uniapp vue 在main.js里为vue创建了一个方法函数，怎么把这个函数的this指向每个调用它的页面？

Question

Vue.prototype.modal = {
    show: true
};

Vue.prototype.showModal= function({title = "提示", content = "是否确认"}){
    console.log(this);
    return new Promise((resolve, reject)=>{
        this.modal = {
            show: true,
            title,
            content,
            callback: resolve,
            catch: reject
        };
    });
};