TLDR; This is what you want, code below:
The proper place to do this would be in your ViewModel. Your CheckBox can have three states, all of which you want to make use of:
- Checked - Every item is checked
- Unchecked - No item is checked
- Indeterminate - Some items are checked, some are not
You will want to update the CheckBox whenever an item is checked/unchecked and update all items whenever the CheckBox was changed - implementing this only one way will leave the CheckBox in an invalid state which might have a negative impact on user experience. My suggestion: go all the way and implement it properly. To do this you need to be aware of which caused the change - the CheckBox of an entry or the CheckBox in the header.
Here is how I would do it:
First you need a ViewModel for your items, I've used a very simplified one here that only contains the IsChecked property.
public class Entry : INotifyPropertyChanged
{
private bool _isChecked;
public bool IsChecked
{
get => _isChecked;
set
{
if (value == _isChecked) return;
_isChecked = value;
OnPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
Your main ViewModel will have a collection of all items. Whenever an item's IsChecked property changes, you'll have to check if all items are checked/unchecked and update the CheckBox in the header (or rather the value of its datasource).
public class ViewModel : INotifyPropertyChanged
{
public List<Entry> Entries
{
get => _entries;
set
{
if (Equals(value, _entries)) return;
_entries = value;
OnPropertyChanged();
}
}
public ViewModel()
{
// Just some demo data
Entries = new List<Entry>
{
new Entry(),
new Entry(),
new Entry(),
new Entry()
};
// Make sure to listen to changes.
// If you add/remove items, don't forgat to add/remove the event handlers too
foreach (Entry entry in Entries)
{
entry.PropertyChanged += EntryOnPropertyChanged;
}
}
private void EntryOnPropertyChanged(object sender, PropertyChangedEventArgs args)
{
// Only re-check if the IsChecked property changed
if(args.PropertyName == nameof(Entry.IsChecked))
RecheckAllSelected();
}
private void AllSelectedChanged()
{
// Has this change been caused by some other change?
// return so we don't mess things up
if (_allSelectedChanging) return;
try
{
_allSelectedChanging = true;
// this can of course be simplified
if (AllSelected == true)
{
foreach (Entry kommune in Entries)
kommune.IsChecked = true;
}
else if (AllSelected == false)
{
foreach (Entry kommune in Entries)
kommune.IsChecked = false;
}
}
finally
{
_allSelectedChanging = false;
}
}
private void RecheckAllSelected()
{
// Has this change been caused by some other change?
// return so we don't mess things up
if (_allSelectedChanging) return;
try
{
_allSelectedChanging = true;
if (Entries.All(e => e.IsChecked))
AllSelected = true;
else if (Entries.All(e => !e.IsChecked))
AllSelected = false;
else
AllSelected = null;
}
finally
{
_allSelectedChanging = false;
}
}
public bool? AllSelected
{
get => _allSelected;
set
{
if (value == _allSelected) return;
_allSelected = value;
// Set all other CheckBoxes
AllSelectedChanged();
OnPropertyChanged();
}
}
private bool _allSelectedChanging;
private List<Entry> _entries;
private bool? _allSelected;
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
Demo XAML:
<DataGrid ItemsSource="{Binding Entries}" AutoGenerateColumns="False" IsReadOnly="False" CanUserAddRows="False">
<DataGrid.Columns>
<DataGridCheckBoxColumn Binding="{Binding IsChecked, UpdateSourceTrigger=PropertyChanged}">
<DataGridCheckBoxColumn.HeaderTemplate>
<DataTemplate>
<CheckBox IsChecked="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=local:MainWindow}, Path=ViewModel.AllSelected}">Select All</CheckBox>
</DataTemplate>
</DataGridCheckBoxColumn.HeaderTemplate>
</DataGridCheckBoxColumn>
</DataGrid.Columns>
</DataGrid>
