I have a Listview in Xaml and I need to bind ItemTapped event to my ModelView using MVVM.
My ListView looks like.
<ListView x:Name="list"
ItemsSource="{Binding employeeList}"
ItemTapped= {Binding selectedItem} >
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<ViewCell.View>
</ViewCell.View>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
ItemTapped is an event which, in MVVM, normally corresponds to an ICommand*. If you meant to bind to a normal data property, judging from the attempted binding statement in your XAML snippet, then it would make more sense to do bind ListView's SelectedItem property instead :
<ListView x:Name="list"
ItemsSource="{Binding EmployeeList}"
SelectedItem="{Binding SelectedItem}">
.....
</ListView>
If there is data related action that need to be taken upon ItemTapped event occur, it might instead be implemented on SelectedItem property changed which triggered via binding (as in the XAML binding above) :
private ItemModel _selectedItem;
public ItemModel SelectedItem
{
get { return _selectedItem; }
set
{
if(_selectedItem != value)
{
_selectedItem = value;
OnPropertyChanged();
//this can be placed before or after propertychanged notification raised,
//depending on the situation
DoSomeDataOperation();
}
}
}
*) Xamarin Blog : Turn Events into Commands with Behaviors
Related
Update: Just a reminder, there's a 500 point bonus on this if someone can just show me how to implement this functionality without using Gestures>
I am using a ViewCell and a gesture recognizer to open up a picker with the following code. The ViewCell has a label on the left and a label area on the right that is populated initially when the app starts and later with the picker when the ViewCell is clicked.
XAML
<ViewCell x:Name="ati" Tapped="OpenPickerCommand">
<Grid VerticalOptions="CenterAndExpand" Padding="20, 0">
<Grid.GestureRecognizers>
<TapGestureRecognizer
Command="{Binding OpenPickerCommand}"
CommandParameter="{x:Reference atiPicker}" NumberOfTapsRequired="1" />
</Grid.GestureRecognizers>
<local:LabelBodyRendererClass Text="Answer Time Interval" HorizontalOptions="StartAndExpand" />
<Picker x:Name="atiPicker" IsVisible="false" HorizontalOptions="End" SelectedIndexChanged="atiPickerSelectedIndexChanged" ItemsSource="{Binding Times}"></Picker>
<local:LabelBodyRendererClass x:Name="atiLabel" HorizontalOptions="End"/>
</Grid>
</ViewCell>
<ViewCell x:Name="pti" Tapped="OpenPickerCommand">
<Grid VerticalOptions="CenterAndExpand" Padding="20, 0">
<Grid.GestureRecognizers>
<TapGestureRecognizer
Command="{Binding OpenPickerCommand}"
CommandParameter="{x:Reference ptiPicker}" NumberOfTapsRequired="1" />
</Grid.GestureRecognizers>
<local:LabelBodyRendererClass Text="Phrase Time Interval" HorizontalOptions="StartAndExpand" />
<Picker x:Name="ptiPicker" IsVisible="false" HorizontalOptions="End" SelectedIndexChanged="ptiPickerSelectedIndexChanged" ItemsSource="{Binding Times}"></Picker>
<local:LabelBodyRendererClass x:Name="ptiLabel" HorizontalOptions="End"/>
</Grid>
</ViewCell>
C# This works for different pickers (ati, bti, pti etc) with CommandParameter
public SettingsPage()
{
InitializeComponent();
BindingContext = new CommandViewModel();
}
void atiPickerSelectedIndexChanged(object sender, EventArgs e)
{
var picker = (Picker)sender;
int selectedIndex = picker.SelectedIndex;
if (selectedIndex != -1)
{
App.DB.UpdateIntSetting(Settings.Ati, selectedIndex);
atiLabel.Text = AS.ati.Text();
}
}
void ptiPickerSelectedIndexChanged(object sender, EventArgs e)
{
var picker = (Picker)sender;
int selectedIndex = picker.SelectedIndex;
if (selectedIndex != -1)
{
App.DB.UpdateIntSetting(Settings.Pti, selectedIndex);
ptiLabel.Text = AS.pti.Text();
}
}
public class CommandViewModel: ObservableProperty
{
public ICommand openPickerCommand;
public CommandViewModel()
{
openPickerCommand = new Command<Picker>(PickerFocus);
//openPickerCommand = new Command(tapped);
}
public ICommand OpenPickerCommand
{
get { return openPickerCommand; }
}
void PickerFocus(Picker param)
{
param.Focus();
}
}
I would like to remove the use of TapGestureRecognizers but I still want to retain the functionality and layout.
