.search], selectMatchingTodos);
+
+  updateSearch = function(e) {
+    this.setState({ search: e.target.value });
+  }
+
   render() {
     console.log(this.props);
     // Injected by connect() call:
-    const { dispatch, visibleTodos, visibilityFilter, currentTheme } = this.props
+    const { dispatch, visibilityFilter, currentTheme } = this.props
     return (
       <div className={currentTheme}>
+        Search: <input type="text" onChange={this.updateSearch.bind(this)}/><br/>
         <AddTodo
           onAddClick={text =>
             dispatch(addTodo(text))
           } />
         <TodoList
-          todos={visibleTodos}
+          todos={this.matchingTodos()}
           onTodoClick={index =>
             dispatch(completeTodo(index))
           } />
@@ -33,7 +64,7 @@ class App extends Component {
 }
 
 App.propTypes = {
-  visibleTodos: PropTypes.arrayOf(PropTypes.shape({
+  todos: PropTypes.arrayOf(PropTypes.shape({
     text: PropTypes.string.isRequired,
     completed: PropTypes.bool.isRequired
   })),
@@ -44,22 +75,11 @@ App.propTypes = {
   ]).isRequired
 }
 
-function selectTodos(todos, filter) {
-  switch (filter) {
-    case VisibilityFilters.SHOW_ALL:
-      return todos
-    case VisibilityFilters.SHOW_COMPLETED:
-      return todos.filter(todo => todo.completed)
-    case VisibilityFilters.SHOW_ACTIVE:
-      return todos.filter(todo => !todo.completed)
-  }
-}
-
 // Which props do we want to inject, given the global state?
 // Note: use https://github.com/faassen/reselect for better performance.
 function select(state) {
   return {
-    visibleTodos: selectTodos(state.todos, state.visibilityFilter),
+    todos: state.todos,
     visibilityFilter: state.visibilityFilter,
     currentTheme: state.currentTheme
   }

memoize.js

@@ -0,0 +1,29 @@
+// This memoizes a function by remembering its last args and its last result
+// and returning the last result if the args are the same as the last call.
+export function memoize(func) {
+  var lastArgs = null;
+  var lastResult;
+
+  function argsDifferent(args) {
+    return lastArgs === null ||
+           lastArgs.length != args.length ||
+           args.some((arg, idx) => { return arg !== lastArgs[idx] });
+  }
+
+  return function(...args) {
+    if(argsDifferent(args)) {
+      lastArgs = args;
+      lastResult = func(...args);
+    }
+    return lastResult
+  }
+}
+
+// This memoizes `func` and returns a function that calls
+// the memoized `func` with the arguments returned by `argFunc`
+export function createMemoizedFunction(argFunc, func) {
+  var memoized = memoize(func);
+  return function() {
+    return memoized(...argFunc());
+  }
+}