WPF Discussion Boards

21-Jan-14 6:24

This isn't working:

XML

<GridViewColumn Header="Category">
    <GridViewColumn.CellTemplate>
        <DataTemplate>
            <Grid>
                <TextBlock Name="txtBlah" Style="{StaticResource GridBlockStyle}" Text="{Binding Category.Name}"></TextBlock>
                <ComboBox ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}},Path=DataContext.Categories}" DisplayMemberPath="Name" SelectedValuePath="Id" SelectedValue="{Binding Category.Id, Mode=TwoWay}" Style="{StaticResource GridEditStyle}" />
            </Grid>
        </DataTemplate>
    </GridViewColumn.CellTemplate>
</GridViewColumn>

When I change the combobox in edit mode it seems to work ok, but the textblock is not being updated. WTWaffles? This particular piece of functionality in WPF has always vexed me.

Cheers, --EA

Jason Gleim21-Jan-14 7:52

21-Jan-14 7:52

Check the output window of the debugger for a binding error. Without the entire context of what you are doing I would guess the problem is the binding of the textblock. Inside the data template, the data context is the current row of the data set repeated for each item in the set.

So if you had a data context for the page which had a collection of categories and a collection of objects which had a category and a name, your combobox source is correct and will bind to the collection of categories. However, the selected value will bind to <row>.Category.Id where row is an object in the collection holding the rows withing the data context. Your bindings should be relative to that.

If your row contains a "category" object, your bindings are correct and the problem is likely that you aren't raising PropertyChanged for the Name property when you are changing the Id property.

If I could see the class that is your DataContext I could be more specific.

HTH

eddieangel21-Jan-14 8:07

21-Jan-14 8:07

Funny, I just answered another question with this same suggestion. I am not seeing a binding path error of any sort.

The DataContext does have a category object and that object has an ID and a name.

So how do I implement the INPC for this scenario? I realize I could create a public category "SelectedCategory" and bind it to the SelectedItem of the ComboBox and then raise the OnPropertyChanged event when it changes, but what do I need to do in order to update the category name property of the SelectedItem of the ListView? Do I need to raise the OnPropertyChanged("SelectedItem") or what?

Jason Gleim21-Jan-14 8:48

21-Jan-14 8:48

Assuming that your category object has members like "Id", "Name", "Description", etc I'm guessing that you want to change the Id via the combobox and have the rest of the properties follow suit. There are a couple of ways of doing it but knowing that it all starts with the property setter for Id is the key.

When the user selects an item in the combobox, it sets the SelectedItem property to the value or object you have specified. If it is a binding (as in your case) the setter gets called for that property. A normal setter, as you know, usually looks like:

set
{
  if(_internalVar != value)
  {
    _internalVar = value;
    RaisePropertyChange("PropertyName");
  }
}

That works fine and raising property change there notifies any controls bound to that property that there is a change in it they should grab. But it doesn't do anything else. Even if you added a SelectedCategory property and bound that to the combobox then exposed name and ID through the internal variable, you still wouldn't fire the property change on the name property when the whole object was changed. That is because the binding system just isn't that smart. It must be told there is a change.

So you have a few options...

- If the combobox changes the ID, then in the setter for the ID property, change the name property as well. If you do it like Name = newValue then prop change gets raised in the setter for Name. If you use the internal variable like _name = newValue then you must follow that up by explicitly raising propertychanged passing Name as the property that has changed.

- You could expose just an ID property and in the setter call a method that sets the values on the category object. Again, if you use the public property setters you will get the event raised. If you use the internal versions, you must raise the event.

If you breakpoint on the Id property setter, property getter, and the same for the Name property, you will see how those things get called. (Basically setter followed by the getter in response to the event) To get the Name to update on the control, that PropertyChanged event needs to be raised carrying "Name" as the property name. How you get there is up to you.

Unfortunately, debugging bindings isn't always the easiest thing... in fact it can be pretty infuriating. A lot of times I'll break on those property setter/getter and start working backwards to figure out why something isn't working. And don't discount typos either. I can't tell you how many times a stupid typo when raising propertychanged or in a binding has wasted hours of my time.

eddieangel21-Jan-14 9:14

21-Jan-14 9:14

Thank you, I failed to implement the INPC at the model level, so none of the screen elements were updating as intended. I am still not getting the dropdown to update correctly, though. How should I implement that combobox?

I tried binding the Combobox directly to the Category like this:

HTML

<ComboBox ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}},Path=DataContext.Categories}" DisplayMemberPath="Category" SelectedValuePath="Id" SelectedItem="{Binding Category}" Style="{StaticResource GridEditStyle}" />

But when I do that the textblock updates correctly but there is no item selected initially when I click on the field to edit it.

