Re: How to pass a new instance of a viewmodel to an existing usercontrol? - WPF Discussion Boards

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

26-May-11 22:08

Strictly speaking, you wouldn't pass the data in. The reason that this is a no-no is that you've introduced a coupling here where the parent ViewModel has to know what views are associated with it, and hence the ViewModels that are associated with those views. This is a real no-no.

Remember that MVVM is based on the concept of a ViewModel wiring up to a Model; this tells you that your child VM needs to hook into the same data as the top-level VM. There's no real reason though, that this has to be a database. What you could do is have a top-level entity which is responsible for reading a whole bunch of data in that your top level VM uses, and your child VMs could also use this source of data.

Forgive your enemies - it messes with their heads

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Duke Carey27-May-11 0:42

Duke Carey

27-May-11 0:42

Thanks Pete. A little history is that this started out as a single SL page with one way for users to select parameters that filtered the data to be returned. Then came the suggestion that some users would want a markedly different way of filtering but the additional controls ate up lots of screen space. No problem, thinks I, refactor out the common portions of the screen (the portion that deals with presenting and editing the data) and create a user control with its own VM with all the editing logic, etc., and drop the user control onto each of the 2 pages.

OK, at the page level my VM ("ParentVM") reacts to the user's filtering steps, retrieves the data, and populates an ObservableCollection that is a property of the ParentVM. It's that ObservableCollection property that I want the ChildVM to access, so that the usercontrol can present the data to the user and the ChildVM can take over. Where I'm stumbling is getting the ChildVM to see the ParentVM's ObservableCollection property.

Am I looking at this bass-ackwards? Or is the approach OK but I'm overlooking something elementary?

Thanks

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

Pete O'Hanlon27-May-11 2:01

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

27-May-11 2:01

A simple mechanism to achieve this is to have the parent level VM manage the population of the data in a model. As long as the child can see the data in the same instance of the model, then it can retrieve and manage it's own ObservableCollection based on this data. All you would need to do would be to hook into the CollectionChanged event on the model to know that you had to retrieve the new data items (although I would normally look at using a more explicit event because the CollectionChanged gets raised on every change to the collection, and you might want to wait for the data to be updated completely before you react to the change).

A mechanism such as making the model class static would ensure that all VMs could see the same instance, and react to changes in it.

So, why am I recommending this approach rather than binding to the VM? After all, you could achieve a similar effect that way. By going with this approach, I've looked at completely decoupling the VMs - ensuring that they don't need to know about each other and, more importantly, ensuring that issues such as you have faced here are, to a large extent, minimised as you change the VMs. Remember, the VM matches what's in the View, so you don't want to have to rejig things around again just because you move something from one view to another.

Forgive your enemies - it messes with their heads

My blog | My articles | MoXAML PowerToys | Mole 2010 - debugging made easier - my favourite utility

Mycroft Holmes27-May-11 13:54

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

27-May-11 13:54

I use a different approach and I have the nasty feeling it may be incorrect.

I have a navigate frame where the filter lives on the parent (which is itself a navigation frame) and when the user makes a selection the required control is displayed. Where I am probably most in error is that my VMs are static and the child VM gets some information from the parent VM, SelectedItem etc.

I have also been known to have the VM service multiple view (view and dialog).

Never underestimate the power of human stupidity
RAH

Duke Carey31-May-11 0:37

Duke Carey

31-May-11 0:37

Thanks for the input Mycroft. I'm not a programmer - but I'm closer to being one than anybody else in our department so this stuff falls to me. One of the things I've found frustrating in learning MVVM is that there's plenty of info on the web about how to do this task or that task in MVVM, but not much that explains WHY, at least not in terms that a novice can get their head wrapped around.

I thought about trying an approach very similar to yours but stalled on pursuing it. The downside to not doing this full time is that doing things by trial and error is REALLY time consuming because, for me, there's lots more error than there would be for somebody more fluent and accomplished.

