影子根 getElementsByClassName

我正在使用 LitElement 创建自定义 Web 组件。我对此相当陌生，决定尝试制作图像幻灯片。我用了W3Schools 幻灯片作为参考 https://www.w3schools.com/howto/howto_js_slideshow.asp同时将其修改为 LitElement。 问题是，当我尝试使用 document.getElementByClassName 时，我没有得到任何东西。我很熟悉这个问题，因为我正在使用 Shadow DOM，所以我将其更改为 this.shadowRoot.getElementsByClassName。不幸的是，我被告知我想要使用的不是一个函数。当我使用 LitElement 和 Shadow dom 时，如何通过类名获取元素？如果您想看看我的组件是什么样子，这里是代码：

import { LitElement, html} from '@polymer/lit-element';

class ImageGalleryElement extends LitElement {

static get properties() { return {
    slideIndex: { type: Number },
  }};

constructor(){
    super();

    this.slideIndex=1;
    this.showSlides(this.slideIndex);
}

// Next/previous controls
plusSlides(n) {
    this.showSlides(this.slideIndex += n);
}

// Thumbnail image controls
currentSlide(n) {
    this.showSlides(this.slideIndex = n);
}

showSlides(n) {
    var i;
    console.dir(this.shadowRoot);
    var slides = this.shadowRoot.getElementsByClassName("mySlides");
    console.dir(slides);
    var dots = this.shadowRoot.getElementsByClassName("dot");
    if (n > slides.length) {this.slideIndex = 1} 
    if (n < 1) {this.slideIndex = slides.length}
    for (i = 0; i < slides.length; i++) {
        slides[i].style.display = "none"; 
    }
    for (i = 0; i < dots.length; i++) {
        dots[i].className = dots[i].className.replace(" active", "");
    }
    slides[this.slideIndex-1].style.display = "block"; 
    dots[this.slideIndex-1].className += " active";
}


render(){
return html`
    <style>
        * {box-sizing:border-box}

        /* Slideshow container */
        .slideshow-container {
            max-width: 1000px;
            position: relative;
            margin: auto;
        }

        /* Hide the images by default */
        .mySlides {
            display: none;
        }

        /* Next & previous buttons */
        .prev, .next {
            cursor: pointer;
            position: absolute;
            top: 50%;
            width: auto;
            margin-top: -22px;
            padding: 16px;
            color: white;
            font-weight: bold;
            font-size: 18px;
            transition: 0.6s ease;
            border-radius: 0 3px 3px 0;
            user-select: none;
        }

        /* Position the "next button" to the right */
        .next {
            right: 0;
            border-radius: 3px 0 0 3px;
        }

        /* On hover, add a black background color with a little bit see-through */
        .prev:hover, .next:hover {
            background-color: rgba(0,0,0,0.8);
        }

        /* Caption text */
        .text {
            color: #f2f2f2;
            font-size: 15px;
            padding: 8px 12px;
            position: absolute;
            bottom: 8px;
            width: 100%;
            text-align: center;
        }

        /* Number text (1/3 etc) */
        .numbertext {
            color: #f2f2f2;
            font-size: 12px;
            padding: 8px 12px;
            position: absolute;
            top: 0;
        }

        /* The dots/bullets/indicators */
        .dot {
            cursor: pointer;
            height: 15px;
            width: 15px;
            margin: 0 2px;
            background-color: #bbb;
            border-radius: 50%;
            display: inline-block;
            transition: background-color 0.6s ease;
        }

        .active, .dot:hover {
            background-color: #717171;
        }

        /* Fading animation */
        .fade {
            -webkit-animation-name: fade;
            -webkit-animation-duration: 1.5s;
            animation-name: fade;
            animation-duration: 1.5s;
        }

        @-webkit-keyframes fade {
            from {opacity: .4} 
            to {opacity: 1}
        }

        @keyframes fade {
            from {opacity: .4} 
            to {opacity: 1}
        }
    </style>
    <!-- Slideshow container -->
    <div class="slideshow-container">

        <!-- Full-width images with number and caption text -->
        <div class="mySlides fade">
            <div class="numbertext">1 / 3</div>
            <img src="../../img/img-snow-wide" style="width:100%">
            <div class="text">Caption Text</div>
        </div>

        <div class="mySlides fade">
            <div class="numbertext">2 / 3</div>
            <img src="../../img/img-nature-wide" style="width:100%">
            <div class="text">Caption Two</div>
        </div>

        <div class="mySlides fade">
            <div class="numbertext">3 / 3</div>
            <img src="../../img/img-mountains-wide" style="width:100%">
            <div class="text">Caption Three</div>
        </div>

        <!-- Next and previous buttons -->
        <a class="prev" @click="${this.plusSlides(-1)}" >&#10094;</a>
        <a class="next" @click="${this.plusSlides(1)}">&#10095;</a>
    </div>
    <br>

    <!-- The dots/circles -->
    <div style="text-align:center">
        <span class="dot" @click="${this.currentSlide(1)}"></span> 
        <span class="dot" @click="${this.currentSlide(2)}"></span> 
        <span class="dot" @click="${this.currentSlide(3)}"></span> 
    </div>
`;
}
} 
// Register the element with the browser
customElements.define('image-gallery-element', ImageGalleryElement);

The getElementsByClassName()方法仅适用于 HTML 文档或元素。

The shadowRoot是一个文档片段通过继承 https://dom.spec.whatwg.org/#interface-shadowroot，不是文档，也不是 HTML 元素。

相反，你应该使用querySelectorAll().

对于以下情况，行为相同：

• getElementsByTagName()
• getElementsByClassName()

Note 1

getElementById()在使用以下命令创建的（内存中）节点上不可用document.createElement

测试选择器方法：https://jsfiddle.net/WebComponents/9yrtn8vb/ https://jsfiddle.net/WebComponents/9yrtn8vb/

Note 2

请注意，分档内容是不为所动对于 Shadow DOM，它仍然保留在 light DOM 中

当你move从 lightDOM（在主 DOM 中）到 ShadowRoot 的元素，
Live Nodelist 将被清空

let pieces = document.getElementsByTagName("[TAGNAME here]");
console.warn(tags);// lists all tags
this.shadowRoot.append(...this.children);
console.warn(tags);// empty