I have a picker control which is optional field and the user can set the selected item of picker to be empty if he wants.
Is it possible to have Select as the additional option in itemsource of xamarin forms picker control.
Code for Custom picker:
<controls:CustomPicker
Grid.Row="1"
Grid.Column="1"
Margin="0,5,0,0"
Image="Downarrow"
ItemDisplayBinding="{Binding abb}"
ItemsSource="{Binding StateList}"
Placeholder="Select"
SelectedIndex="{Binding StateSelectedIndex}"
Style="{StaticResource Key=PickerHeight}" />
StateSelectedIndex = -1;
I tried setting selected index = -1. That works only when nothing is selected in the picker control, but once if an item is selected from the picker, then option "Select" can not be chosen (disappears).
I tried referring below url Default value for Picker but this did not help.
Any help is appreciated.
Solution 1:
You should set the binding mode of SelectedIndex
SelectedIndex="{Binding StateSelectedIndex,Mode=TwoWay}"
Solution 2:
You could binding the value of SelectedItem in ViewModel .
public class YourViewModel: INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
public ObservableCollection<string> MyItems { get; set; }
private string selectItem;
public string SelectItem
{
get
{
return selectItem;
}
set
{
if(value!=null)
{
selectItem = value;
NotifyPropertyChanged("SelectItem");
int SelectIndex = MyItems.IndexOf(value);
// this will been invoked when user change the select , do some thing you want
}
}
}
Related
I have the following situation:
I have an app, where I want to create notes using a special page for it. I have VM with ObservableCollection NoteItems with all the notes. When I want to create a new note, I add a new note to this ObservableCollection and pass this new note using BindingContext
var editingPage = new EditingPage();
editingPage.BindingContext = NoteItems[NoteItems.Count - 1].Text;
Application.Current.MainPage.Navigation.PushAsync(editingPage);
In the editing page I have an Entry field
<Entry
x:Name="EdtingPageEntryField"
Text="{Binding Text}"
Placeholder="Enter a note"
/>
, which is Binding his text with the Text parameter of a NoteItem.
The problem is that if I change the text in entry field, it does not automatically apply to a Text parameter of a NoteItem. That is why I want to pass this text when I close this EditingPage(go back to the MainPage). So the question is, how can I pass this Text parameter to a NoteItem element from NoteItems ObservableCollection, which is located in VM.
UPD. The value of NoteItem, which is located in NoteItems does not change
UPD2. I was wrong about the value of NoteItem, it has been changed, but the new value does not display on MainPage, that is why I used INotifyPropertyChanged, but it did not work.
Here is Note Item class
public class NoteItem
{
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
private string text;
public string Text
{
get { return text; }
set
{
if(text != value)
{
text = value;
NotifyPropertyChanged();
}
}
}
And MainPage.xaml:
<ContentPage.BindingContext>
<local:NoteItemViewModel/>
</ContentPage.BindingContext>
<FlexLayout>
<ListView ItemsSource="{Binding NoteItems}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Label Text="{Binding Text}"/>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</FlexLayout>
'''
in order to dynamically update the UI, your model must implement INotifyPropertyChanged
public class NoteItem : INotifyPropertyChanged
{
...
}
simply adding a PropertyChanged method is not the same as implementing INotifyPropertyChanged. You must add the interface to the class definition so that the binding mechanism knows that your class has implemented it
I have a Xamarin.Forms application, and it uses FreshMvvm. I have two picker controls for selecting countries and states/provinces. The picker of countries is populated initially, but the list of states/provinces should be populated on the fly based on the selected country. I cannot find how it can be done using command and not code-behind event handling.
Here are my controls in MyPage.xaml:
<Picker Title="Choose Country..."
ItemsSource="{Binding Countries}"
ItemDisplayBinding="{Binding Value}"
SelectedItem="{Binding SelectedCountry}"
Margin="0, 0, 0, 5" />
<Picker Title="Choose State..."
ItemsSource="{Binding States}"
ItemDisplayBinding="{Binding Value}"
SelectedItem="{Binding SelectedState}"
Margin="0, 0, 0, 5" />
What should I put in MyPageModel.cs?
using Freshmvvm you can make use of the WhenAny method and listen to changes on the SelectedCountry property. When this happens you will get filter the collection of the states by country using the SelectedCountry and update your States collection with the result.
