我有一个想要在事件发生时运行函数的类.每当函数调用时,该类实例的函数就不会运行.

class Player {
    constructor() {
        window.addEventListener('keypress', this.key_press);
        this.keys_pressed = 0;
    }

    key_press(key_press) {
        if(key_press.key == "w") {
            console.log(key_press);
            this.keys_pressed += 1;
        }
        console.log(this.keys_pressed);
    }
}

每当调用this.key_press时,它将注销NaN.似乎该类的方法没有在运行,而是在运行copy(？).我也尝试过在key_press()中运行另一个实例函数,但它表示该函数未定义.

任何帮助表示赞赏.

解决方法:

将事件侦听器添加到窗口或DOM的任何元素时,此值指向该元素而不是Player实例.

因此,由于窗口不具有keys_pressed属性,而您将得到NaN,并且keys_pressed = 1被视为keys_pressed = undefined 1,即NaN.

为了解决这个问题,我们需要显式绑定到Player实例：

const input = document.querySelector("#input");
class Player {
    constructor() {
        input.addEventListener('keypress', this.key_press.bind(this));
        this.keys_pressed = 0;
    }

    key_press(key_press) {
        if(key_press.key == "w") {
            this.keys_pressed += 1;
        }
        console.log(this.keys_pressed);
    }
}
let p = new Player();

Type here: <input type="text" id="input">

我们还可以使用箭头(=>)函数从当前上下文中捕获this并将其指向Player对象的当前实例：

const input = document.querySelector("#input");
class Player {
    constructor() {
        input.addEventListener('keypress', (event) => this.key_press(event));
        this.keys_pressed = 0;
    }

    key_press(key_press) {
        if(key_press.key == "w") {
            this.keys_pressed += 1;
        }
        console.log(this.keys_pressed);
    }
}
let p = new Player();

Type here: <input type="text" id="input">