Since Pete responded, I have had only a few minutes to even think about what he suggested, so no real progress on it.

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

Mycroft Holmes31-May-11 13:10

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

31-May-11 13:10

Duke Carey wrote:
I'm not a programmer

Considering this I then ask myself why you decided to use MVVM, as it is strongly focused on testing which is very much a professional programmers criteria.

I don't have a requirement to do a lot of testing myself but have found that MVVM has numerous other benefits and it does dictate a nicely structured solution. Getting the structure just right is the real challenge.

Sometimes (often) the why will not make sense unless you have the same requirement and some people make stuff overly complex just for the sake of complexity. I'm pretty sure I have bastardised the MVVM concepts but then I just want the bloody thing to work and I do like simple, even if it means breaking a design principle (one of the reasons I am no longer writing articles).

Never underestimate the power of human stupidity
RAH

_Maxxx_31-May-11 13:32

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

31-May-11 13:32

Mycroft Holmes wrote:
I do like simple, even if it means breaking a design principle (one of the reasons I am no longer writing articles).

I think knowingly breaking a principle for a good reason is a good thing - after all, someone decided upon a principle, it wasn't handed down on some rock tablet from a burning bush (sorry, religion not my strong point - but you see what I mean)

So if you have need to break a design, with knowledge of the design and knowing you are breaking it, then I think you should not only do so, but you should also write articles specifically about it.

I know I have seen people using MVVM write pages of XAML and C# code because they don't want to write any code-behind - although the code-behind is probably three lines and makes perfect sense. I say, break the principle by all means- but break it with the knowledge that you are breaking it, and why you are breaking it, and do so with head held high. (cue stirring music(

MVVM# - See how I did MVVM my way
___________________________________________
Man, you're a god. - walterhevedeich 26/05/2011

.\\axxx
(That's an 'M')

Pete O'Hanlon31-May-11 21:37

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

31-May-11 21:37

_Maxxx_ wrote:
I say, break the principle by all means- but break it with the knowledge that
you are breaking it, and why you are breaking it

Exactly so. As I've often told people, MVVM is just a tool to use. It should never become dogma - if it does that, then it's failed in it's intended purpose of making WPF/SL development easier to accomplish.

Forgive your enemies - it messes with their heads

My blog | My articles | MoXAML PowerToys | Mole 2010 - debugging made easier - my favourite utility

_Maxxx_29-May-11 15:35

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

29-May-11 15:35

Pete O'Hanlon wrote:
the parent ViewModel has to know what views are associated with it, and hence the ViewModels that are associated with those views. This is a real no-no.

I don't understand why you say this?

Lets say I will have a page showing a list of customers, and also the individual fields from the currently selected customer.

In MVVM I would create a VM for the page, and also (because the functionality is self-contained) a VM for the list of customers and a VM for the individual Customer fields (that is, ONE VM with properties for each Customer field)

Now, my Page VM is nonsense without having a CustomerListVM and a CustomerEditVM somehow contained within it.

It certainly doesn't know about the VIEWs but it MUST know about the ViewModels

Similarly neither the customerListVM nor the CustomerEditVM can know about the PageVM - they need to exist independently.

so surely my PageVM legitimately can say "I want to display this list of customers" - and give the list to the CustomerListVM?

MVVM# - See how I did MVVM my way
___________________________________________
Man, you're a god. - walterhevedeich 26/05/2011

.\\axxx
(That's an 'M')

Pete O'Hanlon30-May-11 23:05

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

30-May-11 23:05

_Maxxx_ wrote:
I don't understand why you say this?

Lets say I will have a page
showing a list of customers, and also the individual fields from the currently
selected customer.

In MVVM I would create a VM for the page, and
also (because the functionality is self-contained) a VM for the list of
customers and a VM for the individual Customer fields (that is, ONE VM with
properties for each Customer field)

Now, my Page VM is nonsense
without having a CustomerListVM and a CustomerEditVM somehow contained within
it.

