As far as I know, the default way to use a ObservableCollection that is bound to a listview is with model classes as elements (ObservableCollection<MyModel>). So when a listview element is selected, we use NavigateAsync and pass the model, which then can be used by the ViewModel to "fill itself".
The problem with this approach is, that it's not possible to use ViewModel properties for binding in the listview.
For example:
I have a View, ViewModel and Model "PickList", which contains a collection of "PickLine" objects - each having a View, ViewModel and Model themselves. The PickLine object contains a property "PickedQuantity" and a property "OpenQuantity". Now in my PickList view, I don't want to bind these two to separate items (e.g. two labels), but I want to have one label to display both I a format like for example "PickedQuantity / OpenQuantity". I know this example can be solved by using multi binding or something like this. But that's not the meaning of it all.
My PickLine ViewModel already has a property "QuantityString", that I want to bind to the label of a listview element via DataTemplate. But how can I do this. Is it even possible?
Make a property that combines the two other properties and bind to that. E.g.:
public string FullQuantity {get {return $"{PickedQuantity} / {OpenQuantity}";}}
Then in the setter for PickedQuantity and OpenQuantity, you will want to call whatever PropertyChanged method you have set up to notify the bindings of a property change and pass in the FullQuantity property name so elements that are bound to FullQuantity get updated when either PickedQuantity or OpenQuantity are changed.
This way, you are only binding one label's text to one property and that label would get updated when either of the two quantity properties are changed.
Note: I am unfamiliar with Prism, but this approach should work regardless of the Mvvm framework in use.
Your PickListViewModel should expose a collection property whose items are of type PickLineViewModel (not PickLine).
Whether you need an ObservableCollection<PickLineViewModel> depends on where changes can happen - in service / model that initially created the PickLines or in the GUI or both. In any way, you have to make sure the changes are propagated from one side (the collection of view models) to the other (the collection of models). Google wrapping observable collection as a starter (hint: avoid two-way sync if possible). These blog posts are old but still relevant and make a good reading. A trivial wrapping is described in this answer.
Related
I have a property (text box) which is a part of a list. And that list is a property in another list (class). Now when I write something in that text box , the ancestor one should get updated. I have tried RaisePropertyChanged() but its not working out. Can anyone provide me with right syntax??
When binding lists you should take a look at INotifyCollectionChanged. RaisePropertyChanged propagates that the list itself (not just one item in it) changed. If bound to a GUI the GUI will check for a change and sees the same old list in place, so it takes no further action.
I have multiple of views (user controls), each with its own ViewModel. To navigate between them I am using buttons. Buttons display image and text from corresponding view model and also need column and row (because there are like 10 views: 10 columns with different number of rows each).
Right now buttons are created dynamically (I made a Navigator control for this) and for view models I have base class to hold text, image, column and row. Number of views available will be different (depends on user level and certain settings), that's why it's I need control here.
Question: how shall my control get data from view models?
Right now I have interface INavigator, defined in (lol) control itself. And view models implement it. I could go opposite, let my control to know about view models. Both looks wrong.
There is a single Navigator control what has, lets say, Items bound to a list of view models. It can cast each view model to INavigator or ViewModelBase (common for all pages) to obtain specific view model image, text, column and row. So either view model knows about control (to implement INavigator) or control knows about ViewModelBase.. And this is a problem, both solution bind tight control and view models, which is bad in mvvm.
Schematically
The way you've drawn your diagram answers your own question as to how you should structure the code for this.
What you need is one VM (let's call it MainVM) which contains an ObservableCollection<VMBase> of the other VMs (using your base type so that they can all happily live in the same collection).
Your View needs an ItemsControl (bound to your ObservableCollection<VMBase>) where you specify a DataTemplate for the Button using the properties exposed by the VMBase type only. Set the Command property in the Button to call SwitchCommand, CommandParameter is set to the item itself (i.e. {Binding .}).
Your View also needs a ContentControl bound to a SelectedVM property on MainVM which you can populate.
Implement SwitchCommand to set the SelectedVM property based on the value from the CommandParameter.
public void ExecuteSwitchCommand(object parameter)
{
var vmBase = parameter as VMBase;
if (vmBase != null)
SelectedVM = vmBase;
}
All properties mentioned here should be INotifyPropertyChanged enabled so that the View registers when they change and updates the UI.
To get the different UIs for the ContentControl, add type-specific DataTemplates for each of your specific VM types to the Resources file of your View (or if you're smart and are building a custom plug-in framework, merge the Resource Dictionaries).
A lot of people forget with MVVM that the whole point is that there is a purposeful separation of View from ViewModel, thus meaning you can potentially have many Views for a single ViewModel, which is what this demonstrates.
I find it's easiest to think of MVVM as a top-down approach... View knows about it's ViewModel, ViewModel knows about its Model, but Model does not know about its ViewModel and ViewModel does not know about its View.
I also find a View-first approach to development the easiest to work with, as UI development in XAML is static (has to be).
I think a lot of people get to wrapped up in 'making every component (M, V, VM) standalone and replaceable', myself included, but I've slowly come to the conclusion that is just counter-productive.
Technically, sure you could get very complicated and using IoC containers, create some ViewLocator object which binds a View-type to a ViewModel-type, but... what exactly does that gain you besides more confusion? It makes it honestly harder (because I've done this at one point) to develop because now you've lost design-time support first and foremost, among other things; and you're still either binding to a specific view model interface in your view or creating the binding at run-time. Why complicate it?
This article is a good read, and the first Note: explicitly talks about View vs. ViewModel. Hopefully, it will help you draw your own conclusions.
