I'm trying to set seletected value to a pre-loaded combobox using silverlight with MVVM.
I load these combobox items before selecting value.
For example I have a combobox to select a country. My first step is to load a List which is bound to the combobox source. This is loading perfectly.
After this, I have a "SelectedCountry" object bound to the selectedItem of the combobox in a two-way binding.
This is working perfect when I select any of the combobox values and my SelectedCountry object is correctly selected.
The problem comes when I try to assign the selected value in my ViewModel. This way, the combobox selecteditem is not updated.
I suppose this is because, on fact, they are not the same object (they have the same values but they are diferent references).
Should this work if I re-implement the equals method? Or should I find the same object by searching into the List?? This would be very easy because this two countries would be the same if they have the same id... but I can have more complex objects and I think the equals method would be better.
Thanks in advance!!
Edit for adding some code example:
<ComboBox Grid.Column="7" Margin="6,0" Name="cBTipoPoliza" VerticalAlignment="Center" TabIndex="4" ItemsSource="{Binding TiposPolizas, Mode=OneWay}" SelectedItem="{Binding TipoPoliza, Mode=TwoWay}">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding nombre_tipo}" />
</DataTemplate>
</ComboBox.ItemTemplate>
Usually I just override the .Equals() method to check if they are equal by ID or Name
You should try to avoid having multiple copies of the same object in memory at the same time. One method to do this is to have your VM be the source of objects. When you load the list have the VM do it and expose an AvailableCountries ObservableCollection property on the VM that your ComboBox can bind to.
If your objects are semantically equal based on ID, definitely override Equals and == and != and hashcode. However be careful because if you're enabling people to update the objects you can run into collisions (even within the same instance of the app) where one screen is holding onto stale data.
Related
WPF, MVVM
I'm finding that if I use a CollectionViewSource with my ComboBox, when I close the window, an extra call to the SelectedValue Setter is executing, if SelectedValue is bound to a string property. If I set the ItemsSource binding directly to the VM, this call does not happen. The extra call is causing values to change in the VM, resulting in incorrect data. I have other ComboBoxes setup the same way, but they bind to integer values.
CollectionViewSource definition:
<CollectionViewSource x:Key="AllClientsSource" Source="{Binding AllClients}" >
<CollectionViewSource.SortDescriptions>
<scm:SortDescription PropertyName="ClientName" />
</CollectionViewSource.SortDescriptions>
</CollectionViewSource>
ComboBox with CollectionViewSource:
<ComboBox Grid.Column="2"
ItemsSource="{Binding Source={StaticResource AllClientsSource}}"
DisplayMemberPath="ClientName" SelectedValuePath="ClientId"
SelectedValue="{Binding Path=ClientId}"
Visibility="{Binding Path=IsEditingPlan, Converter={StaticResource BoolVisibility}}" />
ComboBox direct to VM (Forgoing sorting):
<ComboBox Grid.Column="2" ItemsSource="{Binding AllClients}"
DisplayMemberPath="ClientName" SelectedValuePath="ClientId"
SelectedValue="{Binding Path=ClientId}"
Visibility="{Binding Path=IsEditingPlan, Converter={StaticResource BoolVisibility}}" />
Can anyone tell me why there is an extra setter call using the CollectionViewSource? What's different about the string binding? Is there a way to properly work around it?
EDIT: I tried changing it up and using the SelectItem property on the ComboBox. Same result. So it seems that if the item is a scalar data type, it works as expected. If it's an object, you get an extra setter call with a null value. Again, if I remove the CollectionViewSource from the equation, it works as expected.
EDIT, AGAIN: I added a link to a sample project that illustrates the issue. Targets .Net 4.5.
Run the project.
Click to display View One
Select a Client and the client's name will display on the right.
Click to display View Two
Go back to View One - Note that the selected client is no longer selected.
Click to display View Three
Select a Region and the region's name is displayed on the right.
Go back to View Two
Go back to View Three - Note that the selected region is still selected.
The only difference between the views is that One and Two use a CollectionViewSource. Three binds directly to the ViewModel. When you move to a new tab from One or Two, the setter for the selected item is getting called with a null value. Why? What's the best work-around?
Thanks.
Apparently this is caused when the CollectionViewSource is removed from the visual tree... I moved the CollectionViewSource to the ViewModel and exposed it as a property and the issue is effectively worked-around.
