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How can I wtite my code behind for checkbox in MVVM?

I have a code for checkbox. Please tell me how to write it in MVVM?
There is a function that I can choose only one checkbox. In general I understand that I must to write command.
XAML:
<StackLayout>
<!-- Place new controls here -->
<ListView ItemsSource="{Binding Items}" ItemSelected="ListView_ItemSelected">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Horizontal">
<CheckBox HorizontalOptions="Start" Color="Black" CheckedChanged="CheckBox_CheckedChanged"
IsChecked="{Binding IsSelected}"
/>
<Label Text="meow" TextColor="Gray"></Label>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>
CODE BEHIND
public partial class MainPage : ContentPage
{
public MainPage()
{
InitializeComponent();
BindingContext = new MainPageViewModel();
}
Model previousModel;
private void CheckBox_CheckedChanged(object sender, CheckedChangedEventArgs e)
{
if (previousModel != null)
{
previousModel.IsSelected = false;
}
Model currentModel = ((CheckBox)sender).BindingContext as Model;
previousModel = currentModel;
if (currentModel.IsSelected)
{
var viewModel = BindingContext as MainPageViewModel;
int index = viewModel.Items.IndexOf(currentModel);
}
}
private void ListView_ItemSelected(object sender, SelectedItemChangedEventArgs e)
{
if (previousModel != null)
{
previousModel.IsSelected = false;
}
Model currentModel = e.SelectedItem as Model;
currentModel.IsSelected = true;
previousModel = currentModel;
}
}
ViewModel
public class MainPageViewModel
{
public List<Model> Items { set; get; }
public MainPageViewModel()
{
List<Model> list = new List<Model>();
for (int i=0; i<10; i++)
{
list.Add(new Model { IsSelected = false });
}
Items = list;
}
}
Model
public class Model : INotifyPropertyChanged
{
bool isSelected;
public bool IsSelected
{
set
{
isSelected = value;
onPropertyChanged();
}
get => isSelected;
}
public event PropertyChangedEventHandler PropertyChanged;
void onPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}

For an event to command use Corcav.Behavior nuget
https://github.com/corradocavalli/Corcav.Behaviors
...
xmlns:corcav="clr-namespace:Corcav.Behaviors;assembly=Corcav.Behaviors"
...
<CheckBox>
<corcav:Interaction.Behaviors>
<corcav:BehaviorCollection>
<corcav:EventToCommand EventName="CheckedChanged" Command="{Binding Path=CheckBoxChangedCommand}" Commandparameter="{Binding .}"/>
</corcav:BehaviorCollection>
</corcav:Interaction.Behaviors>
</CheckBox>
Add this command in ViewModel and write your logic
public ICommand CheckBoxChangedCommand{ get; set; }
...
CheckBoxChangedCommand= new Command<object>(CheckBoxChanged);
...
private void CheckBoxChanged(object obj)
{
//set all list/collection element to false with linq
if(obj is Model model)
{
model.IsSelected = true;
}
}

For now, CheckBox do not support Command. This issue has reported on Github and have not fixed. We could follow this enhancement. https://github.com/xamarin/Xamarin.Forms/issues/6606
You could use the InputKit instead. Install Xamarin.Forms.InputKit on NuGet.
It provides CheckChangedCommand.
CheckChangedCommand: (Command) Bindable Command, executed when check changed.
<input:CheckBox HorizontalOptions="Start" Color="Black" CheckChangedCommand="{Binding CheckBoxChangedCommand}">

Xamrin forms Charge a page before going to it

I am learning Xamaring forms , I want to do 4 content pages. One will display my to do listand images.
I would like to know if there is a way to charge my todo list before going to the last page from any of my 3 pages.
Knowing that I am going through pages like this :
var page = new LastPage();
MainView.Content = page.Content;
Thanks for your help

