We Keep Coding

Re-render the screen when an object changes

I would like to slightly modify the code generated when creating a maui project to implement the following
add an object to Meetings in MainPage.xaml.cs when the button is clicked
display the contents of that Meetings
I wrote the following code for this purpose, but there is no change in the output content. One possible reason for this is that adding data to the object does not re-render the screen. How can I solve this problem?
Views/MainPage.xaml
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:App.Views"
x:Class="App.Views.MainPage">
<ScrollView>
<VerticalStackLayout
Spacing="25"
Padding="30,0"
VerticalOptions="Center">
<Image
Source="dotnet_bot.png"
SemanticProperties.Description="Cute dot net bot waving hi to you!"
HeightRequest="200"
HorizontalOptions="Center" />
<Label
Text="Hello, World!"
SemanticProperties.HeadingLevel="Level1"
FontSize="32"
HorizontalOptions="Center" />
<Label
Text="Welcome to .NET Multi-platform App UI"
SemanticProperties.HeadingLevel="Level2"
SemanticProperties.Description="Welcome to dot net Multi platform App U I"
FontSize="18"
HorizontalOptions="Center" />
<Button
x:Name="CounterBtn"
Text="Click me"
SemanticProperties.Hint="Counts the number of times you click"
Clicked="OnCounterClicked"
<ListView ItemsSource="{Binding Meetings}" />
</VerticalStackLayout>
</ScrollView>
</ContentPage>
Views/MainPage.xaml.cs
namespace App.Views;
using App.Models;
public partial class MainPage : ContentPage
{
int count = 0;
public MainPage()
{
InitializeComponent();
BindingContext = new Models.AllMeetings();
}
private void OnCounterClicked(object sender, EventArgs e)
{
count++;
if (count == 1)
CounterBtn.Text = $"Clicked {count} time";
else
CounterBtn.Text = $"Clicked {count} times";
SemanticScreenReader.Announce(CounterBtn.Text);
((Models.AllMeetings)BindingContext).Meetings.Add(new Models.Meeting() { Name = "foo" });
}
}
Modes/AllMeetings
namespace App.Models;
internal class AllMeetings
{
public List<Meeting> Meetings { get; set; }
}
Models/Meetings.cs
namespace App.Models;
internal class Meeting
{
public string Name { get; set; }
}
Updates
Models/AllMeetings.cs
using System.Collections.ObjectModel;
namespace ailia_speech_gui.Models;
internal class AllMeetings
{
public ObservableCollection<Meeting> Meetings { get; set; }
public void Add_Meeting(Meeting meeting)
{
this.Meetings.Add(meeting);
}
}

I made a demo on my side. You can refer to my demo to change your project.
Here is the code in my Model named Products.cs:
namespace ListViewDelete.Models
{
public class Products
{
public string Name
{
get; set;
}
public double Price
{
get; set;
}
}
}
Then you need to create a viewmodel to realize the delete and add method and create the ObservableCollection to load the data.
Here is the code in my ViewModel:
namespace ListViewDelete.ViewModels
{
internal class ProductsViewModels
{
public ObservableCollection<Products> Products
{
get; set;
}
public Command<Products> RemoveCommand
{
get
{
return new Command<Products>((Product) => {
Products.Remove(Product);
});
}
}
public Command<Products> AddCommand
{
get
{
return new Command<Products>((Product) => {
Products.Add(Product);
});
}
}
public ProductsViewModels()
{
Products = new ObservableCollection<Products> {
new Products {
Name = "name1",
Price = 100
},
new Products {
Name = "name2",
Price = 100
},
new Products {
Name = "name3",
Price = 100
}
};
}
}
}
Last, you need to create the ListView or the CollectionView in the MainPage.xaml. Here is the code in the MainPage.xaml:
<StackLayout>
<Button Text="add" Clicked="Button_Clicked"></Button>
<CollectionView ItemsSource="{Binding Products}">
<CollectionView.ItemTemplate>
<DataTemplate>
<StackLayout>
<Label Text="{Binding Name}" />
<Label Text="{Binding Price}" />
<Button Text="Remove" Clicked="Remove_Clicked" />
</StackLayout>
</DataTemplate>
</CollectionView.ItemTemplate>
</CollectionView>
</StackLayout>
Here is the code in MainPage.xaml.cs:
namespace ListViewDelete
{
public partial class MainPage : ContentPage
{
public MainPage()
{
InitializeComponent();
// bind the viewmodel to the Mainpage
BindingContext = new ProductsViewModels();
}
//delete the item from the observablecollection
public void Remove_Clicked(object sender, EventArgs e)
{
var button = sender as Button;
var product = button.BindingContext as Products;
var vm = BindingContext as ProductsViewModels;
vm.RemoveCommand.Execute(product);
}
//add the new item to the observablecollection
private void Button_Clicked(object sender, EventArgs e)
{
var product = new Products()
{
Name =" new name",
Price = 100
};
var vm = BindingContext as ProductsViewModels;
vm.AddCommand.Execute(product);
}
}
}

