KnockoutJS：通过自定义绑定将属性添加到ViewModel

Question

I'm adding a custom binding to my page. Within it, I'd like to save one of the objects I create as a property of the viewModel. When I do this, I get "undefined" when I try and access it outside of the binding. Why? Here is a reduced down example:

HTML:

<div id = "myDiv" data-bind = "fooAdd: myFoo"></div>

JavaScript:

ko.bindingHandlers.fooAdd = {
  init: function(element, valueAccessor, allBindingsAccessor, viewModel) {
    var foo = 1;
    viewModel._foo = foo;
  }
};

var ViewModel = function(){
  var self = this;
  self.myFoo = ko.observable({});
  console.log(self); //Here I can expand the object returned in 
                     //the console and see that _foo is equal to 1.
  console.log(self._foo); //returns undefined
};

ko.applyBindings(new ViewModel());

Answer 1

From your sample, you're setting the new property in the init, which gets called only at the time of applying the bindings, but you are trying to access it in the constructor of the view model. Try console.log(viewmodel._foo) after the applyBindings call.