Develop Reference

How to bind a property of a XAML view to a public variable in the code behind every time the property changes

I am trying to take a property from a XAML control, specifically the TranslationX property, and store it in a public variable every time the value is changed.
I have tried using data binding by implementing the INotifyPropertyChanged interface and binding the TranslationX property to the public variable from my interface implementation, but had no luck
Essentially, I am needing the TranslationX property of a control to trigger function calls depending on the total displacement, ex. if the control is dragged to -200 in the X direction, it triggers function "Y". I cannot seem to access this translation value in a way that allows me to check if it is above or below a certain value.
I am very new to C# and Xamarin, so any advice is much appreciated.
EDIT:
Here is my current ViewModel class:
public class ReceiptPageViewModel : INotifyPropertyChanged
{
double shift = 0;
public double Shift
{
get => shift;
set
{
if (shift == value)
return;
else
{
shift = value;
OnPropertyChanged(nameof(Shift));
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
void OnPropertyChanged(string name)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}
}
And here is my syntax for my Binding:
TranslationX="{Binding Shift}"

in your XAML
<SomeElement ... TranslationX="{Binding TransX}" ... />
in your ViewModel
double transX;
public double TransX {
get { return transX; }
set {
transX = value;
if (transX > somethresholdvalue) {
...
}
}
}

Follow the MVVM pattern.
Create a Base View Model with the INotifyPropertyChanged.
Then your custom view model is going to inherit from that base class.
BaseViewModel
public class BaseViewModel : INotifyPropertyChanged
{
bool isBusy = false;
public bool IsBusy
{
get { return isBusy; }
set { SetProperty(ref isBusy, value); }
}
string title = string.Empty;
public string Title
{
get { return title; }
set { SetProperty(ref title, value); }
}
protected bool SetProperty<T>(ref T backingStore, T value,
[CallerMemberName]string propertyName = "",
Action onChanged = null)
{
if (EqualityComparer<T>.Default.Equals(backingStore, value))
return false;
backingStore = value;
onChanged?.Invoke();
OnPropertyChanged(propertyName);
return true;
}
#region INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
{
var changed = PropertyChanged;
if (changed == null)
return;
changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
#endregion
Your custom class:
ReceiptPageViewModel
public class ReceiptPageViewModel : BaseViewModel
{
double shift = 0;
public double Shift
{
get { return shift; }
set { SetProperty(ref shift, value); }
}
}
And in your Xamarin Page set the BindingContext to the ViewModel
(This is an example)
public partial class Page1 : ContentPage
{
private ReceiptPageViewModel viewModel;
public Page1()
{
BindingContext = viewModel = new ReceiptPageViewModel();
InitializeComponent();
}
}
Now you can set the property in the XAML view:
<SomeElement ... TranslationX="{Binding Shift}" ... />
Here you can view a full episode about MVVM Pattern with #JamesMontemagno as host.

C# How to edit cell value in gridview?

I have a gridview shown as below in XAML
<ListView x:Name="listTasks">
<ListView.View>
<GridView x:Name="gridTasks">
<GridViewColumn Header="ID" HeaderStringFormat="Lowercase" Width ="26" DisplayMemberBinding="{Binding id}"/>
<GridViewColumn Header="Something" Width="113" DisplayMemberBinding="{Binding something}"/>
<GridViewColumn Header="State" Width="179" DisplayMemberBinding="{Binding currentState}"/>
</GridView>
</ListView.View>
</ListView>
and i have a button which adds to this gridview using the below
m.myList.Add(new mylistview.myitems
{
id = m.id,
something= m.something,
currentState = m.currentState,
});
This button works perfectly by adding the row into the gridview. However I would like to modify theCurrentState using a method that is running. How would I locate for example, ID = "8" and then modify theCurrentState for that row?
UPDATED CODE SHOWN
I've now replaced my list<Task> with ObservableCollection and managed to get it to add to my listview when I click onto my button. However, I am struggling to implement the iNotifyPropertyChanged into my code and getting it to work correctly... Below is my listview class
public class mylistview : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([System.Runtime.CompilerServices.CallerMemberName] string propertyName = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
private string _currentState;
public string currentState
{
get { return _currentState; }
set
{
_currentState = value;
OnPropertyChanged();
}
}
public ObservableCollection<myitems> _myList = new ObservableCollection<myitems>();
public ObservableCollection<myitems> myList
{
get { return _myList; }
}
private static int _id = 0;
public class myitems
{
public int id { get; set; }
public string something{ get; set; }
public string currentState { get; set; }
}
public int id
{
get { return _id; }
set { _id = value; }
}
}

