Test Class for Call back class which implements Metadata.DeployCallback interface

Hi,

When we create a class to insert metadata using Metadata class then we write callback class to send notifications or another process about the status of deployment or insert of metadata.

 Callback class:
public class PostInstallCallback implements Metadata.DeployCallback{
public void handleResult(Metadata.DeployResult result,
        Metadata.DeployCallbackContext context) {
       
        if (result.status == Metadata.DeployStatus.Succeeded) {
            // Deployment was successful, take appropriate action.     
           Messaging.SingleEmailMessage  msgObj = new Messaging.SingleEmailMessage ();
            msgObj.setSubject('MetaData insertion succeeded');         
            List<String> emailAddresses = new List<String>{UserInfo.getUserEmail()};
            Metadata.DeployDetails deployDetailsList = result.details;
            List<Metadata.DeployMessage> deploymentMessagesList = deployDetailsList.componentSuccesses;   
            String errorMessageBody = '';
            for(Metadata.DeployMessage messageObj:deploymentMessagesList){
                if(messageObj.success==true && !String.valueOf(messageObj.fullName).split('\\.')[1].containsIgnoreCase('xml')){
                    errorMessageBody+=String.valueOf(messageObj.fullName).split('\\.')[1]+'\n';
                }
            }
            errorMessageBody +=' records created';
            msgObj.setPlainTextBody(errorMessageBody);
            msgObj.setToAddresses(emailAddresses);
            Messaging.sendEmail(new List<Messaging.Email>{msgObj});         
        } else {
            // Deployment wasn’t successful, take appropriate action.
            Messaging.SingleEmailMessage  msgObj = new Messaging.SingleEmailMessage ();
            msgObj.setSubject('MetaData insertion failed');
            msgObj.setPlainTextBody(result.details+'='+result.errorMessage);
            List<String> emailAddresses = new List<String>{UserInfo.getUserEmail()};     
            Metadata.DeployDetails deployDetailsList = result.details;
            List<Metadata.DeployMessage> deploymentMessagesList = deployDetailsList.componentFailures;   
            String errorMessageBody = '';
            for(Metadata.DeployMessage messageObj:deploymentMessagesList){
                if(messageObj.success==false){
                    errorMessageBody+=messageObj.problem+'\n';
                }
            }
            msgObj.setPlainTextBody(errorMessageBody);
            msgObj.setToAddresses(emailAddresses);
            Messaging.sendEmail(new List<Messaging.Email>{msgObj});
        }
    }
}


Test Class:

@isTest
private class PostInstallCallbackTest {
@IsTest
    static void testMyCallback() {
       
        // Instantiate the callback.
        Metadata.DeployCallback callback = new PostInstallCallback();     
        // Create test result and context objects.
        Metadata.DeployResult result = new Metadata.DeployResult();
        result.status = Metadata.DeployStatus.Succeeded;
        Metadata.DeployMessage messageObj = new Metadata.DeployMessage();
        messageObj.changed = true;
        messageObj.success = true;
        messageObj.fullName='';
        messageObj.componentType ='CustomMetadata';
        messageObj.fullName = 'VAT_Rate.SreeVardhan3';
        Metadata.DeployDetails deployDetailsObj = new Metadata.DeployDetails();
        deployDetailsObj.componentSuccesses.add(messageObj);
        result.details = deployDetailsObj;     
        Metadata.DeployCallbackContext context = new Metadata.DeployCallbackContext();
       
        // Invoke the callback's handleResult method.
        callback.handleResult(result, context);
    }
    @IsTest
    static void testMyCallbackErrorMessage() {
       
        // Instantiate the callback.
        Metadata.DeployCallback callback = new PostInstallCallback();     
         // Create test result and context objects.
        Metadata.DeployResult result = new Metadata.DeployResult();
        result.status = Metadata.DeployStatus.Failed;
        Metadata.DeployMessage messageObj = new Metadata.DeployMessage();
        messageObj.changed = false;
        messageObj.success = false;   
        messageObj.componentType ='CustomMetadata';
        messageObj.fullName = 'VAT_Rate.SreeVardhan3';
        messageOBj.problem = 'Default: value not of required type: 1';
        messageObj.problemType = Metadata.DeployProblemType.Error;
        Metadata.DeployDetails deployDetailsObj = new Metadata.DeployDetails();
        deployDetailsObj.componentFailures.add(messageObj);
        result.details = deployDetailsObj;     
        Metadata.DeployCallbackContext context = new Metadata.DeployCallbackContext();       
        // Invoke the callback's handleResult method.
        callback.handleResult(result, context);
    }
}


References:
https://trailhead.salesforce.com/en/content/learn/modules/apex_metadata_api/apex_metadata_api_testing