<input type="text" data-bind="value:count,valueUpdate: 'afterkeydown'" />
  <span data-bind="text:countLen"></span>/10
  <span style="color: red;" data-bind="validationMessage:count"></span>

        /*computed valueUpdate等属性的学习*/
        self.count = ko.observable().extend({
            maxLength: { params: 10, message: "最大长度为10" },
            required: {
                params: true,
                message: "请输入字符..."
            }
        });
        self.count.subscribe(function (o) {//实现当大于某个长度时,只绑定指定长度的字符
            if (o.length > 10)
                self.count(o.substr(0, 10));
        });

        self.countLen = ko.computed(function () {
            return self.count() ? self.count().trim().length : 0;
        });