It's been suggested to me that it would be better if I used the Tapped event of the ViewCell like this:
Tapped="OnTapped"
Can someone explain in some detail how I could wire this up in C#. Would I be best to code something into the CommandViewModel as well as in the C# backing code. Also can the view model have one method that takes an argument so it could be used to open up different pickers?
An example of how I could do this would be very much appreciated. Note that I don't particularly need to use the CommandViewModel if there is a way that I could do this by coding just in the .cs backing code.
(Sorry for the poor english)
Despite not being best practice, I guess you can do something like this, dismissing the viewmodel:
XAML:
<ViewCell x:Name="ati" Tapped="OpenPickerCommand">
<Grid VerticalOptions="CenterAndExpand" Padding="20, 0">
<local:LabelBodyRendererClass Text="Answer Time Interval" HorizontalOptions="StartAndExpand" />
<Picker x:Name="atiPicker"
IsVisible="false"
HorizontalOptions="End"
SelectedIndexChanged="atiPickerSelectedIndexChanged"
ItemsSource="{Binding Times}">
</Picker>
<local:LabelBodyRendererClass x:Name="atiLabel" HorizontalOptions="End"/>
</Grid>
</ViewCell>
<ViewCell x:Name="pti" Tapped="OpenPickerCommand">
<Grid VerticalOptions="CenterAndExpand" Padding="20, 0">
<local:LabelBodyRendererClass Text="Phrase Time Interval" HorizontalOptions="StartAndExpand" />
<Picker x:Name="ptiPicker" IsVisible="false" HorizontalOptions="End" SelectedIndexChanged="ptiPickerSelectedIndexChanged" ItemsSource="{Binding Times}"></Picker>
<local:LabelBodyRendererClass x:Name="ptiLabel" HorizontalOptions="End"/>
</Grid>
</ViewCell>
C#:
private void OpenPickerCommand(object sender, System.EventArgs e)
{
if (sender != null)
{
Picker pkr = sender == ati ? atiPicker : ptiPicker;
pkr.Focus();
}
}
Answering your question "Can the view model have one method that takes an argument?", it is exactly what you're already doing using the 'OpenPickerCommand' method. The problem is that using the ViewCell's public event 'Tapped', you can't set parameters to the delegate handler.
Let me know if it works for you or if you do need some more information.
I hope it helps.
You can solve this with attached properties. Simply define a "behavior" class for ViewCell that adds the Command/Parameter properties.
public static class TappedCommandViewCell
{
private const string TappedCommand = "TappedCommand";
private const string TappedCommandParameter = "TappedCommandParameter";
public static readonly BindableProperty TappedCommandProperty =
BindableProperty.CreateAttached(
TappedCommand,
typeof(ICommand),
typeof(TappedCommandViewCell),
default(ICommand),
BindingMode.OneWay,
null,
PropertyChanged);
public static readonly BindableProperty TappedCommandParameterProperty =
BindableProperty.CreateAttached(
TappedCommandParameter,
typeof(object),
typeof(TappedCommandViewCell),
default(object),
BindingMode.OneWay,
null);
private static void PropertyChanged(BindableObject bindable, object oldValue, object newValue)
{
if (bindable is ViewCell cell)
{
cell.Tapped -= ViewCellOnTapped;
cell.Tapped += ViewCellOnTapped;
}
}
private static void ViewCellOnTapped(object sender, EventArgs e)
{
if (sender is ViewCell cell && cell.IsEnabled)
{
var command = GetTappedCommand(cell);
var parameter = GetTappedCommandParameter(cell);
if (command != null && command.CanExecute(parameter))
{
command.Execute(parameter);
}
}
}
public static ICommand GetTappedCommand(BindableObject bindableObject) =>
(ICommand)bindableObject.GetValue(TappedCommandProperty);
public static void SetTappedCommand(BindableObject bindableObject, object value) =>
bindableObject.SetValue(TappedCommandProperty, value);
public static object GetTappedCommandParameter(BindableObject bindableObject) =>
bindableObject.GetValue(TappedCommandParameterProperty);
public static void SetTappedCommandParameter(BindableObject bindableObject, object value) =>
bindableObject.SetValue(TappedCommandParameterProperty, value);
}
After that reference your behavior namespace in XAML and specify the property values using fully qualified names:
<ViewCell StyleId="disclosure-indicator"
behaviors:TappedCommandViewCell.TappedCommand="{Binding BrowseCommand}"
behaviors:TappedCommandViewCell.TappedCommandParameter="https://www.google.com">
<StackLayout Orientation="Horizontal">
<Label Text="Recipient"
VerticalOptions="Center"
Margin="20,0"/>
<Label Text="{Binding LedgerRecord.Recipient}"
HorizontalOptions="EndAndExpand"
VerticalOptions="Center"
Margin="0,0,20,0"/>
</Label>
</StackLayout>
</ViewCell>
The above will allow you to use MVVM and no Tap Gesture Recognizers.