That should look like this:
[PropertyChanged.AddINotifyPropertyChangedInterface]
public class MyViewModel : FreshBasePageModel
{
public ObservableCollection<Country> Countries { get; set; }
public ObservableCollection<State> States { get; set; }
// This would be the collection where you have all the States
private List<State> _allStatesCollection = new List<State>();
public Country SelectedCountry { get; set; }
public MyViewModel()
{
// Listening for changes on the `SelectedCountry`
this.WhenAny(OnCountryChanged, o => o.SelectedCountry);
}
//Method called when a new value is set in the `SelectedCountry` property
private void OnCountryChanged(string property)
{
//Filter the collection of states and set the results
var states = _allStatesCollection.Where(a => a.CountryCode == SelectedCountry.Code).ToList();
States = new ObservableCollection<State>(states);
}
}
Note: The code above expects you to be using the Fody INotifyPropertyChanged Nuget package. In case you are not using it, you can either install it or implement your properties PropertyChanged manually. That won't change the rest of the code.
Hope this helps.-
I have a ListView which is bound to an ObservableCollection.
Is there a way to update a single cell whenever a property of a SomeModel item changed, without reloading the ListView by changing the ObservableCollection?
(Question is copied from https://forums.xamarin.com/discussion/40084/update-item-properties-in-a-listviews-observablecollection, as is my answer there.)
As I can see you are trying to use MVVM as a pattern for your Xamarin.Forms app. You are already using the ObservableCollection for displaying a list of the data. When a new item is added or removed from collection UI will be refreshed accordingly and that is because the ObserverbleCollection is implementing INotifyCollectionChanged.
What you want to achieve with this question is next behaviour, when you want to change the particular value for the item in the collection and update the UI the best and simplest way to achieve that is to implement INotifyPropertyChanged for a model of the item from your collection.
Bellow, I have a simple demo example on how to achieve that, your answer is working as I can see but I am sure this example would be nicer for you to use it.
I have simple Button with command and ListView which holds my collection data.
Here is my page, SimpleMvvmExamplePage.xaml:
<StackLayout>
<Button Text="Set status"
Command="{Binding SetStatusCommand}"
Margin="6"/>
<ListView ItemsSource="{Binding Cars}"
HasUnevenRows="True">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Vertical"
Margin="8">
<Label Text="{Binding Name}"
FontAttributes="Bold" />
<StackLayout Orientation="Horizontal">
<Label Text="Seen?"
VerticalOptions="Center"/>
<CheckBox IsChecked="{Binding Seen}"
Margin="8,0,0,0"
VerticalOptions="Center"
IsEnabled="False" />
</StackLayout>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>
The basic idea from this demo is to change the value of the property Seen and set value for the CheckBox when the user clicks on that Button above the ListView.
This is my Car.cs class.
public class Car : INotifyPropertyChanged
{
private string name;
public string Name
{
get { return name; }
set
{
name = value;
OnPropertyChanged();
}
}
private bool seen;
public bool Seen
{
get { return seen; }
set
{
seen = value;
OnPropertyChanged();
}
}
// Make base class for this logic, something like BindableBase
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
In the full demo example which is on my Github, I am using my BindableBase class where I handle raising the INotifyPropertyChanged when some property value is changed with this SetProperty method in the setter of the props.
You can find the implementation here: https://github.com/almirvuk/Theatrum/tree/master/Theatrum.Mobile/Theatrum.Mobile
The last thing to show is my ViewModel for this page, and inside of the ViewModel, I will change the value of Seen property to True for items in the collection when the user clicks on the Button above the ListView. Here is my SimpleMvvmExamplePageViewModel.cs
public class SimpleMvvmExamplePageViewModel
{
public ObservableCollection<Car> Cars { get; set; }
public ICommand SetStatusCommand { get; private set; }
public SimpleMvvmExamplePageViewModel()
{
// Set simple dummy data for our ObservableCollection of Cars
Cars = new ObservableCollection<Car>()
{
new Car()
{
Name = "Audi R8",
Seen = false
},
new Car()
{
Name = "BMW M5",
Seen = false
},
new Car()
{
Name = "Ferrari 430 Scuderia",
Seen = false
},
new Car()
{
Name = "Lamborghini Veneno",
Seen = false
},
new Car()
{
Name = "Mercedes-AMG GT R",
Seen = false
}
};
SetStatusCommand = new Command(SetStatus);
}
private void SetStatus()
{
Car selectedCar = Cars.Where(c => c.Seen == false)
.FirstOrDefault();
if (selectedCar != null)
{
// Change the value and update UI automatically
selectedCar.Seen = true;
}
}
}
This code will help us to achieve this kind of behaviour: When the user clicks on the Button we will change value of the property of the item from collection and UI will be refreshed, checkbox value will be checked.
