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I got this asked in an interview recently . He wanted to know the order of instantiation of Model View and ViewModel what the precise order of instantiation would be ?
I thought the view is always instantiated first and then comes the viewmodel and then comes the model. was i wrong ??
I thought the view is always instantiated first and then comes the viewmodel and then comes the model. was i wrong ??
There is no single standard. There are, in general, two approaches:
View-First - The View will be instantiated first, and in turn instantiate the ViewModel, which will likely create the underlying model. This typically means the order of instantiation is View->ViewModel->Model.
ViewModel-First - The ViewModel is created, which in turn instantiates the Model. The View is generated by the system based on DataTemplates after the ViewModel. This would mean the order of instantiation would be ViewModel->Model, then View (indirectly from XAML).
Most frameworks which are geared heavily towards a designer-first approach tend to do View-First construction. This makes it easier to work with the designer (in general).
Many frameworks which are geared heavily towards developer-focused scenarios will often do ViewModel first. This approach can actually lead to even less coupling, and simpler "code-only" construction of everything from the ViewModel level.
This is an open ended question because you can look at it conceptually, in which case it follows the acronym. If you look at it in practice (particularly referring to WPF or WinStore Apps) its a bit different.
Conceptually
Model should be instantiated first because all ensuing decisions of the application will be based on the model on which the app was designed to operate on. Then the view model, because views depend on view models, not the other way around. One VM can have multiple views, but one view generally does not have multiple view models (generally!). Then the view(s) that present the data.
Practice (In WPF and WinStore Apps)
The App class is instantiated first, which fits in some odd portion of the VM-M area. But that's not completely relevant because it's outside the scope of the pattern. The View is usually created and attached to the visual tree first. Then the ViewModel is instantiated in the code-behind, at which point the model is loaded. Then a massive UI refresh occurs that displays everything that was loaded initially. From then on out, everything in the 'conceptually' portion holds true.
This question may get closed due to opinions, as there is no definite answer. but this is what I've seen, read, and experienced.
Well that's a strange interview question. In my opinion and in general, I would agree with you. The view model would instantiate the model and the view would come first, instantiating the view model. But of course, it very much depends on the architecture of the application. The beauty of WPF enables these things to be done in different ways. Then you also have dependency injection, so I would say that the answer should really be 'it depends'.
The question is a bit silly, because it's limited to a simple scenario where each layer is a single class. Suppose one view model provides another one. If we decide "view comes first", do we need to create another view before we are allowed to call that function on the original view model? What if the view must be chosen based on the returned view model? And on the flip side, if we decide "viewmodel comes first", what if the new viewmodel must be chosen based on parameters input from the view?
A layered architecture is about dependencies. MVVM says V depends on VM and VM depends M. It doesn't say anything about instantiation order. You might decide that dependencies should be passed into constructors, meaning that instantiation order needs to be M-VM-V, but I don't see any practical reason to try to enforce such a small detail throughout an entire application
IMHO it is a Trojan horse question to see how one thinks more than an actual answer to see if one can quantify their experience with actual MVVM projects.
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I have 3 ViewModels, let s say ViewModelA, ViewModelB and ViewModelC.
Each view Model has a corresponding View.
In ViewModelA I have a Public Event that I`m using send some information.
I want ViewModelB and ViewModelC to subscribe to that Event in the current/Running instance of ViewModelA?
How can I do this?
If a new a ViewModelA I will have a different instance of ViewModelA, so I need a reference to the current ViewModelA...
Note: I`m not using MVVM Light or nay other framework (yet), because I did not learn them, yet :)
Thank you.
If you are creating new views from the view of ViewModelA you can pass the reference to another view like this
var viewModelA = DataContext as ViewModelA;
var newWindow = new ViewB(viewModelA);
Then you would need to have a property in your ViewModelB
public ViewModelA MyViewModelA { get; set; }
And in your new view:
public ViewB(ViewModelA viewModelA)
{
InitializeComponents();
var viewModelB = DataContext as ViewModelB;
viewModelB.MyViewModelA = viewModelA;
}
And then you can access your ViewModelA via MyViewModelA.