To directly answer your question, I think having your ViewModels implement an INavigator interface of some sort is probably ideal. Remember your VM is 'glue' between your view and model/business logic, its job is to transform business data into data that is consumable by your views, so it exists somewhere between both your UI and business layers.
This is why there are things like Messengers and View Services, which is where your navigator service on the ViewModels can fit in nicely.
I think the design has led to a no way out situation.
I believe that creating a custom button control where the dependency properties tie the image, the row and column actually provide a way for the page, which it resides on ,to get that information to them; whether they are dynamically created or not.
Continuing on with that thought. There is no MVVM logic applied to a custom control, the control contains what it needs to do its job and that is through the dependency properties as mentioned. Any functionality of the button should be done by commanding; all this makes the button data driven and robust enough to use in a MVVM methodology or not.
Question: how shall my control get data from view models?
There should only one viewmodel which is the page the control resides on. The control is simply bound to information which ultimately resides on that VM. How it gets there, that is up to the programmer. If the button is going to contain state data, that is bound from its dependency property in a two way fashion back to the item it is bound to.
By keeping VMs out of the buttons and only having one VM that is the best way to segregate and maintain the data. Unless I am really missing something here....
Same as others here I find it a bit hard to actually understand what you are asking, so this is quite general. The answer to the question header is simply: the Control gets the data from the ViewModel through bindings, always. You set the DataContext of your Control to the corresponding ViewModel, and from there you keep the ViewModel and the Control synchronized:
If you add an ItemsControl containing buttons to the View, you add an ObservableCollection<ButtonViewModel> to the ViewModel and bind the ItemsSource of the ItemsControl to this.
If you allow the user to dynamically add content to the View, the actual code that does it resides in the ViewModel, e.g. when the user clicks on a button "Add Button", you use the Command property to call a ViewModel method that adds a ButtonViewModel to the collection and the View will automatically reflect your changes.
There do exist complicated cases that are impossible to code exclusively in the ViewModel, I have found Behaviors to be the missing link there, but I'll get into that when you show me the specific case.
If you'd like to get a working example, please provide as much code as you can, with your exact expectations of what it should do.
I have a C# Windows Phone 7.1 project that uses the MVVM Light toolkit. On one application page in the app I have a Telerik data bound list box that is bound to a property in my view model. At first that property had a return type of:
List<string>
With that return type the property did not show up in the list of candidate elements in the Path list box, when I activated the Create Data Binding dialog box in order to assign the list box's ItemsSource property. However, when I changed that property's return type to:
ObservableCollection<string>
it showed up immediately. In the future, how can I determine the correct return type for a particular control's ItemsSource or other data bindable property?
Your question is little confusing. Both are completely okay and work well for different scenarios. You use list<string> when you don't plan to change collection, and ObservableCollection otherwise.
Now, you can use MSDN(see ItemsSource property to see what property needs to be what.
As you can see from documentation, ItemsSorce has to be IEnumerable, meaning all types that derive from it, can be used.
Now as for the real question, why didn't your property show up in the IDE, I can only guess that Microsoft wants you to use ObservableCollection always, but I don't agree with this. So that's why it's best to manually do the bindings to properties through Xaml.
Say you have a xaml page that takes two inputs such as DATE and some TEXT value and displays the result on a DataGrid, its easy to have a class with with 3 properties of dataTypes DateTime, string and some Collection and implement the same in MVVM.
I have a xaml page with a DataGrid and few input controls(like textbox or datepicker) generated dynamically based on a certain criteria. I want to implement this in a MVVM pattern but not sure how to go about. Any pointers will be helpful.
In my experience, MVVM is not answer for all presentation scenarios, sometimes you just have to use codebehind or build custom control. I would suggest here building custom control that takes (binds with dependency property) object or list of objects, and generates controls (text boxes with input validation/verification, for ints, doubles, date picker, etc), and binds given object to them. In the end you will have one smart control that takes object and renders input fields, and binds to them. Your VM class would just expose object or list of objects.
It is always good to separate the view and the model; however this cannot always be done with simple data binding of simple types.
I would be looking at something like a custom control that could databind to a list of {name, value, spec} pairs when the “spec” is the logical type of control to use.
Thinking about it; that is like what a property grid does if you pass it a custom collection of property descriptors….
Devexpess has a “vertical grid” that may help, also look at 3rd party property grids.
I have two data-bound text boxes. One is bound to a string and the other to a number. The 'default' binding is set in XAML. Under some circumstances I need to reverse the bindings at runtime (the string is usually a prefix but sometimes it's a suffix).
I have the following code in my view model, called when the window is loaded:
Binding stringBinding = BindingOperations.GetBinding(view.seqLeft, TextBox.TextProperty);
Binding numberBinding = BindingOperations.GetBinding(view.seqRight, TextBox.TextProperty);
view.seqLeft.SetBinding(TextBlock.TextProperty, numberBinding);
view.seqRight.SetBinding(TextBlock.TextProperty, stringBinding);
After that the code loads the properties to which the binding refers.
The problem is that the 'new' binding doesn't seem to work. What have I missed? Is there a better way?
I might consider exposing Prefix and Suffix strings to which View can bind, then use logic within the ViewModel, or whatever backing object you're using, to fill those strings accordingly. This option neatly segments the business concern from the visual and simplifies what you have to keep track of in your view.
Why monkey around with the bindings at all? If you want to have a TextBox that's bound to one of two different things, create two TextBoxes, put them in the same location, and toggle their visibility based on whatever your swap condition is.
The only thing wrong with my code was the TextBlock.TextProperty in the SetBinding calls! They should, of course, have been TextBox.TextProperty but I'd messed with it so long I wasn't seeing the wood for the trees.