When I'm writing my Button, which is inherited from the button, I set the ControlTemplate using an image, the source is x:bind to a string property in code-behind, like the screenshot below:
<Grid x:Name="rectangle1">
<Image Source="{x:Bind aCheckImage}" //Error Here
Stretch="UniformToFill"
HorizontalAlignment="Center"
VerticalAlignment="Center">
</Image>
</Grid>
Why it throws an error that is "Object reference not set to an instance of an object" in compile. How to fix it?
#Vijay Nirmal gives me a way to solve it.
If you want to bind the image source to other value that you should sure your value in the code. I will write it below.
public static readonly DependencyProperty aCheckImageProperty = DependencyProperty.Register(
"aCheckImage", typeof(BitmapSource), typeof(MainPage), new PropertyMetadata(default(BitmapSource)));
public BitmapSource aCheckImage
{
get { return (BitmapSource) GetValue(aCheckImageProperty); }
set { SetValue(aCheckImageProperty, value); }
}
I should set it in constructor.
public MainPage()
{
InitializeComponent();
aCheckImage = new BitmapImage(new Uri("ms-appx:///Assets/LockScreenLogo.scale-200.png"));
}
If you want to know how to write uri, please see https://msdn.microsoft.com/zh-cn/library/windows/apps/xaml/hh965322(v=win.10).aspx
And then I can bind it in XAML.
<Grid x:Name="rectangle1">
<Image Source="{x:Bind aCheckImage}"
Stretch="UniformToFill"
HorizontalAlignment="Center"
VerticalAlignment="Center">
</Image>
</Grid>
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 am creating a ListView with ImageCell. I want to exicute a command on ImageCell click. But, debugger not call command method in ViewModel.
(I do the same for button command , that is working.)
View:
<ListView x:Name="lvLocation" ItemsSource="{Binding LocationList}">
<ListView.ItemTemplate>
<DataTemplate>
<ImageCell ImageSource="edit.png" Height="40" Text="{Binding StoreName}" Detail="{Binding CityName}" Command="{Binding OnImageListCommand}" CommandParameter="{Binding CityName}"/>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
View Model:
public class ClassName: MvvmBaseClass
{
public ICommand OnImageListCommand { get; set; }
public ClassName()
{
OnImageListCommand = new Command<string>( OnImageListClick );
}
private void OnImageListClick(string _commandParamenter)
{
//Write code here.
}
}
Your code looks good. What is the item's type in LocationList? Make sure that LocationList contains the items of type ClassName
The ImageCell will inherit the BidningContext from lvLocation's ItemSource
<TextBlock Name="currency" TextWrapping="Wrap" VerticalAlignment="Center"/>
<TextBlock Margin="5,0" Text="{Binding Text, ElementName=currency" TextWrapping="Wrap" VerticalAlignment="Center" FontSize="22" />
I am using the above code for binding property of one field to another in my WP7 application.
i want to do the similar task from back-end. any suggestions??
Bindings are working in a specified data context. You can set the data context of your layout root to the page instance, then you can bind to any of your properties. (DataContext is inherited through the child FrameworkElements.) If you want your binding to update its value whenever you change your property from code, you need to implement INotifyPropertyChanged interface or use Dependency properties.
<Grid x:Name="LayoutRoot">
<TextBox Text="{Binding Test, Mode=TwoWay}" />
</Grid>
public class MainPage : PhoneApplicationPage, INotifyPropertyChanged
{
private string test;
public string Test
{
get { return this.test; }
set
{
this.test = value;
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("Test"));
}
}
public MainPage()
{
InitializeComponents();
LayoutRoot.DataContext = this;
}
public event PropertyChangedEventHandler PropertyChanged;
}
This is a stupid example since you can access your TextBox any time from MainPage, this has much more sense if you are displaying model objects with DataTemplates.
(I typed this on phone, hope it compiles..)
i got my solution as: var b = new Binding{ElementName = "currency", Path = new PropertyPath("Text")}; Textblock txt = new TextBlock(); txt.SetBinding(TextBlock.TextProperty, b);
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'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.