checked绑定是关联到checkable的form表单控件到view model上 - 例如checkbox()或者radio button()
<p>Send me spam: <input type="checkbox" data-bind="checked: wantsSpam"/></p>
<script type="text/javascript">
var viewModel = {
wantsSpam: ko.observable(true) // Initially checked
};
// ... then later ...
viewModel.wantsSpam(false); // The checkbox becomes unchecked
</script>