import type { NextRequest } from 'next/server';
/**
 * A function that sends data to the client.
 * @param data - The data to send to the client.
 * @param eventName (optional) - The name of the event.
 * @returns void
 * @example
 * send({ message: 'test' });
 * send({ message: 'test' }, 'testEvent');
 */
type SendFunction = (data: any, eventName?: string) => void;
/**
 * A function that cleans up resources when the connection is closed.
 * @returns void | Promise<void>
 * @example
 * return () => {
 *   console.log('Cleanup');
 * };
 */
type Destructor = (() => void) | Promise<() => void> | (() => Promise<void>);
/**
 * A function that handles the Server-Sent Events (SSE) connection.
 * @param send - A function to send data to the client.
 * @param close - A function to close the SSE connection.
 * @returns void | Promise<void> | (() => void)
 * The callback can return a cleanup function that will be called when the connection is closed.
 * **HINT:** See also {@link Destructor}, the createSSEHandler {@link SSEOptions} and {@link SSECallback} context `onClose`.
 */
type SSECallback = (send: SendFunction, 
/**
 * A function to close the SSE connection.
 * @returns void
 * @example
 * close();
 */
close: () => void, 
/**
 * An object containing the last event ID received by the client. Not null if the client has been reconnected.
 * @property lastEventId - The last event ID from the client.
 * @property onClose - A function to set a cleanup function that will be called when the connection is closed. If the cleanup function is already set, a warning will be logged. The cleanup function set by the `onClose` function will be called even if the cleanup function returned by the callback is not called.
 * @example
 * { lastEventId: '12345' }
 */
context: {
    lastEventId: string | null;
    onClose: (destructor: Destructor) => void;
}) => void | Promise<void> | Destructor;
/**
 * An optional object to configure the Server-Sent Events (SSE) handler.
 * @property onClose - A function that will be called when the connection is closed even if the SSECallback has not been called yet.
 * @example
 * { onClose: () => console.log('SSE connection has been closed and cleaned up.') }
 */
type SSEOptions = {
    onClose?: Destructor;
};
/**
 * Creates a Server-Sent Events (SSE) handler for Next.js.
 *
 * @param callback - A function that handles the SSE connection. It takes two arguments: {@link SSECallback}
 *   - `send`: A function to send data to the client. See {@link SendFunction}.
 *   - `close`: A function to close the SSE connection.
 *   - `context`: An object containing the last event ID received by the client. Not null if the client has been reconnected.
 *   The callback can return a cleanup function that will be called when the connection is closed.
 *
 * **HINT:**
 * Be sure to **NOT** await long running operations in the callback, as this will block the response from being sent initially to the client. Instead wrap your long running operations in a async function to allow the response to be sent to the client first.
 *
 * @example
  ```
   export const GET = createSSEHandler((send, close) => {
      const asyncStuff = async () => {
        // async
      };

      asyncStuff();
   });
```
 *
 * @param options - An optional object to configure the handler. {@link SSEOptions}
 *  - `onClose`: A function that will be called when the connection is closed even if the SSECallback has not been called yet.
 * @returns A function that handles the SSE request. This function takes a `NextRequest` object as an argument.
 *
 * The returned function creates a `ReadableStream` to handle the SSE connection. It sets up the `send` and `close` functions,
 * and listens for the `abort` event on the request signal to close the connection.
 *
 * The response is returned with the appropriate headers for SSE:
 * - `Content-Type`: `text/event-stream`
 * - `Cache-Control`: `no-cache, no-transform`
 * - `Connection`: `keep-alive`
 */
export declare function createSSEHandler(callback: SSECallback, options?: SSEOptions): (request: NextRequest) => Promise<Response>;
export {};