agent-skills/skills/vercel-composition-patterns/rules/state-context-interface.md

---
title: Define Generic Context Interfaces for Dependency Injection
impact: HIGH
impactDescription: enables dependency-injectable state across use-cases
tags: composition, context, state, typescript, dependency-injection
---

## Define Generic Context Interfaces for Dependency Injection

Define a **generic interface** for your component context with three parts:
`state`, `actions`, and `meta`. This interface is a contract that any provider
can implement—enabling the same UI components to work with completely different
state implementations.

**Core principle:** Lift state, compose internals, make state
dependency-injectable.

**Incorrect (UI coupled to specific state implementation):**

```tsx
function ComposerInput() {
  // Tightly coupled to a specific hook
  const { input, setInput } = useChannelComposerState()
  return <TextInput value={input} onChangeText={setInput} />
}
```

**Correct (generic interface enables dependency injection):**

```tsx
// Define a GENERIC interface that any provider can implement
interface ComposerState {
  input: string
  attachments: Attachment[]
  isSubmitting: boolean
}

interface ComposerActions {
  update: (updater: (state: ComposerState) => ComposerState) => void
  submit: () => void
}

interface ComposerMeta {
  inputRef: React.RefObject<TextInput>
}

interface ComposerContextValue {
  state: ComposerState
  actions: ComposerActions
  meta: ComposerMeta
}

const ComposerContext = createContext<ComposerContextValue | null>(null)
```

**UI components consume the interface, not the implementation:**

```tsx
function ComposerInput() {
  const {
    state,
    actions: { update },
    meta,
  } = use(ComposerContext)

  // This component works with ANY provider that implements the interface
  return (
    <TextInput
      ref={meta.inputRef}
      value={state.input}
      onChangeText={(text) => update((s) => ({ ...s, input: text }))}
    />
  )
}
```

**Different providers implement the same interface:**

```tsx
// Provider A: Local state for ephemeral forms
function ForwardMessageProvider({ children }: { children: React.ReactNode }) {
  const [state, setState] = useState(initialState)
  const inputRef = useRef(null)
  const submit = useForwardMessage()

  return (
    <ComposerContext
      value={{
        state,
        actions: { update: setState, submit },
        meta: { inputRef },
      }}
    >
      {children}
    </ComposerContext>
  )
}

// Provider B: Global synced state for channels
function ChannelProvider({ channelId, children }: Props) {
  const { state, update, submit } = useGlobalChannel(channelId)
  const inputRef = useRef(null)

  return (
    <ComposerContext
      value={{
        state,
        actions: { update, submit },
        meta: { inputRef },
      }}
    >
      {children}
    </ComposerContext>
  )
}
```

**The same composed UI works with both:**

```tsx
// Works with ForwardMessageProvider (local state)
<ForwardMessageProvider>
  <Composer.Frame>
    <Composer.Input />
    <Composer.Submit />
  </Composer.Frame>
</ForwardMessageProvider>

// Works with ChannelProvider (global synced state)
<ChannelProvider channelId="abc">
  <Composer.Frame>
    <Composer.Input />
    <Composer.Submit />
  </Composer.Frame>
</ChannelProvider>
```

**Custom UI outside the component can access state and actions:**

The provider boundary is what matters—not the visual nesting. Components that
need shared state don't have to be inside the `Composer.Frame`. They just need
to be within the provider.

```tsx
function ForwardMessageDialog() {
  return (
    <ForwardMessageProvider>
      <Dialog>
        {/* The composer UI */}
        <Composer.Frame>
          <Composer.Input placeholder="Add a message, if you'd like." />
          <Composer.Footer>
            <Composer.Formatting />
            <Composer.Emojis />
          </Composer.Footer>
        </Composer.Frame>

        {/* Custom UI OUTSIDE the composer, but INSIDE the provider */}
        <MessagePreview />

        {/* Actions at the bottom of the dialog */}
        <DialogActions>
          <CancelButton />
          <ForwardButton />
        </DialogActions>
      </Dialog>
    </ForwardMessageProvider>
  )
}

// This button lives OUTSIDE Composer.Frame but can still submit based on its context!
function ForwardButton() {
  const {
    actions: { submit },
  } = use(ComposerContext)
  return <Button onPress={submit}>Forward</Button>
}

// This preview lives OUTSIDE Composer.Frame but can read composer's state!
function MessagePreview() {
  const { state } = use(ComposerContext)
  return <Preview message={state.input} attachments={state.attachments} />
}
```

The `ForwardButton` and `MessagePreview` are not visually inside the composer
box, but they can still access its state and actions. This is the power of
lifting state into providers.

The UI is reusable bits you compose together. The state is dependency-injected
by the provider. Swap the provider, keep the UI.