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Stopwatch State

3:06 with Guil Hernandez

The stopwatch will have two main states visible to the user. It will be either in a running state, or a stopped state. The buttons on the interface should change based on the running state. We'll start by initializing state in the Stopwatch component.

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Conditional rendering with if...else:

render() {
  let button;
  if (this.state.isRunning) {
    button = <button>Stop</button>;
  } else {
    button = <button>Start</button>;
  }

  return (
    <div className="stopwatch">
      <h2>Stopwatch</h2>
      <span className="stopwatch-time">...</span>
      { button }
      <button onClick={this.handleReset}>Reset</button>
    </div>
  );
}

The stopwatch will have two main states visible to the user. 0:00

It will be either in a running state or a stop state. 0:03

The buttons on the interface should change based on the running state. 0:06

For example, 0:09

start changes to stop when we click the start to stopwatch and vice versa. 0:10

Let's build this dynamic UI first. 0:14

We'll start by initializing state in the stopwatch component. 0:16

Add a state property on the Stopwatch class. 0:19

Inside the state object, 0:22

we'll add a property named isRunning which will be a Boolean. 0:24

It will be true if the Stopwatch is running, false if not. 0:28

We'll initialize it to false. 0:32

Now we're going to do some conditional rendering in our buttons. 0:34

We need to render a start or stop button based on the value of isRunning. 0:37

You could do this a number of different ways, for 0:42

example with an if, else statement in the render method. 0:44

You can see a method of this in the teacher's notes. 0:47

I'll teach you how to write this logic in a more concise way 0:49

using the JavaScript conditional or ternary operator. 0:52

The ternary expression will read the value of isRunning and 0:55

set the button's text to stop if it's true and start if it's false. 0:59

In the start button let's replace the text with curly braces 1:04

to write a jsx expression. 1:09

Inside the curly braces type this.state.isRunning followed 1:11

by a question mark, the string stop, a colon, and the string start. 1:16

So the expression is saying if is running is true, display the text Stop, 1:22

otherwise, display the text Start. 1:26

If you're still not quite sure how ternary operators work, 1:29

be sure to watch the Treehouse videos posted in the teacher's notes. 1:33

Over in the scoreboard, we see the text start, and 1:36

that's correct because isRunning is false by default. 1:39

Next, lets add an onClick event to our start and stop button. 1:42

The onClick event will call a function named handleStopwatch, 1:46

that will start or stop the timer based on the isRunning state. 1:50

So lets pass it, this.handleStopwatch. 1:54

Now we'll write this function below the state object, 1:59

create an error function named handleStopwatch. 2:03

Inside the function, 2:11

we'll update the isRunning state with this.setState passing it an object. 2:13

And we're going to toggle the isRunning state by setting it to the opposite 2:18

of its current true or false values using the logical not operator. 2:24

So let's add the isRunning property and set its value to not this.state.isRunning. 2:30

All right, let's have a look at our stopwatch in the browser. 2:39

If I click Start, the button changes to Stop, and clicking Stop changes it back. 2:41

You can also see the isRunning state changing from true to false or 2:48

false to true in React Dev tools when you select the stopwatch component. 2:52

Great, our stopwatch is taking shape. 2:57

Next we'll need to make the timer actually tick in seconds and 2:59

create the event handler for the reset button. 3:03

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Stopwatch State

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