Develop Reference

Xamarin Forms ListView stays empty after filling the source

I have a problem. I created a ListView with as itemsource a List called unknownDeviceList from my ViewModel. Here is my ViewModel:
public class VM_AddDeviceList : BindableObject
{
private List<UnknownDevice> _unknownDeviceList;
public List<UnknownDevice> unknownDeviceList
{
get
{
return _unknownDeviceList;
}
set
{
if (_unknownDeviceList != value)
{
_unknownDeviceList = value;
OnPropertyChanged();
}
}
}
public List<UnknownDevice> deviceList_raw;
public VM_AddDeviceList()
{
deviceList_raw = new List<UnknownDevice>();
unknownDeviceList = new List<UnknownDevice>();
MyHandler();
}
private async Task LoadUnknownDeviceList()
{
deviceList_raw = await App.RestService.GetDevices();
foreach (UnknownDevice device in deviceList_raw)
{
bool containsItem = App.knownDeviceList.Any(item => item.MAC == device.MAC);
if (!containsItem)
{
unknownDeviceList.Add(device);
}
}
}
public Task MyHandler()
{
return LoadUnknownDeviceList();
}
}
Now I can see that unknownDeviceList gets filled in the foreach, but on the screen the ListView stays empty. What am I doing wrong?
Something with the async and await?

You are raising PropertyChanged when setting unknownDeviceList to inform the view that the list has changed. Anyway, there is no way for the view to know that there were items added to unknownDeviceList.
The most idiomatic way to solve the issue would be to use an ObservableCollection<string> instead.
private ObservableCollection<string> _unknownDevices = new ObservableCollection<string>();
public ObservableCollection<string> UnknownDevices => _unknownDevices;
Please note that I've used the expression body syntax for read-only properties for UnknownDevices, it's not a field.
Since ObservableCollection<string> implements INotifyCollectionChanged which can be subscribed to by the binding to UnknownDevices the view is informed about the changes in UnknownDevices and will be updated when any items are added or removed.

Filtering a xaml list box

Hi I have a list box with some customer names, I want to filter the list based on the text entered in the TextBox. After researching a bit I heard that we can use CollectionViewSourse and ICollectionView but did not get a stage where I could get it working.
Could you please suggest on how to achieve this.
Your help is much appreciated.
XAML
<TextBox x:Name="txtSearch"/>
<ListBox x:Name="lbCustomers">
XAML.cs
List<string> customerList;
public MainPage()
{
this.InitializeComponent();
customerList = new List<string>();
customerList.Add("Andrew");
customerList.Add("bravo");
customerList.Add("Carol");
customerList.Add("Dela");
customerList.Add("Eva");
customerList.Add("family");
customerList.Add("George");
customerList.Add("Health");
customerList.Add("Illa");
customerList.Add("Jack");
customerList.Add("Andrew");
lbCustomers.ItemsSource = customerList;
CollectionViewSource collectionViewSource = new CollectionViewSource();
collectionViewSource.Source = customerList;
ICollectionView collectionView = collectionViewSource.View;
}
Edit:
I can not access 'CollectionViewSource.GetDefaultView' and 'view.Filter'. I get an error : 'collectionviewsource does not contain a definition for getdefaultview'
When I looked into the definition i did not find the 'GetDefaultView' and 'Filter dependency properties'
public sealed class CollectionViewSource : DependencyObject, ICollectionViewSource
{
public CollectionViewSource();
public static DependencyProperty IsSourceGroupedProperty { get; }
public static DependencyProperty ItemsPathProperty { get; }
public static DependencyProperty SourceProperty { get; }
public static DependencyProperty ViewProperty { get; }
public System.Boolean IsSourceGrouped { get; set; }
public PropertyPath ItemsPath { get; set; }
public System.Object Source { get; set; }
public ICollectionView View { get; }
}

Try getting the default collection view for your collection. Every time your txtSearch changes you have to change the filter.
ICollectionView view = CollectionViewSource.GetDefaultView(customerList);
view.Filter = obj =>
{
string item = obj as string;
return (item.ToLower().Contains(YourFilter));
};

I would advice you to read about data binding and how it use it to bind listboxes and textboxes and manage your collections in your viewmodels.
But to fix your problem as it is.
Define your IcollectionView at a global level just like customerList and in your main change your code to
CollectionViewSource collectionViewSource = new CollectionViewSource();
collectionViewSource.Source = customerList;
collectionView = collectionViewSource.View;
collectionView.Filter = collectionFilter;
lbCustomers.ItemsSource = collectionView;
and add these 2 additional methods
private bool collectionFilter(object obj)
{
if (string.IsNullOrWhiteSpace(txtSearch.Text))
return true;
string name = obj.ToString();
return name.Contains(txtSearch.Text);
}
private void TxtSearch_OnTextChanged(object sender, TextChangedEventArgs e)
{
collectionView.Refresh();
}
change textbox to
<TextBox x:Name="txtSearch" TextChanged="TxtSearch_OnTextChanged"/>
These should be self explanatory changes but if you need any help i am happy to explain
The filter method is where you define your logic of filtering the listbox items on display

