canvas-receiver/ts/app.ts

class CanvasReceiverController {
    private canvasDiv: HTMLDivElement;
    private canvas!: HTMLCanvasElement;
    private ctx!: CanvasRenderingContext2D | null;
    private ws: WebSocket;
    private coordinates?: Coordinates;

    /**
     * Represents whether the app is waiting for the first coordinates of a draw path. If lineTo
     * is called with the first pair of coordinates, every path will be connected. Therefore this
     * variable is needed to prevent drawing a line to the first pair of coordinates.
     */
    private firstCoordinates: boolean = false;

    constructor() {
        this.canvasDiv = document.querySelector('#canvas') as HTMLDivElement;

        this.createCanvas();

        this.ws = new WebSocket('ws://localhost:8081', 'json');

        this.ws.onmessage = message => this.processMessage(message);
        this.ws.onopen = () => this.ws.send('RECEIVER');
    }

    private createCanvas() {
        this.canvas = document.createElement('canvas');
        this.canvasDiv.appendChild(this.canvas);
        this.canvas.width = this.canvas.offsetWidth;
        this.canvas.height = this.canvas.offsetHeight;
        if (!this.canvas.getContext) {
            alert('Canvas API unavailable, drawing will not work!');
        } else {
            this.ctx = this.canvas.getContext('2d');
        }
    }

    processMessage(message: MessageEvent<any>) {
        switch (message.data) {
            case 'CLEAR':
                this.clear();
                break;
            case 'START':
                this.startDrawing();
                break;
            case 'STOP':
                this.stopDrawing();
                break;
            default:
                this.draw(JSON.parse(message.data));
                break;
        }
    }

    startDrawing() {
        console.log('Received START command');
        if (!this.ctx) {
            return;
        }
        this.firstCoordinates = true;
    }

    stopDrawing() {
        if (!this.ctx) {
            return;
        }
    }

    private clear() {
        this.canvasDiv.removeChild(this.canvas);
        this.createCanvas();
    }

    draw(coordinates: Coordinates) {
        if (!this.ctx) {
            return;
        }

        this.ctx.beginPath();

        if (this.coordinates) {
            this.ctx.moveTo(this.coordinates.x, this.coordinates.y);
        }

        if (this.firstCoordinates) {
            this.firstCoordinates = false;
        } else {
            this.ctx.lineTo(coordinates.x, coordinates.y);
        }

        this.ctx.stroke();
        this.coordinates = coordinates;
    }
}


class Coordinates {
    public x: number;
    public y: number;

    constructor(x: number, y: number) {
        this.x = x;
        this.y = y;
    }
}

const app = new CanvasReceiverController();

export { app };