I'm trying to implement some methods for my CoreData models and I'm wondering if the way I'm doing it is a good practice.
I want to implement methods of the type getObjectsWithFieldName. These methods could be used by several views. The way I'm doing it is implementing a Class method in the model as follows:
+(NSArray *)getObjectWithFieldName:(NSString *)fieldName andContext:(NSManagedObjectContext *) context;
Is this a good practice? Or there is another way to do it for iPhone Development?
Short answer, probably not. What class were you planning on adding this to?
If you already have access to the NSManagedObjectModel you can query the model directly to get this information. What class were you planning on adding this to?
Update
That is not going to work very well with Core Data because the designs are different. What is your end goal? You are looking for entities that have a particular property, but why? Since you are writing the code you know what the model looks like, why do you need to query the model to look for the entity?
This design, on its face, does not make sense in Cocoa/Core Data development so some clarification is going to be needed.
Related
I'm over-thinking this and getting myself into a muddle but can't clear my head.
I'm new at WPF and I'm trying to get familiar with MVVM. I understand the theory. I need a view, a model and another model (called the view-model).
However, what happens if my model is constructed by a parameter of the View's constructor.
So, assuming I have a totally empty project, the only thing is I've an overloaded MainWindow constructor which takes the model:
public MainWindow(Logging.Logger logFile)
{
InitializeComponent();
this.DataContext = logFile;
}
The Model is the logFile. Can I still implement the MVVM when I don't have a separate Model class?
Any thoughts would be appreciated.
You're overthinking this.
There are several components to MVVM:
View:
The view provides a, well, view on the data. The view gets its data from a viewmodel
ViewModel:
The viewmodel is there to organise your data so that you can organise one or more sources of data into a coherent structure that can be viewed. The viewmodel may also perform basic validation. The ViewModel has no understanding of the UI so should not contain references to controls, Visibility, etc. A viewmodel gets its data from services.
Services:
Services provide data from external sources. These can be WCF, web services, MQ, etc (you get the idea). The data the service returns may need to be shaped so that it can be displayed in the UI. To do this you would take the raw data from the service and convert it to one or more Model objects.
Model:
A model object is an object that has been created so that it can be easily displayed/work with the UI.
You may find that you don't need to shape your data coming from your services (lucky you), in which case there's no need to create model objects. You may also decided that you don't want your services talking directly to your viewmodels but instead what to have them get their data via a "mediator" object. That's also good in some situations (usually when you're receiving a continuous stream of data from a source/multiple sources).
MVVM is a bit like porridge: there are lots of potential garnishes you can add, but you don't necessarily need to add them all. Or want to.
Does this help?
Edit: just stumbled on this: a more in-depth expression of what MVVM is:Mvvm Standardisation. This may also be useful
The Model is something the ViewModel will know about but not the View. If you need to present info about a Logger, you can certainly have a LoggerViewModel that knows about a Logger, and in turn the View winds up getting to know about the ViewModel. There are several ways to do that, and setting the DC in the view constructor is one of them.
After that basic understanding of who knows about who, what really makes the MVVM architecture pattern, IMO, is that ViewModel communicates to the View via databinding. Nothing more and nothing less. Lots of goodies come out of this, but that is the crux of it that makes it different than other separation of concerns patterns (like Presentation Model, MVP, etc)
That said, you need to get a feel for it by working through some sample projects. Asking questions here on SO is fantastic when you get stuck on something, but you must realize your question here is a bit fuzzy at best. Also, unless you are really looking to present logging info in your view, logging is not an MVVM concern. Its interesting alright but not MVVM.
Google Josh Smith's MVVM demo on MSDN for a perfectly meaty yet approachable beginning sort of project. And ask more questions or refine the one here as they come up!
HTH,
Berryl
forget the view! at least at the beginning ;)
try to think about what you want and what you need. what i understand is that you wanna handle a logfile. so you need a viewmodel for that.
public class LoggerViewmodel{}
you can put the logfile as a parameter to the vm ctor. now you have to think about what you wanna do with your logfile? for everything you want create a property (LastModified, LastRow, whatever) on your viewmodel.
btw there a two different ways to do mvvm, first is view first and the other is viewmodel first. i do both in my projects and take the appraoch wich fits better (viewmodel first the most time ;)) to my needs.
pls edit your questions and add what you wanna do with your logfile, then we can give you a better answer.
edit:
Can I still implement the MVVM when I don't have a separate Model class?
to answer your question the short way - yes you can. you have to seperate the view and viewmodel and use binding to bind the view to the datacontext(viewmodel).
I have read several resources so far about MVC design pattern for iPhone application development. However I have one question related Model part of MVC in iPhone development context.
Firstly, I would like to say what I think of MVC,
Model : is related to a data which we have regarding our application's objects. Example : Application of a Train simulator than, Each train on simulation will be having it'e own attributes and that will be saved inside Train model.
