Stabilization treatment of slot content

Stabilization of Slot Content

Vue 3's slot mechanism undergoes a complex processing flow during compilation and runtime, where stabilization is a key technique to ensure efficient updates of dynamic slot content. When a component has conditional slots or dynamic slot names, the renderer needs to establish stable node relationships to avoid unnecessary re-rendering.

// Dynamic slot example
const DynamicSlot = defineComponent({
  setup() {
    const slotName = ref('header')
    return () => h('div', [
      h('child', {}, {
        // Dynamic slot name
        [slotName.value]: () => h('span', 'Dynamic content')
      })
    ])
  }
})

Compilation Phase Handling of Slot Nodes

The template compiler converts slot content into special code generation structures. For default slots and named slots, the compiler generates different render function code blocks:

// Compiled slot function example
function renderSlot(_ctx, _cache, $props, $setup, $data, $options) {
  return (_openBlock(), _createBlock(_ctx.child, null, {
    default: _withCtx(() => [
      _createVNode("span", null, "Default content")
    ]),
    footer: _withCtx(() => [
      _createVNode("div", null, "Footer content")
    ]),
    _: 1 /* STABLE */
  }))
}

The key lies in the _: 1 marker, an internal flag indicating that the slot object should be treated as a stable structure during component updates. The compiler decides whether to add this marker based on whether the slot content contains dynamic nodes.

Runtime Stabilization Strategies

When the renderer handles component updates, it compares old and new slot objects through the updateComponent function. Stabilization is primarily reflected in the following scenarios:

1. Dynamic Slot Name Stability: When only the slot name changes while the content remains the same, Vue reuses the existing VNode.
2. Conditional Slot Optimization: Slots maintain internal state without loss when toggling between show/hide.
3. Scoped Slot Caching: Slot content with the same scope parameters is reused.

// Simplified runtime slot comparison logic
function updateSlots(instance, nextSlots) {
  if (nextSlots._ === 1 /* STABLE */) {
    // Reuse stable slots directly
    instance.slots = nextSlots
    return
  }
  
  // Non-stable slots require deep comparison
  for (const key in nextSlots) {
    if (!isSameSlot(instance.slots[key], nextSlots[key])) {
      instance.slots[key] = nextSlots[key]
    }
  }
}

Special Handling of Scoped Slots

Stabilization of scoped slots requires additional consideration of changes to scope parameters. Vue 3 uses a caching mechanism to optimize performance:

const CachedSlot = defineComponent({
  setup(_, { slots }) {
    const count = ref(0)
    return () => h('div', [
      slots.default({
        // Scope parameters
        count: count.value
      })
    ])
  }
})

The compiler generates cached render functions for scoped slots:

function renderScopedSlot(_ctx, _cache, $props) {
  return _cache[1] || (
    _cache[1] = $props => [
      _createVNode("span", null, $props.count)
    ]
  )
}

Conflict Resolution Between Dynamic Child Nodes and Stabilization

When slot content contains dynamic child nodes, stabilization requires special handling. Vue 3 addresses this through the block tree mechanism:

<!-- Slot containing both static and dynamic content -->
<template #default="{ item }">
  <div class="static">Fixed structure</div>
  <div v-for="i in item.list" :key="i.id">{{ i.text }}</div>
</template>

The compiled output generates code blocks with PatchFlags:

function render(_ctx, _cache) {
  return (_openBlock(), _createBlock(_ctx.child, null, {
    default: _withCtx(({ item }) => [
      _createVNode("div", { class: "static" }, "Fixed structure"),
      (_openBlock(true), _createBlock(_Fragment, null, _renderList(item.list, i => (
        _createVNode("div", { key: i.id }, i.text, 1 /* TEXT */)
      )), 256 /* UNKEYED_FRAGMENT */))
    ]),
    _: 1 /* STABLE */
  }))
}

Handling Forced Update Scenarios

In some cases, stabilized slots need to be forcibly updated. Vue 3 provides manual control methods:

import { useSlots, watchEffect } from 'vue'

const ForceUpdateComponent = defineComponent({
  setup() {
    const slots = useSlots()
    watchEffect(() => {
      // Manually mark the slot as unstable
      slots._ = 0
    })
  }
})

This mechanism is commonly used in the following scenarios:

• Slot content depends on global state.
• Slot updates within dynamic components.
• Intermediate layer processing when higher-order components pass through slots.

Differences in Server-Side Rendering

Slot stabilization in SSR environments differs, primarily in:

1. No runtime block markers are generated.
2. Slot content is directly serialized into strings.
3. Stabilization relationships are reconstructed during hydration.

// SSR slot render function example
function renderSlotSSR() {
  return {
    default: _ssrRenderWithContext(() => _ssrRenderTag('div', null, 'Server content')),
    _: 1
  }
}