Do you want to achieve the result like following GIF?
If so, you need achieve it by MVVM and INotifyPropertyChanged
First of all, you should create a model to achieve the INotifyPropertyChanged.
public class MyModel: INotifyPropertyChanged
{
string name;
public string Name
{
set
{
if (name != value)
{
name = value;
OnPropertyChanged("Image");
}
}
get
{
return name;
}
}
string count;
public string Count
{
set
{
if (count != value)
{
count = value;
OnPropertyChanged("Count");
}
}
get
{
return count;
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
Then you need the ViewModel to push the data.
public class MyViewModel
{
public ObservableCollection<MyModel> myModels { get; set; }
public MyViewModel() {
myModels = new ObservableCollection<MyModel>();
myModels.Add(new MyModel() { Count = "0", Name = "test1" });
myModels.Add(new MyModel() { Count = "1", Name = "test2" });
myModels.Add(new MyModel() { Count = "2", Name = "test3" });
}
}
In the First page and end page, you should binding same viewmodel that use bindingcontext like following code format.
MainPage.xaml
<StackLayout>
<!-- Place new controls here -->
<Button Text="Next" Clicked="Button_Clicked"></Button>
<ListView x:Name="mylistview" ItemsSource="{Binding myModels}" HasUnevenRows="True" >
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell >
<StackLayout>
<Label Text="{Binding Name}"
FontAttributes="Bold"
FontSize="Large"
HorizontalOptions="Center"
VerticalOptions="Center" />
<Label Text="{Binding Count}"
LineBreakMode="WordWrap"
HorizontalOptions="Center" />
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>
MainPage.xaml.cs
public partial class MainPage : ContentPage
{
MyViewModel viewModel;
public MainPage()
{
InitializeComponent();
viewModel = new MyViewModel();
BindingContext = viewModel;
}
private void Button_Clicked(object sender, EventArgs e)
{
Navigation.PushAsync(new Page1(viewModel));
}
}
Here is my demo, you can download it.
https://github.com/851265601/XFormsMvvmChange
Here is a helpful article about it, you can refer to it.
https://learn.microsoft.com/en-us/xamarin/xamarin-forms/xaml/xaml-basics/data-bindings-to-mvvm

Why this picker is not selecting an item as expected?

I'm trying to do a picker that loads ItemSource from a List and depending on an external event, change its SelectedIndex based on Local.id, but what I've been trying so far didn't works.
C# code:
public class Local
{
public string cidade { get; set; }
public int id { get; set; }
}
public int CidadeSelectedIndex{ get; set; }
string jsonCidades;
public async void CarregaCidades()
{
try
{
using (WebClient browser = new WebClient())
{
Uri uriCidades = new Uri("xxxxxxx.php");
jsonCidades = await browser.DownloadStringTaskAsync(uriCidades);
}
var ListaCidades = Newtonsoft.Json.JsonConvert.DeserializeObject<List<Local>>(jsonCidades);
PickerCidades.ItemsSource = ListaCidades;
}
catch (Exception)
{
throw;
}
}
//In some moment of the execution, this code is called:
Local localizacao = JsonConvert.DeserializeObject<Local>(json);
if (localizacao.GetType().GetProperty("id") != null)
{
/*CidadeSelectedItem = localizacao;
I tried that before with SelectedItem="{Binding CidadeSelectedItem, Mode=TwoWay}" */
CidadeSelectedIndex = localizacao.id; // now trying this
}
Before I was trying to bind using ItemDisplayBinding="{Binding ListaCidades.cidade, Mode=OneWay}" but since it was not working I start to use ItemSources=ListaCidades
My XAML code:
<Picker x:Name="PickerCidades"
SelectedIndex="{Binding CidadeSelectedIndex, Mode=TwoWay}"
Grid.Column="1" Grid.Row="0"
SelectedIndexChanged="PickerCidades_SelectedIndexChanged">
</Picker>
I think it's not working because I'm setting the items using ItemsSource. I think I need to bind it using xaml. Would be nice have some help.

