I am trying to implement a system that would enhance WPF's DataBinding engine.
My prime concern right now is the following:
I would like to be able to 'freeze' the DependencyProperty - stop it from updating from the Model - once the user has started to input something in the UI.
Consider the following example:
I am binding a TextBox.TextProperty to some property on my ViewModel.
The user started typing inside the textbox, and the moment he starts to type, I want to prevent the ViewModel from updating the View.
Only after commiting the change, the user will see the update in the model.
I am trying to create some kind of MultiBinding using a a bool-flag which will tell me whether we need to update the GUI or not, but other than that I don't know how to continue.
Any help would be appreciated!
You can do that by setting UpdateSourceTrigger to PropertyChanged in binding and then set the flag accordingly in the setter of bounded property. PropertyChanged value Updates the binding source immediately as user will input any character.
xaml
<TextBox Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}"/>
ViewModel
public class ViewModel:INotifyPropertyChanged
{
bool stopUpdate;
string name;
public string Name
{
get
{ return name;}
set
{
name = value;
stopUpdate=true;
OnPropertyChanged("Name");
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string propName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propName));
}
Related
I'm in a situation where I need to empty a property in my model whenever someone changes a value in a combobox.
A side effect of this is, that whenever I change the value of the Combobox-Bound variable, the Combobox SelectionChanged event is triggered.
Is there anyway to know who is triggering this event. I'd like to know if it is triggered manually or by binding.
I'm looking in to the sender, but they look about the same.
Thank you,
This is a prime example of why WPF developers should use the MVVM design pattern. By relying on SelectionChanged events to control the flow of your code, you're losing your own control over what should happen and are resorting to having to know how an event is being triggered so that you can respond. That's reactive, not pro-active. As a developer, you should always know what can and will affect your code flow.
Rather, set your WPF page or control's DataContext to a viewmodel class that wraps your model and bind your combobox to the ViewModel properties, specifically SelectedItem in this case. This will simplify your coding immensely.
ViewModel
public class ViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public ObservableCollection<string> Names = new ObservableCollection<string>();
private string _selectedName;
private YourModel _model;
public ViewModel(YourModel model)
{
_model = model;
}
public string SelectedName
{
get { return _model.SelectedName; }
set
{
_model.SelectedName = value;
OnPropertyChanged();
}
}
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
}
XAML
<ComboBox ItemsSource="{Binding Names}" SelectedItem="{Binding SelectedName}" />
Dropodownclosed is the event I best use here instead of selectionchanged. This will make sure that the event is triggered not by the data that is bound but by user interface interaction.
Editing this entire post to clarify... I cannot seem to nail this:
BackgroundWorker receives data from a WCF service that is a list of objects. The service reference is configured to be ObservableCollection.
I pass the ObservableCollection via a delegate into my main UI thread and set it equal to the UI threads Local Collection.
A listbox is bound to this local collection and does not update. I've added the following to my collection:
public ObservableCollection<EmployeeData> _empData { get; set; }
public event PropertyChangedEventHandler PropertyChanged;
public ObservableCollection<EmployeeData> EmpData
{
get { return _empData ; }
set
{
_empData = value;
OnPropertyChanged("EmpData");
}
}
private void OnPropertyChanged(string p)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(p));
}
This even fires but the PropertyChanged is always null. My XAML listbox has a binding declared as:
ItemsSource="{Binding Path=EmpData}"
No matter what I do EmpData updates but the ListBox does not, I've tried several other methods but nothing ever changes in the listbox, its always just null.
I've been working on this for over a day now, I cannot seem to get this whole automatic updating thing to 'click'.
I'm not sure that I understand exactly what you are doing, but here are a couple of suggestions.
Have a single ObservableCollection
Bind your itemcollection (or listbox, or whatever) to this
Depending on the user, clear and fill that observablecollection with list data
Have the background worker update the list and refresh the observable collection if anything has changed.
Ideally your EmployeeData class will implement the INotifyPropertyChanged interface, so that property changes will get automatically updated in your view.