How to add TreeViewItems to a TreeView Control

This question is a follow up to this question. I've taken some advice from this site and decided to start learning MVVM implementation for my work with TreeViews. With that being said I am VERY new to MVVM and I'm still getting familiar with the syntax and implementation.
I have a TreeView that displays integer-type data but I would like it to work with strings instead. The tree also allows the user to add to any level by selecting the TreeViewItem and then typing in the new integer header into the textBox, and then clicking a button.
I'd like for a parent, child, and grandchild to be available at start-up with pre-defined names. Another thing to note is that that user will only be able to add to the TreeView at the level of the child (so they'll only be able to add grandchildren).
Model
public class TreeViewModel : PropertyChangedBase
{
public string Value { get; set; }
public ObservableCollection<TreeViewModel> Items { get; set; }
public CollectionView ItemsView { get; set; }
public TreeViewModel(string value)
{
Items = new ObservableCollection<TreeViewModel>();
ItemsView = new ListCollectionView(Items)
{
SortDescriptions =
{
new SortDescription("Value",ListSortDirection.Ascending)
}
};
Value = value;
}
}
ViewModel
public class SortedTreeViewWindowViewModel : PropertyChangedBase
{
private string _newValueString;
public string NewValueString
{
get { return _newValueString; }
set
{
_newValueString = value;
OnPropertyChanged("NewValueString");
}
}
public TreeViewModel SelectedItem { get; set; }
public ObservableCollection<TreeViewModel> Items { get; set; }
public ICollectionView ItemsView { get; set; }
public SortedTreeViewWindowViewModel()
{
Items = new ObservableCollection<TreeViewModel>();
ItemsView = new ListCollectionView(Items) { SortDescriptions = { new SortDescription("Value", ListSortDirection.Ascending) } };
}
public void AddNewItem()
{
ObservableCollection<TreeViewModel> targetcollection;
//Insert the New Node as a Root node if nothing is selected.
targetcollection = SelectedItem == null ? Items : SelectedItem.Items;
if (_newValueString != null)
{
targetcollection.Add(new TreeViewModel(_newValueString));
NewValueString = string.Empty;
}
}
}
View Code-Behind
public partial class Window1 : Window
{
public SortedTreeViewWindowViewModel ViewModel { get { return DataContext as SortedTreeViewWindowViewModel; } set { DataContext = value; } }
public Window1()
{
InitializeComponent();
ViewModel = new SortedTreeViewWindowViewModel()
{
Items = {new TreeViewModel("Test")}
};
}
private void AddNewItem(object sender, RoutedEventArgs e)
{
ViewModel.AddNewItem();
}
private void OnSelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
ViewModel.SelectedItem = e.NewValue as TreeViewModel;
}
}
Thanks so much for the help. I'm hoping that going through this will help me understand how to edit and build ViewModels so that I can learn to improvise a little more in the future.
UPDATE
The TreeView is now composed of strings, so that part is solved. I still need help with the default nodes though. I have updated my code to reflect this change.

Here are my suggestions
Change the places where you have int to string. The TreeView should handle that change.
In the constructor of your ViewModel, manually insert your default nodes. Make sure you understand how to work with a TreeView as that will affect the design of your Model and ViewModel and naturally improve the implementation.
Here is a very simple example how to fill a tree in the ViewModel which is bound to a TreeView in the View:
View
<Window x:Class="TreeViewExample.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">
<StackPanel>
<StackPanel>
<TreeView ItemsSource="{Binding Tree}"/>
</StackPanel>
</StackPanel>
</Window>
View Code-Behind
namespace TreeViewExample
{
using System.Windows;
public partial class MainWindow : Window
{
public MainWindow()
{
DataContext = new MainWindowViewModel();
InitializeComponent();
}
}
}
ViewModel
namespace TreeViewExample
{
using System.Collections.ObjectModel;
using System.Windows.Controls;
class MainWindowViewModel
{
public ObservableCollection<TreeViewItem> Tree { get; set; }
public MainWindowViewModel()
{
Tree = new ObservableCollection<TreeViewItem>();
Tree.Add(GetLoadedTreeRoot());
}
private TreeViewItem GetLoadedTreeRoot()
{
TreeViewItem parent = new TreeViewItem() { Header = "Parent" };
TreeViewItem child1 = new TreeViewItem() { Header = "Child 1" };
TreeViewItem child2 = new TreeViewItem() { Header = "Child 2" };
TreeViewItem grandchild1 = new TreeViewItem() { Header = "Grandchild 1" };
TreeViewItem grandchild2 = new TreeViewItem() { Header = "Grandchild 2" };
child1.Items.Add(grandchild1);
child2.Items.Add(grandchild2);
parent.Items.Add(child1);
parent.Items.Add(child2);
return parent;
}
}
}
Produces:
Parent
Child 1
Grandchild 1
Child 2
Grandchild 2
Additional thoughts:
To clean up your code-behind, you might look up a Command implementation, of which there are many. Although you sometimes need it, avoid code in the code-behind when possible. I really like this example because it shows you a general MVVM implementation without getting into advanced Command-related topics (ItemTemplates, Interactivity namespace, etc.).

