docs(attach): add documentation for attach prop

docs/.vitepress/config/en.ts

@@ -45,6 +45,7 @@ export const enConfig: LocaleSpecificConfig<DefaultTheme.Config> = {
           { text: 'Extending', link: '/advanced/extending' },
           { text: 'Primitives', link: '/advanced/primitive' },
           { text: 'Scaling Performance 🚀', link: '/advanced/performance' },
+          { text: 'Attach', link: '/advanced/attach' },
           {
             text: 'Caveats',
             link: '/advanced/caveats',

docs/advanced/attach.md

@@ -0,0 +1,190 @@
+# `attach` 🖇
+
+Using the `attach` prop, you can tell Tres exactly where you want to insert a child into its parent.
+
+:::info
+
+The `attach` prop is not required for many common cases. For instance:
+
+* adding a single `<Material>` to a `<Mesh>`
+* adding a `<Geometry>` to a `<Mesh>`
+* adding one or more `<Mesh>`s to a parent `<Mesh>`
+
+:::
+
+## Background
+
+Tres tries to automatically determine where to insert child tag into its parent. For example, in this code, Tres will:
+
+* automatically insert the geometry into `parent.geometry`
+* automatically insert the material into `parent.material`
+
+```vue
+<template>
+  <TresMesh name="parent">
+    <TresBoxGeometry />
+    <TresMeshNormalMaterial />
+  </TresMesh>
+</template>
+```
+
+## Problem
+
+Tres covers common cases, like above. But it doesn't cover every possible case.
+
+When Tres doesn't automatically choose the proper insertion location for a child, one solution is to fall back to procedural code in `<script>`.
+
+Here's how you might add multiple materials to a mesh using `<script>`:
+
+```vue
+<script setup lang="ts">
+import { MeshBasicMaterial } from 'three'
+import { onMounted, shallowRef } from 'vue'
+
+const meshRef = shallowRef()
+onMounted(() => {
+  meshRef.value.material = [
+    new MeshBasicMaterial({ color: 'red' }),
+    new MeshBasicMaterial({ color: 'orange' }),
+    new MeshBasicMaterial({ color: 'yellow' }),
+    new MeshBasicMaterial({ color: 'green' }),
+    new MeshBasicMaterial({ color: 'blue' }),
+    new MeshBasicMaterial({ color: 'purple' }),
+  ]
+})
+</script>
+
+<template>
+  <TresMesh ref="meshRef">
+    <TresBoxGeometry />
+  </TresMesh>
+</template>
+```
+
+But this workaround means:
+
+* your materials aren't managed by Tres
+* your code is imperative, not declarative
+* your code is non-reactive by default
+
+## Solution
+
+The `attach` prop lets you specify where an object will be added to the parent object using declarative code.
+
+## Usage
+
+Here's the example above, rewritten declaratively using `attach`:
+
+```vue
+<template>
+  <TresMesh>
+    <TresBoxGeometry />
+    <TresMeshBasicMaterial color="red" attach="material-0" />
+    <TresMeshBasicMaterial color="orange" attach="material-1" />
+    <TresMeshBasicMaterial color="yellow" attach="material-2" />
+    <TresMeshBasicMaterial color="green" attach="material-3" />
+    <TresMeshBasicMaterial color="blue" attach="material-4" />
+    <TresMeshBasicMaterial color="purple" attach="material-5" />
+  </TresMesh>
+</template>
+```
+
+## "Pierced" `attach`
+
+You can deeply attach a child to a parent by "piercing" – i.e., using a kebab-case string.
+
+### Pseudocode
+
+First, here are a few simple pseudocode examples. This will attach `bar` at `foo.ab.cd`:
+
+```html
+<foo>
+  <bar attach="ab-cd" />
+</foo>
+```
+
+This will attach `bar` at `foo.ab.cd.ef`:
+
+```html
+<foo>
+  <bar attach="ab-cd-ef" />
+</foo>
+```
+
+### Usage
+
+As a concrete example, you can used "pierced" `attach` to add custom `BufferAttribute`s:
+
+```vue
+<script setup lang="ts">
+import { TresCanvas } from '@tresjs/core'
+
+const positions = new Float32Array([-1.0, -1.0, 1.0, 1.0, -1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0, -1.0, 1.0])
+</script>
+
+<template>
+  <TresCanvas clear-color="gray">
+    <TresMesh :scale="0.3333">
+      <TresBufferGeometry>
+        <TresBufferAttribute
+          attach="attributes-position"
+          :count="positions.length / 3"
+          :array="positions"
+          :itemSize="3"
+        />
+      </TresBufferGeometry>
+      <TresMeshBasicMaterial color="red" />
+    </TresMesh>
+  </TresCanvas>
+</template>
+```
+
+## Arrays
+
+You can attach within arrays by using array indices in the `attach` string.
+
+### Usage
+
+For example, you can use array indices to attach `THREE` post-processing passes to the `THREE.EffectComposer.passes` array:
+
+```vue
+<script lang="ts" setup>
+import { EffectComposer } from 'three/examples/jsm/postprocessing/EffectComposer'
+import { RenderPass } from 'three/examples/jsm/postprocessing/RenderPass'
+import { OutputPass } from 'three/examples/jsm/postprocessing/OutputPass'
+import { UnrealBloomPass } from 'three-stdlib'
+import { extend, useLoop, useTres } from '@tresjs/core'
+import { shallowRef } from 'vue'
+
+extend({ EffectComposer, OutputPass, UnrealBloomPass, RenderPass })
+const { renderer, scene, camera, sizes } = useTres()
+const composer = shallowRef<EffectComposer>()
+
+useLoop().render(() => {
+  if (composer.value) {
+    composer.value!.render()
+  }
+})
+</script>
+
+<template>
+  <TresEffectComposer
+    ref="composer"
+    :args="[renderer]"
+    :set-size="[sizes.width.value, sizes.height.value]"
+  >
+    <TresRenderPass
+      :args="[scene, camera]"
+      attach="passes-0"
+    />
+    <TresUnrealBloomPass
+      :args="[undefined, 0.5, 0.1, 0]"
+      attach="passes-1"
+    />
+    <TresOutputPass
+      attach="passes-2"
+      :set-size="[sizes.width.value, sizes.height.value]"
+    />
+  </TresEffectComposer>
+</template>
+```