I have a DataTemplate(well two data templates) that I want to use as views for some
basic form viewmodels(that that contain a value and and boolean indicating whether I want to use the value).
I want to use the datatemplate(s) several times for separate form items. I think the right way to do this is to set it as the ContentControl's ContentTemplate (in that case it will have the same data context right?) but I also want to pass the label string and since the label string is part of the ui and doesn't change it seems wrong to put it in the viewmodel object. How do I give access of the label string to the DataTemplate instance?
Just like its name, a DataTemplate is used to template the Data... For example, if you have a class called MyItem which has a Name and Value and you want this shown in a specific way, you'll set a datatemplate for Item and use it whenever needed.
In your case, you're speaking about having very similar views, with only a minor change between them. This minor change (if I understood your question correctly) is not something that comes from the model or from the viewmodel but something which is entirely view-oriented (a different title for the page, for instance).
If you plan on using a different viewmodel for every view, and each viewmodel has a different purpose - I don't see a problem with adding a Title property to the VM and bind to that too (Remember, MVVM is a set of guidelines, not rules...)
If you still rather have it separated from the viewmodel, then you can use an Attached Property. Create an Attached Property called TemplateTitle, for instance, and have each contentcontrol in each view change it. The label, of course, will bind to that Attached Property.
Related
My app has a background thread that periodically retrieves data from an external source, in the form of key/value pairs. I would like to expose this data for binding, presumably by storing them in some kind of static(?) model, as the data will be needed by numerous views throughout my app. There are potentially hundreds of these keys, and may be different for each customer, so I can't simply create an INotifyPropertyChanged model with a property for each value.
The app has multiple views visible at any one time, and each of these will have numerous controls (usually textboxes) that I want to bind to individual items in the above collection. When a value in the collection is updated, any controls bound to only that item should change to reflect the new value. I'm assuming an ObservableCollection wouldn't be suitable here, as a change to a single item will result in all controls updating, regardless of which item they are bound to?
To throw a further complexity into the mix, some values (which are numeric) will need formatting for display, e.g. number of decimal places, or adding a suffix such as "volts". The formatting rules are user-defined so I can't hardcode them into (say) the XAML binding's StringFormat expression. Ideally I should be able to access both the raw value (e.g. for calculations), and the formatted version (for display purposes). I'm sure it must be possible to achieve the latter using some clever WPF feature!
I would appreciate any pointers on how I can solve these requirements.
Edit: it's worth mentioning that I've previously tried implementing the model as some kind of collection. The problem is that it won't be initially populated with all values, and these only get added some time later. When they do eventually get added, a bound control doesn't update - presumably because it wasn't initially able to bind to the missing value.
I would take a different approach, namely a variation of Event Aggregation. I would have a single class that manages the overall collection (probably a singleton class like franssu suggested), but instead of binding directly to the collection in that class you create smaller models that are more specific to the individual views.
When your main model receives a new item, it publishes an event, which is consumed by the smaller models who can inspect the new item and determine whether or not they should add that item to their internal collection (the one the individual views are bound to). If it doesn't "belong" to their view, they can simply ignore the event.
You could use similar event publishing for updates to items and such, although if you're binding to the actual items you probably don't need that.
Just implement the INotifyCollectionChanged Interface and the INotifyPropertyChanged and you ll get a Collection like the ObservableCollection.
But rember if you select a Item from your Collection (as example a ObservableCollection) and you change that item your other controls won t update. So if you have a class Person in your Collection and you change the name of one person the other controls won t get the new name of the person.
Inside the Person object you still have to implement the INotifyPropertyChanged Interface and raise the event when your name changes.
So what I want to tell you is: A Collection with the interface INotifyCollectionChanged will only tell the bound controls: There is a new Item, there has been a item removed or a items index changed BUT not if the item itself changes.
So you ll need a Collection that provides the points above and a Item contained by the collection that raises events if a property of it changes.
ObservableCollection is perfect here. You should find that a standard ItemsControl bound to an ObservableCollection will only update the controls of the items that have changed, not every item in the collection.
This is the reason ObservableCollection exists - the events that it raises specifically identify items that have changed, so that the UI can handle them sensibly.
I've tested this locally with a small WPF app and it works fine. Worth noting, though, that a virtualised items panel would probbaly appear to break this behaviour when it scrolls...
EDIT: rereading your question, you actually say "When a value in the collection is updated..." If your collection contains instances of a class, and you update properties on the class, you don't even need ObservableCollection for this to work - you just need the class to implement INotifyPropertyChanged.
I am trying to do things the .net way and declare my menu items for my context menu like a good citizen in xaml :).
I have a listbox that contains a bunch of list items based of ItemsSource.
QUESTION: How can I dynamically set the visibility of a particular menu item based off a function to be defined in the Page class?
I could just data bind this to a property in my DataContext class, but there are a few things that are more ui-specific that it really should not know about, so I'd like to keep it clean.
Can I somehow bind the visibility to a function in the page class and pass it the data context and have it return the appropriate visibility value required? Or is there another way?
Thanks!
swine
Bindings are useful because the UI updates whenever the bound value changes. Since a function doesn't update, even if you could bind to a function, the UI would never change. The UI would also never know when to call the bound function.
Why don't you just bind to a bool DataProperty and then use the function to update the bool. If you explain more of what you are trying to do, then I can give you a more detailed solution.
I'm new to wpf and now I have a problem. I have a model class say Customer and I've created a DataTemplate with TargetType property set to Customer. It works good. But I actually want two different templates like one for just displaying the record and another for in-place editing. Is it possible to specify two different templates for same datatype based on some creteria?
