Why target update before source ? A strange problem when using binding.

Question

A data class MyTestClass has a property IsTestChecked. Have a toggle in the data template of this class and binding the IsChecked property of the toggle button to IsTestChecked.

<ToggleButton
...
IsChecked="{Binding IsTestChecked, Mode=TwoWay}"
...
/>

I find a strange thing on this binding, that when i clicke at the toggle button, the IsTestChecked property of MyTestClass

changes before the IsChecked property of ToggleButton. In my understanding, the ToggleButton.IsChecked should be changed

firstly and then change the MyTestClass.IsTestChecked because of the binding. I am confused about this issue.

Can anyone give me some suggestion?

Thank you very much!

Answer 1

 Can anyone give me some suggestion?


Thanks

Answer 2

Why is that causing a problem?

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John Fenton

Answer 3

ClarkZeng said:

 Can anyone give me some suggestion?


Thanks

Is there any problem due to this behavior? I guess since ToggleButton has a property change callback set for the IsCheckedProperty, it fires before the value is updated and this causes the binding to update before you can see the value of the property update itself

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MVP Client App

Answer 4

I just verified the behavior by writing my own dependency property as something like below

        public static readonly DependencyProperty IsButtonCheckedProperty =

            DependencyProperty.Register("IsChecked", typeof(bool), typeof(ComboTest),

            new FrameworkPropertyMetadata(false,

                FrameworkPropertyMetadataOptions.Journal | FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,

                new PropertyChangedCallback(OnIsCheckedChanged)));

 

        private static void OnIsCheckedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)

        {

            //this is just a dummy code to see when this method is called

            string str = "hello world";

        }

 

        public bool IsButtonChecked

        {

            get { return (bool)GetValue(IsButtonCheckedProperty); }

            set { SetValue(IsButtonCheckedProperty, value); }

        }

I put break points in both the SetValue call and also the OnIsCheckedChanged method. On execution, I found that the breakpoint on SetValue is hit first, but before it is completed, it goes to the OnIsCheckedChanged method, finishes executing it and then comes back to SetValue method.

If we take the ToggleButton example, it also has similar behavior. Hence before the value of IsChecked DP is updated, the call to OnIsCheckedChanged causes the event to fire, and this will cause the binding to update the value. Finally after this is done, the control comes back to SetValue call and the property assignment is completed. 

Hope this helps !  

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MVP Client App

Answer 5

Isn't that by design.

DependencyPropertyChangedEventArgs e
e.NewValue <-- contains the value you should be checking.

Still not sure why this would be a problem?

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John Fenton

Nice demo of how it works BTW!

Answer 6

Atul Gupta said:

I just verified the behavior by writing my own dependency property as something like below

        public static readonly DependencyProperty IsButtonCheckedProperty =

            DependencyProperty.Register("IsChecked", typeof(bool), typeof(ComboTest),

            new FrameworkPropertyMetadata(false,

                FrameworkPropertyMetadataOptions.Journal | FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,

                new PropertyChangedCallback(OnIsCheckedChanged)));

 

        private static void OnIsCheckedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)

        {

            //this is just a dummy code to see when this method is called

            string str = "hello world";

        }

 

        public bool IsButtonChecked

        {

            get { return (bool)GetValue(IsButtonCheckedProperty); }

            set { SetValue(IsButtonCheckedProperty, value); }

        }

I put break points in both the SetValue call and also the OnIsCheckedChanged method. On execution, I found that the breakpoint on SetValue is hit first, but before it is completed, it goes to the OnIsCheckedChanged method, finishes executing it and then comes back to SetValue method.

If we take the ToggleButton example, it also has similar behavior. Hence before the value of IsChecked DP is updated, the call to OnIsCheckedChanged causes the event to fire, and this will cause the binding to update the value. Finally after this is done, the control comes back to SetValue call and the property assignment is completed. 

Hope this helps !  

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MVP Client App

 Thank you very much!

Actually, i want to control my IsChecked property with a IsCheckable flag in UI (just like MenuItem), not in data class.  That is because that i don't want user can change IsChecked by Click or Enter, but developer can change it by using the property of data class.

Answer 7

For read only on a checkbox, you just set IsEnabled="false" you probably will also want to override the template to make it display the way you want.

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John Fenton