Just wondering really if there's a consensus on the 'right' way to do this, for MVVM, DDD, and other philosophies . . .
So I've got a login screen, represented by a ViewModel, LoginViewModel. It can take a name and password. It also takes in through dependency injection a LoginService, that implements the logic of taking the username and password, and retrieving the Employee object.
My question is what's the 'right' way to get this information to the next view model? Let's say it's AccountSettings, which needs to know about the logged in employee. How do we encapsulate that? I've got an AccountSettingsViewModel, but should it require
a) An instance of the LoginViewModel?
b) An instance of the LoginService, which keeps a reference to the logged in employee
c) A shared object or field on a global object, like App or something?
Thanks in advance!
Personally all my view models in DDD or otherwise are simple data containers, used to restrict the data that gets sent from the application to the UI/view. I might include some code in my view models that's specific to transforming data for that view. I also consider my view models to be coupled to my views (I only mention this because I've seen 2 teams put them in their own separate project/assembly away from the views!).
If I have anything copying data, or performing actions to get the data needed for the view model, this would live in either my domain model or my application layer, probably in a service. I wouldn't ever inject a service into a view model.
Related
I'm building a .NET Core MVC application. It has a single endpoint that retrieves an imdb id of a movie by scraping the imdb site. So my question is, where do I put the logic to get the id? My original project structure is shown below.
+--Controller
+--Entry point api call
+--Logic
+--Class that retrieves imdbId
+--Models
+--Models
+--Context
So I was originally going to put the logic where it retrieves the id in the "Logic" folder and call it from the Controller. I was also going to instantiate the model and store it in the DB here. I also do request validation in the controller and make sure the given movie title and release year are correct format.
I'm starting to think this is incorrect though? Should I put request validation and id retrieval in the model layer? Any help on how to approach this would be appreciated.
So I was originally going to put the logic where it retrieves the id in the "Logic" folder and call it from the Controller.
This is what I would do too. ID retrieval is not a concern of the controller (the presentation layer does not care how you retrieve the ID) so it should be placed in a separate layer.
Should I put request validation and id retrieval in the model layer?
No, because this does not pertain to the models. The model layer should just contain the classes for your models. I would put request validation in the controller (presentation layer).
My suggestion is that your original project structure works fine. Within the logic layer, I would further separate concerns among different services, so that the ID retrieval functionality would reside in a separate service from the DB storage functionality (DB management could also be a separate layer on its own).
I would suggest you put it in the Logic class, so you can unit test the logic outside of the model. Your models should be super simple, just properties, and if there is some other internal logic they need.
The http call you will need to make I would put in your logic and ensure you are using some interface for your http client so you can create moqs for easier unit testing.
It appears that all the existing breezejs examples are passing entity models to and from the BreezeController.
But almost all our pages built are using some form of view models. In the days we have no BreezeJs, we retrieve the data (domain model) from a repository to populate (using AutoMapper or manually) a view model, which contains only the necessary data for that view. The WebAPI sends only the view model data over to the browser where we can populate a client-side view model (usually a knockout observable).
When saving data, we collect data from a <form> to populate an input view model, send only that data over to the server, where data in the input view model is mapped to the domain model. The update is saved by calling SaveChanges() on the DbContext entity in the repository.
Now, BreezeJs is to take over all our repository code by creating an EFContextProvider. The examples I have seen usually retrieve the domain model data and then pass it all to the client side.
[HttpGet]
public IQueryable<Item> Items() {
return _contextProvider.Context.Items;
}
It is the client-side javascript's job to build a view model.
Of course it is possible for us to build the view model at the server side:
[HttpGet]
public List<ItemViewModel> Items() {
var items = _contextProvider.Context.Items
.Include("RelatedEntity")
.ToList();
var model = new List<ItemViewModel>();
.... some code to build model from items ....
return model;
}
The benefit is that less data is transferred across the network, and we can do many manipulations on the server side. But I don't know if it is a good practice to modify this BreezeController like that. But at least, it returns data needed to list all the items.
The real trouble came when I tried to POST data back.
In the BreezeJs examples I found, they use a ko.observableArray() to store all the domain model data, let's say vm.items. Then the new record newItem is build by manager.createEntity into a domain model. After validating the data, item.entityAspect.validateEntity(), the newItem is pushed into vm.items and manager.saveChanges() is called, which somehow invokes SaveChanges() on the BreezeController.