Do you want to achieve the result like following GIF?
My xaml layout like following code.
<StackLayout>
<!-- Place new controls here -->
<Picker x:Name="PickerCidades"
ItemsSource="{ Binding locals}"
SelectedIndex="{Binding CidadeSelectedIndex, Mode=TwoWay}"
ItemDisplayBinding="{Binding cidade}"
Grid.Column="1" Grid.Row="0"
SelectedIndexChanged="PickerCidades_SelectedIndexChanged">
</Picker>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Label Text="CidadeSelectedIndex: " Grid.Column="0" Grid.Row="0"/>
<Label Text="{Binding CidadeSelectedIndex}" Grid.Column="1" Grid.Row="0"/>
</Grid>
</StackLayout>
Layout background code.
public partial class MainPage : ContentPage
{
MyViewModel myViewModel;
public MainPage()
{
InitializeComponent();
myViewModel= new MyViewModel();
BindingContext = myViewModel;
}
private void PickerCidades_SelectedIndexChanged(object sender, EventArgs e)
{
var picker = (Picker)sender;
int selectedIndex = picker.SelectedIndex;
myViewModel.CidadeSelectedIndex = selectedIndex;
}
}
MyViewMode code.I use static data for testing. You can achieve the INotifyPropertyChanged interface to change dynamically.
public class MyViewModel : INotifyPropertyChanged
{
int _cidadeSelectedIndex=1;
public int CidadeSelectedIndex
{
set
{
if (_cidadeSelectedIndex != value)
{
_cidadeSelectedIndex = value;
OnPropertyChanged("CidadeSelectedIndex");
}
}
get
{
return _cidadeSelectedIndex;
}
}
public ObservableCollection<Local> locals { get; set; }
public MyViewModel()
{
locals = new ObservableCollection<Local>();
locals.Add(new Local() { cidade= "xxx0" , id= 0 });
locals.Add(new Local() { cidade = "xxx1", id = 1 });
locals.Add(new Local() { cidade = "xxx2", id = 2 });
locals.Add(new Local() { cidade = "xxx3", id = 3 });
locals.Add(new Local() { cidade = "xxx4", id = 4 });
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}

If the goal is to change the User Interface from code you need to have a ViewModel that implements INotifyPropertyChanged (or inherits from a base that does). Then instead of SelectedIndex bound property being a simple get; set as below it fires off the PropertyChanged event.
public int CidadeSelectedIndex{ get; set; }
Needs to fire notification event. Something along these lines
public class MyViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
// This method is called by the Set accessor of each property.
// The CallerMemberName attribute that is applied to the optional propertyName
// parameter causes the property name of the caller to be substituted as an argument.
private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
private int _cidadeSelectedIndex;
public int CidadeSelectedIndex
{
get => _cidadeSelectedIndex;
set {
_cidadeSelectedIndex = value;
NotifyPropertyChanged();
}
}
}

