I'm relatively new to Flutter and Cubit pattern and I'm trying to figure out which are the best ways to work with them. Recently my colleague and I have been struggling to reach an agreement where we should inject the cubit and the repositories.
Reading the bloc/cubit documentation, it is not very clear about where we should do it.
IMO, everything that we need to instantiate, should be injected as high in the tree as possible where two different components that will use this information have in common.
For my colleague, each widget can instantiate one cubit, meaning that each widget will have its own instance of the cubit.
I would like to discuss about what are the community thoughts and best practices regarding the dependency injection and architecture regarding cubit.
There is no single answer to that question. It all depends on your project structure and architecture. In general though:
It's OK to create a few cubits/blocs in one screen/widget. Some widgets or screens contain more than one business logic stuff. Cubits are just classes that help you maintain the state, but it's no different than having many animation controllers or text editing controllers, it's just that it serves a more high-level state management. Let's say that you have a comments section in your app. You may have:
a cubit for the comments themselves, to load them, load more on scroll, report error when loading failed etc.
a cubit for each comment that manages the "Like" button under a comment
It's perfectly valid to have it that way.
It's OK to have global cubits for the whole app. There are some things that you need to have access to from the whole application. It usually is navigation (Navigator), and some theme management (Theme), why not something more business-logic related then, like authentication logic, current user context, user's app preferences, etc.? :)
IMO, everything that we need to instantiate, should be injected as high in the tree as possible where two different components that will use this information have in common.
This is a good approach. Most frequently it will be above your routes, so somewhere above your MaterialApp. If you make use of nested Navigators, then this common place could be above this nested Navigator.
On a more technical side, how will you manage the dependencies used in those cubits/blocs is up to you and your liking. I find some of the options:
Instantiating all repositories and other dependencies in main.dart method and then passing them in constructors to your blocs/cubits in Providers.
To reload those dependencies you will need a Hot Restart though, Hot Reload won't be enough.
Putting your dependencies in the widget's tree with Provider, just like blocs/cubits.
Using riverpod instead of provider.
Using a Service Locator pattern with get_it and injectable combo.
The most correct choice will be something that you (and your colleagues) are most comfortable developing with and that scales well.
One of the objectives of Repository pattern is supposed to decouple business logic with data access. Then why is it that database context is not created in repository class itself instead of being provided to it by service layer or controller?
You pass the context in because you may have a transaction that spans repositories. The context is the unit of work for the repositories.
Database Context represents your Unit Of Work in most cases. With this in mind, there are at least two reasons to inject it in repositories instead of creating it in repositories.
Using same Context across multiple repositories
Many articles say Transaction is responsibility of Repository. But, in practical world, this may not be the case every time. One may need to span the transaction across multiple repositories. To achieve this, there is no better way than to inject it.
Manage Context at some higher level (Context per Request in Web may be) which is not accessible to Repository
This is not much different than above. Many implementations manage Context at higher level which is not accessible to Repositories. Context per Request is popular example. In this case, Context must be separated somehow from Repositories and handled differently. But, repositories does need the Context; so just inject it.
Contexts exposed by full ORMs implement change tracking at good scale. To take better advantage of this feature, it is better if Context is separated from Repositories.
Imagine the case where your repository call is successful but you do not want to flush because your other file action failed.
By separating Context from Repository, you better implement SRP. Context handles responsibility of Change Tracking and Flushing. Repository hold in-memory representation of data with changes if any and allows to read/write this in-memory data.
(Note: My question has very similar concerns as the person who asked this question three months ago, but it was never answered.)
I recently started working with MVC3 + Entity Framework and I keep reading that the best practice is to use the repository pattern to centralize access to the DAL. This is also accompanied with explanations that you want to keep the DAL separate from the domain and especially the view layer. But in the examples I've seen the repository is (or appears to be) simply returning DAL entities, i.e. in my case the repository would return EF entities.
So my question is, what good is the repository if it only returns DAL entities? Doesn't this add a layer of complexity that doesn't eliminate the problem of passing DAL entities around between layers? If the repository pattern creates a "single point of entry into the DAL", how is that different from the context object? If the repository provides a mechanism to retrieve and persist DAL objects, how is that different from the context object?
Also, I read in at least one place that the Unit of Work pattern centralizes repository access in order to manage the data context object(s), but I don't grok why this is important either.
I'm 98.8% sure I'm missing something here, but from my readings I didn't see it. Of course I may just not be reading the right sources... :\
I think the term "repository" is commonly thought of in the way the "repository pattern" is described by the book Patterns of Enterprise Application Architecture by Martin Fowler.
