daggerheart/module/applications/multiclassDialog.mjs

const {HandlebarsApplicationMixin, ApplicationV2} = foundry.applications.api;

export default class DhpMulticlassDialog extends HandlebarsApplicationMixin(ApplicationV2) {
    constructor(actorName, actorClass, resolve){
        super({});

        this.actorName = actorName;
        this.actorClass = actorClass;
        this.resolve = resolve;

        this.classChoices = Array.from(game.items.reduce((acc, x) => {
            if(x.type === 'class' && x.name !== actorClass.name){
                acc.add(x);
            }
           
            return acc;
        }, new Set()));
        this.subclassChoices = [];
        this.domainChoices = [];

        this.data = {
            class: null,
            subclass: null,
            domain: null,
        };
    }

    get title(){
        return `${this.actorName} - Multiclass`; 
    }

    static DEFAULT_OPTIONS = {
        classes: ["daggerheart", "views", "multiclass"],
        position: { width: 600, height: 'auto' },
        actions: {
            selectClass: this.selectClass,
            selectSubclass: this.selectSubclass,
            selectDomain: this.selectDomain,
            finish: this.finish,
        },
    };
      
    static PARTS = {
        form: {
            id: "levelup",
            template: "systems/daggerheart/templates/views/multiclass.hbs"
        }
    }

    async _prepareContext(_options) {
        const context = await super._prepareContext(_options);
        context.classChoices = this.classChoices;
        context.subclassChoices = this.subclassChoices;
        context.domainChoices = this.domainChoices;
        context.disabledFinish = !this.data.class || !this.data.subclass || !this.data.domain; 
        context.data = this.data;

        return context;
    }

    static async selectClass(_, button) {
        const oldClass = this.data.class;
        this.data.class = this.data.class?.uuid === button.dataset.class ? null : await fromUuid(button.dataset.class);
        if(oldClass !== button.dataset.class){
            this.data.subclass = null;
            this.data.domain = null;
            this.subclassChoices = this.data.class ? this.data.class.system.subclasses : [];
            this.domainChoices = this.data.class ? this.data.class.system.domains.map(x => {
                const config = SYSTEM.DOMAIN.domains[x];
                return { name: game.i18n.localize(config.name), id: config.id, img: config.src, disabled: this.actorClass.system.domains.includes(config.id) };
            }) : [];
        }

        this.render(true);
    }

    static async selectSubclass(_, button) {
        this.data.subclass = this.data.subclass?.uuid === button.dataset.subclass ? null : this.subclassChoices.find(x => x.uuid === button.dataset.subclass);
        this.render(true);
    }

    static async selectDomain(_, button) {
        const domain = this.data.domain?.id === button.dataset.domain ? null : this.domainChoices.find(x => x.id === button.dataset.domain);;
        if(domain?.disabled) return;

        this.data.domain = domain; 
        this.render(true);
    }

    static finish(){
        this.close({}, this.data);
    }

    async close(options={}, data=null) {
        this.resolve(data);
        super.close(options);
    }
}