I'm trying to understand WPF binding. As simple as it gets:
I have a ClassWithProperty that has a public uint Prop1.
The main window has a public ClassWithProp object and uses it for data context. This is set in the main Windows's constructor:
this.ClassWithProp = new ClassWithProp();
this.DataContext = this.ClassWithProp;
ClassWithProp's default constructor sets Porp1 value to 1.
The main windows contains a label:
<Label Content="{Binding Prop1}" ... />
It also contains a button that, when click, sets the ClassWithProp.Prop1 to 2.
When the window first appears, the label correctly shows 1. When the button is clicked the property's value is changed to 2, but the lable does not refresh.
Sorry - probably obvious but I'm a novice in WPF:
Why doesn't the bound label update when the undelying property changes?
Your ClassWithProperty needs to implement the INotifyPropertyChanged interface (which has just the one event on it, PropertyChanged), this way the WPF binding subsystem can listen for property changes and update the value. When you have changed the value of a property, you raise the event.
Here is an example:
pulic class ClassWithProperty : INotifyPropertyChanged
{
public uint Prop1
{
get { return _prop1; }
set
{
_prop1 = value;
OnPropertyChanged("Prop1");
}
}
protected void OnPropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
private uint _prop1;
}
Implement INPC.
Also read the overview, it probably answers more than 90% of questions people have about data binding.
I have a small silverlight app, where i have a list of objects with a Name and a Description. I databind them to a listbox, and show them, no problems.
However i want to be able to change the name or the description from my codebehind (updated through a webservice) and make the UI update, how to make the ui reflect the change of e.g. my name?
EDIT:
Made the binding twoway, implemented the interface INotifyPropertyChanged interface, still not working. Debugging shows that the PropertyChanged event is not assigned
public string Name
{
get
{
return name;
}
set
{
OnPropertyChanged("Name");
}
}
and the OnPropertyChanged method
private void OnPropertyChanged(string property)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(property));
}
}
I never get into the 'if', the event is null i.e. not assigned by anyone ???
SOLUTION:
Updated the setter to use the instance variable of 'name', tried it first with the property 'Name' this gave a stackoverflow :-)
Make the binding mode to be TwoWay. See here: http://msdn.microsoft.com/en-us/library/cc278072%28VS.95%29.aspx#direction_of_the_data_flow
I have a WPF Toolkit DataGrid bound to an ObservableCollection of Car in my view model. Car has a PropertyChanged event and the setters of each of its two string properties fire the event. I also have the grid's SelectedItem property bound to a property in the view model of type Car, also called SelectedItem.
On the same window as the grid, I have buttons for add, modify and delete. Add and modify open a dialog window with two textboxes, one for each Car property. Delete just shows a confirm dialog then does the delete.
For add and delete, I add or delete an item from the ObservableCollection and the grid updates itself as expected. However, for modify it does not. At first, my Car did not use PropertyChanged and after some searching I found that it needed to for the grid to update when an individual item's properties changed. But now that I am using PropertyChanged, the grid still doesn't update.
I've tried changing the values of the SelectedItem in my view model as well as directly changing the item on the collection.
What am I doing wrong?
Make sure you're implementing INotifyPropertyChanged and not just raising a PropertyChanged event. Also, when raising PropertyChanged, you must pass "this" as the sender, otherwise WPF will ignore the event.
Below is a simple base class that implements INotifyPropertyChanged.
public class Person : INotifyPropertyChanged {
private string name;
public string Name {
get { return name; }
set {
if (name != value) {
name = value;
OnPropertyChanged("Name");
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName) {
var handler = PropertyChanged;
if (handler != null) {
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
Sounds like the classic problem with ObservableCollection. ObservableCollection only notifies of additions, deletions, etc. on it's self. It will NOT notify of changes to properties of whatever you have stored in it. This is why your add/delete operations work as expected.
What you should do is use a CollectionView and bind to that:
ObservableCollection<MyObject> myCollection = new ObservableCollection<MyObject>();
ICollectionView view = CollectionViewSource.GetDefaultView(myCollection);
using this method also has the benifit that grouping and sorting are built into the view.