[HttpPost]
public SaveResult SaveChanges(JObject saveBundle) {
return _contextProvider.SaveChanges(saveBundle);
}
I find too many things have been taken over! (Laugh at me if you disagree.) My questions are:
Can I just createEntity and then saveChanges?
I only have an empty form to fill in and submit. There is certainly no need to build a whole items array on the client-side.
Can I pass an input view model as a JObject and do some server-side processing before calling _contextProvider.SaveChanges()?
It turns out to be a super long post again. Thank you for reading it all through. Really appreciate it!
Good questions. Unfortunately, our demo code seems to have obscured the real capabilities of Breeze on both client and server. Breeze is not constrained in the ways that your fear.
I don't want to repeat everything that is in our documentation. We really do talk about these issues. We need more examples to be sure.
You are describing a CQRS design. I think it over-complicates most applications. But it's your prerogative.
If you want to send ItemViewModel instead of Item, you can. If you want that to be treated as an entity on the Breeze client - have the EntityManager turn it into a KO observable and manage it in cache, change track it, validate it -, you'll have to provide metadata for it ... either on server or client. That's true for Breeze ... and every other system you can name (Ember, Backbone, etc). Soon we will make it easier to create metadata on the server for an arbitrary CLR model; that may help.
You have complete control over the query on the server, btw, whether Item or ItemViewModel. You don't have to expose an open-ended query for either. You seem to know that by virtue of your 2nd example query.
On to the Command side.
You wrote: "[the examples] use a ko.observableArray() to store all the domain model data, let's say vm.items"
That is not precisely true. The items array that you see in examples exists for presentation. The items array isn't storing anything from a Breeze perspective. In fact, after a query, the entities returned in the query response (if they are entities) are already in the manager's cache no matter what you do with the query result, whether you put them in an array or throw them away. An array plays no role whatsoever in the manager's tracking of the entities.
You wrote: "Can I just createEntity and then saveChanges?"
Of course! The EntityManager.createEntity method puts a new entity in cache. Again, the reason you see it being pushed into the items array is for presentation to the user. That array has no bearing on what the manager will save.
You wrote: "Can I pass an input view model ... and do some server-side processing before calling _contextProvider.SaveChanges()?"
I don't know what you mean by "an input viewmodel". The Breeze EntityManager tracks entities. If your "input viewmodel" is an entity, the EntityManager will track it. If it has changed and you call saveChanges, the manager will send it to the controller's SaveChanges method.
You own the implementation of the controller's SaveChanges method. You can do anything you want with that JObject which is simply a JSON.NET representation of the change-set data. I think you'll benefit from the work that the ContextProvider does to parse that object into a SaveMap. Read the topic on Customizing the EFContextProvider. Most people feel this provides what they need for validating and manipulating the client change-set data before passing those data onto the data access layer ... whether that is EF or something else.
If instead you want to create your own, custom DTO to POST to your own custom controller method ... go right ahead. Don't call EntityManager.saveChanges though. Call EntityManager.getChanges() and manipulate that change array of entities into your DTO. You'll be doing everything by hand. But you can. Personally, I'd have better things to do.
I am relatively new to developing Cocoa applications on the Mac and come from a .NET C# background. I was wondering if a Cocoa Model object should contain its own data access methods such as Create, Update and Delete etc. Apples documentation seems to lean towards the Model doing everything but it doesn't seem right to have a Model (ie UserModel) which has a method named GetUsers which returns a collection of UserModels!
In ASP.NET MVC all my Models are just a representation of a Business object (ie a User) or a View. Using the example from above it would be the controllers responsibility to call a service (Business Layer or something of that nature) and get back a list of UserModel objects. The same controller would also populate a UserModel with data and pass that as a parameter to some other service which could then perform an Update or a Delete.
Any thoughts on this subject would be greatly appreciated as example code from Apple tend to be rather simple and don't really touch on CRUD type operations.
Thanks in advance.
I also come from a .NET background and I agree that Apple sometimes confuse things a bit. I tend to keep my domain models clean and implement a data access service. The only time I do it differently is if I am using CoreData in which my domain level objects are also CoreData objects (so they have underlying data persistence) HOWEVER I still use a Storage Service / Data Access Service to retrieve and save through.
If you want an example of a Storage Service / DAL I use then one of my blog posts contains it....CoreData Example
I am new to programming (6 weeks now). i am reading a lot of books, sites and blogs right now and i learn something new every day.