I've always done it like this and haven't seen any problems so far.
1) You can implement some kind of simple Publisher/subscriber like this one on codeproject. You will be one step ahead because most of frameworks has something similar:
In MVVM light it is called Messenger:
In Prism there is EventAggregator
2) Ugly solution would be to create static event in ViewModelA, this way you won't need a reference
While you can pass references between ViewModels, it makes your app tightly coupled and not particularly scalable. Also if you decide to make a change in the future the amount of refactoring quickly grows making managing changes a lot more difficult.
Have a look at a PubSub Event framework. These are all included in MVVM Frameworks such as PRISM or MVVM-Light that you mention, but you can always add your own version if you don't want or need the full frameworks mentioned above.
Have a look here for a simple no nonsense implementation that you should be able to adapt to your own requirements.
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I'm writing a simple code generator in C# for automating common tasks on bussiness applications such as data binding, model and viewmodel generation and record updating.
The generated code uses a data mapper that implements equallity by reference comparision (without id) and flag properties for transient state (if the object was created but not persisted).
For updating the object properties I have 3 options:
On the property setter call an UPDATE for one column only immediatly. This would provide instant persistence without any other mecanism managed by the final programmer, but it would requiere and unnecessary number of UPDATE calls
Mantain a Frozen state on all entities wich would prevent any property set, and BeginModification and EndModification methods, wich would enable property setters and UPDATE all modified columns on the EndModification. This woud requiere the programmer to call this methods wich is undesirable for the code generator, because code simplicity and diminishing programmer intervention is its primary goal
Mantain a timer for each entity (wich can be implemented as a global timer and local counters), and give certain "dirty time" to entities, when a property is setted, its dirty time is resetted to 0 and when its local clock gets to certain values, columns UPDATE would be made. This wouldn't require any extern final programmer code and woud group several property sets on a single UPDATE, because contiguos property sets have almost 0 time between.
The timer aproach can be combined with a CommitChanges method that will call the UPDATE immediatly if desired
My prefered way is the local dirty timer because the posibility of zero programmer intervention besides property sets, the question is: It is posible that this timer aproach would lead to data inconsistency?
If you're writing this as an educational exercise or as a means for further honing your design skills, then great! If you're writing this because you actually need an ORM, I would suggest that looking at one of the many existing ORM's would be a much wiser idea. These products--Entity Framework, NHinbernate, etc.--already have people dedicated to maintaining them, so they provide a much more viable option than trying to roll your own ORM.
That said, I would shy away from any automatic database updates. Most existing ORM's follow a pattern of storing state information at the entity level (typically an entity represents a single row in a table, though entities can relate to other entities, of course), and changes are committed by the developer explicitly calling a function to do so. This is similar to your timer approach, but without the...well...timer. It can be nice to have changes committed automatically if you're writing something like a Winforms application and the user is updating properties through data binding, but that is generally better accomplished by having a utility class (such as a custom binding list implementation) that detects changes and commits them automatically.
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I have an application that requires the use of a lot of different enumerations. The application can be devided into various layers. For the sake of the example let's assume three layers: a 3rd party analytics library, the application business logic and the UI/presentation logic.
In many cases all layers may have the need for an enum representing the same concept. Let's take a payment frequency for instance. (e.g. annual, semi-annual, quarterly etc...).
The 3rd party library provides its own enum, the various classes in the business logic layer would need a similar enum, and finally the UI layer may need it for presenting various choices in dropdowns etc...
Now, normally I'd like to shield the user of each layer from its internal dependencies and implementation details by not exposing types from internal dependcies in the public interfaces of the layer. That means that even though the interaction with the 3rd party lib requires the use of its own "Frequency" enum, I'd need to create an equivalent "Frequency" enum for the business layer and potencially another one for the UI layer...
All of this requires a lot of mapping back and forth, with potencially lot of additional mapper classes. The adventage on the other hand is that each layer may decide to exclude values it does not need or support from its own version of the enum...
Now since I have to deal with a lot of enums, I was just wondering if this is generally a good thing to do, or am I just overcomplicating things?