Meeting collection must be somewhere initialized before calling any operation on collestion (be it on property level or in constructor):
public class AllMeetings
{
public ObservableCollection<Meeting> Meetings { get; } = new ObservableCollection<Meeting>();
public void Add_Meeting(Meeting meeting)
{
this.Meetings.Add(meeting);
}
}
And ListView must have some data template to tell UI how data should be presented:
<ListView ItemsSource="{Binding Meetings}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Label Text="{Binding Name}"/>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>

OnPropertyChanged not fire with label text

I am in a Xamarin app I which I have to make a to-do list. I am using listview. I have to set a label text dynamically every time I add a new to-do like so: Number of tasks 2/4 where 2 are the done tasks and 4 are the total ones. Everything goes right but I found some issues when I have to update the label text. I am using the MVVM pattern. In XAML I bind the text value to SetInfoDoneText. In MainPage, I have the bindigContext set to VM(TodoListViewModel). I use INotifyPropertyChanged with OnPropertyChanged. I made a method in which the value of setInfoDoneText is changed. The problem is that the set and get is called only once and when the setInfoDoneText is updated by the method OnPropertyChanged does not fire again. Here is the code.
THE PROBLEM IS WHEN I TRY TO UPDATE lblDoneInfo text (
)
class TodoListViewModel: INotifyPropertyChanged
{
public ObservableCollection<TodoItem> todoItems { get; set; }
public event PropertyChangedEventHandler PropertyChanged;
public String setInfoDoneText;
public String SetInfoDoneText
{
get => setInfoDoneText;
set
{
setInfoDoneText = value;
OnPropertyChanged("SetInfoDoneText");
}
}
protected void OnPropertyChanged(string propertyName = null)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
int doneTask = 0;
int totalTask = 0;
public TodoListViewModel()
{
this.todoItems = new ObservableCollection<TodoItem>();
setInfoDoneText = "Number of tasks: " + doneTask + "/" + totalTask;
}
public ICommand AddTodoCommand => new Command(AddTodoItem);
public String newTodoInputValue { get; set; }
public String selectedItem { get; set; }
public bool highPriority { get; set; }
public bool midPriority { get; set; }
public bool lowPriority { get; set; }
Color newColor = new Color();
public void AddTodoItem()
{
if (highPriority)
{
newColor = Color.Red;
AddNewItem(newColor);
highPriority = false;
}
if (midPriority)
{
newColor = Color.Orange;
AddNewItem(newColor);
midPriority = false;
}
if (lowPriority)
{
newColor = Color.Yellow;
AddNewItem(newColor);
lowPriority = false;
}
}
public TodoItem AddNewItem(Color newColor)
{
TodoItem newItem = new TodoItem(newTodoInputValue,
false,
highPriority,
midPriority,
lowPriority,
newColor);
todoItems.Add(newItem);
UpdateDoneInfo();
return newItem;
}
public ICommand RemoveTodoCommand => new Command(RemoveTodoItem);
public void RemoveTodoItem(object o)
{
TodoItem todoItemBeingRemoved = o as TodoItem;
todoItems.Remove(todoItemBeingRemoved);
}
public ICommand EditTodoCommand => new Command(EditTodoItem);
public void EditTodoItem(object o)
{
TodoItem todoItemBeingEdited = o as TodoItem;
int newIndex = todoItems.IndexOf(todoItemBeingEdited);
todoItems.Remove(todoItemBeingEdited);
TodoItem updatedTodo = AddNewItem(newColor);
//todoItems.Add(updatedTodo);
int oldIndex = todoItems.IndexOf(updatedTodo);
todoItems.Move(oldIndex, newIndex);
}
public String UpdateDoneInfo()
{
totalTask = todoItems.Count;
foreach (TodoItem item in todoItems)
{
if (item.complete) doneTask++;
}
return setInfoDoneText = "Number of tasks: " + doneTask + "/" + totalTask;
}
}
<ContentPage.BindingContext>
<local:TodoListViewModel/>
</ContentPage.BindingContext>
<StackLayout>
<Entry
x:Name="inputField"
Text="{Binding newTodoInputValue}"
Placeholder="Enter a todo..."
/>
<Label x:Name="lblDoneInfo" Text="{Binding SetInfoDoneText, Mode=TwoWay }">
</Label>
<FlexLayout AlignItems="Center" JustifyContent="SpaceBetween">
<input:CheckBox x:Name="highP"
IsChecked="{Binding highPriority}"
CheckChangedCommand="{Binding AddTodoCommand}"
Margin="0,0,20,0" />
<Label Text="High Priority" FontSize="Medium"/>
<input:CheckBox x:Name="midP"
IsChecked="{Binding midPriority}"
CheckChangedCommand="{Binding AddTodoCommand}"
Margin="0,0,20,0" />
<Label Text="Medium Priority" FontSize="Medium"/>
<input:CheckBox x:Name="lowP"
IsChecked="{Binding lowPriority}"
CheckChangedCommand="{Binding AddTodoCommand}"
Margin="0,0,20,0" />
<Label Text="Low Priority" FontSize="Medium"/>
</FlexLayout>
<ListView x:Name="todoList" ItemsSource="{Binding todoItems}" SelectedItem="{Binding selectedItem}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell Height="20">
<FlexLayout JustifyContent="SpaceBetween" AlignItems="Center" Padding="20,0">
<ContentView>
<FlexLayout AlignItems="Center">
<input:CheckBox IsChecked="{Binding complete}" Margin="5" />
<Label x:Name="todoText" TextColor="{Binding color}" Text="{Binding todoText}" FontSize="Large"/>
</FlexLayout>
</ContentView>
<ImageButton
Source="editar_24.png"
BackgroundColor="Transparent"
WidthRequest="100"
HeightRequest="100"
Margin="0,0,20,0"
Command="{Binding Path=BindingContext.EditTodoCommand,
Source={x:Reference todoList}}"
CommandParameter="{Binding .}"/>
<ImageButton
Source="basura_24.png"
BackgroundColor="Transparent"
WidthRequest="100"
HeightRequest="100"
Command="{Binding Path=BindingContext.RemoveTodoCommand,
Source={x:Reference todoList}}"
CommandParameter="{Binding .}"/>
</FlexLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>

