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This stage we'll build the stopwatch component! We'll cover working with React's useEffect hook and learn to use conditional rendering to update your UI based on the current state.
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[MUSIC]
0:00
In the previous stage, we added two
new components to our Scoreboard app,
0:04
the Stats component and
the AddPlayerForm component.
0:09
We passed the player state down
to the Stats component, but
0:13
before we could display
the total number of players,
0:17
and the total score,
we needed to calculate their values.
0:20
We used the player's array and
0:25
the reduce method to calculate the total
number of players and the total score.
0:27
After working on the Stats component,
we created our first controlled component.
0:33
As you may recall, a controlled component
is a component that renders a form
0:39
element whose value is
controlled by React.
0:44
Our AddPlayerForm component renders
a form element that allows users to
0:48
add a new player to the scoreboard.
0:53
AddPlayerForm is a controlled component
because a value entered in the text
0:57
field is controlled by React through
a state that we named value.
1:02
We used the state hook to add
value to the AddPlayerForm,
1:07
and learned that when working with hooks,
1:11
you'll often use a named import to
input hooks at the top of the file.
1:14
To relay the new players name back
up to the main App component,
1:19
we created an event handler in App.js and
1:24
passed the event handler down to
the AddPlayerForm component.
1:27
The event handler is a function that
takes, as a parameter, the value state.
1:32
It then uses the spread operator to
add a new player to the player state
1:37
without modifying the original players.
1:42
In this stage, we're gonna build
a stopwatch into our Scoreboard app.
1:45
It will be a stateful component
that counts in seconds and
1:49
allows users to start,
stop, and reset the time,
1:54
which could be useful in games for
limiting players time length.
1:57
We'll create a button to start and
stop the timer.
2:04
If the time is stopped and started again,
2:08
it should continue counting
from where it left off.
2:11
Hitting the reset button should
return the timer to zero.
2:15
We'll start by building the stopwatch
component and its elements.
2:19
The first bit of functionality we'll
implement is the start and stop button.
2:24
The button will display start when
the stopwatch isn't running, and
2:29
stop when it is.
2:33
We'll use a ternary operator to
conditionally render the button's name
2:35
based on the state.
2:40
So we'll create a state that equals
true when the stopwatch is running and
2:41
false when it's not.
2:46
Lastly, we'll tackle
the functionality of the stopwatch.
2:49
In React, there are times we want
some side effects to happen after our
2:53
component has rendered.
2:57
Fetching data from an API,
manually changing the DOM, and
2:59
setting up a timer are all
examples of side effects.
3:03
React provides a mechanism called effect
hook that lets you execute an arbitrary
3:08
function, which it calls side effect
inside your function components.
3:14
In our Scoreboard app, the side effect
will be the stopwatch functionality.
3:20
When the user clicks the start button,
3:26
the effect hook will run our side
effect that will create a timer
3:28
to continuously add the value one
to our stopwatch time every second.
3:32
And when the user clicks the stop button,
the effect hook will pause the timer.
3:38
Often side effects create resources
such as a subscription, or
3:43
a timer ID that need to be cleaned
up before the next effect is called,
3:47
or when the component leaves the screen.
3:52
Cleaning up these resources helps
prevent memory leaks that can
3:55
negatively impact performance.
4:00
The effect hook can be
intimidating at first, but
4:02
don't worry I'll break it down and
walk you through the whole process.
4:05
Let's get started.
4:09
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