wpf Observable 集合替换项目
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Observable Collection replace item
提问by thewayman
I have a ObservableCollection, I can add and remove item from the collection. But I can't replace an existing item in the collection. There is a way to replace an item and reflect that on my bound components.
我有一个ObservableCollection，我可以从集合中添加和删除项目。但我无法替换集合中的现有项目。有一种方法可以替换一个项目并将其反映在我绑定的组件上。
System.Collections.Specialized.NotifyCollectionChangedAction.Replace
Can anyone please show me how to accomplish this?
谁能告诉我如何做到这一点？
回答by SLaks
collection[someIndex] = newItem;
回答by Mikhail Tumashenko
Updated: Indexer uses overridden SetItem and notifies about changes.
更新：索引器使用覆盖的 SetItem 并通知更改。
I think the answer about using indexer may be wrong, because the question was about replace and notify.
我认为关于使用索引器的答案可能是错误的，因为问题是关于替换和通知的。
Just to clarify: ObservableCollection<T>uses indexer from its base Collection<T>class, which in turn is a wrapper of List<T>, which is a wrapper of simple array of T. And there's no override for indexer method in ObservableCollectionimplementation.
只是为了澄清：ObservableCollection<T>使用来自其基Collection<T>类的索引器，它又是 的包装器List<T>，它是T.的简单数组的包装器。在ObservableCollection实现中没有覆盖索引器方法。
So when you use indexer to replace an item in ObservableCollectionit invokes the following code from Collectionclass:
因此，当您使用索引器替换ObservableCollection 中的项目时，它会调用Collection类中的以下代码：
public T this[int index] {
get { return items[index]; }
set {
if( items.IsReadOnly) {
ThrowHelper.ThrowNotSupportedException(ExceptionResource.NotSupported_ReadOnlyCollection);
}
if (index < 0 || index >= items.Count) {
ThrowHelper.ThrowArgumentOutOfRangeException();
}
SetItem(index, value);
}
It just checks boundaries and calls SetItem that uses indexer of underlying Listclass:
它只是检查边界并调用使用底层List类索引器的 SetItem ：
protected virtual void SetItem(int index, T item) {
items[index] = item;
}
During assignment there is no call to the CollectionChangedevent, because underlying collections know nothing of it.
在分配期间，不会调用该CollectionChanged事件，因为底层集合对此一无所知。
But when you use SetItemmethod, it is called from ObservableCollection class:
但是当你使用SetItem方法时，它是从 ObservableCollection 类中调用的：
protected override void SetItem(int index, T item)
{
CheckReentrancy();
T originalItem = this[index];
base.SetItem(index, item);
OnPropertyChanged(IndexerName);
OnCollectionChanged(NotifyCollectionChangedAction.Replace, originalItem, item, index);
}
After assignment it calls OnCollectionChangedmethod, which fires CollectionChangedevent with NotifyCollectionChangedAction.Replaceaction parameter.
分配后它调用OnCollectionChanged方法，该方法CollectionChanged使用NotifyCollectionChangedAction.Replace动作参数触发事件。
protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
if (CollectionChanged != null)
{
using (BlockReentrancy())
{
CollectionChanged(this, e);
}
}
}
As a conclusion: the idea with custom class inherited from ObservableCollection and Replacemethod that calls base.SetItem()worth a try.
总结：继承自 ObservableCollection 的自定义类和Replace调用方法的想法base.SetItem()值得一试。