Right now i am using coldfusion (job). I have read many of the oop and cf related articles on the web and i am planning to get into mxunit next and after that to look at some frameworks.
One thing bothers me and i am not able to find a satisfactory answer. Beans are sometimes described as DataTransferObjects, they hold Data from one or many sources.
What is the recommended practice to handle this data?
Should i use a separate Object that reads the data, mutates it and than writes it back to the bean, so that the bean is just a storage for data (accessible through getters) or should i implement the methods to manipulate the data in the bean.
I see two options.
1. The bean is only storage, other objects have to do something with its data.
2. The bean is storage and logic, other objects tell it to do something with its data.
The second option seems to me to adhere more to encapsulation while the first seems to be the way that beans are used.
I am sure both options fit someones need and are recommended in a specific context but what is recommended in general, especially when someone does not know enough about the greater application picture and is a beginner?
Example:
I have created a bean that holds an Item from a database with the item id, a name, and an 1d-array. Every array element is a struct that holds a user with its id, its name and its amount of the item. Through a getter i output the data in a table in which i can also change the amount for each user or check a user for deletion from this item.
Where do i put the logic to handle the application users input?
Do i tell the bean to change its array according to the user input?
Or do i create an object that changes the array and writes that new array into the bean?
(All database access (CreateReadUpdateDelete) is handled through a DataAccessObject that gets the bean as an argument. The DAO also contains a gateway method to read more than one record from the database. I use this method to get a table of items, which i can click to create the bean and its data.)
You're observing something known as "anemic domain model". Yes, it's very common, and no, it's not good OO design. Generally, logic should be with the data it operates on.
However, there's also the matter of separation of concerns - you don't want to stuff everything into the domain model. For example, database access is often considered a technically separate layer and not something the domain models themselves should be doing - it seems you already have that separated. What exactly should and should not be part of the domain model depends on the concrete case - good design can't really be expressed in absolute rules.
Another concern is models that get transferred over the network, e.g. between an app server and a web frontend. You want these to contain only the data itself to reduce badnwidth usage and latency. But that doesn't mean they can't contain logic, since methods are not part of the serialized objects. Derived fields and caches are - but they can usually be marked as transient in some way so that they are not transferred.
Your bean should contain both your data and logic.
Data Transfer Objects are used to transfer objects over the network, such as from ColdFusion to a Flex application in the browser. DTOs only contain relevant fields of an object's data.
Where possible you should try to minimise exposing the internal implementation of your bean, (such as the array of user structs) to other objects. To change the array you should just call mutator functions directly on your bean, such as yourBean.addUser(user) which appends the user struct to the internal array.
No need to create a separate DAO with a composed Gateway object for your data access. Just put all of your database access methods (CRUD plus table queries) into a single Gateway object.
I've got this Silverlight Prism application that is using MVVM. The model calls a WCF service and a list of data is returned.
The ViewModel is bound to the View, so the ViewModel should have a List property.
Were should I keep data returned by a WCF service in MVVM?
Should the List property be calling into the Model using its getter? Where the model has a ReturnListOfData() method that returns the data stored in the model.
Or does the ViewModel stores the data after the Model is done with calling the server?
This is a follow up on Where to put the calls to WCF or other webservices in MVVM?
Generally if I need to keep the Model objects around (I consider most things coming back from a WCF service a Model object) I will store it in my ViewModel in a "Model" property.
I've seen people go so far as to create a standard Model property on their base ViewModel type, like this (I don't do this, but it's nice):
public class ViewModel<ModelType> : INotifyPropertyChanged ...
{
//Model Property
public ModelType Model
{
...
}
}
It's really up to you. Keeping them as close to their related ViewModels is probably the thing to take away here.
It really depends on other aspects of your application. E.g. how's the data returned by ReturnListOfData() used? Are there other components interested in it? Does user update elements in the list? Can it create new elements that he'll want to save later? etc.
In the simplest case you'd just have a List property exposed by your viewmodel to view, and you'd reset that list to whatever ReturnListOfData() returned. It will probably work for a case when user simply performs a search, doesn't do anything to the data later on, and there's only one view that is interested in that data.
But suppose a user wants to be able to modify elements of that list. Clearly, you'll have to somehow track the changes in that original list, so then when user clicks save (or cancel), you'd send to the server only elements that were changed (or added) or restore the original elements if user clicks cancel. In this case you'd need a Model object, that would keep the original data, so then your viewmodel contains only its copy.