React Using Callbacks to Pass Information Lab

Objectives

1. Practice using state, props, and events
2. Practice using callbacks to effect change up and down the component tree

Introduction

In React, props are used to pass information down the component tree, from parents to children. In order to propagate information in the opposite direction, we can use callback functions that are passed from parent components to children.

This allows the callback to be owned by a different component than the one invoking it. Once invoked, the callback can effect change in the component that owns it, instead of the component that called it.

For this lab, we have three components:

└── Matrix
    ├── ColorSelector
    └── Cell (x1000)

We would like to implement the following behavior: when a Cell is clicked, its background color becomes the current selected color. With our ColorSelector component providing the user interface to select a specific color, we need some way for Cell to know about the currently selected color. To do this, we will have the parent component, Matrix, keep track of the current selected color.

Because passing information between children components is taboo in React, (i.e. passing information between ColorSelector and a Cell), we need to make sure that the following is implemented:

• ColorSelector has a way to set some 'selected color' in Matrix when a user selects a color
• Cell has a way to know what the current selected color is when it is clicked

Using callbacks, we can do both of these things pretty easily!

Deliverables

Matrix

• provide a way for Matrix to keep track of the selected color (think state!)
• write a method that takes in a single argument of a hex color string (i.e. '#fff') and sets the selected color to that
• consider what should be done with that method once it is written. Who needs it? How can we get it to them? Use the skills you already have with React to solve this

ColorSelector

• update makeColorSwatches to provide the <div>s it is rendering with some way to fire off that method we wrote for Matrix (hint: consider using an event listener and controlling the argument when an event handler is fired)

Cell

• find a way to access the currently selected color from Matrix's state without passing the value explicitly as a prop. (Consider passing a method from Matrix to Cell instead. You will want to create an additional method in Matrix to do this.)
• implement a handleClick method that that updates the Cell's state with the current selected color

Once Finished

Assert the following behavior:

You should see 100x100 cells rendering with a color selection <div> at the top of the screen. When any of the color swatches in the color selection <div> are clicked, subsequently selected cells should change to that color.

Resources

View React Using Callbacks to Pass Information Lab on Learn.co and start learning to code for free.