代理

//Subject: a pointer to a function plays the virtual draw()
typedef struct Graphic {
    const char *fileName;
    void (*draw)(struct Graphic *self);
} Graphic;

//RealSubject: it is costly to create
typedef struct {
    Graphic base;
} Image;

//Request()
void imageDraw(Graphic *self) {
    printf("draw %s\n", self->fileName);
}

Image *imageNew(const char *fileName) {
    printf("load %s\n", fileName);
    Image *image = malloc(sizeof *image);
    *image = (Image){{fileName, imageDraw}};
    return image;
}

//Proxy: the same Subject, the real image is made on the first draw()
typedef struct {
    Graphic base;
    Image *image;
} ImageProxy;

Image *getImage(ImageProxy *proxy) {
    if (proxy->image == nullptr) {
        proxy->image = imageNew(proxy->base.fileName);
    }
    return proxy->image;
}

void proxyDraw(Graphic *self) {
    Graphic *image = &getImage((ImageProxy *)self)->base;
    image->draw(image);
}

//Client
ImageProxy proxy = {{"1.png", proxyDraw}, nullptr};
Graphic *graphic = &proxy.base;
//operation without creating a RealSubject
const char *fileName = graphic->fileName;
//forwarded to the RealSubject
graphic->draw(graphic);
//printed: load 1.png
//         draw 1.png
graphic->draw(graphic);
//printed: draw 1.png

printf("fileName: %s\n", fileName);
free(proxy.image);