依赖项属性绑定-分配新值

Question

I've implemented a simple DependencyProperty on my WPF control to use it in Binding.

public static readonly DependencyProperty PollingProperty = DependencyProperty.Register("Polling", typeof(Polling), typeof(ConverterView), new UIPropertyMetadata(null));

public Polling Polling
{
    get { return (Polling)GetValue(PollingProperty); }
    set { SetValue(PollingProperty, value); }
}

The control itself is set as DataContext, so in XAML I just use it like this:

<ProgressBar Height="25" Value="{Binding Path=Polling.Progress, Mode=OneWay}" />

Polling.Progress is an integer property that implements INoftiyPropertyChanged, so changes get promoted to the UI. Everything works fine and changes on "Progress" are shown in the ProgressBar as expected.

However there is a certain point in my application where a new "Polling"-instance is applied to the DependencyProperty.

Polling = new Polling(); Polling.Start();

After that, the binding is no longer evaluated and the ProgressBar stays at the last value of the old instance.

Update:

As my case is maybe a little bit specific I'll explain a bit more.

A Button on my ConverterView WPF-Control allows the user to start an operation:

private void cmdAusformatieren_Click(object sender, RoutedEventArgs e)
{
    Polling = Document.Converter(ConvertFinished);
}

The Convert-method takes a delegate as parameter, which will be invoked after the operation is finished. (The whole process runs asynchronous.) The Convert-Method returns a Polling -object which provides an integer property Progress, that provides the information I want to display in a ProgressBar. (What may be misleading here, Polling is the name of my DependencyProperty as well as the name of my class).

As far as this, everything seems ok and the Binding to Polling.Progress works.

In my event ConvertFinished() which triggers after the first operation is done, I get a new Polling instance returned and want to use this in Binding from then on.

private void ConvertFinished(object result)
{           
    Polling = Document.Format((byte[])result, FormatFinished);
}

After this assignment to my Polling DependencyProperty, the Binding is no longer updated and stays with its previous value.

Answer 1

Instead of

Polling = new Polling () ;

you need to write

SetCurrentValue (PollingProperty, new Polling ()) ;

When you assign a depencency property directly, all bindings on it are lost. OTOH SetCurrentValue does not affect bindings, it just sets the current value (duh) and propagates notifications.

Answer 2

The simpliest solution, in this case, is reassign a binding , at this time from the code at runtime. Cause, as you noted too, the binding holds a reference to the object it was first assigned.

Answer 3

Ok, could you try this:

BindingOperations.ClearBinding(this.YouProgressBarName, ProgressBar.ValueProperty); BindingOperations.SetBinding(this.YouProgressBarName, ProgressBar.ValueProperty);

first and check if it works. I predict it won't.

There's one thing I'd suggest to change - get rid of subproperties (I understand that you use complex objects to reduce the number parameters in your dels). They've limited support by WPF framework.

Another thing I'd try is forcing your Polling property value to 0 just before assigning a new value to it

private void cmdAusformatieren_Click(object sender, RoutedEventArgs e)
{
    this.Polling.Progress = 0; // see if make any difference
    this.Polling = Document.Converter(ConvertFinished);

    if (this.Polling.Progress != 0) { throw new Exception(); }
}