Tag Archives: calc

Creating a Car Game in React – Part 1 – Drawing and Moving

Since I started looking at React, I’ve wondered whether it would be possible to create a multi-user game. The game would look a little like a Spectrum game that I used to play called: Trans-Am. I’m guessing most people reading this are not going to be old enough to remember this. Essentially, it marks the peak of car game development, and everything has been down hill ever since.

If you have no idea what I’m talking about then there’s a demo of the game here.

I’m not going to try and emulate this exactly, I thought I’d use it as a basis to make a multi-player car game.

The GitHub repository for this post can be found here.

Create a React Application

We’ll start by creating a new React Application (see here for details):

Now we have the application, we’ll need some game assets. If you want to use the same assets as me then feel free to pull my repository. However, at this stage, all you’ll need is a square box and a green screen.

Game Layout

The next stage is to design the game layout; because this is React, we’ll start with App.js. We’ll delegate all of our game logic to a component called Game:

import React from 'react';
import './App.css';
import Game from './Components/Game';
function App() {
	return (
		<div className="App">
			<Game />
		</div>
		);
}
export default App;

If you want to see, comprehensively what Game.Jsx looks like, then have a look at the latest version on GitHub. However, some of the highlights are the render method:

render() { 
	return <div onKeyDown={this.onKeyDown} tabIndex="0">
		<Background backgroundImage={backgroundImg}
		windowWidth={this.state.windowWidth} windowHeight={this.state.windowHeight} /> 
	<Car carImage={carImg} centreX={this.state.playerX} 
		centreY={this.state.playerY} width={this.playerWidth}
		height={this.playerHeight} /> 
	</div>
}

This will probably change as to game progresses, but at the minute, it just renders to two constituent components. We’re also responding to KeyDown here, so let’s have a look at onKeyDown:

onKeyDown(e) {
	switch (e.which) {
		case 37: // Left
			this.playerMove(this.state.playerX - this.SPEED, this.state.playerY); 
			break;
		case 38: // Up
			this.playerMove(this.state.playerX, this.state.playerY - this.SPEED);
			break;
		case 39: // Right
			this.playerMove(this.state.playerX + this.SPEED, this.state.playerY); 
			break;
		case 40: // Down
			this.playerMove(this.state.playerX, this.state.playerY + this.SPEED);
			break;
		default:
			break;
	}
} 

playerMove(x, y) {
	this.setState({
		playerX: x,
		playerY: y
	}); 
}

We’re storing the players position in state; as I detailed here, this enables us to update the state and have React update the screen as it detects a change in the Virtual DOM.

Game Components

In an effort to stay as close as possible to React’s preferred architecture, the components of the game (the background and the cars for now) will be, well, components. Background is easy:

import React from 'react';
function Background(props) {
	const bgStyle = { 
		width: `calc(${props.windowWidth}px)`, 
		height: `calc(${props.windowHeight}px)`, 
		top: 0,
		left: 0,
		position: 'absolute' 
	};
	return (
		<img src={props.backgroundImage} style={bgStyle} />
	);
}
export default Background;

We’re basically just displaying an image here. One thing that’s worth noting is that the windowWidth and windowHeight are properties, not state. They do exist as state in the Game component and, when they change, are updated there, and so updated here. The React guys call this Lifting State.

The car component is exactly the same idea:

import React from 'react';
function Car(props) {
	const left = Math.round(props.centreX - (props.width / 2));
	const top = Math.round(props.centreY - (props.height / 2));
	const carStyle = { 
		width: `calc(${props.width}px)`, 
		height: `calc(${props.height}px)`, 
		top: `calc(${top}px)`,
		left: `calc(${left}px)`, 
		position: 'absolute',
		zIndex: 1 
	};
	return (
		<img src={props.carImage} style={carStyle} />
	);
}
export default Car;

There are a number of advantages to this idea of maintaining the state in a higher component; for example, this way, you can share a single state between components; however, the biggest advantage for us is that, while the components are, effectively, intelligent sprites, you can easily create an “EnemyCar” version of the Car component.

It’s worth bearing in mind that, because the position of the car doesn’t exist in this component as state, we wouldn’t be able to change it here, even if we wanted to. The strategy to get around this is to have an update function passed in as a property (effectively a function pointer that you can call from within the child component).

In the next post, I’m going to update the movement so it’s a little more car-like, and introduce some obstacles.

References

https://reactjs.org/docs/components-and-props.html

https://stackoverflow.com/questions/43503964/onkeydown-event-not-working-on-divs-in-react

https://stackoverflow.com/questions/37440408/how-to-detect-esc-key-press-in-react-and-how-to-handle-it/46123962

https://reactjs.org/docs/lifting-state-up.html

https://stackoverflow.com/questions/36862334/get-viewport-window-height-in-reactjs

A C# Developer’s Guide to: ReactJS – Part 2 – Moving Controls

Following on from my previous post, I’m going to extend our ReactJS application by adding some boxes, and allowing the user to re-arrange them on the screen.

Concepts

There are two key concepts to consider when working with React, and we’ll cover one of them in this post; that is: state.

React has a special property of each component known as state. If you use that to bind any of the UI to, then React will refresh that component when the state changes.

Moving a UI Element

Okay – that sounds great, but how would we do this in practice?

Imagine that you have a HTML box drawn on the screen; you might have something like this:

<div style="height:100px; width:200px; background:#0000FF" />

We can draw a box. If you use this code, then your box will be on the top left of the screen; so we can tell it not to be by specifying the left and top; let’s try defining a CSS style:

<div style="left:10px; top:20px;height:100px; width:200px; background:#0000FF" />

With React, we can set those values to a value derived from the state; for example:

render() {
    const top = this.state.newY;
    const left = this.state.newX;

    const myStyle = {
        height: '100px',
        width: '200px', 
        top: `calc(${top}px)`,
        left: `calc(${left}px)`, 
        borderStyle: 'solid',
        borderColor: 'blue', 
        position: 'absolute',
        zIndex: 1,
        background: 'blue'
    };

    return ( 
        <div> 
            <div style={myStyle}>source</div>
        </div>
    );
}

What this means is that every time I change the state values, the values in the style will update.

To illustrate this, if we write a React application like this (only the timeout function is new):

render() {
    const top = this.state.newY;
    const left = this.state.newX;
    const myStyle = {
        height: '100px',
        width: '200px', 
        top: `calc(${top}px)`,
        left: `calc(${left}px)`, 
        borderStyle: 'solid',
        borderColor: 'blue', 
        position: 'absolute',
        zIndex: 1,
        background: 'blue'
    };

    setTimeout(() => {
        this.setState({newX: this.state.newX + 1, newY: this.state.newY + 1});
    }, 50);

    return ( 
        <div> 
            <div style={myStyle}></div>
        </div>
    );
}

We can make out box move diagonally across the screen:

Screenshot included because you couldn’t possibly imaging what a blue rectangle looks like.

Start the app using VS Code

A quick note on VS Code. If you select Terminal -> New from the menu, you can run the React application directly from the IDE; and the best part is that if there’s something already running on the port, VS Code will just give you a new one: