knockoutjs抛出无法解析绑定属性

Question

我正在尝试使用knockoutjs 1.2.l并使用以下代码

$(function() {
  var viewModel = {
    categories: ko.observableArray([  
        {"Selected": false, "Text": "Rooms", "Value": "1"},
        {"Selected": false, "Text": "Automobile", "Value": "2"},
        {"Selected": false, "Text": "Buy & Sell", "Value": "3"},
        {"Selected": false, "Text": "Tutions", "Value": "4"},
        {"Selected": false, "Text": "Immigration", "Value": "5"}
    ]),
    initialData: {
        "Description": null,
        "SubCategoryId": 0,
        "Title": null,
        "UserId": 0,
        "AdTypeId": 0,
        "AddressId": null,
        "SubCategory": null,
        "User": null,
        "AdType": null,
        "Address": null,
        "Id": 0,
        "CreatedOn": "\/Date(1307627158991)\/",
        "CreatedBy": 0,
        "LastUpdatedOn": "\/Date(1307627158991)\/",
        "LastUpdatedBy": 0
    },
    chosenCategory: ko.observable()
  };
  ko.applyBindings(viewModel); // Makes Knockout get to work
});

Follwing是html

<div id="createAdDiv">

<form action="/Ads/Create" method="post">        <p>
        You've chosen: <b data-bind="text: chosenCategory().Text"></b>(Value: <span data-bind='text: chosenCategory().Value'></span>)
    </p>
    <div data-bind="visible: chosenCategory"> <!-- Appears when you select something -->
You have chosen a country with population 
<span data-bind="text: chosenCategory() ? chosenCategory().Text : 'unknown'"></span>.
</div>
    <fieldset>
        <div class="editor-label">

            <label for="SubCategoryId">Choose a Sub Category</label>
        </div>
        <div class="editor-field">
            <select data-bind="options: categories,optionsCaption:&#39;Choose...&#39;,optionsText: &#39;Text&#39;,value:chosenCategory" data-val="true" data-val-number="The field Choose a Sub Category must be a number." data-val-required="The Choose a Sub Category field is required." id="SubCategoryId" name="SubCategoryId"></select>

            <span class="field-validation-valid" data-valmsg-for="SubCategoryId" data-valmsg-replace="true"></span>
        </div>

    </fieldset>
</form></div>

抛出异常.

无法解析绑定属性.消息:TypeError:selectedCategory()未定义; 属性值:text:selectedCategory().文本

但是,如果我将javascript更改为跟随它的工作原理

$(function() {
    var viewModel = {
        categories: ko.observableArray( [{"Selected":false,"Text":"Rooms","Value":"1"},{"Selected":false,"Text":"Automobile","Value":"2"},{"Selected":false,"Text":"Buy & Sell","Value":"3"},{"Selected":false,"Text":"Tutions","Value":"4"},{"Selected":false,"Text":"Immigration","Value":"5"}] )
            ,initialData: {"Description":null,"SubCategoryId":0,"Title":null,"UserId":0,"AdTypeId":0,"AddressId":null,"SubCategory":null,"User":null,"AdType":null,"Address":null,"Id":0,"CreatedOn":"\/Date(1307628565958)\/","CreatedBy":0,"LastUpdatedOn":"\/Date(1307628565958)\/","LastUpdatedBy":0}
    };
    viewModel.chosenCategory = ko.observable(viewModel.categories);
      ko.applyBindings(viewModel); // Makes Knockout get to work

});

我只关注来自knockout.js网站的一个例子.

Answer 1

您将要在第一个段落标记中检查null,如:

<p>
    You've chosen: <b data-bind="text: chosenCategory() ? chosenCategory().Text : 'unknown'"></b>(Value: <span data-bind="text:chosenCategory() ? chosenCategory().Value : 'unknown'"></span>)
</p>

在你的第二段代码中,它正在工作,因为它是viewModel.categories的读取Text和Value属性,它们只是空的.如果你想设置默认值,那么你会想做类似的事情viewModel.chosenCategory = ko.observable(viewModel.categories()[0]);

另一种方法是使用该部分的模板并传入selectedCategory,因为它们处理空值而无需任何额外的工作.虽然,它不会渲染该部分,而不是显示"未知"之类的东西