Knockout计算列与模型

Question

我的MVC C＃解决方案中有一个具有以下属性的模型

public class RegistrationRequirementModel
{
    public string LoadIntent { get; set; }
    public string Francophone { get; set; }
    public string Gender { get; set; }

    public RegistrationRequirementModel(L09RegistrationRequirement requirement)
    {
        LoadIntent = requirement.LoadIntent;
        Francophone = requirement.Francophone;
        Gender = requirement.Gender;
    }
}

在我的javascript中，我可以调用模型并显示数据，但是当涉及到使用一些失败的计算函数时。

使用Javascript

    var registrationRequirementModel = {
        frenchData:  ko.observable(""),
        genderData:  ko.observable(""),
        loadIntentData:  ko.observable(""),

        isMissingData: ko.computed(function () {
            if (this.frenchData() == "") { return true };
            if (this.genderData() == "") { return true };
            if (this.loadIntentData() == "") { return true };
            return false;
        },this),

    }

   $(document).ready(function () {

        ko.applyBindings(registrationRequirementModel, document.getElementById("RegistrationSurveyContent"));

        $.ajax({
            url: getStudentRegRequirementsUrl,
            type: "GET",
            contentType: jsonContentType,
            dataType: "json",
            success: function (data) {
                if (!account.handleInvalidSessionResponse(data)) {
                    registrationRequirementModel.frenchData(data.Francophone);
                    registrationRequirementModel.genderData(data.Gender);
                    registrationRequirementModel.loadIntentData(data.LoadIntent);
                }
            },
            error: function (jqXHR, textStatus, errorThrown) {
                if (jqXHR.status != 0)
                    $('#notificationHost').notificationCenter('addNotification', { message: "Unable to retrieve registration requirement.", type: "error" });
            }
        });
    });

HTML

<table style="width:100%">
    <tbody>
        <tr>
            <td data-bind="text: loadIntentData"></td>
            <td data-bind="text: frenchData"></td>
            <td data-bind="text: genderData"></td>
        </tr>
    </tbody>
</table>

目的是在缺少数据时显示html。但是，当我激活此代码时，计算列仍然说法语数据不是函数。我的观点可以在我的html data-bind="visible: isMissingData"中使用。但不幸的是。我可以从我的数据中读取事件。

这是我对api的呼唤

 public async Task<JsonResult> GetRegistrationRequirementAsync()
 {
     string StudentID = CurrentUser.PersonId;
     try
     {
         var requirement = await ServiceClient.L09GetRegistrationRequirementAsync(StudentID);
         RegistrationRequirementModel registrationRequirementModel = new RegistrationRequirementModel(requirement);
         return Json(registrationRequirementModel, JsonRequestBehavior.AllowGet);
      }
      catch (Exception e)
      {}
}

Answer 1

frenchData is not a function控制台错误源于KnockoutJS ViewModel的设置方式。本质上，正常可观测量下面的计算函数isMissingData具有this的新内部范围上下文，其不反映registrationRequirementModel对象的相同外部范围。

要解决此问题，您应该从使用object literal切换到constructor function，以便您可以将this ViewModel范围分配给self/that变量，从而减轻范围问题。然后通过KO Apply Bindings实例化你新存储的ViewModel，你现在可以在AJAX成功后获得访问权限：

function registrationRequirementModel() {
  var self = this;
  self.frenchData = ko.observable("");
  self.genderData = ko.observable("");
  self.loadIntentData = ko.observable("");

  self.isMissingData = ko.computed(function() {
    if (self.frenchData() == "") {
      return true
    };
    if (self.genderData() == "") {
      return true
    };
    if (self.loadIntentData() == "") {
      return true
    };
    return false;
  }, this);
}

$(document).ready(function() {
  var vm = new registrationRequirementModel();
  ko.applyBindings(vm, document.getElementById("RegistrationSurveyContent"));

  // replace with endpoint
  var jsonData = {
    Francophone: "Francophone",
    Gender: "Male",
    LoadIntent: "LoadIntent"
  };

  if (handleInvalidSessionResponse(jsonData)) {
    vm.frenchData(jsonData.Francophone);
    vm.genderData(jsonData.Gender);
    vm.loadIntentData(jsonData.LoadIntent);
  }
});

function handleInvalidSessionResponse(data) {
  if (typeof data !== "undefined") return true;
  return false;
}

下面是场景http://jsfiddle.net/ajxrw39m/3/的模拟JSFiddle

Answer 2

当你定义你的viewmodel时，this没有指向新创建的对象，它指向你正在创建它的上下文中的任何this（可能是window）。

var vm = {
  computedUsingThis: ko.computed(function() {
    return this;
  }, this)
}

console.log(
  vm.computedUsingThis() === vm,     // false
  vm.computedUsingThis() === window  // true
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>

有许多方法可以避免这个问题。您可以使用构造函数和new关键字，或为viewmodel创建工厂方法：

const VM = () => {
  const a = ko.observable("a");
  const b = ko.observable("b");
  
  const ab = ko.pureComputed(
    () => a() + b()
  );
  
  return { a, b, ab };
}

const vm = VM();
vm.ab.subscribe(console.log);

setTimeout(
  () => {
    vm.a("A");
  },
  500
)

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>