And I want to switch this template based on some property on ViewModel like when IsEditmode is True.
Or am I doing it wrong? Should I use styles instead?
Your approach seems to be perfectly fine.
You can create a DataTemplateSelector which will allow you to choose a data template based on arbitrary criteria from code behind.
I often use these to decide which template to use based on a enum-type property.
There are two easy ways I can think of, ofcourse there are other ways based on the complexity and architecture you want to follow.
Define DataTemplate with 'Key' and specifically call that either using StaticResource/DynamicResource Binding.
You can have a DataTrigger inside the datatemplate which makes some parts of the template visible/collapsed based on your 'EditMode' property
I'm working in a WPF project, I'm using the MVVM patter in my project.
I created a user control (also in WPF) and I want to use it in my project, now, my problem is that I have a method in my user control that I need to call from my View Model, but I don't know how to do that, how to bind to the method inside my control from the view model.
If I use code behind, obviously there is no problem since I have a direct reference to my control, so I can do "mycontrol.MyMethod();"m, but of course, doing in this way will go against the logic of the MVVM pattern.
I tried to use a Dependency Property in my user control, and use that Dependency Property to bind to it in the xaml of my project but it didn't worked, the compiler says that the property was not found or is not serializable.
So I will appreciate if someone can share some light about how can I accomplish this.
Edited
As far as I understand you have the view, which is all the GUI, then you have the model, which is all the logic, and them you have the view-model which is like an intermediate layer used to bind the view with the model, right?
In this way I have developed my project, however I came to the problem that I need a custom control, a TextBox that remember what the user entered, and when he start typing, if there are words that start with that letter, those words are shown as a suggestion, as Google does it.
This TextBox is used as a search filter; so I created a user control to do this, I added a method to my user control to allow whatever application that uses my control to add items to an internal array that holds all the strings that the user has entered.
I created a user control because I couldn't find any control that behaves the way I want.
So my problem is when I add my user control to the main project, because I need to someway be able to call the method that add the items to the internal array, but maybe I'm doing things the wrong way, so if any of you has a better idea, I will appreciate if you shared it with me.
You should never call View methods from ViewModel, and vice versa.
Make a property (ObservableCollection?) on your ViewModel, it will have CollectionChanged event, subscribe to it to monitor changes (if needed).
When you add an item to the collection in your ViewModel, GUI will be updated accordingly (you have to perform the Add() operation on GUI thread, btw).
If you need to change the current position in your list, there are colections for that (CollectionViewSource, etc).
If you really really need to pass a string to your control, make a DependencyProperty and bind it OneWay to your ViewModel's property. When you set the value, it will call PropertyChangedCallback on your DependencyProperty.
Why does the consumer of the user control need to maintain the control's internal array? That seems like you've exposed an implementation detail that you don't need to.
Why not simply make that array a dependency property (and an IEnumerable<string> or ObservableCollection<string> besides)? Then you can simply create the corresponding property in your view model and bind it to the control. It also makes the control considerably more versatile.
You shouldn't call something in the View from the ViewModel since that breaks the model.
If the reason you want to call the method in the user control is to do with UI only, I don't see anything wrong with doing it from the view - the view's cs and the view's xaml are in the same "space" in the model. You can be overly-purist in wanting to have lean and mean view cs files.
I'm using PRISM (and thus the MVVM pattern). I've got a complex DateTime picker view with radio buttons.
The user can pick today, yesterday, a date, a week of a year, etc. I use radio buttons for the different choices.
What's the best way to do that in MVVM?
I really can't think of a clean way. I could create lots of custom behaviors to add to each item to track them but it doesn't seem maintainable.
I'm going to put some code-behind but I really don;t like that and to me it breaks the MVVM principle (put everything in the XAML).
Does anyone have a better idea on how to go about that?
Keep the RadioButtons, add an enum type to your VM that can return things like "Today" "Yesterday" or "Tomorrow." On the UI side create a ValueConverter that takes a parameter like "Tomorrow" and compares it with the bound value on VM, then returns the bool? needed by IsChecked.
Put it in code behind.
The M-V-VM pattern is not "put everything in xaml" it's "separate concerns". Your VM wants a DateTime right? In which case it doesn't care how that DateTime is being chosen it just needs a DateTime.
Putting View logic in the ViewModel isn't a good idea as you're now giving the VM knowledge about the workings of the View. The flow is meant to be View knows about the ViewModel which knows about the Model. The reverse is not normally true. (As with all things computer related there is always exceptions)
Hope this helps.
How about a ComboBox instead of the RadioButtons presenting a list of TimeOffset (custom class) bound to your ViewModel, with a corresponding SelectedTimeOffest property.
If you need to display extra information depending on the type of TimeOffset, e.g. a WeekOffset (subclass of TimeOfset) with a WeekCount property, have a content control with several DataTemplates customized by the type of TimeOFfset.
Just an idea...
you could create a enum, bind the values of the enum to a listbox, retemplate the ListboxItems
this post is using silverlight, but something similar should work
http://leeontech.wordpress.com/2009/03/18/creating-radiobuttonlist/
The ViewModel is designed to present the model in a way the view can consume.
In this case, you could have a boolean property for each button in the VM, and when a button updates it just sets all the other properties to false. Then in your View you can bind each properties IsChecked to the corresponding property in the ViewModel.
Also, be aware there is currently a bug in binding radio buttons in WPF. Here's a potential solution.