So I see you're using data bindings already, that's good. But your question makes me think you haven't quite grasped everything it can do for you yet.
My recommendation would be to forget about adding items directly to listOfTasks.Items. Instead you should make an ObservableCollection to hold that list and bind the listOfTasks to it. Like so:
ObservableCollection tasks = new ObservableCollection<mylistview.myitems>();
ListOfTasks.ItemsSource = tasks;
With that binding in place you should be able to simply add new items to the tasks list when they click your button:
tasks.Add(new mylistview.myitems
{
id = theId,
something= something,
currentState = theCurrentState,
});
and it should automatically update the GUI.
The last step is to make sure that the class mylistview.myitems implements INotifyPropertyChanged. This is easier than it sounds; you just need to have it trigger an event any time the property is set. Something like so:
public class exampleProperties: INotifyPropertyChanged
{
//this is the event you have to emit
public event PropertyChangedEventHandler PropertyChanged;
//This is a convenience function to trigger the event.
//The CallerMemberName part will automatically figure out
//the name of the property you called from if propertyName == ""
protected void OnPropertyChanged([System.Runtime.CompilerServices.CallerMemberName] string propertyName = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName);
}
}
//Any time this property is set it will trigger the event
private string _currentState = "";
public string currentState
{
get { return _currentState; }
set
{
if (_currentState != value)
{
_currentState = value;
OnPropertyChanged();
}
}
}
}
Now that the gridview is bound to an ObservableCollection and the items held in that collection can notify interested GUI controls that their properties have changed, you should simply be able to update the GUI simply by changing the appropriate item in the collection.
And here's an example of a form that uses the whole technique: https://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged(v=vs.110).asp
edit
I forgot that you specifically need to bind to the ItemSource property of the ListView. The way I have done it in the past is to set ItemsSource={binding} in the ListView's xaml and then assign an ObservableCollection to ListView.DataContext. However I have found an easier way and updated the original post with it. Here's a reference: http://www.wpf-tutorial.com/listview-control/listview-with-gridview/
Edit 2
Aha, you're adding the iPropertyChangedNotify to the wrong thing. It goes on the myitems class like so:
public class myitems : iNotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([System.Runtime.CompilerServices.CallerMemberName] string propertyName = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
private int _id;
public int id
{
get { return _id; }
set
{
_id = value;
OnPropertyChanged();
}
}
public string something{ get; set; }
public string currentState { get; set; }
}
I leave updating the current state and something properties as an excersize. They also need to trigger the OnPropertyChanged event when their value is set.

Maybe with
listOfTasks.Items.Cast<ListViewItem>().First(item => item.ID == "8").theCurrentState = newState;
//I'm not sure about the Cast stuff, because I don't know what types the ListView uses for its items
Of course you could iterate through the items with a loop and check manually for the ID as well.

UWP MVVM Data Binding for dummies (textbox.text from String)

Well, having a go at MVVM with UWP template 10. I have read many pages, and although everyone tries to say its really easy, I still can't make it work.
To put it into context, OCR is being run on an image, and I would like the text to be displayed in textbox automatically.
Here is my Model:
public class TextProcessing
{
private string _ocrText;
public string OcrText
{
get { return _ocrText; }
set
{
_ocrText = value;
}
}
}
Here is my ViewModel:
public class ScanPageViewModel : ViewModelBase, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private TextProcessing _ocrTextVM;
public ScanPageViewModel()
{
_ocrTextVM = new TextProcessing();
}
public TextProcessing OcrTextVM
{
get { return _ocrTextVM; }
set {
_ocrTextVM = value;
this.OnPropertyChanged("OcrTextVM");
}
}
public void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
Here is my View:
<TextBox x:Name="rtbOcr"
Text="{Binding OcrTextVM.OcrText, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
Firstly, that is not working. Could someone try to show where I am going wrong?
Then, the data is coming from a Services file, how would the Services update the value? What would be the correct code?
Thanks in advance.