Controller : is something which controls UI updates. It keeps a reference to Train model and checks for any changes in model, If there is than change the View of that particular train. And it keeps checking for any UI input so it can change data inside Model.
View : This is fairly obvious View is all about UIView, What we see on screen.
Now, Question is...
Does Model has to be persistent to be considered as a Model? or I can
have a Class which has variables without any persistency. Would that
be considered as Model as well, Or Model must be stored somewhere like
in CoreData or .Txt file etc....
Thanks for any input!
Does Model has to be persistent to be considered as a Model?
It may vary or transform as long as your controller can support it.
Model must be stored somewhere like in CoreData or .Txt file
Not at all.
You can use any Model if applicable. Model is just another abstraction of your actual logic/database/network access/blablabla...
No, the idea behind MVC does not actually relate to databases, although that is usual. The model simply should capture all the business logic. If you are building a calculator, view is the display and the buttons, model is the part of code that knows how to add and subtract, and controller is the one that connects the two. No persistence involved.
No, a model doesn't have to be persistent.
In theory a good way to start your project would be to use a non-persistent model for simplicity. Then when you change your model to become persistent, you should not have to make any changes to your view or controller, since you will design the interface to be unaware of the details of the model.
In practice that's not a great idea on iPhone if you're planning to use Core Data for your model when you make it persistent. Core Data requires you to design your classes in a little bit of a different way. Even though you shouldn't have to make many changes to your view and controller code when you change your model, you will have to make a lot of code changes to your model. If you know you will be using Core Data, better to start right off with it.
I am currently using caliburn.micro in my WP7 project and I am quite happy with it. My application is very data-heavy, so I took a look at Agfx (http://agfx.codeplex.com), seems it can save me a lot of time on data requesting and caching.
But the problem here is that agfx also provide a base view model, while I've already had one which inherits Screen of caliburn.micro. Of course I can encapsulate a new view model base which inherits ModelItemBase from agfx, and implements IScreen. But I kinda don't like this, is there any better soultion or best practice you can share with me about how to integrate the 2 great frameworks?
Best Regards,
-Peng
I am actually using AgFx with another UI framework which has its own ViewModelBase. My own understanding is, the ModelItemBase that's provided by AgFx is a model base rather than a viewmodel base. It basically takes care of the data.
My viewmodel which inherits from my ViewModelBase, does a lot more stuff like Tombstoning, application bar bindings, etc. It's designed for displaying the data on the view.
I think it fits in mvvm and works out really well. Hope this helps. :)
Guess we have simple model, e.g. let it be a Person { Name, Age }.
Now we want to display a list of persons.
Persons are read-only
We don't need to edit them
We don't need any additional stuff like presentation properties, etc.
Now question is if it is a good practice do not create PersonViewModel class that will probably be a copy of model class or will delegate all its properties? Is it a good idea to simply bind listbox to list of persons, not to their view models? It looks DRY enough, but what about idea of MVVM?
I have no issue with bypassing the VM and using the M directly in the View. Sometimes the models are so small and static that loading them into a wrapping VM is wasteful.
I've created standalone ViewModels, but generally not standalone models. The reason for this is DataBinding -- most POCOs don't implement the INotifyPropertyChanged interface and adding them in to make them pseudo-Models seems to defeat the purpose of re-using a simple class AND respecting the MVVM pattern.
Now, if you KNOW you're never going to be editing them, it may not be a bad idea. A VM with simple property redirects seems a bit pointless to me.
As far as I'm concerned, if you implement INotifyPropertyChanged, then it becomes a ViewModel, and you can bind to it. :) When I wrote SoapBox Core which is all MVVM, I took the approach that everything is a ViewModel. The only Model objects were classes from third party libraries that I brought in and wrapped in a ViewModel of my own.
I wouldn’t create a ViewModel for the Person business object when the ViewModel doesn’t introduce new required functionality.
You might be interested that there is a second approach in the MVVM community: Create a ViewModel per View and not per Business Object.
Sample applications that follow this approach can be found on the WPF Application Framework (WAF) website.
I'm developing a program which allows users to input some information which then gets stored and dynamically creates an image based on it.
I was going to use the Entity Framework to do the work with the data, but then I obviously need a way to generate the image. My thinking was that the "correct" way to do this was to somehow extend the data entity to include a function call like "CreateImage", or alternatively, to create a separate class not in the EF called "DataImage" which would have a "generate" method.
Extending the EF seems the "pure" way to do this, but I'm not sure how or if it's more practical than using the separate class.
Any thoughts on the best way to do this and how to do it using EF?
Putting this functionality in the EF would be a major violation of SRP. Breaking SRP has cascading negative effects as your application grows.
The approach you most likely want to take instead is a totally separate, encapsulated image generation service which takes interfaces that your EF entities implement. This decouples your image service from your data access completely; you get complete testability and zero dependencies right away.