Listview doesn't refresh the item view correctly

I'm developing an app with xamarin forms and the MVVM pattern. I have a page with a listview that has three buttons but all the time with only 2 visibles and change the visibility of two of them when I press a button. The problem is that for the first ten items it works like supposed to be, press the button and dissapear and appear the other, but after the 10th item when I press the button it dissapear but the other doesn't appear until I scrool the list view to a position where the item is out of the screen. When the item is out of the screen and come back to be on the screen, the button appear. The visibility of the buttons is controlled changing a boolean property that is binded to the IsVisible property of the button and one of them with a converter to negate the value of the property. This is a repository that you can clone and see the code and test, maybe is something with my Visual Studio.
Initially, I thought it could be for a race condition and made the method that change the variable synchronous but it doesn't work.
This is my list view
<ListView ItemsSource="{Binding Items}"
HasUnevenRows="True"
SeparatorVisibility="None"
IsRefreshing="False">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout>
<Label Text="{Binding Name}"/>
<StackLayout Orientation="Horizontal">
<Button Text="One"
HorizontalOptions="CenterAndExpand"
TextColor="Green"
BackgroundColor="White"
BorderColor="Green"
BorderWidth="1"
WidthRequest="150" />
<Button Text="Two"
HorizontalOptions="CenterAndExpand"
BackgroundColor="Green"
TextColor="White"
Command="{Binding TestCommand}"
WidthRequest="150"
IsVisible="{Binding TestVariable, Converter={StaticResource negate}}" />
<Button Text="Three"
HorizontalOptions="CenterAndExpand"
BackgroundColor="Red"
Command="{Binding TestCommand}"
TextColor="White"
WidthRequest="150"
IsVisible="{Binding TestVariable}" />
</StackLayout>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
The viewmodel
public class ListViewTestModel : BaseViewModel
{
private List<ListItemTestModel> items;
public List<ListItemTestModel> Items
{
get => items;
set
{
SetValue(ref items, value);
}
}
public ListViewTestModel()
{
List<ListItemTestModel> itemList = new List<ListItemTestModel>();
for (int i = 0; i < 40; i++)
{
itemList.Add(new ListItemTestModel { Name = "Test" });
}
Items = itemList;
}
}
And another view model that is binded to each item in the listView
public class ListItemTestModel : BaseViewModel
{
private bool testVariable;
public string Name { get; set; }
public bool TestVariable
{
get
{
return testVariable;
}
set
{
SetValue(ref testVariable, value);
}
}
public Command TestCommand { get; set; }
public ListItemTestModel()
{
TestCommand = new Command(() =>
{
TestMethod();
});
}
public void TestMethod()
{
TestVariable = !TestVariable;
}
}
the BaseViewModel
public class BaseViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
protected void SetValue<T>(ref T backingField, T value, [CallerMemberName] string propertyName = null)
{
if (EqualityComparer<T>.Default.Equals(backingField, value))
{
return;
}
backingField = value;
OnPropertyChanged(propertyName);
}
}
And the codebehind of the page
public partial class MainPage : ContentPage
{
public ListViewTestModel ViewModel { get; }
public MainPage()
{
ViewModel = new ListViewTestModel();
BindingContext = ViewModel;
InitializeComponent();
}
}

I suggest listview Caching Strategy may case this issue, the default value is RetainElement for ListView, so using CachingStrategy="RecycleElement" in ListView.
About listview Caching Strategy, you can take a look:
https://learn.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/listview/performance#caching-strategy

You should definitely go to ObservableCollection type for your items thus you'll be able to observe and display any changes
private ObservableCollection<ListItemTestModel> items;
public ObservableCollection<ListItemTestModel> Items
{
get => items;
set => SetValue(ref items, value);
}
And you should set your BindingContext AFTER the InitializeComponent() method or property changed will be propagate before your view is initialized.
public MainPage()
{
InitializeComponent();
BindingContext = new ListViewTestModel();;
}
public ListViewTestModel()
{
List<ListItemTestModel> itemList = new List<ListItemTestModel>();
for (int i = 0; i < 40; i++)
{
itemList.Add(new ListItemTestModel { Name = "Test" });
}
Items = new ObservableCollection<ListItemTestModel>(itemList);
}

EASY way to refresh ListBox in WPF?

I have created a simple form that inserts/updates/deletes a values for Northwind Customers.
Everything works fine, except in order to see a results, I have to close it, and reopen again.
My form looks like this :
I've searched tens of articles on how to refresh ListBox, but all of those use interface implementing, or using DataSets, and stuff I have never heard of and cannot implement. It's a very simple project, using simple procedures. Is there an easy way to refresh the list of customers without adding many lines of code?

