proxy_pattern_example.ts

interface Extent {
    width: number;
    height: number;
}

abstract class Graphic {
    protected fileName: string;
    abstract getExtent(): Extent;
    abstract draw(): void;
}

class Img extends Graphic {
    private extent: Extent;
    constructor(fileName, width, height) {
        super();
        this.fileName = fileName;
        this.extent = {
            width,
            height
        }
        console.log('- Load image data');
    }
    draw(): void {
        console.log('- Draw image');
    }
    getExtent(): Extent {
        console.log('- Get extent from real image meta');
        return this.extent;
    }
}

class ImgProxy extends Graphic {
    private imgInstance: Img;
    private extent: Extent;
    constructor(fileName, width, height) {
        super();
        this.fileName = fileName;
        this.extent = {
            width,
            height
        }
        console.log(`- Doesn\'t have image ${this.fileName} currently`);
    }

    draw() {
        if (!this.imgInstance) {
            console.log('- Initiate img instance');
            this.imgInstance = new Img(this.fileName, this.extent.width, this.extent.height);
        }
        this.imgInstance.draw();
    }

    getExtent(): Extent {
        if (this.imgInstance) {
            return this.imgInstance.getExtent();
        }
        console.log('- Get extent from proxy cache');
        return this.extent;
    }
}

const img = new ImgProxy('a.jpg', 100, 60);
img.getExtent();
img.draw();
img.getExtent();

/**
    - Doesn't have image a.jpg currently
    - Get extent from proxy cache
    - Initiate img instance
    - Load image data
    - Draw image
    - Get extent from real image meta
 */