概述
在上面一篇 中，我们对 DataGrid 控件做了一个概览的介绍，今天开始我们会做进一步的详细分享。
按照概述中分析代码结构的顺序，今天我们先对 CollectionViews 文件夹中的类做详细的分析。
下面是 Windows Community Toolkit Sample App 的示例截图和 code/doc 地址：
Namespace: Microsoft.Toolkit.Uwp.UI.Controls; Nuget: Microsoft.Toolkit.Uwp.UI.Controls.DataGrid;
开发过程
首先再来看一下 CollectionViews 文件夹的代码结构：
4 个类中，CollectionView 是 EnumerableCollectionView 和 ListCollectionView 的基类，而 CollectionViewsError 是和 DataGrid 数据源中错误的处理类，接下来我们来分别看一下：
1. CollectionView
CollectionView 类是 DataGrid 数据相关处理的基类，这个类里的处理方法和属性设置很多，同时还针对 FILTER，SORT 和 GROUP 特性做了处理，下面先来看看类中定义的属性：
- Count - 表示 DataGrid 控件数据的数量，在 OnCollectionChanged 事件处理中，非 Replace 情况下触发；
- IsEmpty - 表示 DataGrid 控件中数据是否为空，同样在 OnCollectionChanged 事件处理中，空和非空状态切换时触发；
- Culture - 表示 DataGrid 控件的区域性信息，在 Culture 变化时，包括名称，日历系统，字符排序等会发生变化；
- CurrentPosition - 表示 DataGrid 控件的当前位置，在子类的 RaiseCurrencyChanges 和 LoadSnapshot 事件中被使用；
- CurrentItem - 表示 DataGrid 控件当前选中的元素，同样在子类的 RaiseCurrencyChanges 和 LoadSnapshot 事件中被使用；
- IsCurrentBeforeFirst - 表示 DataGrid 控件中当前选中是否在首个元素之前；
- IsCurrentAfterLast - 表示 DataGrid 控件中当前选中是否在最后一个元素之后；
接下来看几个重要的方法：
1). CollectionView()
CollectionView 类的构造方法，可以看到方法中创建了监听器，对时间的 Action 调用和卸载做了定义，对于集合改变事件做了绑定，并对布尔类型的属性做了初始设置；
public CollectionView(IEnumerable collection){ _sourceCollection = collection ?? throw new ArgumentNullException("collection"); // forward collection change events from underlying collection to our listeners. INotifyCollectionChanged incc = collection as INotifyCollectionChanged; if (incc != null) { _sourceWeakEventListener = new WeakEventListener (this) { // Call the actual collection changed event OnEventAction = (source, changed, arg) => OnCollectionChanged(source, arg), // The source doesn't exist anymore OnDetachAction = (listener) => incc.CollectionChanged -= _sourceWeakEventListener.OnEvent }; incc.CollectionChanged += _sourceWeakEventListener.OnEvent; } _currentItem = null; _currentPosition = -1; SetFlag(CollectionViewFlags.IsCurrentBeforeFirst, _currentPosition < 0); SetFlag(CollectionViewFlags.IsCurrentAfterLast, _currentPosition < 0); SetFlag(CollectionViewFlags.CachedIsEmpty, _currentPosition < 0);} 2). OnCollectionChanged()
集合变化的处理，包括对变化动画的判断，当变化不是替换时，触发 count 属性变化；以及对于集合空的判断，空和为空切换时，触发 isEmpty 属性变化，前面在属性说明中我们提提到了；
protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs args){ if (args == null) { throw new ArgumentNullException("args"); } unchecked { // invalidate enumerators because of a change ++_timestamp; } CollectionChanged?.Invoke(this, args); // Collection changes change the count unless an item is being // replaced or moved within the collection. if (args.Action != NotifyCollectionChangedAction.Replace) { OnPropertyChanged(CountPropertyName); } bool isEmpty = IsEmpty; if (isEmpty != CheckFlag(CollectionViewFlags.CachedIsEmpty)) { SetFlag(CollectionViewFlags.CachedIsEmpty, isEmpty); OnPropertyChanged(IsEmptyPropertyName); }} 3). SetCurrent()
根据当前选择的元素，当前位置和元素数量设置当前选中；新元素不为空时，设置 IsCurrentBeforeFirst 和 IsCurrentAfterLast 属性为 false；当集合为空时，设置两个属性为 true，设置新的选中位置为 -1；否则，根据 newPosition 的值来设置这两个属性；
protected void SetCurrent(object newItem, int newPosition, int count){ if (newItem != null) { // non-null item implies position is within range. // We ignore count - it's just a placeholder SetFlag(CollectionViewFlags.IsCurrentBeforeFirst, false); SetFlag(CollectionViewFlags.IsCurrentAfterLast, false); } else if (count == 0) { // empty collection - by convention both flags are true and position is -1 SetFlag(CollectionViewFlags.IsCurrentBeforeFirst, true); SetFlag(CollectionViewFlags.IsCurrentAfterLast, true); newPosition = -1; } else { // null item, possibly within range. SetFlag(CollectionViewFlags.IsCurrentBeforeFirst, newPosition < 0); SetFlag(CollectionViewFlags.IsCurrentAfterLast, newPosition >= count); } _currentItem = newItem; _currentPosition = newPosition;}
2. EnumerableCollectionView