Here are the important model pieces:

private SCategory category;
        public SCategory Category 
        {
            get { return category; }
            set
            {
                if (category == value)
                    return;
                category = value;
                OnPropertyChanged("Category");
            }
        }

And then the SCategory:

public class SCategory : Base
    {
        private int id;
        public int Id
        {
            get
            {
                return id;
            }
            set
            {
                if (value == id)
                    return;
                id = value;
                OnPropertyChanged("Id");
                OnPropertyChanged("Category");
            }
        }

        
        private string category;
        public string Category
        {
            get
            {
                return category;
            }
            set
            {
                if (string.IsNullOrEmpty(value) || value == category)
                    return;
                category = value;
                OnPropertyChanged("Category");
            }
        }
    }

If I bind the combobox using SelectedValuePath and SelectedValue I am not getting the right field in the textblock, but I am getting the correct field in the combobox.

Cheers, --EA

Jason Gleim21-Jan-14 9:42

21-Jan-14 9:42

Instead of

public SCategory Category
{
    get { return category; }
    set
    {
        if (category == value)
            return;
        category = value;
        OnPropertyChanged("Category");
    }
}

Make it this:

public SCategory Category
{
    get { return category; }
    set
    {
        if (category == value)
            return;
        category.Id = value.Id;
        category.Category = value.Category;
        OnPropertyChanged("Category");
    }
}

By doing that you will invoke PropertyChange on the Id and Category properties and items bound to those properties should get the event and refresh their values.

Also, you have a bad naming convention by calling your derived class Category and a property of the base class Category. That won't do you any favors when you are trying to unwind the bindings and class relationships. You might want to rename the class... maybe CategoryClass or CategoryObject.

HTH
- Jason

eddieangel21-Jan-14 10:12

21-Jan-14 10:12

The naming convention is not as bad as it looks, I just had to make some changes to post it due to proprietary information disclosure and such.

There is still something I am missing here. After setting that in the type class (Which contains the Category class) the setter is not getting hit again.

If I bind the combobox using SelectedValuePath and SelectedValue={Binding Category.Id} the setter property is hit within the Category class on the Id property. However, if I bind the combobox using SelectedItem={Binding Category, Mode=TwoWay} I do not get a display in the combobox and the setter is not getting hit on the Type object Category property.

I am going to need stronger meds if I spend too much more time in WPF land I think.

Jason Gleim21-Jan-14 10:25

21-Jan-14 10:25

Just out of curiosity... is the Category property not null when the initial binding occurs? Meaning you set the Category property the combobox is bound against to an instance of your class either at the declaration of the backing variable or in the datacontext constructor?

There is an annoying "feature" that if you try to bind to a property that exposes an object and that object is null when the binding occurs, then eventing doesn't work. For example, if you bind a combobox itemsource to a collection that is initially null, even if you set the property to a fully-populated collection later, the combobox will not pickup the change. There has to be an instance of the class exposing the PropertyChanged event at the time of the binding for it to work. Especially helpful in these cases is that there are no errors. Raising the property changed event just goes nowhere.

eddieangel21-Jan-14 11:29

21-Jan-14 11:29

I flattened the model due to some database design problems that I can't (not allowed to) fix. As such I am exposing the categoryId and categoryName property on the type object. I can use most of the same framework, the question is how do I cleanly update the categoryName when the Id is updated?

I could put a lookup into the PropertyChanged event of the categoryId, but it seems dirty to have to visit the database from the model object. Seems dirty, thoughts?

Jason Gleim22-Jan-14 4:28

22-Jan-14 4:28

Sorry for the delay getting back...

I understand your concerns. You didn't say that you are using MVVM but since you are talking about a model class I'm guessing you are, in fact, using it. If that is the case, you should put all of your database code in the model. The idea of the model is to hide the details of the backing data store so you could switch it out if you needed. So it may feel dirty but that is exactly what you are supposed to be doing.

I have fallen into a pattern where I usually create two classes for data. I'll call them DataObject and DataModel (really xxxxObject and xxxxModel as appropriate). DataObject represents the actual data and is typically nothing more than a class with a bunch of public properties. I usually inherit INotifyPropertyChanged and INotifyDataError on that class so that it can alert to value changes and it can alert to bad values.

The second class, DataModel, is where I put the data access functions. This class is data source specific and usually implements an interface I have specified with the functions I'll need. In your case, this class might expose a function like:

public CategoryBase GetCategoryById(int Id)

which would obviously return a category based on an Id. In your ViewModel you would have a property which would take a category ID and in the setter you would call the GetCategoryById then use the results to set the category and Id on the property exposing that data object. As long as the underlying DataObject implements INPC and you have bound to a non-null category object at startup, then the category and Id should follow along with changes in the Id sourced from the combobox.

The grid complicates things a bit because you have a collection of the data objects but that is pretty easy to work around. If you are uncomfortable wiring it up from the property setter for the Id property, remember that Combobox exposes a selection changed event. You could bind a command to that and use that as a jump point to call the DB fetch method and set the category data object.

Ultimately though... I suggest you take MVVM as a suggestion and not a set of hard and fast rules. Like a lot of stuff in software development, there is always some scenario that doesn't fit the rules so we end up breaking them to make it work. If you figure out a system that works, then consider it a success. Just document the code so the next guy can see why you did what you did and why you broke the rule. $Shucks | :-\$

eddieangel22-Jan-14 5:34