The first problem is that you're mixing the code-behind and MVVM
approaches in the same code. It is confusing and certainly not the
right way to code what you want to achieve. So, all commanding must
be in the ViewModel attached to the View, no code-behind apart some
code only used for UI effects.
There is no need to define a gesture recognizer for all visual items since you just want to detect the tap on all the surface of the viewcell. To achieve this you must define all children of the ViewCell with InputTransparent=true. So the tap will not be detected and will be trapped by the parent ViewCell (you
must indicate the InpuTransparent because there is no tap event
bubbling in X.Forms).
Showing and Hidding the picker is a View problem not a ViewModel one. So here you can use some code-behind to create an event handler for the ViewCell Tapped event. This handler will just set visible=true on the picker.
The picker selected event must be connected to a corresponding Command in the ViewModel. So each time the picker is displayed and a value is selected your viewmodel will be aware of the action. This is the only command you need in your viewmodel. Depending of XForms version the picker has no bindable command, so you can use one of the numerous "bindablepicker" implementation you can find on the web or you can also use a XAML EventToCommand Behavior.
So there is two different problems : showing/hidding the picker which can be achieved directly in XAML or with the help of a bit of code-behind; and the picker item selection that must be managed using a Command in the viewmodel.
Hoping this will help you
I'm using MVVM in this project, I have a listbox which bind to a collection of Customers. I want to create an event to navigate a detailsPage using id of the elementselected:
<ListBox ItemsSource="{Binding Customers}" x:Name="state_list" SelectionChanged="state_list_SelectionChanged">
<i:Interaction.Triggers>
<i:EventTrigger EventName="selectionchanged">
<cmd:EventToCommand Command="{Binding stateSelectedCommand}" PassEventArgsToCommand="True" />
</i:EventTrigger>
</i:Interaction.Triggers>
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Vertical">
<TextBlock Text="{Binding nom}" />
<!--TextBlock Text="{Binding LastName}" />
<TextBlock Text="{Binding Text, ElementName=tbCount}" /-->
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
I can't figure out how to get the selected item to add it to the uri and then use it to get data. An example or tutorial would be helpful. Thanks :)
I would create a "SelectedCustomer" property in the ViewModel (next to you Customers property) and bind it to the SelectedItem. Then, on the setter of that property you can navigate to your desired page. This way you eliminate the messy events and command.
<ListBox x:Name="state_list
ItemsSource="{Binding Customers}"
SelectedItem="{Binding SelectedCustomer, Mode=TwoWay}">
...
public Customer SelectedCustomer
{
get
{
return _selectedCustomer;
}
set
{
if (value != null)
{
_selectedCustomer = value;
//Navigate to your page here, either with Navigator class or any other mechanism you have in place for changing content on screen
}
}
}
AlexDrenea gives you a good way of binding SelectedItem to a property on your viewmodel. If you are wanting to navigate based on this in an MVVM architecture, I would suggest using messaging to get it done.