ListBox ItemsSource Doesn't Update

I am facing a ListBox's ItemsSource related issue. I am implementing MVVM with WPF MVVM toolkit version 0.1.
I set one ListBox itemSource to update when a user double clicks on some other element (I handled the event in the code behind and executed the command there, since binding a command to specific events are not supported). At this point through the execution of the command a new ObservableCollection of items get generated and the ListBox's ItemsSource is intended to get updated accordingly. But it is not happening at the moment. ListBox does not update dynamically. What can be the problem? I am attaching relvent code for your reference.
XAML:
List of items which is doubled click to generate the next list:
<ListBox Height="162" HorizontalAlignment="Left" Margin="10,38,0,0" Name="tablesViewList" VerticalAlignment="Top" Width="144" Background="Transparent" BorderBrush="#20EEE2E2" BorderThickness="5" Foreground="White" ItemsSource="{Binding Path=Tables}" SelectedValue="{Binding TableNameSelected, Mode=OneWayToSource}" MouseDoubleClick="tablesViewList_MouseDoubleClick"/>
Second list of items which currently does not get updated:
<ListBox Height="153" HorizontalAlignment="Left" Margin="10,233,0,0" Name="columnList" VerticalAlignment="Top" Width="144" Background="Transparent" BorderBrush="#20EEE2E2" BorderThickness="5" Foreground="White" ItemsSource="{Binding Path=Columns, Mode=OneWay}" DisplayMemberPath="ColumnDiscriptor"></ListBox>
Code Behind:
private void tablesViewList_MouseDoubleClick(object sender, MouseButtonEventArgs e)
{
MainViewModel currentViewModel = (MainViewModel)DataContext;
MessageBox.Show("Before event command is executed");
ICommand command = currentViewModel.PopulateColumns;
command.Execute(null);
MessageBox.Show(currentViewModel.TableNameSelected);
//command.Execute();
}
View Model:
namespace QueryBuilderMVVM.ViewModels
{
//delegate void Del();
public class MainViewModel : ViewModelBase
{
private DelegateCommand exitCommand;
#region Constructor
private ColumnsModel _columns;
public TablesModel Tables { get; set; }
public ControllersModel Operators { get; set; }
public ColumnsModel Columns {
get { return _columns; }
set {
_columns = value;
OnPropertyChanged("Columns");
}
}
public string TableNameSelected{get; set;}
public MainViewModel()
{
Tables = TablesModel.Current;
Operators = ControllersModel.Current;
Columns = ColumnsModel.ListOfColumns;
}
#endregion
public ICommand ExitCommand
{
get
{
if (exitCommand == null)
{
exitCommand = new DelegateCommand(Exit);
}
return exitCommand;
}
}
private void Exit()
{
Application.Current.Shutdown();
}
//Del columnsPopulateDelegate = new MainViewModel().GetColumns;
//Method to be assigned to the delegate
//Creates an object of type ColumnsModel
private void GetColumns() {
ColumnsModel.TableNameParam = TableNameSelected;
Columns = ColumnsModel.ListOfColumns;
}
private ICommand _PopulateColumns;
public ICommand PopulateColumns
{
get {
if (_PopulateColumns == null) {
_PopulateColumns = new DelegateCommand(GetColumns); // an action of type method is passed
}
return _PopulateColumns;
}
}
}
}
Model:
public class ColumnsModel : ObservableCollection<VisualQueryObject>
{
private DataSourceMetaDataRetriever dataSourceTableMetadataObject;// base object to retrieve sql data
private static ColumnsModel listOfColumns = null;
private static object _threadLock = new Object();
private static string tableNameParam = null;
public static string TableNameParam
{
get { return ColumnsModel.tableNameParam; }
set { ColumnsModel.tableNameParam = value; }
}
public static ColumnsModel ListOfColumns
{
get
{
lock (_threadLock)
if (tableNameParam != null)
listOfColumns = new ColumnsModel(tableNameParam);
return listOfColumns;
}
}
public ColumnsModel(string tableName)
{
ColumnsModel.tableNameParam = tableName;
Clear();
try
{
dataSourceTableMetadataObject = new DataSourceMetaDataRetriever();
List<ColumnDescriptionObject> columnsInTable = new List<ColumnDescriptionObject>();
columnsInTable = dataSourceTableMetadataObject.getDataTableSchema("Provider=SQLOLEDB;Data Source=.;Integrated Security=SSPI;Initial Catalog=LogiwizUser", ColumnsModel.tableNameParam);
//List<String> listOfTables = dataSourceTableMetadataObject.getDataBaseSchema("Provider=SQLOLEDB;Data Source=.;Integrated Security=SSPI;Initial Catalog=LogiwizUser");
//List<String> listOfTables = dsm.getDataBaseSchema("G:/mytestexcel.xlsx", true);
//ObservableCollection<VisualQueryObject> columnVisualQueryObjects = new ObservableCollection<VisualQueryObject>();
foreach (ColumnDescriptionObject columnDescription in columnsInTable)
{
VisualQueryObject columnVisual = new VisualQueryObject();
columnVisual.ColumnDiscriptor = columnDescription;
columnVisual.LabelType = "column";
Add(columnVisual);
}
}
catch (QueryBuilderException ex)
{
/* Label exceptionLabel = new Label();
exceptionLabel.Foreground = Brushes.White;
exceptionLabel.Content = ex.ExceptionMessage;
grid1.Children.Add(exceptionLabel);*/
}
}
}
Any help is greatly appreciated. Thanks in advance.