The simple answer is: myListBox.Items.Refresh();

Are you using ObservableCollection and does your model implement INotifyPropertyChanged these two things will automaticly update the ListBox on any change. no need to explicitly refresh the list.
Here is a small example of using ObservableCollection and INotifyPropertyChanged, obviously you will populate your ObservableCollection from your SQL database.
Window:
public partial class MainWindow : Window, INotifyPropertyChanged
{
private ObservableCollection<MyModel> _list = new ObservableCollection<MyModel>();
private MyModel _selectedModel;
public MainWindow()
{
InitializeComponent();
List.Add(new MyModel { Name = "James", CompanyName = "StackOverflow"});
List.Add(new MyModel { Name = "Adam", CompanyName = "StackOverflow" });
List.Add(new MyModel { Name = "Chris", CompanyName = "StackOverflow" });
List.Add(new MyModel { Name = "Steve", CompanyName = "StackOverflow" });
List.Add(new MyModel { Name = "Brent", CompanyName = "StackOverflow" });
}
public ObservableCollection<MyModel> List
{
get { return _list; }
set { _list = value; }
}
public MyModel SelectedModel
{
get { return _selectedModel; }
set { _selectedModel = value; NotifyPropertyChanged("SelectedModel"); }
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string property)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(property));
}
}
}
Xaml
<Window x:Class="WpfApplication11.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525" Name="UI">
<Grid>
<ListBox ItemsSource="{Binding ElementName=UI, Path=List}" SelectedItem="{Binding ElementName=UI, Path=SelectedModel}" Margin="0,0,200,0" DisplayMemberPath="DisplayMember" SelectedIndex="0" />
<StackPanel HorizontalAlignment="Left" Height="100" Margin="322,10,0,0" VerticalAlignment="Top" Width="185">
<TextBlock Text="Name" />
<TextBox Height="23" TextWrapping="Wrap" Text="{Binding ElementName=UI, Path=SelectedModel.Name, UpdateSourceTrigger=PropertyChanged}" />
<TextBlock Text="Company Name" />
<TextBox Height="23" TextWrapping="Wrap" Text="{Binding ElementName=UI, Path=SelectedModel.CompanyName, UpdateSourceTrigger=PropertyChanged}" />
</StackPanel>
</Grid>
</Window>
Model
public class MyModel : INotifyPropertyChanged
{
private string _name;
private string _companyName;
public string Name
{
get { return _name; }
set { _name = value; NotifyPropertyChanged("Name"); }
}
public string CompanyName
{
get { return _companyName; }
set { _companyName = value; NotifyPropertyChanged("CompanyName"); }
}
public string DisplayMember
{
get { return string.Format("{0} ({1})", Name, CompanyName); }
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string property)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(property));
PropertyChanged(this, new PropertyChangedEventArgs("DisplayMember"));
}
}
}
In this case any edit to properties will Update your list instantly, also will update when new Items are added/removed.

How about calling ListBox.UpdateLayout?
Of course you also need to update the particular item(s) so that it returns the updated string from the ToString method.
UPDATE: I think you also need to call ListBox.InvalidateArrange before you call ListBox.UpdateLayout.

Use INotifyPropertyChanged is the best way, refresh the entire list is not a good idea.
Main entrance:
public partial class MainWindow : Window
{
private BindingList<FoodModel> foodList = new BindingList<FoodModel>();
public MainWindow()
{
InitializeComponent();
}
private void Button1_Click(object sender, RoutedEventArgs e)
{
foodList.Add(new FoodModel { foodName = "apple1" });
foodList.Add(new FoodModel { foodName = "apple2" });
foodList.Add(new FoodModel { foodName = "apple3" });
FoodListBox.ItemsSource = foodList;
}
private void Button2_Click(object sender, RoutedEventArgs e)
{
foodList[0].foodName = "orange";
}
private void RefreshButton_Click(object sender, RoutedEventArgs e)
{
FoodListBox.Items.Refresh();
}
}
Model:
public class FoodModel: INotifyPropertyChanged
{
private string _foodName;
public string foodName
{
get { return _foodName; }
set
{
if (_foodName != value)
{
_foodName = value;
PropertyChanged(this, new PropertyChangedEventArgs("foodName"));
}
}
}
public event PropertyChangedEventHandler PropertyChanged = delegate { };
}
XAML:
<ListBox HorizontalAlignment="Center" Name="FoodListBox" VerticalAlignment="Top" Width="194" Height="150">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding foodName}" />
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>