A Repository mediates between the domain and data mapping layers,
acting like an in-memory domain object collection. Client objects
construct query specifications declaratively and submit them to
Repository for satisfaction. Objects can be added to and removed from
the Repository, as they can from a simple collection of objects, and
the mapping code encapsulated by the Repository will carry out the
appropriate operations behind the scenes.
On the surface, Entity Framework accomplishes all of this, and can be used as a simple form of a repository. However, there can be more to a repository than simply a data layer abstraction.
According to the book Domain Driven Design by Eric Evans, a repository has these advantages:
They present clients with a simple model for obtaining persistence objects and managing their life cycle
They decouple application and domain design from persistence technology, multiple database strategies, or even multiple data sources
They communicate design decisions about object access
They allow easy substitution of a dummy implementation, for unit testing (typically using an in-memory collection).
The first point roughly equates to the paragraph above, and it's easy to see that Entity Framework itself easily accomplishes it.
Some would argue that EF accomplishes the second point as well. But commonly EF is used simply to turn each database table into an EF entity, and pass it through to UI. It may be abstracting the mechanism of data access, but it's hardly abstracting away the relational data structure behind the scenes.
In simpler applications that mostly data oriented, this might not seem to be an important point. But as the applications' domain rules / business logic become more complex, you may want to be more object oriented. It's not uncommon that the relational structure of the data contains idiosyncrasies that aren't important to the business domain, but are side-effects of the data storage. In such cases, it's not enough to abstract the persistence mechanism but also the nature of the data structure itself. EF alone generally won't help you do that, but a repository layer will.
As for the third advantage, EF will do nothing (from a DDD perspective) to help. Typically DDD uses the repository not just to abstract the mechanism of data persistence, but also to provide constraints around how certain data can be accessed:
We also need no query access for persistent objects that are more
convenient to find by traversal. For example, the address of a person
could be requested from the Person object. And most important, any
object internal to an AGGREGATE is prohibited from access except by
traversal from the root.
In other words, you would not have an 'AddressRepository' just because you have an Address table in your database. If your design chooses to manage how the Address objects are accessed in this way, the PersonRepository is where you would define and enforce the design choice.
Also, a DDD repository would typically be where certain business concepts relating to sets of domain data are encapsulated. An OrderRepository may have a method called OutstandingOrdersForAccount which returns a specific subset of Orders. Or a Customer repository may contain a PreferredCustomerByPostalCode method.
Entity Framework's DataContext classes don't lend themselves well to such functionality without the added repository abstraction layer. They do work well for what DDD calls Specifications, which can be simple boolean expressions sent in to a simple method that will evaluate the data against the expression and return a match.
As for the fourth advantage, while I'm sure there are certain strategies that might let one substitute for the datacontext, wrapping it in a repository makes it dead simple.
Regarding 'Unit of Work', here's what the DDD book has to say:
Leave transaction control to the client. Although the REPOSITORY will insert into and delete from the database, it will ordinarily not
commit anything. It is tempting to commit after saving, for example,
but the client presumably has the context to correctly initiate and
commit units of work. Transaction management will be simpler if the
REPOSITORY keeps its hands off.
Entity Framework's DbContext basically resembles a Repository (and a Unit of Work as well). You don't necessarily have to abstract it away in simple scenarios.
The main advantage of the repository is that your domain can be ignorant and independent of the persistence mechanism. In a layer based architecture, the dependencies point from the UI layer down through the domain (or usually called business logic layer) to the data access layer. This means the UI depends on the BLL, which itself depends on the DAL.
In a more modern architecture (as propagated by domain-driven design and other object-oriented approaches) the domain should have no outward-pointing dependencies. This means the UI, the persistence mechanism and everything else should depend on the domain, and not the other way around.
A repository will then be represented through its interface inside the domain but have its concrete implementation outside the domain, in the persistence module. This way the domain depends only on the abstract interface, not the concrete implementation.
That basically is object-orientation versus procedural programming on an architectural level.
See also the Ports and Adapters a.k.a. Hexagonal Architecture.
Another advantage of the repository is that you can create similar access mechanisms to various data sources. Not only to databases but to cloud-based stores, external APIs, third-party applications, etc.
You're right,in those simple cases the repository is just another name for a DAO and it brings only one value: the fact that you can switch EF to another data access technique. Today you're using MSSQL, tomorrow you'll want a cloud storage. OR using a micro orm instead of EF or switching from MSSQL to MySql.
In all those cases it's good that you use a repository, as the rest of the app won't care about what storage you're using now.