该类是 CollectionView 类的子类，支持枚举类型的数据集合。下面我们主要分享它基于 CollectionView 的特殊实现部分：
1). EnumerableCollectionView()
先看看构造方法，首先根据数据源设置当前元素和位置等，绑定集合改变，属性改变和当前的改变和改变后事件；重点说一下 OnCurrentChanging 和 OnCurrentChanged 事件，分别可以在改变前做干预处理，改变后做对应处理；
internal EnumerableCollectionView(IEnumerable source) : base(source){ _snapshot = new ObservableCollection 2). ProcessCollectionChanged()
处理集合变化事件的方法，主要对改变做了 Add，Remove，Replace 和 Reset 四种情况的处理；分别看一下处理内容：
- Add - Add 操作后，对 snapshot 集合做对应变化，当新增索引 < 0 或小于当前开始索引时，加到集合开始位置，否则插入对应位置；
- Remove - Remove 操作后，在 snapshot 集合中删除对应位置的元素；
- Replace - Replace 操作后，在 snapshot 集合中替换对应位置的元素；
- Reset - Reset 操作后，对应重置 snapshot 集合；
protected override void ProcessCollectionChanged(NotifyCollectionChangedEventArgs args){ // Apply the change to the snapshot switch (args.Action) { case NotifyCollectionChangedAction.Add: if (args.NewStartingIndex < 0 || _snapshot.Count <= args.NewStartingIndex) { // Append for (int i = 0; i < args.NewItems.Count; ++i) { _snapshot.Add(args.NewItems[i]); } } else { // Insert for (int i = args.NewItems.Count - 1; i >= 0; --i) { _snapshot.Insert(args.NewStartingIndex, args.NewItems[i]); } } break; case NotifyCollectionChangedAction.Remove: if (args.OldStartingIndex < 0) { throw CollectionViewsError.EnumerableCollectionView.RemovedItemNotFound(); } for (int i = args.OldItems.Count - 1, index = args.OldStartingIndex + i; i >= 0; --i, --index) { if (!object.Equals(args.OldItems[i], _snapshot[index])) { throw CollectionViewsError.CollectionView.ItemNotAtIndex("removed"); } _snapshot.RemoveAt(index); } break; case NotifyCollectionChangedAction.Replace: for (int i = args.NewItems.Count - 1, index = args.NewStartingIndex + i; i >= 0; --i, --index) { if (!object.Equals(args.OldItems[i], _snapshot[index])) { throw CollectionViewsError.CollectionView.ItemNotAtIndex("replaced"); } _snapshot[index] = args.NewItems[i]; } break; case NotifyCollectionChangedAction.Reset: LoadSnapshot(SourceCollection); break; }} 3). LoadSnapshot()
加载 snapshot 方法，根据重新加载的元素集合，判断以下属性是否需要响应变化：IsCurrentAfterLast，IsCurrentBeforeFirst，CurrentPosition 和 CurrentItem。
private void LoadSnapshot(IEnumerable source){ // Force currency off the collection (gives user a chance to save dirty information). OnCurrentChanging(); // Remember the values of the scalar properties, so that we can restore // them and raise events after reloading the data object oldCurrentItem = CurrentItem; int oldCurrentPosition = CurrentPosition; bool oldIsCurrentBeforeFirst = IsCurrentBeforeFirst; bool oldIsCurrentAfterLast = IsCurrentAfterLast; // Reload the data LoadSnapshotCore(source); // Tell listeners everything has changed OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset)); OnCurrentChanged(); if (IsCurrentAfterLast != oldIsCurrentAfterLast) { OnPropertyChanged(new PropertyChangedEventArgs(IsCurrentAfterLastPropertyName)); } if (IsCurrentBeforeFirst != oldIsCurrentBeforeFirst) { OnPropertyChanged(new PropertyChangedEventArgs(IsCurrentBeforeFirstPropertyName)); } if (oldCurrentPosition != CurrentPosition) { OnPropertyChanged(new PropertyChangedEventArgs(CurrentPositionPropertyName)); } if (oldCurrentItem != CurrentItem) { OnPropertyChanged(new PropertyChangedEventArgs(CurrentItemPropertyName)); }}
3. ListCollectionView
该类是 CollectionView 类的子类，支持列表类型的数据集合。下面我们也会主要分享它基于 CollectionView 的特殊实现部分：
1). ListCollectionView()
ListCollectionView 类的构造方法，当支持编辑行为时，需要刷新可增加，可删除，可取消编辑的判断；然后设置当前位置和元素；当支持分组时，注册分组描述，分组改变和分组依据的变化处理事件；
public ListCollectionView(IList list) : base(list){ _internalList = list; #if FEATURE_IEDITABLECOLLECTIONVIEW RefreshCanAddNew(); RefreshCanRemove(); RefreshCanCancelEdit(); #endif if (InternalList.Count == 0) { // don't call virtual IsEmpty in ctor SetCurrent(null, -1, 0); } else { SetCurrent(InternalList[0], 0, 1); } #if FEATURE_ICOLLECTIONVIEW_GROUP _group = new CollectionViewGroupRoot(this); _group.GroupDescriptionChanged += new EventHandler(OnGroupDescriptionChanged); ((INotifyCollectionChanged)_group).CollectionChanged += new NotifyCollectionChangedEventHandler(OnGroupChanged); ((INotifyCollectionChanged)_group.GroupDescriptions).CollectionChanged += new NotifyCollectionChangedEventHandler(OnGroupByChanged); #endif} 2). ProcessCollectionChangedWithAdjustedIndex()
