# useQueuer

# Function: useQueuer()

```ts
function useQueuer<TValue, TSelected>(
   fn, 
   options, 
selector): ReactQueuer<TValue, TSelected>;
```

Defined in: [react-pacer/src/queuer/useQueuer.ts:194](https://github.com/TanStack/pacer/blob/main/packages/react-pacer/src/queuer/useQueuer.ts#L194)

A React hook that creates and manages a Queuer instance.

This is a lower-level hook that provides direct access to the Queuer's functionality without
any built-in state management. This allows you to integrate it with any state management solution
you prefer (useState, Redux, Zustand, etc.) by utilizing the onItemsChange callback.

For a hook with built-in state management, see useQueuedState.

The Queuer extends the base Queue to add processing capabilities. Items are processed
synchronously in order, with optional delays between processing each item. The queuer includes
an internal tick mechanism that can be started and stopped, making it useful as a scheduler.
When started, it will process one item per tick, with an optional wait time between ticks.

By default uses FIFO (First In First Out) behavior, but can be configured for LIFO
(Last In First Out) by specifying 'front' position when adding items.

## State Management and Selector

The hook uses TanStack Store for reactive state management. You can subscribe to state changes
in two ways:

**1. Using `queuer.Subscribe` HOC (Recommended for component tree subscriptions)**

Use the `Subscribe` HOC to subscribe to state changes deep in your component tree without
needing to pass a selector to the hook. This is ideal when you want to subscribe to state
in child components.

**2. Using the `selector` parameter (For hook-level subscriptions)**

The `selector` parameter allows you to specify which state changes will trigger a re-render
at the hook level, optimizing performance by preventing unnecessary re-renders when irrelevant
state changes occur.

**By default, there will be no reactive state subscriptions** and you must opt-in to state
tracking by providing a selector function or using the `Subscribe` HOC. This prevents unnecessary
re-renders and gives you full control over when your component updates.

Available state properties:
- `executionCount`: Number of items that have been processed by the queuer
- `expirationCount`: Number of items that have been removed due to expiration
- `isEmpty`: Whether the queuer has no items to process
- `isFull`: Whether the queuer has reached its maximum capacity
- `isIdle`: Whether the queuer is not currently processing any items
- `isRunning`: Whether the queuer is active and will process items automatically
- `items`: Array of items currently waiting to be processed
- `itemTimestamps`: Timestamps when items were added for expiration tracking
- `pendingTick`: Whether the queuer has a pending timeout for processing the next item
- `rejectionCount`: Number of items that have been rejected from being added
- `size`: Number of items currently in the queue
- `status`: Current processing status ('idle' | 'running' | 'stopped')

## Unmount behavior

By default, the hook stops the queuer when the component unmounts.
Use the `onUnmount` option to customize this. For example, to flush pending items instead:

```tsx
const queue = useQueuer(fn, {
  started: true,
  wait: 1000,
  onUnmount: (q) => q.flush()
});
```

## Type Parameters

### TValue

`TValue`

### TSelected

`TSelected` = \{
\}

## Parameters

### fn

(`item`) => `void`

### options

[`ReactQueuerOptions`](../interfaces/ReactQueuerOptions.md)\<`TValue`, `TSelected`\> = `{}`

### selector

(`state`) => `TSelected`

## Returns

[`ReactQueuer`](../interfaces/ReactQueuer.md)\<`TValue`, `TSelected`\>

## Example

```tsx
// Default behavior - no reactive state subscriptions
const queue = useQueuer(
  (item) => console.log('Processing:', item),
  { started: true, wait: 1000 }
);

// Subscribe to state changes deep in component tree using Subscribe HOC
<queue.Subscribe selector={(state) => ({ size: state.size, isRunning: state.isRunning })}>
  {({ size, isRunning }) => (
    <div>Queue: {size} items, {isRunning ? 'Processing' : 'Idle'}</div>
  )}
</queue.Subscribe>

// Opt-in to re-render when queue size changes at hook level (optimized for displaying queue length)
const queue = useQueuer(
  (item) => console.log('Processing:', item),
  { started: true, wait: 1000 },
  (state) => ({
    size: state.size,
    isEmpty: state.isEmpty,
    isFull: state.isFull
  })
);

// Opt-in to re-render when processing state changes (optimized for loading indicators)
const queue = useQueuer(
  (item) => console.log('Processing:', item),
  { started: true, wait: 1000 },
  (state) => ({
    isRunning: state.isRunning,
    isIdle: state.isIdle,
    status: state.status,
    pendingTick: state.pendingTick
  })
);

// Opt-in to re-render when execution metrics change (optimized for stats display)
const queue = useQueuer(
  (item) => console.log('Processing:', item),
  { started: true, wait: 1000 },
  (state) => ({
    executionCount: state.executionCount,
    expirationCount: state.expirationCount,
    rejectionCount: state.rejectionCount
  })
);

// Example with custom state management and scheduling
const [items, setItems] = useState([]);

const queue = useQueuer(
  (item) => console.log('Processing:', item),
  {
    started: true, // Start processing immediately
    wait: 1000,    // Process one item every second
    onItemsChange: (queue) => setItems(queue.peekAllItems()),
    getPriority: (item) => item.priority // Process higher priority items first
  }
);

// Add items to process - they'll be handled automatically
queue.addItem('task1');
queue.addItem('task2');

// Control the scheduler
queue.stop();  // Pause processing
queue.start(); // Resume processing

// Access the selected state (will be empty object {} unless selector provided)
const { size, isRunning, executionCount } = queue.state;
```