I cover this in a blog post I did a while back, but the short summary of doing this within MVVMLight, is to create a Navigator class that sits at the application level.
public class Navigator
{
private PhoneApplicatoinFrame RootFrame;
public Navigator(PhoneApplicationFrame frame)
{
RootFrame = frame;
RegisterMessages();
}
private void RegisterMessages()
{
Messenger.Default.Register<ShowTrackerMessage>(this, ShowTracker);
}
private void ShowTracker(ShowTrackerMessage msg)
{
RootFrame.Navigate(new Uri("/Views/ItemLocationCompassView.xaml", UriKind.RelativeOrAbsolute));
}
}
Then, as part of your application start-up, create it and pass it a reference to your RootFrame:
private static Navigator _navigator;
public static Navigator Nav
{
get { return _navigator; }
}
...
_navigator = new Navigator(this.RootFrame);
Then, you have a couple choices on how you send the Navigation message.
Option 1: In your ViewModel, hook into the PropertyChanged event (part of INotifyPropertyChanged), and send the appropriate message when your SelectedItem property changes.
Option 2: Tie into the SelectionChanged event of your ListBox. I use the MVVMLight's EventToCommand to send that event to a RelayCommand in my ViewModel, then react appropriately to send the message to the Navigator object.
I cover this in more detail at: http://www.smartchitecture.com/?p=27
This question already has an answer here:
Closed 11 years ago.
Possible Duplicate:
Binding between Usercontrol with listbox and parent control (MVVM)
I’m trying to bind a UserControl to property on my main page’s viewmodel
The code looks like this:
UserControl xaml:
<UserControl x:Class="myUserControl" ....>
<Grid x:Name="LayoutRoot" Background="{StaticResource PhoneChromeBrush}" >
<ListBox Name="myListBox" ItemsSource="{Binding Path=myItemsSource}"/>
</Grid>
</UserControl>
the codebehind looks like this:
public partial class myUserControl : UserControl
{
public static DependencyProperty ItemsSourceProperty =
DependencyProperty.Register("myItemsSource",
typeof(IEnumerable), typeof(myUserControl), null);
public IEnumerable myItemsSource
{
get
{
return (IEnumerable)GetValue(ItemsSourceProperty);
}
set
{
SetValue(ItemsSourceProperty, value);
}
}
}
The UC is used like this in the main page:
<phone:PhoneApplicationPage DataContext="{Binding myViewModel, Source={StaticResource Locator}}" ....>
<Grid x:Name="ContentPanel">
<uc:myUserControl x:Name="ucList" myItemsSource="{Binding Path=DataList}"/>
</Grid>
</phone:PhoneApplicationPage>
and the viewModel for the main page look like this:
public class myViewModel : ViewModelBase
{
public ObservableCollection<myObject> DataList
{
get
{
return _datalist;
}
set
{
if (_dataList != value)
{
_dataList = value;
RaisePropertyChanged("DataList");
}
}
}
}
But when the DataList property is set, the uc List in not populated.
What i'm I missing ?
It's still early and I haven't had my coffee yet, but it looks to me like you might need a data template for your list items. You've defined the ItemsSource but not told the control how to render the items themselves.
What does your list contain? Try binding a data template to one of the list item properites, like this:
<UserControl x:Class="myUsercontrol" ....>
<Grid x:Name="LayoutRoot" Background="{StaticResource PhoneChromeBrush}" >
<ListBox Name="myListBox" ItemsSource="{Binding Path=myItemsSource}">
<ListBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding SomeListItemProperty}" />
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
EDIT: Are you by chance using MVVM Light? I saw that you mentioned {StaticResource = Locator} in one of the comments. That would be helpful to know.
One other thing you might try is checking if your UserControl is in its own namespace. If you put it in a UserControl folder, it might be. I've had some issues with binding a UserControl when it wasn't in the same namespace as the ViewModel.