Following code is cite from code.msdn (How to achieve MVVM design patterns in UWP), it will be helpful for you:
Check you code step by step.
1.ViewModel implemented interface INotifyPropertyChanged,and in property set method invoked PropertyChanged, like this:
public sealed class MainPageViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private string _productName;
public string ProductName
{
get { return _productName; }
set
{
_productName = value;
if (PropertyChanged != null)
{
PropertyChanged.Invoke(this, new PropertyChangedEventArgs(nameof(ProductName)));
}
}
}
}
2.Initialize you ViewMode in you page, and set DataContext as the ViewMode, like this:
public sealed partial class MainPage : Page
{
public MainPageViewModel ViewModel { get; set; } = new MainPageViewModel();
public MainPage()
{
...
this.DataContext = ViewModel;
}
}
3.In you xaml, binding data from viewMode, like this:
<TextBox Text="{Binding Path=ProductName,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="ProductNameTextBox" TextChanged="ProductNameTextBox_TextChanged" />

Your OnPropertyChanged call on OcrTextVM isn't actually called in your case, since you set the value in the constructor to its backing field and bypass the property.
If you set the value via the property, it should work:
public ScanPageViewModel()
{
OcrTextVM = new TextProcessing();
}
Of course your view needs to know that ScanPageViewModel is its DataContext. Easiest way to do it is in the constructor of the code-behind of your view:
public OcrView()
{
DataContext = new ScanPageViewModel();
InitializeComponent();
}
Assuming your OCR service is returning a new TextProcessing object on usage, setting the property of OcrTextVM should suffice:
public class ScanPageViewModel : ViewModelBase, INotifyPropertyChanged
{
//...
private void GetOcrFromService()
{
//...
TextProcessing value = OcrService.Get();
OcrTextVM = value;
}
}
On a note, the OcrTextVM name doesn't really reflect what the property is doing, since it doesn't look like it's a viewmodel. Consider renaming it.

Actually, it is very easy once I manage to understand. Here is the code needed to update a TextBox.Text
In the Models:
public class DisplayText : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private string _text;
public string Text
{
get { return _text; }
set
{
_text = value;
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Text)));
}
}
}
In the XAML file:
<TextBox Text="{Binding Helper.Text, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" ... />
In the ViewModels:
private DisplayText _helper = new DisplayText();
public DisplayText Helper
{
get { return _helper; }
set
{
_helper = value;
}
}
Then any mod from the ViewModels:
Helper.Text = "Whatever text, or method returning a string";

WPF Databind control in write-only mode

I'm trying to fiddle a little with WPF bindings, so I created a simple project.
Here's the code:
public class Person : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public int Age {
get { return age; }
set {
age = value;
FirePropertyChanged("Age");
}
}
public string Name
{
get { return name; }
set
{
name = value;
FirePropertyChanged("Name");
}
}
private void FirePropertyChanged(string v)
{
if(PropertyChanged !=null)
PropertyChanged(this, new PropertyChangedEventArgs(v));
}
private int age;
private string name;
}
My viewmodel contains ObservableCollection of Person, and single Person to track selected Person.
I've bound listbox's ItemsSource to ObservableCollection, and SelectedItem to single Person, called CurrentPerson. Also, I've bound TextBox to CurrentPerson.Name.
Code works fine, but whenever I change content of TextBox - my listbox also changes. And no matter what combination of "OneWay, TwoWay, OneWayToSource" binding modes on listbox\selecteditem I cannot prevent listbox from updating from CurrentPerson.
How can I prevent this behavior? I'd like to update listbox from CurrentPerson only by using ICommand interface from VM.

There is only one copy of the Person object which is being used in both ListBox.ItemsSource and TextBox.Text, so naturally updating that object from one location will reflect the change in the other as well.
Two easy solutions would be
Change the BindingMode on TextBox.Text to Explicit, so it doesn't update the Person object until you tell it to
Use a separate string property for TextBox.Text and copy it over to your SelectedPerson.Name whenever the command executes
Personally I prefer the second option because I'm not a big fan of bindings that don't accurately reflect the data object behind the UI component, and it would allow the user to change the SelectedItem without resetting the TextBox value.
For an example of the second option, your ViewModel might look like this :
public class MyViewModel()
{
ObservableCollection<Person> People { get; set; }
Person SelectedPerson { get; set; }
string NewPersonName { get; set; }
ICommand UpdatePersonName { get; }
}
where the UpdatePersonName command would execute
SelectedPerson.Name = NewPersonName;
and the CanExecute would only return true if
SelectedPerson != null
&& !NewPersonName.IsNullOrWhiteSpace()
&& NewPersonName != SelectedPerson.Name

