了解customElements的处理顺序

Question

在某个时间点上，父级自定义元素可以访问其子级，然后再为其子级赋予其自定义方法。

class CustomParent extends HTMLElement {
  connectedCallback() {
    // works
    this.children[0].textContent = "bar";

    // works
    setTimeout(() => this.children[0].test(), 0);

    // throws a Type error
    this.children[0].test();
  }
}

customElements.define("custom-parent", CustomParent);


class CustomChild extends HTMLElement {
  test() {
    this.textContent = "baz";
  }
}

customElements.define("custom-child", CustomChild);

document.body.innerHTML = `
<custom-parent>
  <custom-child>foo</custom-child>  
</custom-parent>
`;

这怎么可能？推迟this.children[0].test()是否安全？

Answer 1

归因于自定义元素的upgrading process。>

第一步

：执行document.body.innerHTML = '<custom-parent><custom-child>foo</custom-child></custom-parent>'时，会将2个元素作为unknown elements插入页面中。

第二步

：父元素首先是升级。它可以作为unknown元素访问其子级（然后更新其textContent属性）。但是它无法访问访问自定义元素test()方法...，因为它还不是自定义元素！

第三步

：子元素紧随其后是升级，现在获得了test()方法。

第四步

：延迟的test()调用在逻辑上有效:-)

请参见下面的示例。它使用querySelectorAll( ':not(:defined)' )显示该子项在其父项之后被升级。

class CustomParent extends HTMLElement {
  constructor() { super() ; console.log( 'parent upgraded') }
  connectedCallback() {
    console.log( 'parent connected', this.children[0].outerHTML )
    // works
    this.children[0].textContent = 'bar'    
    // works
    setTimeout( () => this.children[0].test() )
    // throws a Type error
    try { 
       this.children[0].test() 
    } catch ( e ) { 
      //this shows the parent is upgraded, but not its child 
      var not_upgraded = document.querySelectorAll( ':not(:defined)' )
      console.log( 'not upgraded: ', ...not_upgraded )    
    }    
  }
}

customElements.define( 'custom-parent', CustomParent )

class CustomChild extends HTMLElement {
  constructor() { super() ; console.log( 'child upgraded') }      
  test() { this.textContent = 'baz' }
}

customElements.define( 'custom-child', CustomChild ) 

document.body.innerHTML = `
  <custom-parent>
    <custom-child>foo</custom-child>  
  </custom-parent>`