demo

Creating a State Driven UI

In React data travels two directions, top down in the form of state propagating throughout controls, and bottom up, as interacting with the UI flows back up to modify the state. When writing an application it's often helpful to think of these two directions as separate parts of the development process.

demo

Step #3 of Thinking in React suggests finding the "minimal set of mutable state" that your application requires. So in this demo we are going to add that "minimal state" to our application and drive our UI off of that data. With that done the next step will be to create ways to modify that state, which will in turn cascade down through our UI. This reconciliation process, figuring out what in your UI needs to change based on changing state, is what React excels in.

Adding State to App

For our minimal state, we're going to include just two keys: todos and filter. We don't need to worry about a remaining value because we can calculate that by looking at the number of unchecked todos.

So here is our full constructor

constructor(props) {
  super(props);
  this.state = {
    todos: {
      '04': {
        label: 'Todo 4',
        completed: true
      },
      '03': {
        label: 'Todo 3',
        completed: false
      },
      '02': {
        label: 'Todo 2',
        completed: false
      },
      '01': {
        label: 'Todo 1',
        completed: false
      }
    },
    filter: 'all'
  };
}

You could use an array to represent your todos. Array manipulation can be easier in some cases, but this object approach simplifies other functionality and will ultimately be more performant.

Passing State Through to UI

To avoid reaching into state over and over, we once again use deconstruction to pull out the pieces we need.

const { filter, todos } = this.state;

Now we can pass filter and todos into our components.

return (
  <div>
    <TodoHeader filter={filter} />
    <TodoList todos={todos} filter={filter} />
    <TodoFooter todos={todos} />
  </div>
);

State Driven TodoList

I've already pulled out our props into filter and todos variables, and written a bit of JS that will return an array of filtered todo ids. We'll be using that filtered array to render out todo items.

{
  filteredTodos.map(id => <TodoListItem key={id} id={id} {...todos[id]} />);
}

• map: A JavaScript map takes in an array (filteredTodos) and transforms it into a new array (our rendered TodoListItems)
• key: We use the id from the filterTodos array as the list key to help React track each item as state changes.
• id: The key is not actually passed into the component, so we pass the key in as id as well. This will help us out later.
• todos[id]: Lastly we use the id to grab the todo from our todos object, then use the spread operator to pass through the todo's label and completed values.

  This spread operator is the same as saying label={todos[id].label} completed={todos[id].completed}. Pretty obvious why spread is so handy!

State Driven and Stateful Header

Within the header we've got a situation where we not only want to pass filter state down to it, but we also want to maintain state within the control. Fortunately, this is no problem at all for React. First off let's deal with the incoming state.

Conditional ClassNames

In CSS based styling, visual states are applied by adding and removing classes. We can use the filter value to conditionally add a class thereby lighting up the correct filter button.

<nav className="filter">
  <button className={filter == 'all' ? 'selected' : ''}>all</button>
  <button className={filter == 'active' ? 'selected' : ''}>active</button>
  <button className={filter == 'completed' ? 'selected' : ''}>completed</button>
</nav>

Ternary operators are very popular in React code as each expression could be a string for a className, or even a JSX element.

Creating a Controlled Input

In tradition HTML forms users interact with the form, and on submit, those values are captured and transmitted. Those are called uncontrolled inputs. A controlled input is one whos value is defined by state, and interaction with that input updates state with each keystroke. This round trip process might sound inefficient, but in reality it has little to no impact, and it enables some advanced form functionality.

To create a controlled component, we need two things, which our demo already provides:

1. A state variable to hold the input's value

this.state = { labelInput: '' };

2. A function to update that value

_onChange = evt => {
  this.setState({ labelInput: evt.target.value });
};

With those two pieces in place, we can update our uncontrolled input to being controlled.

<input value={this.state.labelInput} onChange={this._onChange} className="textfield" placeholder="add todo" />