Javascript上下文问题

Question

我刚开始使用Javascript，第二天就开始了，但我遇到了一个小问题。我正在使用Phaser API并且我正在使用状态，基本上我的问题在于：

        game.input.keyboard.addCallbacks(this, null, null, this.keyPress);

我在下面的代码处于相同的状态

keyPress: function(char) {
    console.log("pressed");
    if(this.selected == 1) {
        this.userFieldText.text += char;
    } else if(this.selected == 2) {
        this.passFieldText.text += char;
    }
}

控制台中没有错误，但从不调用该方法。

文档here

---

全班

var loginButton;

var userField;
var userFieldText;

var passField;
var passFieldText;

var selected;

var stateLogin = {

    preload: function() {
        game.stage.backgroundColor = '#000';
        game.load.image('text-field', 'assets/ui/text-box.png');
        game.load.image('login-button', 'assets/ui/login-button.png');
    },

    create: function() {
        this.selected = 0;

        var userLabel = game.add.text(200, 200, 'Enter a username: ');
        userLabel.fontSize = 20;
        userLabel.fill='#fff';

        var passLabel = game.add.text(200, 250, 'Enter a password: ');
        passLabel.fontSize = 20;
        passLabel.fill='#fff';

        var userField = game.add.sprite(350, 195, 'text-field');
        userField.inputEnabled = true;
        userField.events.onInputUp.add(function() { this.selectField('user') }, this);

        var passField = game.add.sprite(350, 245, 'text-field');
        passField.inputEnabled = true;
        passField.events.onInputUp.add(function() { this.selectField('pass') }, this);

        this.userFieldText = game.add.text(360, 200, '');
        this.userFieldText.fontSize = 16;
        this.userFieldText.fill = "#fff";

        this.passFieldText = game.add.text(360, 250, '');
        this.passFieldText.fontSize = 16;
        this.passFieldText.fill = "#fff";

        game.input.keyboard.addCallbacks(this, null, null, this.keyPress);

    },

    update: function() {
        console.log('Selected: ' + this.selected + ' Text: ' + this.userFieldText.text);
    },

    selectField: function(field) {
        if(field == 'user') {
            this.selected = 1;
        } else if(field == 'pass') {
            this.selected = 2;
        }
    },

    keyPress: function(char) {
        console.log("pressed");
        if(this.selected == 1) {
            this.userFieldText.text += char;
        } else if(this.selected == 2) {
            this.passFieldText.text += char;
        }
    }

};