Normally when you want a databound control to 'update,' you use the "PropertyChanged" event to signal to the interface that the data has changed behind the scenes.
For instance, you could have a textblock that is bound to the datacontext with a property "DisplayText"
<TextBlock Text="{Binding Path=DisplayText}"/>
From here, if the DataContext raises the PropertyChanged event with PropertyName "DisplayText," then this textblock's text should update (assuming you didn't change the Mode of the binding).
However, I have a more complicated binding that uses many properties off of the datacontext to determine the final look and feel of the control. To accomplish this, I bind directly to the datacontext and use a converter. In this case I am working with an image source.
<Image Source="{Binding Converter={StaticResource ImageConverter}}"/>
As you can see, I use a {Binding} with no path to bind directly to the datacontext, and I use an ImageConverter to select the image I'm looking for. But now I have no way (that I know of) to tell that binding to update. I tried raising the propertychanged event with "." as the propertyname, which did not work.
Is this possible? Do I have to wrap up the converting logic into a property that the binding can attach to, or is there a way to tell the binding to refresh (without explicitly refreshing the binding)?
Any help would be greatly appreciated.
Thanks!
-Adam
The workaround here was to add a property to my object (to be used as the datacontext) called "Self" , which simply returned
public Object Self { get { return this; }}
Then in the binding I used this property:
<Image Source="{Binding Path=Self, Converter={StaticResource ImageConverter}}"/>
Then when I call
PropertyChanged(this, new PropertyChangedEventArgs("Self"))
it works like a charm.
Thanks all.
I don't believe there is a way of accomplishing exactly what you need with your current converter. As you mentioned, you could do the calculation in your ViewModel, or you could change your converter into an IMulitValueConverter.
From your specific scenario (the converter tied to a ViewModel class, and a few of its properties), I would lean towards implementing the logic in the ViewModel.
Hmm, you don't show the full implementation. But I think it should update, if the value bound to the GUI provides the PropertyChanged-Event.
Regards
Related
I want to call controls inside view like button and item template inside viewmodel. Please tell how can I do that. My view contains following
<ItemsControl Name="cDetails"
Width="395"
ItemTemplate="{DynamicResource Test}"
ItemsSource="{Binding ViewModels}"
Visibility="{Binding IsLoaded,
Converter={StaticResource visibilityConverter}}">
<Button Name="btnComplete"
Grid.Column="1"
HorizontalAlignment="Center"
Command="{Binding AuditCommand}"
CommandParameter="1">
Complete
</Button>
Please tell how can I call these items in my viewmodel using vb.net.
Thanks
Accessing your view components from inside your viewmodel is not the way to do things in MVVM. Because it is specifically not designed to work this way, you will have to go out of your way to make it work. You should probably investigate how to accomplish your goals using MVVM properly, or forego using MVVM at all and do the work in your code-behind.
Since you have not described what your goal is, it is hard to provide specific recommendations. In general when using MVVM, you manipulate things in your viewmodel and set properties. Your view binds to these properties so that it updates appropriately as they are being set. Your viewmodel does not directly manipulate the views themselves, only the viewmodel properties that they are bound to.
For example, let's say you are updating the text on a TextBlock. You could do something like this in xaml:
<TextBlock Text="{Binding SomeText}" />
Then, your viewmodel (which should implement the INotifyPropertyChanged interface) defines this property and sets it as desired.
public string SomeText
{
get { return _someText; }
set
{
if (_someText != value)
{
_someText = value;
NotifyPropertyChanged("SomeText");
}
}
}
private string _someText;
...
// At any time, you can set the property, and the
// binding will update the text in the control for you.
SomeText = "Some text";
If you absolutely need to manipulate your views from code (or if you are not using MVVM), the appropriate place for that sort of code is the "xaml.cs" file next to your view (the code-behind). You can assign a name to anything in your xaml using syntax like <TextBlock x:Name="SomeTextBlock" /> and then access it from the code-behind as a member variable with the same name. For example, you could do SomeTextBlock.Text = "Some text". However, this is not usually necessary for the vast majority of use cases if you are using MVVM.
You shouldn't try to access controls directly from the ViewModel. The ViewModel must not know about the View implementation.
