/// <summary>
/// Binds the property on many elements into a combination for which <paramref name="handler"/> is invoked whenever its value changes.
/// This is useful for instance when you have a property on a collection that is the reduction of a property of collection element (in which case you can use the handler to set the reduced value).
/// </summary>
/// <typeparam name="TElement"> The type of the elements of the specified observable collection. </typeparam>
/// <typeparam name="TElementProperty"> The type of the property on <typeparamref name="TElement"/> whose name is <paramref name="propertyName"/> and whose values are to be reduced. </typeparam>
/// <param name="collection"> The collection to monitor for property changes. </param>
/// <param name="propertyName"> The name of the property on the collection elements to monitor. </param>
/// <param name="handler"> The function handling the change of the reduced value. The first argument is the collection and the second the new value. </param>
/// <param name="combine"> The function taking the previous reduced value and an updated value of one of the properies, and returns their reduction. </param>
/// <param name="valueIfCollectionEmpty"> The value to be taken if the specified collection is empty. </param>
/// <param name="equalityComparer"> The function comparing <typeparamref name="TElementProperty"/>s for equality, determining when to invoke <paramref name="handler"/>. </param>
public static void BindCollective <TElement, TElementProperty>(this INotifyCollectionChanged collection,
string propertyName,
PropertyChangedEventHandler handler,
Func <TElementProperty /*previous resultant*/, TElementProperty /*new property*/, TElementProperty> combine,
Option <TElementProperty> valueIfCollectionEmpty = default,
IEqualityComparer <TElementProperty> equalityComparer = null)
{
Contract.Requires(collection != null);
Contract.Requires(collection is IEnumerable <TElement>);
Contract.Requires(HasProperty <TElement>(propertyName));
Contract.Requires(handler != null);
Contract.Requires(combine != null);
equalityComparer = equalityComparer ?? EqualityComparer <TElementProperty> .Default;
Option <TElementProperty> resultant = Option <TElementProperty> .None;
collection.Bind(propertyName, elementPropertyChanged, typeof(TElement), true, onElementRemoved);
setResultant(recomputeResultant()); // if handler shouldn't be triggered on the initially present elements, set as resultant = recomputeResultant() rather than through setResultant
Option <TElementProperty> recomputeResultant()
{
Option <TElementProperty> result = Option <TElementProperty> .None;
foreach (var element in (IEnumerable <TElement>)collection)
{
var propValue = GetPropertyValue <TElement, TElementProperty>(element, propertyName);
if (!result.HasValue)
{
result = propValue;
}
else
{
result = combine(result.Value, propValue);
}
}
return(result); // returns Option.None when the collection is empty
}
void onElementRemoved(object sender, PropertyChangedEventArgs e)
{
Contract.Assert(resultant.HasValue);
var propValue = GetPropertyValue <TElement, TElementProperty>((TElement)sender, e.PropertyName);
if (equalityComparer.Equals(propValue, resultant.Value)) // if the resultant value was removed from the collection
{
setResultant(recomputeResultant());
}
}
void elementPropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName == propertyName)
{
var newValue = GetPropertyValue <TElement, TElementProperty>((TElement)sender, e.PropertyName);
var newResultant = resultant.HasValue ? combine(resultant.Value, newValue) : newValue;
setResultant(newResultant);
}
}
void setResultant(Option <TElementProperty> newResultant)
{
// newResultant: None means the collection is empty.
TElementProperty newValue;
if (newResultant.HasValue)
{
newValue = newResultant.Value;
}
else
{
if (valueIfCollectionEmpty.HasValue)
{
newValue = valueIfCollectionEmpty.Value;
}
else
{
// don't trigger the handler if there is no value to set
return;
}
}
if (resultant.HasValue)
{
TElementProperty oldValue = resultant.Value;
if (!equalityComparer.Equals(oldValue, newValue))
{
handle(oldValue, newValue);
}
}
else
{
TElementProperty oldValue = valueIfCollectionEmpty.ValueOrDefault();
handle(oldValue, newValue);
}
}
void handle(TElementProperty oldValue, TElementProperty newValue)
{
var arg = new PropertyMutatedEventArgs <TElementProperty>(propertyName, oldValue, newValue);
handler(collection, arg);
}
}