I have this requirement, that I have a collection of items (ObservableCollection), but I only want to display the first item. The requirement comes from the fact that in most of the case, the collection only contains one item. And due to the space limit, even if there is more than one items in the collection, we'd like to display the number of the items, details of the first one (same presentation as prior situation) and a ... symbol to indicate to the user that there is more items. And when the mouse is over the UI element a popup will eventually display all items.
The first solution I can think of (please suggest others if they are better) is to bind to this collection (but not using an ItemsControl) and define a DataTemplateSelector derived class (which return either the DataTemplate to display the only one item, or the DateTemplate which has the ... and the popup for more details, based on the number of items in the collection) and use it as ContentTemplateSelector.
But now my question: how both of my DataTemplate would look like in XAML, so that they can display only the first item in the collection? Obviously I can't have a ItemsControl.
UPDATE:
Now I have managed to make it work and agree this question can be closed (I can't delete it anymore since there is already some answers).
I actually knew how to bind to one certain item in the collection, but this was not where I am confused. I felt I should use ContentControl as one answer suggests. But I thought since I need to bind to the whole collection (not to single indexed item), and use a DataTemplateSelector to select the proper DataTemplate based on the number of items in the collection. The code would look like this:
<ContentControl Content="{Binding MyCollection}"
ContentTemplateSelector="{StaticResource MyTemplateSelector}" />
And in MyTemplateSelector I wasn't sure how to use it since there is no reference to my collection because it is defined as resource and it doesn't have the information of MyCollection. However, it turned out to be very simple, the DataTemplate can refer to an indexed item without knowing the name or any other reference. Simply like this:
<DataTemplate>
<TextBlock Text="{Binding [0].PropertyName}" />
<DataTemplate />
To bind to just one item from a collection, you can use the following syntax:
{Binding Items[0]}
Or to bind to a property of a single item from the collection:
{Binding Items[0].Property}
You can find out more about property path syntax from the Binding.Path Property page at MSDN... from the linked page:
• Indexers of a property can be specified within square brackets following the property name where the indexer is applied. For instance, the clause Path=ShoppingCart[0] sets the binding to the index that corresponds to how your property's internal indexing handles the literal string "0". Multiple indexers are also supported.
Try this
<ContentControl Content="{Binding YourCollection[0]}">
<ContentControl.ContentTemplate>
<DataTemplate>
<TextBlock Text="{Binding Name}"></TextBlock>
</DataTemplate>
</ContentControl.ContentTemplate>
</ContentControl>
Ok, late to the party but I thought I'd share my 2 cents anyway: I'd better go with a dumber (XAML-)view and a view-model closer to your presentation needs.
Translated: instead of mapping your existing view-model (or raw data) and its collection of items directly to the view, I suggest to map that to an appropriate view-model showing something like a YourItemViewModel FirstItem property and a bool HasMore property. That second view-model would be easily unit-testable to make sure it behaves propertly, and would be easily mapped to a view with less logic, so to avoid possible hard-to-test problems in view.
{Binding Items[0].SomeProperty}
{Binding [0].SomeProperty}
{Path=/SomeProperty}
I have question on the checkbox.
First of all,
I have a usercontrol which has a list box like this and this user control will be switched by 2 button and then the data source is changed and then the the displayed officer status will be changed:
When I check the checkbox, Officers[0].IsOnDuty will be changed to true.
The problem is:
When I click another button and switch to another data source, this checked check box is still checked but the Officers[0].IsOnDuty for this data source is false.
How to solve this?
The data context of the list box item is an item for your officers collection, not the collection itself. And using a one way binding is incorrect, as the data source (the officer) will not be updated. So change the DataTemplate to:
<CheckBox IsChecked="{Binding Path=IsOnDuty, Mode=TwoWay}" />
*Here is the list box xaml:
<ListBox ItemsSource="{Binding OfficersCollection}">
<ListBox.ItemTemplate>
<DataTemplate>
<CheckBox IsChecked="{Binding Path=Officers[0].IsOnDuty, Mode=OneWay}" />
*
The problem with your approach is that once you change the ItemsSource (by switching to the next page) your chekcbox is still bound to the item of the first collection. I think this happens because you explicitly use an indexer for the binding Path=Officers[0].IsOnDuty
Your samplelist box xaml does not really make sense. the ItemsSoruce is a OfficerCollection and your ItemTemplate binds to a collection of Officers too. Depending on what you are trying to accomplish you should do one of the following:
If your are just interested in the first officer (as your sample suggest), add a DependencyProperty FirstOfficer (or a INotifyPropertyChanged) property to your collection and bind to it: IsChecked="{Binding Path=Officers.FirstOfficer, Mode=OneWay}"
If you however are interested in all Officers and want checkboxes for all of them you should create a DataTemplate for the Officer type and use this as the ItemTemplate.
