Apizr – Part 5: Requesting with Optional pattern

var optionalUserList = await _reqResOptionalMediator.SendFor(api => api.GetUsersAsync());

var optionalPagedUsers = await _userOptionalMediator.SendReadAllOptionalQuery();

Optional is pretty cool when trying to handle nullables and exceptions, but  I still want to write it shorter to get my request done and managed with as less code as possible.

I mean, even if we use the typed optional mediator or typed crud optional mediator to get things shorter, we still have to deal with the result matching boilerplate.

Fortunately, Apizr provides some dedicated extensions to help the lazy guy I am getting things as short as we can:

OnResultAsync ask you to provide one of these parameters:

• Action<TResult> onResult: this action will be invoked just before throwing any exception that might have occurred during request execution
• Func<TResult, ApizrException<TResult>, bool> onResult: this function will be invoked with the returned result and potential occurred exception
• Func<TResult, ApizrException<TResult>, Task<bool>> onResult: this function will be invoked async with the returned result and potential occurred exception

All give you a result returned from fetch if succeed, or cache if failed (if configured). The main goal here is to set any binded property with the returned result (fetched or cached), no matter of exceptions. Then the Action will let the exception throw, where the Func will let you decide to throw manually or return a success boolean flag.

Here is what our final request looks like:

with Action (auto throwing after invocation on excpetion):

await _reqResOptionalMediator.SendFor(api => api.GetUsersAsync()).OnResultAsync(userList => { users = userList?.Data; });

Or with Func and throw:

await _reqResOptionalMediator.SendFor(api => api.GetUsersAsync()).OnResultAsync((userList, exception) => 
{ 
    users = userList?.Data; 
    
    if(exception != null)
        throw exception;

    return true;
});

Or with Func and success flag:

var success = await _reqResOptionalMediator.SendFor(api => api.GetUsersAsync()).OnResultAsync((userList, exception) => 
{ 
    users = userList?.Data; 
    
    return exception != null;
});

Of course, remember to catch your throwing exceptions at least globaly (look at AsyncErrorHandler).

CatchAsync let you provide these parameters:

• Action<Exception> onException: this action will be invoked just before returning the result from cache if fetch failed. Useful to inform the user of the api call failure and that data comes from cache.
• letThrowOnExceptionWithEmptyCache: True to let it throw the inner exception in case of empty cache, False to handle it with onException action and return empty cache result (default: False)

This one returns result from fetch or cache (if configured), no matter of potential exception handled on the other side by an action callback

var userList = await _reqResOptionalMediator.SendFor(api => api.GetUsersAsync()).CatchAsync(AsyncErrorHandler.HandleException, true);

Here we ask the api to get users and if it fails:

• There’s some cached data?
  • AsyncErrorHandler will handle the exception like to inform the user call just failed
  • Apizr will return the previous result from cache
• There’s no cached data yet!
  • letThrowOnExceptionWithEmptyCache is True? (witch is the case here)
    • Apizr will throw the inner exception that will be catched further by AsyncErrorHander (this is its normal behavior)
  • letThrowOnExceptionWithEmptyCache is False! (default)
    • Apizr will return the empty cache data (null) witch has to be handled further

One line of code to get all the thing done safely and shorter than ever!

When Apizr works all together with MediatR and OptionalAsync, we can write things much shorter than ever, cleaner and consistent, keeping it all loosely coupled from data to views and all safe.

As a reminder, here are all the ways of using OptionalAsync and MediatR with Apizr:

public class MyViewModel
{
    private readonly IMediator _mediator;
    private readonly IOptionalMediator<IReqResService> _reqResOptionalMediator;
    private readonly ICrudOptionalMediator<User, int, PagedResult<User>, IDictionary<string, object>> _userOptionalMediator;
    
    public MyViewModel(IMediator mediator, 
        IOptionalMediator<IReqResService> reqResOptionalMediator,
        ICrudOptionalMediator<User, int, PagedResult<User>, IDictionary<string, object>> userOptionalMediator)
    {
        _mediator = mediator;
       _reqResOptionalMediator = reqResOptionalMediator;
       _userOptionalMediator = userOptionalMediator;
    }
    
    public ObservableCollection<User>? Users { get; set; }

    // This is a dummy example presenting all the ways to play with Optional
    // You should choose only one of it
    private async Task GetUsersAsync()
    {
        //////////////////
        // CLASSIC API
        //////////////////
        
        // The classic api interface way with mediator and optional request
        var optionalUserList = await _mediator.Send(new ExecuteOptionalRequest<IReqResService, UserList>((ct, api) => api.GetUsersAsync(ct)), CancellationToken.None);
        
        // The classic api interface way with typed optional mediator (the same but shorter)
        var optionalUserList = await _reqResOptionalMediator.SendFor(api => api.GetUsersAsync());

        // Handling the optional result for both previous ways
        optionalPagedResult.Match(userList =>
        {
            if (userList.Data != null && userList.Data.Any())
                Users = new ObservableCollection<User>(userList.Data);
        }, e =>
        {
            if (e.CachedResult?.Data != null && e.CachedResult.Data.Any())
                Users = new ObservableCollection<User>(e.CachedResult.Data);
        });
        
        // The classic api interface way with typed optional mediator and OnResultAsync result handling extension
        await _reqResOptionalMediator.SendFor(api => api.GetUsersAsync()).OnResultAsync(userList => 
        { 
            if (userList.Data != null && userList.Data.Any())
                Users = new ObservableCollection<User>(userList.Data);
        });

        // The classic api interface way with typed optional mediator and CatchAsync exception handling extension
        var userList = await _reqResOptionalMediator.SendFor(api => api.GetUsersAsync()).CatchAsync(AsyncErrorHandler.HandleException, true);
        if (userList.Data != null && userList.Data.Any())
            Users = new ObservableCollection<User>(userList.Data);
        
        
        //////////////////
        // CRUD API
        //////////////////
            
        // The crud api interface way with mediator and optional request
        var optionalPagedResult = await _mediator.Send(new ReadAllOptionalQuery<PagedResult<User>>(), CancellationToken.None);
        
        // The crud api interface way with typed crud optional mediator
        var optionalPagedResult = await _userOptionalMediator.SendReadAllOptionalQuery();
        
        // Handling the optional result for both previous ways
        optionalPagedResult.Match(pagedUsers =>
        {
            if (pagedUsers.Data != null && pagedUsers.Data.Any())
                Users = new ObservableCollection<User>(pagedUsers.Data);
        }, e =>
        {
            if (e.CachedResult?.Data != null && e.CachedResult.Data.Any())
                Users = new ObservableCollection<User>(e.CachedResult.Data);
        });
        
        // The crud api interface way with typed optional mediator and OnResultAsync result handling extension
        await _userOptionalMediator.SendReadAllOptionalQuery().OnResultAsync(pagedUsers => 
        { 
            if (pagedUsers.Data != null && pagedUsers.Data.Any())
                Users = new ObservableCollection<User>(pagedUsers.Data);
        });

        // The crud api interface way with typed optional mediator and CatchAsync exception handling extension
        var pagedUsers = await _userOptionalMediator.SendReadAllOptionalQuery().CatchAsync(AsyncErrorHandler.HandleException, true);
        if (pagedUsers.Data != null && pagedUsers.Data.Any())
            Users = new ObservableCollection<User>(pagedUsers.Data);
    }
}

In this article we’ve seen how Apizr could work with OptionalAsync.