In MVVM pattern what is the recommended order of instantiation? [closed]

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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 ??

回答1:

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:

1. 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.

2. 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.

回答2:

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.

回答3:

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'.

回答4:

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

回答5:

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.

来源：https://stackoverflow.com/questions/19187873/in-mvvm-pattern-what-is-the-recommended-order-of-instantiation