Simple case:
<usercontrol>
<Views:UserListView x:Name="settingsTreeView"/>
<Button DataContext="{Binding ElementName=settingsTreeView, Path=SelectedItem}"
Command="{Binding CreateChildCommand}"/>
</usercontrol>
The task just is to bind to button a DataContext which implements CreateChildCommand.
DataContext is the selected item in the treeview.
Nothing to happen.
I have checked the button properties at run time: both Command and DataContext properties of the button are null.
Questions:
Is it valid to change DataContext at runtime?
How to pass selected item to the Command object?
Concerning commanding in Silverlight you can refer to:
http://johnpapa.net/silverlight/5-simple-steps-to-commanding-in-silverlight/
http://community.infragistics.com/silverlight/media/p/125526.aspx
http://houseofbilz.com/archives/2009/05/22/adventures-in-mvvm-commands-in-silverlight/
http://www.silverlightshow.net/items/Silverlight-4-How-to-Command-Control.aspx
something goes wrong
First thing to do would be to check if your bindings are failing. You should see some sort of output in the output log if this is the case. Its valid to change the DataContext at runtime, and you can bind the selected item to the CommandParameter property on the button if you want it to be passed to the command. Let me know if you have errors in your output log or not.
Related
I have a an object created in Xaml:
<Grid>
<MyObject/>
</Grid>
I need someway to bind the object myObject back to a property in my view model. I dont know whether this is possible, everything ive seen so far binds properties together, but any help would be greatly appreciated.
I am assuming what you want is your ViewModel to hold the actual visual control MyObject in it and your Grid to display it via MVVM.
This is possible through ContentControl in WPF.
Assuming your ViewModel has a property MyObjectView which holds MyObject...
<Grid>
<ContentControl Content="{Binding MyObjectView}" />
</Grid>
Having said that you must take caution that same MyObjectView is not bound to any other content control as that will result in an error
"Specified element is already the logical child of another element.
Disconnect it first"
And if that requirement is possible then you must excercise ContentTemplate option.
Let me know if this helps.
It is possible. It kinda breaks mvvm though.
You can attach an InvokeCommandAction to this object, and bind the CommandParameter to it via ElementBinding. Then in the callback of the command which you defined in the viewmodel, you will have a reference to this object from the CommandParameter.
I am looking for a way where a control can be enable when an item from a combo box is selected. Is there a simple way through data binding when a user selects an item from a combo box that it then enables another control to be used?
If you're using MVVM, you can bind the SelectedItem of the combobox to a property in your viewmodel.
Say this is your combobox:
<ComboBox ItemsSource="{Binding widgetlist}" SelectedItem="{Binding Path=selectedwidget, Mode=TwoWay}"></ComboBox>
And this is your control:
<DockPanel IsEnabled="{Binding controlenabled}">
...
</DockPanel>
Then in selectedwidget's setter, you can change the controlenabled property to False or True. Don't forget to notify that the controlenabled property changed (or if you want, make controlenabled a DependencyProperty.)
In summary, you've got 3 properties to bind to:
widgetlist, an ObservableCollection or some other collection that is the source for your combobox
selectedwidget, an item of that collection type that changes to whatever the combobox currently has selected
controlenabled, a bool that the other controls look at to decide if they are enabled/disabled.
Like many examples in MVVM, this way may require slightly more thought and code on the outset, but will be far more maintainable and scalable later. For example, say you want some more controls to also enable/disable themselves based on the same scenario. Piece of cake: add IsEnabled="{Binding controlenabled}"> to them.
Yes. You want to bind to IsEnabled in the target control which you want to dynamically enable or disable, and use a Value Converter to convert a matching string or item from the ComboBox to a true value for being enabled.
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.
Ok, hopefully this is simple but for some reason I can't find a straight answer and I'm not familiar enough with WPF yet to know how to do it.
I have a listview, it gets bound to an observable collection of objects to display. I want to have a context menu with a bunch of options. The options in the context menu are relative to the particular object in the list that was clicked on (things like delete, export, etc).
