init functions integration

src/App.ts

@@ -13,6 +13,7 @@ import {
   matchConstraints,
   onlyCommands,
   matchCommandName,
+  matchFunctionCallback,
   onlyOptions,
   onlyShortcuts,
   onlyEvents,
@@ -578,7 +579,12 @@ export default class App {
     this.listeners.push([onlyCommands, matchCommandName(commandName), ...listeners] as Middleware<AnyMiddlewareArgs>[]);
   }
 
-  public options<Source extends OptionsSource = 'block_suggestion'>(
+  // matches a Slack function callback specifically
+  public function(callbackID: string, ...listeners: Middleware<SlackEventMiddlewareArgs>[]): void {
+    this.listeners.push([matchFunctionCallback(callbackID), ...listeners] as Middleware<AnyMiddlewareArgs>[]);
+  }
+
+  public options<Source extends OptionsSource = OptionsSource>(
     actionId: string | RegExp,
     ...listeners: Middleware<SlackOptionsMiddlewareArgs<Source>>[]
   ): void;

src/middleware/builtin.ts

@@ -261,6 +261,21 @@ function matchesPattern(pattern: string | RegExp, candidate: string): boolean {
   return pattern.test(candidate);
 }
 
+/**
+ * Middleware that filters out any Function that doesn't match callback_id
+ */
+export function matchFunctionCallback(callback_id: string): Middleware<SlackEventMiddlewareArgs> {
+  return async ({ event, next }) => {
+    // Filter out any functions that are not the correct function name
+    if (!event || 'function_executed' !== event.type || !event.function || callback_id !== event.function.callback_id) {
+      return;
+    }
+
+    // TODO: remove the non-null assertion operator
+    await next!();
+  };
+}
+
 /*
  * Middleware that filters out events that don't match pattern
  */

src/types/events/base-events.ts

@@ -75,7 +75,8 @@ export type SlackEvent =
   | WorkflowPublishedEvent
   | WorkflowUnpublishedEvent
   | WorkflowStepDeletedEvent
-  | WorkflowStepExecuteEvent;
+  | WorkflowStepExecuteEvent
+  | FunctionExecutedEvent;
 
 export type EventTypePattern = string | RegExp;
 
@@ -896,3 +897,28 @@ export interface WorkflowStepExecuteEvent {
 
 // NOTE: `user_resourced_denied`, `user_resource_granted`, `user_resourced_removed` are left out because they are
 // deprecated as part of the Workspace Apps Developer Preview
+
+export interface FunctionExecutedEvent {
+  type: 'function_executed';
+  function_execution_id: string;
+  function: {
+    id: string;
+    callback_id: string;
+    name: string;
+    description: string;
+    input_parameters: {
+      name: string;
+      description: string;
+      type: string;
+    }[];
+    output_parameters: {
+      name: string;
+      description: string;
+      type: string;
+    }[];
+  };
+  arguments: {
+    [index: string]: any;
+  };
+  event_ts: string;
+}