It certainly doesn't know about the VIEWs but it MUST know
about the ViewModels

Similarly neither the customerListVM nor the
CustomerEditVM can know about the PageVM - they need to exist
independently.

so surely my PageVM legitimately can say "I want to
display this list of customers" - and give the list to the CustomerListVM?

If you check out the problem that the user describes, it's not the same issue that you are describing here. What you describe is a perfectly acceptable solution where you have one view containing data derived from several VMs, where the containing VM is still being displayed.

What the OP described however was a different scenario. He has moved the implementation out of a single view into multiple views, and there is no real physical interaction between these views other than the fact that they were all originally part of one larger VM. The key point here is that the view/viewmodel interaction here is no longer with the "parent" VM, it's with the child VM. This means that, conceptually, the new VM can exist without knowledge of the old VM. The Model part, which has been entirely forgotten about in this situation, can be the same model (and is the one I would choose). So, rather than having to contort the code to worry about maintaining this disconnected list of VMs, it makes more sense to just have the child VM use the same model. Remember that a lot of people mistake the model for always being the data layer; it doesn't have to be, so a simple containing model makes this scenario a whole lot easier.

Forgive your enemies - it messes with their heads

My blog | My articles | MoXAML PowerToys | Mole 2010 - debugging made easier - my favourite utility

_Maxxx_31-May-11 13:16

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

31-May-11 13:16

Thanks Pete - I was interpreting the OP differently - on re-reading after your post, I follow your logic.

cheers

MVVM# - See how I did MVVM my way
___________________________________________
Man, you're a god. - walterhevedeich 26/05/2011

.\\axxx
(That's an 'M')

Pete O'Hanlon31-May-11 21:37

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

31-May-11 21:37

No problems mate. It took me two readings to get what he wanted right in my head.

Forgive your enemies - it messes with their heads

My blog | My articles | MoXAML PowerToys | Mole 2010 - debugging made easier - my favourite utility

Mycroft Holmes31-May-11 13:14

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

31-May-11 13:14

At this point I introduce my bastardisation and have 1 CustomerVM that would service all 3 of your views.

Never underestimate the power of human stupidity
RAH

_Maxxx_31-May-11 13:27

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

31-May-11 13:27

Mycroft Holmes wrote:
1 CustomerVM that would service all 3 of your views.

that's fine - but what if you want to use the customer lookup view elsewhere - not for editing a customer but for selecting a customer for some other function (say during order entry) You don't want to use that VM with all its overhead do you? soyou would end up with aniother ViewModel supporting the Customer lookup View...

I appreciate you call it a bastardisation - but what you're really doing is treating the containing user control as a View, with a ViewModel, and the other two as user Controls (not views) so they don't have a view model any more than a text box or combo box would. And that's fine - I just find it very confusing if you call something a View that is really just a control - and if it's not just a control, i.e. if it is a View, then it should be modelled by a ViewModel -and a ViewModel's job is to model a view (otherwise they'd be called ViewsModels) Smile | :)

MVVM# - See how I did MVVM my way
___________________________________________
Man, you're a god. - walterhevedeich 26/05/2011

.\\axxx
(That's an 'M')

Mycroft Holmes31-May-11 14:30

Re: How to pass a new instance of a viewmodel to an existing usercontrol?

31-May-11 14:30

Oh I'm flexible, for simple list/add dialog/detail I often have 2 views and a dialog using the same VM. If a view is particularly complex then it gets it's own VM even if that is duplicating info already in an existing VM. And no I have no intention of renaming them VsM Laugh | :laugh:

_Maxxx_ wrote:
I just find it very confusing if you call something a View that is really just a control

The list view is the entry point into the structure, the detail view is distinct, navigated to via a double click from the datagrid on the list view, the add dialog is obviously a childwindow from the list view. All 3 use the same VM, calling the detail view a control is not correct, the dialog maybe.

Never underestimate the power of human stupidity
RAH

_Maxxx_31-May-11 15:59