The final result of this demo could be seen on this gif bellow.
P.S. I could combine this with ItemTapped event from ListView but I wanted to make this very simple so this example is like this.
Hope this was helpful for you, wishing you lots of luck with coding!
Any UI associated with a model item will be refreshed, if replace the item with itself, in the Observable Collection.
Details:
In ViewModel, given property:
public ObservableCollection<Item> Items { get; set; } = new ObservableCollection<Item>();
Where Item is your model class.
After adding some items (not shown), suppose you want to cause item "item" to refresh itself:
public void RefreshMe(Item item)
{
// Replace the item with itself.
Items[Items.IndexOf(item)] = item;
}
NOTE: The above code assumes "item" is known to be in "Items". If this is not known, test that IndexOf returns >= 0 before performing the replacement.
In my case, I had a DataTemplateSelector on the collection, and the item was changed in such a way that a different template was required. (Specifically, clicking on the item toggled it between collapsed view and expanded/detailed view, by the TemplateSelector reading an IsExpanded property from the model item.)
NOTE: tested with a CollectionView, but AFAIK will also work with the older ListView class.
Tested on iOS and Android.
Technical Note:
This replacement of an item presumably triggers a Replace NotifyCollectionChangedEvent, with newItems and oldItems both containing only item.
I am using Picker in xamarin forms. I am binding the below class to Picker:
public string FieldCode{get; set; }
public string FieldValue{get; set; }
The Picker is like below:
<Picker Grid.Row="1" Grid.Column="0" x:Name="pkrMvmtCat" Style="{StaticResource WOFormPicker}" Title="Select" ItemsSource="{Binding FieldCode}" ItemDisplayBinding="{Binding FieldValue}" SelectedIndexChanged="pkrMvmtCat_SelectedIndexChanged"></Picker>
I want to get the FieldCode value, when i change the picker index.
Please me to resolve this issue.
You have to listen to the event, and then cast the selected item from the list. Here is an example:
void OnPickerSelectedIndexChanged(object sender, EventArgs e)
{
var picker = (Picker)sender;
int selectedIndex = picker.SelectedIndex;
if (selectedIndex != -1)
{
var field = (string)picker.ItemsSource[selectedIndex];
}
}
Not sure if I formatted the question appropriately, please let me know if I did not. But I am trying to simply bind a background color to a value in my viewcell. I have this working, actually. The issue is when I update a value, I don't see the change in background color. The implementation is a bit complicated, but here's my code.
ViewCell (OnBindingContextChanged)
...
ShowReadOverlay.SetBinding(Xamarin.Forms.VisualElement.BackgroundColorProperty, new Xamarin.Forms.Binding(".", Xamarin.Forms.BindingMode.TwoWay, new XamarinMobile.Converters.GridCellBackgroundColorConverter(), null, null, null));
...
So essentially I just build my layout. I decided to only post the relevant code that sets the binding in my OnBindingContextChanged method. If anyone needs any other code I'd be glad to add it, just don't know if it's relevant. My ViewCell class is a simple class that just inherits ViewCell.