PropertyChanged only fires when you set the value of the public property SetInfoDoneText. Your code is setting the value of the field setInfoDoneText (lowercase).
It is generally best practice to make the field private to prevent this
private String setInfoDoneText;

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

Xamarin Forms CollectionView stays empty after binding it

I have a problem. I created this class that creates an ImageSource collection ObservableCollection<TemplateSource>:
public class TemplateListViewModel
{
public ObservableCollection<TemplateSource> sourceList { get; set; }
public TemplateListViewModel()
{
sourceList = new ObservableCollection<TemplateSource>();
loadingTemplates += onLoadingTemplates;
LoadTemplateList();
}
private event EventHandler loadingTemplates = delegate { };
private void LoadTemplateList()
{
loadingTemplates(this, EventArgs.Empty);
}
private async void onLoadingTemplates(object sender, EventArgs args)
{
List<Template> templateList = await App.RestService.GetTemplates(App.User);
foreach (var template in templateList)
{
ImageSource source = ImageSource.FromUri(new Uri("mysite.org/myapp/" + template.FileName));
TemplateSource templateSource = new TemplateSource { Id = template.Id, Source = source };
sourceList.Add(templateSource);
}
}
}
And in my XAML I use this code:
<ContentPage.Content>
<StackLayout HorizontalOptions="Fill">
<Frame IsClippedToBounds="True" HeightRequest="45" CornerRadius="5" Padding="0" Margin="15,15,15,0" BackgroundColor="Transparent">
<Entry Placeholder="Search" ReturnType="Done" PlaceholderColor="Gray" x:Name="txtSearch" Margin="5,0,0,0" TextColor="White" />
</Frame>
<CollectionView ItemsLayout="HorizontalList" ItemsSource="{Binding sourceList}">
<CollectionView.ItemTemplate>
<DataTemplate>
<ff:CachedImage
Source="{Binding .}"
VerticalOptions="Center"
HorizontalOptions="Fill" />
</DataTemplate>
</CollectionView.ItemTemplate>
</CollectionView>
</StackLayout>
</ContentPage.Content>
And finally in the page.xaml.cs (code behind):
protected override void OnAppearing()
{
TemplateListViewModel vm = new TemplateListViewModel();
BindingContext = vm;
base.OnAppearing();
}
Now I already got help with this code from #Deczaloth, but he couldn't figure out why the CollectionView stays emtpy after I bind it. Now I already checked, but the sourceList does get filled, so thats not the problem.
What am I doing wrong?

