askFor: function() {
    var done = this.async();

    // Have Yeoman greet the user.
    this.log(yosay('Welcome to the marvelous Myblog generator!'));

    var prompts = [{
        name: 'blogName',message: 'What do you want to call your blog?',default: 'myblog'
    }];

    this.prompt(prompts,function(props) {
        this.blogName = props.blogName;

        done();
    }.bind(this));
}

这是this.async的代码

this.async = function() {
    return function() {};
}