I'm not sure if I've followed the question properly.
So, we have a class Person as
public class Person : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public int Age
{
get { return age; }
set
{
age = value;
FirePropertyChanged("Age");
}
}
public string Name
{
get { return name; }
set
{
name = value;
FirePropertyChanged("Name");
}
}
private void FirePropertyChanged(string v)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(v));
}
private int age;
private string name;
}
And we have a view model as
public class ViewModel : INotifyPropertyChanged
{
public ObservableCollection<Person> List { get; set; }
Person currentPerson;
public Person CurrentPerson {
get { return currentPerson; }
set { currentPerson = value;
FirePropertyChanged("CurrentPerson");
}
}
private void FirePropertyChanged(string v)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(v));
}
public event PropertyChangedEventHandler PropertyChanged;
}
The xaml is
<ListBox ItemsSource="{Binding List}" SelectedItem="{Binding CurrentPerson}">
<ListBox.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding Name}" Width="100" />
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
And I bind the view model to the view via
ViewModel vm = new ViewModel();
vm.List = new ObservableCollection<Person>();
foreach (var i in Enumerable.Range(1,10))
{
vm.List.Add(new Person() { Name = "Test" + i.ToString(), Age= i });
}
vm.CurrentPerson = null;
this.DataContext = vm;
Whenever I change the value at textbox, it updates the name properly. I tried to add a handler for list changed, but it doesn't happen to get triggered.
vm.List.CollectionChanged += List_CollectionChanged;
void List_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
MessageBox.Show(e.Action.ToString());
}
Can you comment if it isn't the same as your problem statement?

If you want to control when and what is saved/updated, you obviously need is a ViewModel for editing your Person model.
When selecting a person in your Listbox, you have to pass the person's id (avoid passing the object itself) to the PersonEditViewModel which is bound to the properties that shall be edited, load the persons data into the PersonEditViewModel and then edit. Once you hit the "Save" button, it should commit the change and update the database or whatever you are using for persistence.
Use either events/messages to pass values/events back and forth, or use a navigation approach (like INavigationAware interface in Prism).

INotifyPropertyChanged.PropertyChanged fired, but UI not updated after form loads

Below is an example of my model , ViewModel and xaml binding. The viewmodel implements INotifyPropertChanged. The problem i'm having is...when the wpf form first loads i set ActiveStock and i see both setter and getter being called and the ui is updated to reflect the data correctly.
However, when i later set StockViewModel.ActiveStock, FirePropertyChanged is invoked but i don't see the getter being called, and consequently the UI does not update to reflect the new data. Any ideas what might be happening here?
The second question i have is whether i also need to raise PropertyChanged for the child properties (PriceData and CompanyData) of my model when ViewModel.ActiveStock is changed?
public class Stock
{
public string Ticker { get; set; }
public StockData PriceData { get; set; }
public StockData CompanyData { get; set; }
}
public class StockData
{
...
}
public class StockViewModel:INotifyPropertyChanged
{
private Stock _activeStock;
public Stock ActiveStock
{
get{ return _activeStock;}
set{ _activeStock = value; FirePropertyChanged("ActiveStock");}
}
...
}
XAML:
<UserControl Template="{StaticResource StockTemplate}" DataContext="{Binding ActiveStock}" Tag="{Binding PriceData}" />
<UserControl Template="{StaticResource StockTemplate}" DataContext="{Binding ActiveStock}" Tag="{Binding CompanyData}" />
Edit:
if i remove the DataContext binding for the UserControl and instead set the DataContext for these two controls in code behind when ActiveStock changes, it works fine. why???

The getter is not being called because as far as I can see nothing is "getting" the value, The only properties used are PriceData and CompanyData and these don't use INotifyPropertyChanged
You will have to implement INotifyPropertyChanged on your Stock class for the UI to reflect the changes.
public class Stock : INotifyPropertyChanged
{
private string _ticker;
private StockData _priceData;
private StockData _companyData;
public string Ticker
{
get { return _ticker; }
set { _ticker = value; NotifyPropertyChanged("Ticker"); }
}
public StockData PriceData
{
get { return _priceData; }
set { _priceData = value; NotifyPropertyChanged("PriceData"); }
}
public StockData CompanyData
{
get { return _companyData; }
set { _companyData = value; NotifyPropertyChanged("CompanyData"); }
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string property)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(property));
}
}
}

You might want to try to specify the mode property on your datacontext bindings.
DataContext="{Binding ActiveStock, Mode=OneWay}"
I'm not sure that OneTime is the default binding for DataContext, but it would explain so if the above helps.
The second question has been answered by sa_ddam213.
HTH