Instead, in WPF we connect View and ViewModel through Bindings. Bindings connect Properties of controls in the View, with Properties in the ViewModel.
Commands are a special type of Property that can be bound as actions for controls like Button.
In your example, you would need to have these properties in your ViewModel:
A collection named ViewModels
A boolean named IsLoaded
And an ICommand named AuditCommand
By managing those properties, you should be able to control what's shown in your View and its behavior.
If you need more control, create more Bindings to other properties, or create some events in your ViewModel and manage them from your View's code-behind.
I am working on a Silverlight application, and I want to bind the simple text property of textblock through a property of string type.
What I did was:
<TextBlock Text="{Binding Name}"/>
Code behind:
public string Name{get;set;}
Name = "Testing..!";
but it will not work.
To expand on anatoliiG's answer (which will work): Data binding refers to properties on the DataContext property of the current element by default. This means that your
<TextBlock Text="{Binding Name}" />
is actually translated to
Set the value of the Text property to this.DataContext.Name
(DataContext is inherited, so if it is not explicitly set on the TextBlock it will check the parent, then the parent of the parent etc etc)
You can resolve your problem in one of two ways:
You can set the value of this.DataContext on the parent to the parent itself (as anatoliiG suggests). This means that when it looks up this.DataContext.Name it will be checking the Page itself, which is where your Name property is found.
You can change your Binding so it looks at the Page instead of Page.DataContext when it is looking up bindings. You can achieve this using the RelativeSource markup extension:
This translates to:
Find the first ancestor of the TextBlock that is of type Page, and bind to the Name property on that object
As a final note, you will also need to implement INotifyPropertyChanged on your DataContext object if you are going to ever change the value of Name.
Oh, and you should be using view models as the DataContext instead of the Page itself!
Answer to your question is: in Page_Loaded event set LayoutRoot.DataContext = this;. But it is more hack, than good practice.
You should take a look into MVVM pattern and INotifyPropertyChanged and create ViewModel which will contain this property.
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.
Here is my XAML code:
<TextBlock x:Name="subTitle" Text="{Binding Name}" />
And my C# code:
PropertyInfo pi = ...;
subTitle.DataContext = pi;
The TextBlock displays simply nothing.
There is no DataContextChanged event but the TextBlock gets notified about the change because if I omit the path "Name" from the binding expression I get the ToString() representation of the DataContext. I can access no property of the bound object. There are no tools for debugging and I spent hours on this tiny but breaking issue.
Please help me solve this problem. Is it a bug or am I missing something? Thank you very much.
I'd guess that PropertyInfo.Name isn't a dependency property and/or it doesn't implement INotifyPropertyChanged.
Create your own viewmodel object (which implements INotifyPropertyChanged) and use that for binding values to the view.
I have a control template with a toggle button. This ToggleButton has it's IsChecked property one way bound to a dependancy property. If i set the dependancy property explicitly the binding works.
The problem is that after I interact with the toggle button in the UI, the bindings don't update the IsChecked property if I set the dependency property explicitly.
I do have a work arround using TwoWay binding which works fine. My question is, why does it behave this way? Am I missing something? Is there a bug in the binding mechanism of Silverlight?
EDIT TO INCLUDE SNIPPET:
The binding in the ControlTemplate looks something like (could be replaced with TemplateBinding)
<ToggleButton x:Name="PlayPause" Grid.Column="0"
IsChecked="{Binding Paused, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}"
HorizontalAlignment="Center"
Width="50" Height="50"/>
The explicit setting of the dependency property is the fairly bog standard:
myComponent.Paused = true;
WPF deletes one way bindings when the target property (IsChecked in this case) is modified. Silverlight used to keep the binding when IsChecked was modified. If Paused was later set, this value would overwrite IsChecked as well.
According to you, it seems Silverlight reverted to WPF behavior. Oh well. Personally, I consider modifying a binded property a bug. If the properties are not meant to be in sync commanding may be a better solution.
You should use TwoWay binding
Make sure that the object that contains your Paused property supports INotifyPropertyChanged.
Make sure that the setter for Paused triggers the PropertyChanged event