绑定下拉列表（选择）列表的初始/默认值

Question

I'm having a small issue with setting the initial value of a dropdown. The code below is the view model definition and the initialization in $(document).ready . I have an array called sourceMaterialTypes and a selectedSourceMaterialType representing the selected value of that array. I am initializing the view model with values from the (ASP.Net MVC) Model and ViewBag.

var viewModel = {
    sourceMaterialTypes : 
        ko.observableArray(@Html.Raw(Json.Encode(ViewBag.SourceMaterialTypes))),
    selectedSourceMaterialType :
        ko.observable(@Html.Raw(Json.Encode(Model.SourceMaterialType))),
    ingredientTypes :
        ko.observableArray(@Html.Raw(Json.Encode(ViewBag.IngredientTypes))),
    selectedIngredientType : ko.observable()
};

$(document).ready(function () {

    ko.applyBindings(viewModel);

    viewModel.selectedSourceMaterialType.subscribe(function(newSourceMaterialType) {
        $.getJSON("/IngredientType/FindByMaterialType",
                  { "id": newSourceMaterialType })
            .success(function (data) {
                viewModel.ingredientTypes($.parseJSON(data));
            })
            .error(function () { alert("error"); });
    });
});

The following is the definition of my dropdown (select) list with the Knockout binding definition.

<select id="SourceMaterialTypeId"
        name="SourceMaterialTypeId"
        data-bind="options: sourceMaterialTypes,
                   optionsText: 'Name',
                   optionsValue : 'Id',
                   value: selectedSourceMaterialType"></select>

This all works fine except for the initially selected value in the source materials dropdown ( selectedSourceMaterialType is bound correctly so when the dropdown selection changes its value is correctly updated, it is only the initial selection I am having a problem with), which is always the first item in the sourceMaterialTypes array on my view model.

I would like the initially selected value to be that which is initialized from the (server-side) model as the value of selectedSourceMaterialType view model property.

Answer 1

我想你需要传递Id而不是传递selectedSourceMaterialType可观察函数中的整个对象 - >

selectedSourceMaterialType: ko.observable(@Model.SourceMaterialType.Id)

Answer 2

The API has the solution for you, you'll just need to add optionsCaption to your select.

<select id="SourceMaterialTypeId"
    name="SourceMaterialTypeId"
    data-bind="options: sourceMaterialTypes,
               optionsText: 'Name',
               optionsValue : 'Id',
               value: selectedSourceMaterialType,
               optionsCaption: 'Please select...'"></select>

Answer 3

As @nEEBz suggested, selectedSourceMaterialType is initialized improperly. In the learn.knockoutjs.com "Lists and Collections" tutorial , they initialize their viewmodel's selected-item property by passing the value of a specific index of the observable array. For example, do this:

selectedSourceMaterialType: ko.observable(sourceMaterialTypes[2])

...instead of this:

selectedSourceMaterialType: ko.observable({"Id":1,"Name":"Coffee Bean" /* ... */});

That way, the value of the selected item is a reference to the item in the same observable array that the dropdownlist items come from.