The setter of property Columns should raise a PropertyChanged event.
Implement INotifyPropertyChanged to do so : MSDN INotifyPropertyChanged
I guess MVVM Toolkit provides a way of doing so easily (perhaps ViewModelBase already implement the interface ...).
EDIT : Implementing INotifyPropertyChanged is not enough, you have to raise the event created by INotifyPropertyChanged. You property should look something like this :
private ColumnsModel _columns;
public ColumnsModel Columns
{
get { return _columns; }
set
{
_columns = value;
PropertyChanged("Columns");
}
}

use an observableCollection<T> instead of a List<T>
MSDN DOC:
WPF provides the ObservableCollection class, which is a built-in implementation of a data collection that exposes the INotifyCollectionChanged interface. Note that to fully support transferring data values from source objects to targets, each object in your collection that supports bindable properties must also implement the INotifyPropertyChanged interface. For more information, see Binding Sources Overview.

How do you bind ObservableCollections to ItemsSource?

DataContextDataContext context1 = new DataContextDataContext();
public MainWindow()
{
InitializeComponent();
DataContext = new ObservableCollection<MyObject>();
RadGridView1.Filtered+=new EventHandler<GridViewFilteredEventArgs>(RadGridView1_Filtered);
ObservableCollection<MyObject> _MyObject = new ObservableCollection<MyObject>();
foreach (var p in context1.Students)
{
_MyObject.Add(new MyObject { ID = p.StudentID, Name = p.StudentFN });
}
}
void RadGridView1_Filtered(object sender, GridViewFilteredEventArgs e)
{
RadGridView1.ItemsSource = ObservableCollection<MyObject>();
}
private void Button_Click(object sender, RoutedEventArgs e)
{
}
}
public class MyObject
{
public int ID { get; set; }
public string Name { get; set; }
}
How do you bind my ObservableCollections to the ItemsSource?

You want to set the ItemSource to the instance of an ObservableCollection you created in the constructor:
RadGridView1.ItemsSource = _MyObject;

You can make the observable collection as a public property in your code-behind/presenter/viewmodel, like
public ObservableCollection<MyObject> MyObjectCollection {get;set;}
then you can populate that and the binding can be code code behind.
ItemsSource is a dependency property you can bind it in XAML or code behind, like suppose you want to bind to ListBox's(say named lstItems) ItemsSource, like (below code is considering that 'MyObjectCollection' is in codebehind
Binding bindingObject = new Binding("MyObjectCollection");
bindingObject.Source = this; //codebehind class instance which has MyObjectCollection
lstItems.SetBinding(ListBox.ItemsSource, bindingObject);
or in XAML,
<ListBox x:Name="lstItems" ItemsSource="{Binding Path=MyObjectCollection}"/>
for both the ways above you need to set the datacontext which is 'this' (for this specific solution).
But maybe you want to look into basic WPF databinding where you can understand Depedency properties, binding objects, binding modes, etc.
http://msdn.microsoft.com/en-us/library/aa480224.aspx
http://msdn.microsoft.com/en-us/library/ms750612.aspx
http://joshsmithonwpf.wordpress.com/2008/05/19/gradual-introduction-to-wpf-data-binding/