在 document.body.scrollTop 上触发链接到 KnockoutObservable 的 Knockout BindingHandler

Question

我试图触发 Knockout BindingHandler 作为 document.body.scrolltop 值大于或等于特定值的直接结果。我试图创建一个基于语句的可观察对象。首先，这可能吗？或者我应该更新布尔结果作为计算的一部分吗？

scrollPosition: KnockoutObservable<boolean> = ko.observable(document.body.scrollTop >= 200)

我也试过：

scrollPosition: KnockoutComputed<boolean> = ko.computed(() => {
    if (document.body.scrollTop >= 100) {
        return true;
    }
    else {
        return false;
    }
});

其余相关代码为：

HTML

<a href="javascript:void(0);" id="topLink" data-bind="topScroll: scrollPosition"><i class="glyphicon glyphicon-stats"></i></a>

CSS

#topLink {
    position: fixed;
    bottom: 40px;
    right: 40px;
    background: rgba(72,72,72,1);
    width: 50px;
    height: 50px;
    display: none;
    text-decoration: none;
    -webkit-border-radius: 35px;
    -moz-border-radius: 35px;
    border-radius: 35px;
    -webkit-transition: all 0.3s linear;
    -moz-transition: all 0.3s ease;
    -ms-transition: all 0.3s ease;
    -o-transition: all 0.3s ease;
    transition: all 0.3s ease;
}

绑定处理程序

ko.bindingHandlers.topScroll = {
    update: function (element, valueAccessor) {
        // This will be called once when the binding is first applied to an element,
        // and again whenever any observables/computeds that are accessed change
        // Update the DOM element based on the supplied values here.

        var value = valueAccessor();

        if (ko.unwrap(value)) {

            $(element).css("display", "block");
        }
    }
};

我的目标是在顶部滚动超出特定值时显示返回顶部样式 link。有人可以指出我哪里出错了吗？

Answer 1

... Firstly, is this possible? Or should I be updating the boolean result as part of a computed?

scrollPosition = ko.observable(document.body.scrollTop >= 200)

您希望 scrollPosition 是动态的。您实际上 想要的是 body.scrollTop 成为可观察的。但事实并非如此。所以严格来说：没有，这是不可能的。

我的建议是创建一个 custom binding handler on the body element that detects when the user is scrolling (akin to this) 并随后更新一个可观察对象。

这是关于如何在视图中使用它的伪代码：

<body data-bind="changeOnScroll: shouldShowLink">

自定义绑定处理程序可能是这样的（伪代码）：

ko.bindingHandlers.changeOnScroll = {
    init: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // This will be called when the binding is first applied to an element
        // Set up any initial state, event handlers, etc. here

        element.onscroll = function() {
            var obs = valueAccessor();
            obs(element.scrollTop > 200); // set the observable
        }
    },
    update: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // This will be called once when the binding is first applied to an element,
        // and again whenever any observables/computeds that are accessed change
        // Update the DOM element based on the supplied values here.

        // Nothing here, since it's a one-way binding from DOM to observable.
    }
};

我已经使用了 onscroll，但如果您有可用于标准化事件处理的 jQuery，您也可以使用 jQuery。

Answer 2

在看到 Jeroen 的回答之前，我已经找到了一个令我满意的解决方案。我已将固定的 true 传递给 BindingHandler 数据绑定。

<a href="javascript:void(0);" id="topLink" data-bind="topScroll: true"><i class="glyphicon glyphicon-stats"></i></a>

然后将BindingHandler修改为如下...

ko.bindingHandlers.topScroll = {
    init: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // This will be called when the binding is first applied to an element
        // Set up any initial state, event handlers, etc. here
        $(window).scroll(function() {

            if ($(this).scrollTop() >= 150) {         // If page is scrolled more than 150px

                $(element).css("display", "block");  // show the arrow
            } else {
                $(element).css("display", "none");   // don't show the arrow
            }
        });

        $(element).click(function () {      // When arrow is clicked
            $('body,html').animate({
                scrollTop: 0                       // Scroll to top of body
            }, 'slow');
        });

    }    
};

在 document.body.scrollTop 上触发链接到 KnockoutObservable 的 Knockout BindingHandler

Trigger Knockout BindingHandler linked to KnockoutObservable on document.body.scrollTop

html

css

jquery

knockout.js

bindinghandlers