KnockoutJS仅在字段部分完成时验证

寻找一种解决方案，仅在部分完成时才验证输入字段，即，如果用户未输入值，则不要验证。http://jsfiddle.net/2H2Bv/

当所有字段都满足其验证要求时，即当附加到每个输入的所有验证消息均已消失并且在单击提交按钮之前，删除顶部的验证摘要消息的解决方案“下面的完整缺少的字段”，也将很有用。 ...

的HTML

<script id="customMessageTemplate" type="text/html">
    <em class="customMessage" data-bind='validationMessage: field'></em>
</script>

<!-- ko if: displayAlert -->
<p class="customMessage" data-bind="text: validationSummary"></p> <br />
<!-- /ko -->

<fieldset>
    <legend>Details</legend>
    <label>First name:
        <input data-bind='value: firstName' />
    </label>
    <label>Last name:
        <input data-bind='value: lastName' />
    </label>
    <div data-bind='validationOptions: { messageTemplate: "customMessageTemplate" }'>
        <label>Email:
            <input data-bind='value: emailAddress' required pattern="@" />
        </label>
</fieldset>
<br>
<button type="button" data-bind='click: submit'>Submit</button>
<br>
<br> <span data-bind='text: errors().length'></span> errors

JS：

ko.validation.rules.pattern.message = 'Invalid.';

ko.validation.configure({
    decorateElement: true,
    registerExtenders: true,
    messagesOnModified: true,
    insertMessages: true,
    parseInputAttributes: true,
    messageTemplate: null
});

var viewModel =  function() {
    this.firstName = ko.observable().extend({
        minLength: 2,
        maxLength: 10
    });
    this.lastName = ko.observable().extend({
        required: true
    });
    this.emailAddress = ko.observable().extend({ // custom message
        required: {
            message: 'Enter your email address.'
        }
    });
    this.validationSummary = ko.observable("Complete missing fields below:");
    this.displayAlert = ko.observable(false);
    this.submit = function () {
        if (this.errors().length == 0) {
            alert('Thank you.');
        } else {
            this.displayAlert(true);
            this.errors.showAllMessages();
        }
    };
    this.errors = ko.validation.group(this);
};

ko.applyBindings(new viewModel());