I can see one potential problem in your code XD:
When you bind the Source property of CachedImage you set the binding to ".", but you should instead bind to the Source property of the TemplateSource class (in your context "." means a TemplateSource item!), that is you should change your code like so:
<ff:CachedImage
Source="{Binding Source}"
VerticalOptions="Center"
HorizontalOptions="Fill" />

Xamarin List View SelectedItem ViewModel binding

Using Visual Studio 2015 proff,
My LoginViewModel Class (Portable Class Library)
public class LoginViewModel : INotifyPropertyChanged, INotifyCollectionChanged
{
LoginPage page;
private ObservableCollection<Employees> _employeeList;
private string _loginName;
public ObservableCollection<Employees> EmployeeList
{
get { return _employeeList; }
set
{
_employeeList = value;
OnPropertyChanged();
OnCollectionChanged(NotifyCollectionChangedAction.Reset);
}
}
public string LoginName
{
get { return _loginName; }
set
{
_loginName = value;
if (_loginName != null)
{
OnPropertyChanged();
}
}
}
public LoginViewModel(LoginPage parent)
{
page = parent;
}
public async void GetEmployees()
{
var loginService = new LoginService();
EmployeeList = await loginService.GetEmployeesAsync();
}
public event PropertyChangedEventHandler PropertyChanged;
public event NotifyCollectionChangedEventHandler CollectionChanged;
protected virtual void OnCollectionChanged( NotifyCollectionChangedAction action)
{
CollectionChanged?.Invoke(this, new NotifyCollectionChangedEventArgs(action));
}
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
My LoginPage.xaml (Portable Class Library)
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="ScannerApp.Views.LoginPage"
xmlns:ViewModels="clr-namespace:ScannerApp.ViewModels;assembly=ScannerApp">
<StackLayout Orientation="Vertical">
<Label Text="Please Login"
VerticalOptions="Start"
HorizontalTextAlignment="Center"
IsVisible="true"
FontSize="Large"
FontAttributes="Bold" />
<ListView x:Name="mylist" ItemsSource="{Binding EmployeeList}"
HasUnevenRows="True" SelectedItem="{Binding LoginName}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Vertical" Padding="12,6">
<Label Text="{Binding Name}"/>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
My LoginPage.xaml.cs Class (Portable Class Library)
public partial class LoginPage : ContentPage
{
public LoginPage()
{
InitializeComponent();
BindingContext = new LoginViewModel(this);
}
protected override void OnAppearing()
{
base.OnAppearing();
LoginViewModel model = new LoginViewModel(this);
model.GetEmployees();
BindingContext = model;
}
public ListView MyList
{
get
{
return mylist;
}
}
}
Question
I get a list of employees, the list on the front end renders this. The user then Selects a name from the list, At this point I would like to detect this and then navigate to my different page.
Currently my property is not hit, I'm wondering if this has anything to do with my Binding on the code behind "OnAppearing"? but I'm not sure.

While what you have may work there are a few tweaks I would suggest.
No need to set your BindingContext in your constructor and in OnAppearing(). Just make your LoginViewModel a class level private property in your code-behind and only assign it to your BindingContext in your constructor. Then call GetEmployees() in OnAppearing().
Also, you should make GetEmployees() return a Task, in order to await as far up the chain as possible.
ViewModel:
....
public async Task GetEmployees()
{
var loginService = new LoginService();
EmployeeList = await loginService.GetEmployeesAsync();
}
....
Code-behind:
public partial class LoginPage : ContentPage
{
private LoginViewModel _model;
public LoginPage()
{
InitializeComponent();
BindingContext = _model = new LoginViewModel(this);
}
protected override async void OnAppearing() //Notice I changed this to async and can now await the GetEmployees() call
{
base.OnAppearing();
await _model.GetEmployees();
}
public ListView MyList
{
get
{
return mylist;
}
}
private async void OnItemSelected(object sender, SelectedItemChangedEventArgs e) {
if (e.SelectedItem == null) return;
await Navigation.PushAsync(new MenuPage());
}
}
XAML:
<!-- Adding OnItemSelected in XAML below -->
<ListView x:Name="mylist"
ItemsSource="{Binding EmployeeList}"
HasUnevenRows="True"
SelectedItem="{Binding LoginName}"
ItemSelected="OnItemSelected">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Vertical" Padding="12,6">
<Label Text="{Binding Name}"/>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>

mylist.ItemSelected += async (sender, e) => {
if (e.SelectedItem == null) return;
await Navigation.PushAsync(new MenuPage());
};
This works, had to set the page to a nav wrap in the App.cs, then apply this event handler to the OnAppearing method.