The internal implementation of the slot mechanism

Basic Concepts of Slot Mechanism

Vue3's slot mechanism allows components to receive template fragments as content and render this content at specific locations within the component. Slots are divided into two basic types: default slots and named slots.

// Parent component using slots
<template>
  <ChildComponent>
    <template v-slot:header>
      <h1>This is the header</h1>
    </template>
    This is the default slot content
    <template v-slot:footer>
      <p>This is the footer</p>
    </template>
  </ChildComponent>
</template>

// Child component defining slots
<template>
  <div>
    <slot name="header"></slot>
    <slot></slot>  <!-- Default slot -->
    <slot name="footer"></slot>
  </div>
</template>

Compilation Process of Slots

Vue3's template compiler converts slot content into special render functions. The compiled code roughly looks like this:

// Compiled parent component render function
function render() {
  return h(ChildComponent, null, {
    header: () => h('h1', 'This is the header'),
    default: () => 'This is the default slot content',
    footer: () => h('p', 'This is the footer')
  })
}

The compiler performs the following key steps:

1. Identifies <template v-slot> syntax
2. Converts slot content into object properties
3. Places default content into the default property
4. Converts named slots into properties with corresponding names

Runtime Implementation of Slots

Vue3 internally manages slot content through the slots object. When a component is instantiated, it processes the slots:

// Simplified internal implementation
function setupComponent(instance) {
  const { vnode } = instance
  if (vnode.children) {
    normalizeObjectSlots(vnode.children, instance.slots)
  }
}

function normalizeObjectSlots(children, slots) {
  for (const key in children) {
    const value = children[key]
    slots[key] = (props) => normalizeSlotValue(value(props))
  }
}

Implementation of Scoped Slots

Scoped slots allow child components to pass data to slots:

// Child component
<template>
  <ul>
    <li v-for="item in items">
      <slot :item="item"></slot>
    </li>
  </ul>
</template>

// Parent component usage
<template>
  <ChildComponent>
    <template v-slot="slotProps">
      <span>{{ slotProps.item.name }}</span>
    </template>
  </ChildComponent>
</template>

The compiled code generates scoped slot functions:

function render() {
  return h(ChildComponent, null, {
    default: (props) => h('span', props.item.name)
  })
}

Dynamic Slot Names

Vue3 supports dynamically specifying slot names:

<template>
  <ChildComponent>
    <template v-slot:[dynamicSlotName]>
      Dynamic slot content
    </template>
  </ChildComponent>
</template>

The compiled code generates dynamic property access:

function render() {
  return h(ChildComponent, null, {
    [dynamicSlotName.value]: () => 'Dynamic slot content'
  })
}

Performance Optimization of Slots

Vue3 includes several performance optimizations for slots:

1. Static hoisting during compilation: Static slot content is hoisted outside the render function
2. Caching slot functions: Avoids unnecessary re-renders
3. Block Tree optimization: Tracks dynamic nodes within slots

// Optimized slot handling
function renderSlot(slots, name, props) {
  const slot = slots[name]
  if (slot) {
    return slot(props)
  }
}

Interaction of Slots with Teleport/KeepAlive

Slots can be used in conjunction with other built-in components:

<template>
  <KeepAlive>
    <ComponentWithSlots>
      <template v-slot:content>
        <Teleport to="#modal">
          Teleportable slot content
        </Teleport>
      </template>
    </ComponentWithSlots>
  </KeepAlive>
</template>

In such cases, Vue maintains the correct slot context relationships to ensure Teleport and KeepAlive function properly.

Update Mechanism of Slots

When slot content changes, Vue3 executes the following update process:

1. Detects changes in the parent component's slot content
2. Generates new slot functions
3. Triggers re-rendering of the child component
4. Compares old and new slot content to perform minimal DOM operations

// Simplified update logic
function updateSlots(instance, newSlots) {
  const oldSlots = instance.slots
  for (const key in newSlots) {
    oldSlots[key] = newSlots[key]
  }
  instance.render() // Triggers re-rendering
}

Handling Edge Cases for Slots

Vue3 handles various edge cases for slots:

1. Default content: Displays default content when no slot content is provided
2. Duplicate slot names: The last definition takes effect
3. Non-reactive content: Static content does not trigger updates

// Default content example
<slot>This is the default content</slot>

// Duplicate slot names
<template v-slot:header>First</template>
<template v-slot:header>Second</template> <!-- This one takes effect -->

TypeScript Support for Slots

Vue3 provides comprehensive type support for slots:

defineComponent({
  slots: {
    default: (props: { item: Item }) => VNode,
    header: () => VNode
  }
})

// Type checking when using components
<MyComponent>
  <template v-slot="props">  <!-- props is automatically inferred as { item: Item } -->
    {{ props.item.name }}
  </template>
</MyComponent>