So I need the object that the user right clicked on in my listview to be passed as a parameter to the command that the context menu executes.
How do I do this?
Edit: I should mention I would prefer a solution that is mostly (if not entirely) xaml - I'm trying to avoid having significant code in the code-behind. If that's the only way to do it though...
Further Edit: More details that I forgot to mention that are important. The command I want executed is on the object bound to the data context of my user control, it is not on the objects in the list view. So I need the context menu's on the list view's items to be bound to a command that is on the user control's data context, and the listview item passed as a parameter into that command.
It depends on whether your ContextMenu is part of the template for individual items, or if it is attached to the ListBox as a whole.
If you are attaching your ContextMenu to the items in the list using a DataTemplate (this is generally the best way to do it), the DataContext on the MenuItem is already set so all you need to do is:
<MenuItem ... CommandParameter="{Binding}" />
On the other hand, if your ContextMenu is attached to the ListBox as a whole, you'll need to access the SelectedItem property of the ListBox:
<MenuItem ... CommandParameter="{Binding SelectedItem, RelativeSource={RelativeSource FindAncestor,ListBox,1}} />
I will try and explain this as concise as possible. I have 2 objects, the first which we will call object A that has an Id property and the second we will call object B, which has a ParentId property. The obvious relationship is that object B's ParentId is set to an object A's Id property. I am using the MVVM pattern, so on the viewmodel I have 2 ObservableCollections, one containing objects A the other objects B. On construction of the viewmodel, I create and fill the ObservableCollection<'A'> named ListItems. My xaml is simple,
<StackPanel>
<ListBox IsSynchronizedWithCurrentItem="True" ItemsSource="{Binding ListItems}">
</ListBox>
<ComboBox SelectedValuePath="ParentId" SelectedValue="{Binding Path=ListItems/Id, Mode=OneWay}" ItemsSource="{Binding ComboItems}">
</ComboBox>
<Button Click="Button_Click" Content="Push Me"/>
</StackPanel>
As you can see the combobox's SelectedValue is bound to the ListItems current item's Id property. So essentially the listbox and combobox are in a master details.
If you press the button, it will fill the ObservableCollection<'B'> name ComboItems, which in turn populates the combobox. Now here is where the oddity begins. When I start the program, if the only thing I do is press the button, and then afterwords select an item in the listbox, the combobox will properly select an item due to the SelectedValue binding. But if I start the program and first select an item in the listbox and then press the button, the current combobox item will not change with the current listbox item. The binding appears to be forever broken. Does anyone know why this happens?
Ps. If I set the ItemsSource on the combobox before I set the SelectedValue/SelectedValuePath, the master/detail binding will never work. I know there is order to xaml, but that seems a little fragile. So if anyone has input on that also, I am all ears.
Thanks, Nate
EDIT -
When binding SelectedValue, it is very fragile. If the binding is working, i.e. have not selected anything in the listbox and then filled the combobox, if you choose an item in the combobox, the binding will break. After much time wasted with this, I chose to bind SelectedItem. This binding does not break in any of the conditions I have previously specified. I would however take any answers as to why SelectedValue binding is so ridiculous. Thanks again to all that have answered or will answer.
Yeah this is a problem we stumble upon quite a lot.
The problem is that after the ItemsSource property gets a new value, the SelectedValue binding will be cleared. Sucks, and until today we have not found a proper solution.
Here are a few workarounds:
Reset the SelectedValue binding in code, as soon as the new ItemsSource has been set. You can do this in a converter, or somewhere you'll know which will replace the ItemsSource binding (like the DataContextChanged event).
Instead of using the Binding on ItemsSource, try using a CollectionViewSource and a Filter. Put all your items in the CollectionViewSource object and filter the items when your combobox changes value.
Manually get your item the old fashion way when your listbox throws a SelectionChanged event.
Mind you, all solutions are not the prettiest in the book. I would go for option 2, its the cleanest IMO ;)
Hope this helps!