I would say you're overcomplicating things. Why not have a separate project for shared entities like this? You can then have each assembly reference your Common project to reference the necessary enums and still have them all totally independent of each other.
Well, i may be missunderstanding you but, actualy, you will just need, in data layer, which is viewd by the logic layer, your enuns. Using your own example, you would have a column names PaymentFrequency in a table. Your table should be mapped in your data layer for this specific field, so you'll create a enum and the type of the column PaymentFrequency - by this moment an attribute of your table class or entity - will be setted as the enum created for this purpose. So, when you point your logic layer to request/access your data layer, you'll need to understand this same enum - so, if your using a DAL layer between logic and data layers, i recommend you to create the enuns in a separate class/file. Well, in your logic layer - by using the enum options - you'll populate a simple Combo component with the enum options, so, you'll be presening the options in the UI by hiding all your layers, in a way that users won't know you are actually using a enum to handle the options.
I hope this help. Sorry - really sorry - for this * bad english!
If this don't help, post some cod examples to be easier to help you =)
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Before down-voting let me explain my question. I have a little experience in designing architectures and try to progress. Ones, when I was fixing a bug, I came up with a conclusion that we need to make our private method to be public and than use it. That was the fastest way to make my job done, and have a bug fixed. I went to my team-leader and said it. After I've got a grimace from him, I was explained that every public method is a very expensive pleasure. I was told that every public method should be supported throughout the lifetime of a project. And much more..
I was wondering. Indeed! Why it wasn't so clearly when I was looking in the code. It wasn't also so evidently when I designed my own architectures. I remember my thoughts about it:
Ahh, I will leave this method public, who knows, maybe it will come usefull when the system grows.
I was confused, and thought that I made scaleable systems, but in fact got tons of garbage in my interfaces.
My question:
How can you explain to yourself if a method is really important and worthy to be public? Are any counterexamples for checking it? How you get trained to make private/public choise without spending hours in astral?
I suggest you read up on YAGNI http://c2.com/cgi/wiki?YouArentGonnaNeedIt
You should write code to suit actual requirements because writing code to suit imagined requirements leads to bloated code which is harder to maintain.
My favourite quote
Perfection is achieved, not when there is nothing more to add, but
when there is nothing left to take away.
-- Antoine de Saint-Exupery French writer (1900 - 1944)
This question need a deep and thorough discussion on OOP design, but my simple answer is anything with public visibility can be used by other classes. Hence if you're not building method for others to use, do not make it public.
One pitfall of unecessarily making private method public is when other classes did use it, it makes it harder for you to refactor / change the method, you have to maintain the downstream (think if this happen to hundreds of classes)
But nevertheless maybe this discussion will never end. You should spend more time reading OOP design pattern books, it will give you heaps more idea
There are a few questions you can ask yourself about the domain in which the object exists:
Does this member (method, property, etc.) need to be accessed by other objects?
Do other objects have any business accessing this member?
Encapsulation is often referred to as "data hiding" or "hiding members" which I believe leads to a lot of confusion. Inexperienced developers would rightfully ask, "Why would I want to hide anything from the rest of my code? If it's there, I should be able to use it. It's my code after all."
And while I'm not really convinced with the way in which your team leader worded his response, he has a very good point. When you have too many connection points between your objects, you end up with too many connections. Objects become more and more tightly coupled and fuse into one big unsupportable mess.
Clearly and strictly maintaining a separation of concerns throughout the architecture can significantly help prevent this. When you design your objects, think in terms of what their public interfaces would look like. What kind of outwardly-visible attributes and functionality would they have? Anything which wouldn't reasonably be expected as part of that functionality shouldn't be public.
For example, consider an object called a Customer. You would reasonably expect some attributes which describe a Customer, such as:
Name
Address
Phone Number
List of orders processed
etc.
You might also expect some functionality available:
Process Payment
Hold all Orders
etc.
Suppose you also have some technical considerations within that Customer. For example, maybe the methods on the Customer object directly access the database via a class-level connection object. Should that connection object be public? Well, in the real world, a customer doesn't have a database connection associated with it. So, clearly, no it should not be public. It's an internal implementation concern which isn't part of the outwardly-visible interface for a Customer.
