In my xaml file I bind button command as follows: ... Command="{Binding MyCommand}" ....
I use the 'well-known' RelayCommand in my view model to create the bound command as follows:
MyCommand = new RelayCommand(param => RunMyCommand())
How can I identify the source button in RunMyCommand()?
Thanks.
Well identify by what exactly? I wouldn't recommend identifying the button itself regardless, so try looking into CommandParameters. Bind or specify something as the CommandParameter and you can receive it into your command.
Related
I have read some thread about how to work on WPF ListView command binding.
Passing a parameter using RelayCommand defined in the ViewModel
Binding Button click to a method
Button Command in WPF MVVM Model
How to bind buttons in ListView DataTemplate to Commands in ViewModel?
All of them suggest write the logic code inside ViewModel class, for example:
public RelayCommand ACommandWithAParameter
{
get
{
if (_aCommandWithAParameter == null)
{
_aCommandWithAParameter = new RelayCommand(
param => this.CommandWithAParameter("Apple")
);
}
return _aCommandWithAParameter;
}
}
public void CommandWithAParameter(String aParameter)
{
String theParameter = aParameter;
}
It is good practice or anyway so I can move the CommandWithAParameter() out of the ViewModel?
In principle, MVVM application should be able to run to its full potential without creating the views. That's impossible, if some parts of your logic are in View classes.
On top of that, ICommand has CanExecute, which will autamagically disable buttons, menu items etc. if the command should not be run.
I understand why with basic RelayCommand implementation it can be hard to see the benefits, but take a look at ReactiveCommand samples.
ReactiveCommand handles async work very well, even disabling the button for the time work is done and enabling it afterwards.
Short example: you have a login form. You want to disable the login button if the username and password are empty.
Using commands, you just set CanExecute to false and it's done.
Using events, you have manualy disable/enable the button, remember that it has to be done in Dispatcher thread and so on - it gets very messy if you have 5 buttons depending on different properties.
As for ListView, commands are also usefull - you can bind current item as command parameter:
<ListView ItemsSource="{Binding MyObjects}">
<ListView.ItemTemplate>
<DataTemplate>
<DockPanel>
<!-- change the context to parent ViewModel and pass current element to the command -->
<Button DockPanel.Dock="Right" Command="{Binding ElementName=Root, Path=ViewModel.Delete}" CommandParameter="{Binding}">Delete</Button>
<TextBlock Text="{Binding Name}"/>
</DockPanel>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
I know how to invoke relay command without parameter using mvvm pattern, but how to do the same with command with parameter?
https://i.stack.imgur.com/o7r5i.jpg
https://i.stack.imgur.com/zNkYR.jpg
https://i.stack.imgur.com/lmw3w.jpg
https://i.stack.imgur.com/iJnF0.jpg
If I understand you correctly, your command requires you to pass the TextEditor object in as a parameter, and you'd like to know how to do this in XAML. Since your TextEditor is named XMLView you'd simply bind this to the command parameter;
<KeyBinding Command="{Binding ValidateXMLCommand}" CommandParameter="{Binding ElementName=XMLView}" Modifiers="Control" Key="V" />
Notice the addition of CommandParameter="{Binding ElementName=XMLView}", this will pass the AvalonEdit TextEditor control instance as a parameter of the command.
Read more; https://stackoverflow.com/a/32064646/8520655
If you instead mean to invoke the RelayCommand from a ViewModel (in normal C#), you'd do the following;
if (ValidateXMLCommand.CanExecute(XMLView))
ValidateXMLCommand.Execute(XMLView);
Also, please do not post images of code, but rather your code formatted using the code style.
The control (e.g. Button / MenuItem) that you're binding your relaycommand to will have a CommandParameter property in addition to the Command property.
See here for an example of usage.
To execute a command from code behind, just call its Invoke() method, with the required parameter.
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.
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.
I'm learning MVVM through a project and I got stuck on something simple.
I have a Button that updates a ListView. I have a command in the ViewModel that make the right things but I want to select the new row and get the focus on a TextBox after I click the Button.
The question is: How do I update my UI after executing a command?
If I need to change my windows Title when an operation have been made, I use a property on the ViewModel that is binded to the Window's title and it's changed when I need it but, I don't know how to get focus on a control when a command has been executed.
Thank you.
To select the new row, add a new property to your ViewModel ("SelectedItem" for instance), and bind the ListView's SelectedItem property to it :
<ListView ItemsSource="{Binding Items}" SelectedItem="{Binding SelectedItem}">...
In the ViewModel, you just have to assign the new item to the SelectedItem property
To focus the TextBox, Mike's idea seems a good one
You could make an attached behavior. I'd suggest using the new Blend behavior framework, ie TriggerAction that contained this custom logic.
For your attached behavior you put on the button, give it a DP for an ICommand and maybe a DP of a ListView type.
On the "protected override void Invoke(object parameter)" of your TriggerAction, execute your ICommand, then you have reference to your ListView. Here you can do your custom code on it, like setting focus.
Your XAML may look something like this:
<Button>
<i:Interaction.Triggers>
<i:EventTrigger EventName="Click">
<Behaviors:CustomBehavior Command="CommandName" ListView="{Binding ElementName=myListView}" />
</i:EventTrigger>
</i:Interaction.Triggers>
<Button/>
I suggest looking at Mike Brown's ExecuteCommandAction behavior (download here), it's about almost 1/2 of what you need.
What about setting focus to the control in the code behind: textBox.Focus()
I consider everything you mention in your question to be GUI logic so I would add a Click event to the button to handle stuff that needs to happend in the GUI.
Hope this helps.
I think you need to use the Mediator pattern. Please see this:
Josh Smith's Mediator Prototype for WPF Apps
This is generally used in communicating with the View from the View-Model. Hope this helps.
In your case you need some way that the ViewModel notifies the View that it should set the focus on a specific control.
This could be done with an IView interface. The view implements this interface and the ViewModel can call a method of the View over this interface. This way you have still the View and ViewModel decoupled of each other.
How this can be done is shown here:
WPF Application Framework (WAF)
http://waf.codeplex.com