Wrong way (Part 1): Let's fetching data from an api and having 2 coun…

…ters for the todos: ongoing and done

src/Todo/TodoList.js

@@ -1,33 +1,73 @@
-import {useState} from 'react';
+import {useEffect, useState} from 'react';
 import {v4} from 'uuid';
 
 function createTodo(title, done) {
   return {id: v4(), title, done};
 }
 
-const TodoList = () => {
-  const [todos, setTodoList] = useState(() => [
+const fetchTodosFromApi = async () => {
+  return [
     createTodo('Frozen yoghurt', false),
     createTodo('Ice cream sandwich', false),
     createTodo('Eclair', false),
     createTodo('Cupcake', false),
     createTodo('Gingerbread', false),
-  ]);
+  ];
+};
+
+const TodoList = () => {
+  const [todos, setTodoList] = useState([]);
+  useEffect(function callApi() {
+    //1. Imagine that we are calling an api to get todos 
+    fetchTodosFromApi().then(list => setTodoList(list));
+  }, []);
+
+  //2. And we have a state for 2 counter
+  const [ongoingCount, setOngoingCount] = useState(0);
+  const [doneCount, setDoneCount] = useState(0);
+
+  // 3. We will update the counters when the list changes
+  useEffect(function refreshCounters() {
+    setDoneCount(todos.filter(t => t.done).length);
+    setOngoingCount(todos.filter(t => !t.done).length);
+  }, [todos]);
+
+  //4. See how many render ?
+  console.log('render with', {total: todos.length, ongoingCount, doneCount});
+  // On first load it renders 3 times:
+  // render with { total: 0, ongoingCount: 0, doneCount: 0 } ==> Before "callApi"
+  // render with { total: 5, ongoingCount: 0, doneCount: 0 } ==> After "callApi"
+  // render with { total: 5, ongoingCount: 5, doneCount: 0 } ==> After "refreshCounters". React is smart an only re-render when the value really changes. Here "doneCount" is "updated" from 0 to 0, so it does not re-render
+
+  // When I set one of the todos as "done", it re-renders only 2 times (but 3 things changed: todos, ongoingCount and doneCount, do you know why?)
+  // render with { total: 5, ongoingCount: 5, doneCount: 0 } ==> when setTodoList is called
+  // render with { total: 5, ongoingCount: 4, doneCount: 1 } ==> when setDoneCount/setOngoingCount is called (after useEffect)
+
+  //2 conclusions: 
+  // - We have too many render!
+  // - React seems to wait the end of the useEffect to re-render the component (smart!)
 
   const addEmptyTodo = () => setTodoList([createTodo('Relax! Edition will come...', false), ...todos]);
+  const markAsDone = (index) => setTodoList([...todos.slice(0, index), {
+    ...todos[index],
+    done: true,
+  }, ...todos.slice(index + 1)]);
+
 
   return <>
     <table>
       <thead>
       <tr>
-        <th rowSpan={2} align="left">My todos <button onClick={addEmptyTodo}>Add</button></th>
+        <th rowSpan={2} align="left">My todos ({ongoingCount} ongoing /{doneCount} done)
+          <button onClick={addEmptyTodo}>Add</button>
+        </th>
       </tr>
       </thead>
       <tbody>
-      {todos.map((todo) => (
+      {todos.map((todo, index) => (
         <tr key={todo.id}>
           <td>{todo.title}</td>
-          <td><input type="checkbox" value="1" checked={todo.done} disabled={true}/></td>
+          <td><input type="checkbox" value="1" checked={todo.done} onChange={() => markAsDone(index)}/></td>
         </tr>
       ))}
       </tbody>