Here's my converter:
public class GridCellBackgroundColorConverter : Xamarin.Forms.IValueConverter
{
#region IValueConverter Members
public object Convert(object value, Type targetType, object parameter,
System.Globalization.CultureInfo culture)
{
try
{
var cell = (XamarinMobile.ViewModels.GridCellViewModel)value;
if(cell.HasRead)
{
//return with shadow
return Xamarin.Forms.Color.FromRgba(0,0,0,0.6);
} else
{
//return no shadow
return Xamarin.Forms.Color.FromRgba(0, 0, 0, 0.0);
}
} catch(System.Exception ex)
{
return null;
}
}
public object ConvertBack(object value, Type targetType, object parameter,
System.Globalization.CultureInfo culture)
{
throw new NotSupportedException();
}
#endregion
}
Simple. It works. Now here's the tricky part. So the grid I'm describing, is a listview that contains cells of stories. A user will click on an image which will take them to a story page. When the user is in the story page, they can either go back to the grid to go to another story, or swipe left or right and they can get to another story that way. When a user goes to a story page from our grid, then the cell gets updated fine. BUT if a user swipes to another story NOT from the grid, that's where my issue is. In my story page I have logic that iterates through the grid cells, and finds the story you're currently on (the story you swiped to) and sees if it's in the grid, if it's in the grid, I update the cell's HasRead property. As such:
//find the cell in the grid (if exists)
ViewModels.GridCellViewModel cell = App.GridCells.Where(x => x.StoryId == App.Story.StoryId).FirstOrDefault();
if (cell != null)
{
cell.HasRead = true;
}
This works but... it doesn't trigger the value converter to change the property. What am I doing wrong? How can I get it so that I can update a property, and have it trigger my value converter?
My guess is that you're converter isn't triggering because you've technically bound to the viewcell itself, not the HasRead property. When you set HasRead, it will (assuming it's implementing INotifyPropertyChanged) fire a PropertyChangedEvent which would trigger the binding and call the value converter. However, since your binding is pointing to the viewcell itself, it will only trigger when that changes and ignore property changes elsewhere on that object.
A possible solution is to change the binding to point to HasRead (instead of '.'), and update your converter to expect the boolean directly rather than taking in a viewcell. This would be a better practice for a converter regardless.
That said, this is not really following the mvvm pattern that is generally recommended for xamarin forms apps. My suggestion would be to have a viewmodel that has a property that holds your story models (wrapped in their own StoryViewModels if you need logic there) and make sure the VM and Model classes implement INotifyPropertyChanged. Make the VM the datacontext for the page, bind the list to your listview source and your listview itemtemplate contents will bind to each individual story. Each story can have a HasRead property that binds to the background color via your updated converter.
Like this:
<ContentPage
x:Class="Stack_Stories.MainPage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:Stack_Stories">
<ContentPage.BindingContext>
<local:StoriesViewModel x:Name="VM" />
</ContentPage.BindingContext>
<ContentPage.Resources>
<ResourceDictionary>
<local:StoryReadBackgroundColorConverter x:Key="HasReadColor" />
</ResourceDictionary>
</ContentPage.Resources>
<ListView ItemsSource="{Binding Stories}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Grid x:Name="StoryGrid" BackgroundColor="{Binding HasRead, Converter={StaticResource HasReadColor}}">
<Button Command="{Binding ToggleReadCommand}" Text="{Binding Name}" />
</Grid>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
public class StoryViewModel : INotifyPropertyChanged
{
private string _name = "";
public string Name
{
get { return _name; }
set { _name = value; OnPropertyChanged(); }
}
private bool _hasRead = false;
public bool HasRead
{
get { return _hasRead; }
set { _hasRead = value; OnPropertyChanged(); }
}
private Command _toggleRead;
public Command ToggleReadCommand
{
get
{
return _toggleRead
?? (_toggleRead = new Command(() => HasRead = !HasRead));
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
public class StoriesViewModel : INotifyPropertyChanged
{
public StoriesViewModel()
{
// add sample stories
Stories.Add(new StoryViewModel { Name = "First Story" });
Stories.Add(new StoryViewModel { Name = "Second Story", HasRead=true });
}
private ObservableCollection<StoryViewModel> _stories = new ObservableCollection<StoryViewModel>();
public ObservableCollection<StoryViewModel> Stories
{
get { return _stories; }
set { _stories = value; OnPropertyChanged(); }
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
public class StoryReadBackgroundColorConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (!(value is bool)) return null;
return (bool)value ? Color.FromRgba(0, 0, 0, 0.6) : Color.FromRgba(0, 0, 0, 0.0);
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}