Implementation Principle of Vue3's Teleport Component

Description:
Teleport is a built-in component introduced in Vue3 that can "teleport" part of the content in a template to other locations in the DOM tree for rendering, while keeping the logical relationships of the components unchanged. This feature is useful for implementing modal boxes, notification prompts, global pop-ups, and other scenarios that need to break through hierarchical constraints.

Detailed Explanation of Implementation Principle:

1. Understanding Core Concepts

• Problem solved by Teleport: Components are constrained by style properties such as overflow:hidden, z-index, and position within the parent DOM hierarchy.
• Core feature: Maintaining logical parent-child relationships of components while physically changing DOM location.
• Required attribute: to specifies the target container (selector or DOM element).

2. Compilation Phase Processing
When the Vue compiler encounters the <Teleport> component:

// Template code
<Teleport to="#modal-container">
  <div class="modal">Content</div>
</Teleport>

// Approximate structure of the compiled render function
function render() {
  return {
    type: Teleport,
    props: { to: '#modal-container' },
    children: [
      { type: 'div', props: { class: 'modal' }, children: 'Content' }
    ]
  }
}

The compiler recognizes Teleport as a special node type, preserving target location and child node information.

3. Mounting Process Analysis
The mounting of Teleport is divided into two stages:

Stage One: Creating Child Nodes

// During component rendering process
function mountTeleport(vnode, container) {
  // 1. Create an empty anchor comment element as placeholder
  const placeholder = document.createComment('teleport')
  vnode.el = placeholder  // Teleport itself corresponds to a comment node
  
  // 2. Get the target container
  const target = document.querySelector(vnode.props.to)
  
  // 3. Mount child nodes to the target container
  vnode.children.forEach(child => {
    mount(child, target)  // Recursively mount child nodes to the target location
  })
  
  // 4. Insert the comment node at the original location
  container.appendChild(placeholder)
}

4. Update Mechanism Implementation
When the props or child nodes of Teleport change:

function patchTeleport(n1, n2) {
  const oldTarget = document.querySelector(n1.props.to)
  const newTarget = document.querySelector(n2.props.to)
  
  // Case 1: Target container changes
  if (oldTarget !== newTarget) {
    // Remove all child nodes from the old container
    n1.children.forEach(child => {
      oldTarget.removeChild(child.el)
    })
    
    // Move child nodes to the new container
    n2.children.forEach(child => {
      newTarget.appendChild(child.el)
    })
  } 
  // Case 2: Only child content changes
  else {
    // Perform normal diff update within the target container
    patchChildren(n1, n2, newTarget)
  }
}

5. Maintaining Lifecycle
The key of Teleport lies in maintaining logical relationships:

// The component instance still belongs to its original parent component
const parentComponent = instance.parent

// Logical relationships such as event handling, provide/inject remain unchanged
// Only the DOM structure is "teleported" to another location

6. Actual DOM Structure Example

<!-- Application root component -->
<div id="app">
  <div class="main-content">
    <!-- teleport start -->  <!-- Comment node placeholder -->
    <!-- teleport end -->
  </div>
</div>

<!-- Target location -->
<div id="modal-container">
  <div class="modal">Teleported content</div>  <!-- Actual rendering location -->
</div>

7. Handling Special Scenarios

• When the target container does not exist: Fall back to rendering at the original location and issue a warning.
• SSR scenario: During server-side rendering, Teleport content is rendered at the original location, and then moved to the target location during client-side hydration.
• Multiple Teleports to the same target: They are appended to the target container in the order of the components.

Technical Summary:

1. Using comment nodes as placeholders to maintain positional relationships.
2. Locating the target container via query selectors.
3. Handling special cases of container changes during the patch process.
4. Keeping the logical relationships of component instances unaffected by DOM location.
5. Providing graceful degradation to ensure functionality.

This implementation approach solves the problem of style hierarchy constraints while maintaining Vue's complete component logical system.