I have a Pivot item template that includes a listbox
<controls:Pivot x:Name="MainPivot" ItemsSource="{Binding PivotItemHeaders}" Title="CLASS TIMETABLE" >
<controls:Pivot.HeaderTemplate>
<DataTemplate>
<TextBlock Text="{Binding Description}"/>
</DataTemplate>
</controls:Pivot.HeaderTemplate>
<controls:Pivot.ItemTemplate>
<DataTemplate>
<ListBox x:Name="Events" ItemsSource="{Binding allEventItems}" ItemTemplate="{StaticResource EventDisplay2}"/>
</DataTemplate>
</controls:Pivot.ItemTemplate>
</controls:Pivot>
In the code behind I want to access the selectedItem of that listbox but I cannot 'get' to the listbox as such because ity is (presumably) within the template
i.e
this.NavigationService.Navigate(new Uri("/View/EventEdit.xaml?selectedEvent=" + Events.SelectedItem, UriKind.Relative));
The Events listbox is not being recognised.
Assuminh I can pass get the object and pass it through as a parameter, what code can I use to retrieve it
I know its starts with
protected override void OnNavigatedTo(NavigationEventArgs e)
{
if (NavigationContext.QueryString.ContainsKey("SelectedEvent"))
{
But I am unsure of the syntax/code to extract out the object from the parameters
Appreciate how I can get the selectedItem from this listbox and the code to get the object being passed through
thanks
Rather than attempt to access the ListBox, you could use the SelectionChanged event to be told when the value changes:
<ListBox x:Name="Events"
ItemsSource="{Binding allEventItems}"
ItemTemplate="{StaticResource EventDisplay2}"
SelectionChanged="Event_SelectionChanged" />
And then in your code behind:
private void Event_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
this.selectedEvent = (EventItem)e.AddedItems[0];
}
You can access the value using NavigationContext.QueryString["selectedEvent"], but you can only store strings in navigation query strings. If your listbox is currently bound to objects, you'll need to select a key and then find that event from the second page using that key.
I've been struggling for a while on this. I'm a bit of a newbie, but i lurked a lot and still couldn't find a solution to my problem, so I decided to post my question here.
I'm binding a pivot to a collection of objects I want to display to create a gallery. Here is my pivot control bound to a list called gallery, where each object contains 2 strings (url and description).
<controls:Pivot ItemsSource="{Binding gallery}" Grid.Row="0" x:Name="galleryPivot">
<controls:Pivot.ItemTemplate>
<DataTemplate>
<StackPanel>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Image Source="{Binding url}" />
<Grid Visibility="{Binding ElementName=galleryPivot, Path=DataContext.ShowDetail}">
<ListBox>
<ListBoxItem>
<StackPanel>
<TextBlock Text="{Binding description}" />
</StackPanel>
</ListBoxItem>
</ListBox>
</Grid>
</Grid>
</StackPanel>
</DataTemplate>
</controls:Pivot.ItemTemplate>
</controls:Pivot>
The datacontext is the viewmodel and initialized in the constructor of the page.
Here is my ViewModel:
public class GalleryViewModel : INotifyPropertyChanged
{
public List<Gallery> gallery
{
get { return Globals.pictures; }
}
private Visibility _showDetail = Visibility.Collapsed;
public Visibility ShowDetail
{
get { return _showDetail; }
set {
_showDetail = value;
RaisePropertyChanged("ShowDetail");
}
}
public GalleryViewModel()
{ }
public event PropertyChangedEventHandler PropertyChanged = delegate { return; };
protected void RaisePropertyChanged(string propertyName)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
The gallery object is a list in my ViewModel, as the ShowDetail property. As ShowDetail is outside the scope, I tried to set ElementName as explained here.
The pivot binds well to the gallery list, but when I change the value of ShowDetail, the grid won't hide. I also tried to set ElementName to LayoutRoot but it still won't work.
My question, how can I bind the visibility when it is outside the scope of the itemtemplate?
Within a DataTemplate the ElementName binding refers only to the names of elements that are within that DataTemplate. The data context within your data template is the Gallery instance, not the GalleryViewModel. You could move the ShowDetail property down to the Gallery class instead.
If you'd rather not do that, then an alternative would be to use a proxy for the data context, which you add as a resource to the page and bind to the page's data context (a GalleryViewModel instance presumably). You can then reference that resource as you would any other resource to get at the parent data context.
If you're not familiar with this proxy concept, then Dan Wahlin's post on the subject should help.