This is a pretty obvious example, of course, but illustrates the point. Whenever you expose a public member, you add to the outwardly-visible "contract" of functionality for that object. What if you need to replace that object with another one which satisfies the same contract? In the above example, suppose you wanted to create a version of the system which stores data in XML files instead of a database. If other objects outside of the Customer are using its public database connection, that's a problem. You'd have to change a lot more about the overall design than just the internal implementation of the Customer.
As a general rule it's usually best to prefer the strictest member visibilities first and open them up as needed. Combine that guideline with an approach of thinking of your objects in terms of what real-world entities they represent and what functionality would be visible on those entities and you should be able to determine the correct course of action for any given situation.
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I'm beging to work on a re-design (software design) of XAML-based application, I wrote 2 months ago. I think I made lot's of architectural mistakes during development, which led to situation when the UI part of app is hard to extend, maintain, code is hard to understand. My app is written using PRISM 4 in MVVM style, but despite the fact that Prism was invented for modular design my App turned out to be very monolithic. I'm going to continue to use PRISM 4 in new design, but this time I want to break my UI part of application in smaller, reusable, extendable building blocks.
Suppose we are designing data input form.Top container contains Save,Cancel buttons and TabControl, which contains 2-3 tabs that contain lot's of grouped input controls.
The are 2 completly different aproaches to UI design I can see: static (compile-time), dynamic(run-time). Static it's when you predefine your UI before compiling, i.e DataGrid with columns defined in XAML. Dynamic it's when you compose UI at runtime, i.e you defined DataGrid in XAML but add columns at runtime based on user asctions.
What rules you use when you decide which aproach to use, sattic or dynamic? What you would choose in this particular example?
Next big question is how to break UI to pieces.
What rules you use when you define UserControls, how you would define UserControls for this example form? Now about ViewModels, would you create single VM for this example form or multiple (explain)? What do you think about situations when ViewModel contains other ViewModels (not simple wrapers around model, but real VM which contains logic).
And now the hardest question at least for me. Extending UI building blocks (UserControls and ViewModels).
Situation when you need to create a copy of some Form but with slightly different interface and|or logic is quite often, especially when you need to integrate authorization (permissions) in UI. Suppose we need to support slighly different version of out example form (doesn't matter how many exactly versions, suppose 2-6).
I can think of these aproaches to solve this problem:
Create duplicates of whole from (usercontrols and viewModels) and modify them (the static way). The good thing all variants are independent, great flexibility, no dependecies, the bad thing code duplicate, if you will need to change something in all variants most likely you will have to modify this everywhere, especially with ViewModels.
Conditional presentartion, you add lot's of conditional code to your ViewModel, like IsThisVisible, IsThatDisabled (the dynamic way). The good thing maximum code reuse, the bad thing code bloat and mess. Your code will be hard to understand,maintain.
Break UI to very small atomic UserControls, compose separate form variants from this UI pieces, and use ViewModel inheritance with virtual members and overrides. I haven't ever used inheritance of ViewModels, and would like to hear your opinion on this subject.
n. Other aproaches I can't think of.
In my experience, the development path tends to work this way:
Design a view in Blend or Kaxaml or whatever, and a view model that backs the view.
Realize that portions of the view need to be dynamic. Implement flags in the view model and styles in the view to show/hide them.
Realize that all the flags are getting out of hand. Get rid of the flags, and refactor the view to present collections of user controls, and the view model to dynamically populate collections of view models.
It sometimes happens that I know well in advance that I'm going to need to use approach #3, and I design for it from the start.
But one of the beauties of WPF and MVVM is that even if I don't design for this from the start, it's not too hard to move in that direction if circumstances demand it. Refactoring a bunch of view model properties into a single collection of view models doesn't take a whole lot of time or effort once you've done it a few times.
I can tell you this, though: making a copy of a XAML object and editing it makes klaxons sound in my head. It's possible to imagine circumstances under which that might be OK, but it's not the way to bet.