There's also the limited case where you get information from multiple sources (db + file system), a repo will act as the facade, but it's still a another name for a DAO.
A 'real' repository is valid only when you're dealing with domain/business objects, for data centric apps which won't change storage, the ORM alone is enough.
It would be useful in situations where you have multiple data sources, and want to access them using a consistent coding strategy.
For example, you may have multiple EF data models, and some data accessed using traditional ADO.NET with stored procs, and some data accessed using a 3rd party API, and some accessed from an Access database living on a Windows NT4 server sitting under a blanket of dust in your broom closet.
You may not want your business or front-end layers to care about where the data is coming from, so you build a generic repository pattern to access "data", rather than to access "Entity Framework data".
In this scenario, your actual repository implementations will be different from each other, but the code that calls them wouldn't know the difference.
Given your scenario, I would simply opt for a set of interfaces that represent what data structures (your Domain Models) need to be returned from your data layer. Your implementation can then be a mixture of EF, Raw ADO.Net or any other type of Data Store/Provider. The key strategy here is that the implementation is abstracted away from the immediate consumer - your Domain layer. This is useful when you want to unit test your domain objects and, in less common situations - change your data provider / database platform altogether.
You should, if you havent already, consider using an IOC container as they make loose coupling of your solution very easy by way of Dependency Injection. There are many available, personally i prefer Ninject.
The domain layer should encapsulate all of your business logic - the rules and requirements of the problem domain, and can be consumed directly by your MVC3 web application. In certain situations it makes sense to introduce a services layer that sits above the domain layer, but this is not always necessary, and can be overkill for straightforward web applications.
Another thing to consider is that even when you know that you will be working with a single data store it still might make sense to create a repository abstraction. The reason is that there might be a function that your application needs that your ORM du jour either does badly (performance), not at all, or you just don't know how to make the ORM bend to your needs.
If you are wrapping your ORM behind a well thought out repository interface, you can easily switch between different technologies as you see fit. It's not uncommon in my repositories to see some methods use EF for their work and others to use something like PetaPoco, or (gasp) ADO.net code. The repository abstraction enables you to use exactly the right tool for the job at hand without leaking these complexities into the client code.
I think there is a big misunderstanding of what many articles call "repository." And that's why there are doubts about what real value those abstractions bring.
In my opinion the repository in it's pure form is IEnumerable, while you and many articles are talking about "data access service."
I've blogged about it here.
I'm starting to use MVVM Light version 4,and i can't understand:
Why should i use DataService and IDataService?
Should i create a dataservice for any class in model?
First of all - as usual - in MVVM you do not have to do something, it is a recommendation or best practice. In general you are free to code the data access into your view model - and to do so might be a good practice for small projects or prototypical implementations.
However, you should consider that this also has some drawbacks. The reason for implementing a data service is that this component is reusable and even might be usable in other scenarios like an MVC application. Furthermore, it separates out the concern of getting model from a data store.
The reason for implementing an IDataService is that you can exchange the implementation when you need to, e.g. for supplying design time data. When you need this, you also have to consider the inversion of control pattern that heavily relies on interfaces. In this case also a IOC container might be interesting, although not necessary.
But, first of all the above are recommendations, patterns, design guidelines, and best practices that give you the freedom to design an application that best fits your requirements.
Edit: size of the data service
The design and scope of your data service depends on your application and it's requirements. It can range from a single data service for all you models to one data service per model. Furthermore, the design of you data service interfaces may be a separate decision. One service class can implement several service interfaces, thus allowing for hiding certain aspects (methods) of the implementation from the user.
When designing a data service you should look into the unit of work and repository patterns. There are several sample implementations around.
If you just need a very simple unit of work pattern that is based on a single query you can have a look at my blog, where I wrote about turning an IQueryable into a unit of work pattern. However, this fits only very simple cases, generally a complete implementation with a repository and a proper unit of work item is more advisably.
I have an ASP.NET MVC 2 application with some complex business rules and I'm trying to decide where to put specific logic.
The logic happens when creating records, based on certain fields of that record other records need to be created.
I'm currently using the repository pattern with an ORM and the easiest place to put this logic would be in my repository class but I feel like this is a pretty feeble location to have important rules, I would put it directly in my partial model classes that have my validation and metadata but I then have to call methods within my controller or repository and that may be extending too much knowledge about implementation to those layers.
What are your best practice tips for me?
Thanks!
You could have a service layer between the controller and the repositories. The repository performs simple CRUD operations with your model. A service method could make use of multiple simple repository calls to compose a business operation. This business operation will be exposed to the controller.