Slack Post Message Invocable Action

oas-sample.json

{
    "openapi": "3.0.0",
    "info": {
        "version": "1.0.0",
        "title": "MuleSoft/Slack Integration Demo"
    },
    "paths": {
        "/slack/message/{channel}/{message}": {
            "post": {
                "summary": "Post a message to a Slack channel",
                "operationId": "postSlackMessage",
                "parameters": [
                    {
                        "name": "channel",
                        "in": "path",
                        "description": "Slack channel to post to",
                        "required": true,
                        "schema": {
                            "type": "string"
                        }
                    },
                    {
                        "name": "message",
                        "in": "path",
                        "description": "Message to be posted",
                        "required": true,
                        "schema": {
                            "type": "string"
                        }
                    }
                ],
                "responses": {
                    "200": {
                        "description": "Operation succeeded"
                    }
                }
            }
        }
    }
}

SlackIntegration.cls

public with sharing class SlackIntegration {
    @future(callout=true)
    public static void postMessage(String url, String message) {
        Map<String, String> body = new Map<String, String>();
        body.put('text', message);
        String bodyJSON = JSON.Serialize(body);

        HttpResponse response = sendRequest(url, 'POST', bodyJSON);
        if (response.getStatusCode() != 200) {
            throw new SlackException('Failed to post Slack Message ' + response.getBody());
        }
    }

    private static HttpResponse sendRequest(String url, String method, String body) {
        HttpRequest request = new HttpRequest();
        request.setEndpoint(url);
        request.setMethod(method);
        request.setHeader('Content-Type', 'application/json;charset=UTF-8');
        request.setBody(body);
        return new Http().send(request);
    }

    public class SlackException extends Exception {
    }
}

SlackPostAction.cls

public with sharing class SlackPostAction {
    @InvocableMethod(label='Post a Slack Message' )
    public static List<OutputParameters> postMessage(List<InputParameters> inputs) {
        List<OutputParameters> outputs = new List<OutputParameters>();
        for (InputParameters input : inputs) {
            OutputParameters output = new OutputParameters();
            try {
                SlackIntegration.postMessage(input.namedCredential, input.message);
                output.isSuccess = true;
            } catch (SlackIntegration.SlackException e) {
                output.isSuccess = false;
                output.errorMessage = e.getMessage();
            }
            outputs.add(output);
        }
        return outputs;
    }

    /**
     * Wrapper class for input parameters
     */
    public class InputParameters {
        @InvocableVariable(required=true)
        public String namedCredential;
        @InvocableVariable(required=true)
        public String message;
    }

    /**
     * Wrapper class for output parameters
     */
    public class OutputParameters {
        @InvocableVariable
        public Boolean isSuccess;
        @InvocableVariable
        public String errorMessage;
    }
}