Generally you can stay out of a lot of trouble if you stick with MVVM and really tailor your ViewModel objects very close to what the View needs so you can bind your View to the ViewModel in the simplest possible way. Think of the ViewModel as the View you want to build but without a visual representation.
I have 2 comboboxes which we will call cbo1 and cbo2. Now, there is a relationship between cbo1 and cbo2. when i select an item at cbo1, the cbo2 ItemsSource is updated (since it is bound to the SelectedItem) anyway, below is the sample XAML code for it.
<ComboBox x:Name="cbo1" Grid.Row="0" Grid.Column="1" Margin="5" SelectedItem="{Binding Path=Brand}"></ComboBox>
<ComboBox x:Name="cbo2" Grid.Row="1" Grid.Column="1" Margin="5" SelectedItem="{Binding Path=Model}" ItemsSource="{Binding ElementName=cbo1, Path=SelectedItem.Models}" DisplayMemberPath="Name"></ComboBox>
the objects used are Brand and Model. Brand has a property named Models which contain a collection of Model objects ( typeof IList ). So basically, a one-many relationship between the 2 classes.
By the way, those 2 classes are used in NHibernate. Now, when I run the app, cbo1 which contains a collection of Brand objects is loaded with the items first. When I select a Brand item, the cbo2 with the Model collection is populated. As you have noticed, both Comboboxes have SelectedItem property bound to the current object properties Brand and Model specifically. When I select a Model on the cbo2, it does not reflect to the current object's Model property. Anything i missed?
typo: the first combo is called cbo1, but the second combo is binding to cbxBrand; but since you say the Models do appear, I'm guessing this is OK in your actual source code, and you renamed it for the Question here?
Anyway, your code completely worked for me, I put a breakpoint on the setter of the Model property and it hit it no probs, so the only thing I can possibly guess at is the Window's DataContext maybe incorrect?
Can you post your code-behind (or ViewModel) ?
I have two WPF list boxes. One is a list of lists (actually a List of ObservableCollection), the other is a list of all known instances of "Thingy".
Here's the datatemplate I'm using for the "thingy" class.
<DataTemplate DataType="{x:Type Model:Thingy}">
<StackPanel Orientation="Horizontal">
<CheckBox x:Name="ThingyInListCheckBox" Click="ThingyInList_Click"></CheckBox>
<TextBlock Text="{Binding Path=ThingyName}"></TextBlock>
</StackPanel>
Here's the XAML for the list boxes:
<ListBox
Name="ListOfGroups"
SelectionMode="Single">
</ListBox>
<ListBox
Name="ListOfThingys"
SelectionMode="Single">
</ListBox>
I have the data binding for the list boxes set up in code, because I'm too tired to figure out how to do it in XAML:
ListOfGroups.ItemsSource = InMemoryCache.ThingyGroups;
ListOfThingys.ItemsSource = InMemoryCache.Thingys;
What I want is the checkbox "ThingyInListCheckBox" to be checked if the 'thingy' object is in the list that is the selected item in the "ListOfGroups" listbox. So basically I need to bind it to the "Contains" method of the "ListOfGroups".SelectedItem while passing it the "ListOfThingys".SelectedItem as a parameter.
I'm tempted to do this all in code, but I'm trying to get a better understanding of XAML data binding because I hate myself and I want me to suffer.
Is this even possible, or have I hit the inevitable "wall of databinding" that exists in every other data binding system in the history of software development?
It is possible, in fact the hard thing is that there are many ways to do this and you have to choose one. None of them is a simple addition to your current code. However there is one way, by which you gain more than solving your problem. Actually, it is more of a pattern, called MVVM (some might argue about the naming).
Here is a small explanation on your example.
Suppose ThingyGroup has an IsSelected property, which is bound to the IsSelected property of the containing ListBoxItem. Again, suppose Thingy has a Group property too. Then you can use Group.IsSelected as a path to bind checkbox. Notice that there is still a small issue that IsSelected is a bool and IsChecked is a nullable bool.
A search on MVVM should give you concrete samples.