处于集合变化和索引调整的方法，首先判断当前动作的类型：Add，Remove 或 Replace，并针对每种不同类型的操作，进行分别的处理；再对 afterLastHasChanged，beforeFirstHasChanged，currentPositionHasChanged 和 currentItemHasChanged 属性进行设置；
private void ProcessCollectionChangedWithAdjustedIndex(EffectiveNotifyCollectionChangedAction action, object oldItem, object newItem, int adjustedOldIndex, int adjustedNewIndex){ EffectiveNotifyCollectionChangedAction effectiveAction = action; if (adjustedOldIndex == adjustedNewIndex && adjustedOldIndex >= 0) { effectiveAction = EffectiveNotifyCollectionChangedAction.Replace; } else if (adjustedOldIndex == -1) { if (adjustedNewIndex < 0) { if (action == EffectiveNotifyCollectionChangedAction.Add) { return; } effectiveAction = EffectiveNotifyCollectionChangedAction.Remove; } } else if (adjustedOldIndex < -1) { ... } else { ... } int originalCurrentPosition = CurrentPosition; int oldCurrentPosition = CurrentPosition; object oldCurrentItem = CurrentItem; bool oldIsCurrentAfterLast = IsCurrentAfterLast; bool oldIsCurrentBeforeFirst = IsCurrentBeforeFirst; NotifyCollectionChangedEventArgs args = null, args2 = null; switch (effectiveAction) { case EffectiveNotifyCollectionChangedAction.Add: // insert into private view#if FEATURE_ICOLLECTIONVIEW_SORT_OR_FILTER#if FEATURE_IEDITABLECOLLECTIONVIEW // (unless it's a special item (i.e. new item)) if (UsesLocalArray && (!IsAddingNew || !object.Equals(_newItem, newItem)))#else if (UsesLocalArray)#endif { InternalList.Insert(adjustedNewIndex, newItem); }#endif if (!IsGrouping) { AdjustCurrencyForAdd(adjustedNewIndex); args = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, newItem, adjustedNewIndex); }#if FEATURE_ICOLLECTIONVIEW_GROUP else { AddItemToGroups(newItem); }#endif break; case EffectiveNotifyCollectionChangedAction.Remove: ... case EffectiveNotifyCollectionChangedAction.Replace: ... case EffectiveNotifyCollectionChangedAction.Move: ... default: Debug.Assert(false, "Unexpected Effective Collection Change Action"); break; } bool afterLastHasChanged = IsCurrentAfterLast != oldIsCurrentAfterLast; bool beforeFirstHasChanged = IsCurrentBeforeFirst != oldIsCurrentBeforeFirst; bool currentPositionHasChanged = CurrentPosition != oldCurrentPosition; bool currentItemHasChanged = CurrentItem != oldCurrentItem; oldIsCurrentAfterLast = IsCurrentAfterLast; oldIsCurrentBeforeFirst = IsCurrentBeforeFirst; oldCurrentPosition = CurrentPosition; oldCurrentItem = CurrentItem; // base class will raise an event to our listeners if (!IsGrouping) { Debug.Assert(!CurrentChangedMonitor.Busy, "Expected _currentChangedMonitor.Busy is false."); CurrentChangedMonitor.Enter(); using (CurrentChangedMonitor) { OnCollectionChanged(args); if (args2 != null) { OnCollectionChanged(args2); } } // Any scalar properties that changed don't need a further notification, // but do need a new snapshot ... } // currency has to change after firing the deletion event, // so event handlers have the right picture if (_currentElementWasRemoved) { int oldCurPos = originalCurrentPosition;#if FEATURE_ICOLLECTIONVIEW_GROUP if (_newGroupedItem != null) { oldCurPos = IndexOf(_newGroupedItem); }#endif MoveCurrencyOffDeletedElement(oldCurPos); // changes to the scalar properties need notification afterLastHasChanged = afterLastHasChanged || (IsCurrentAfterLast != oldIsCurrentAfterLast); beforeFirstHasChanged = beforeFirstHasChanged || (IsCurrentBeforeFirst != oldIsCurrentBeforeFirst); currentPositionHasChanged = currentPositionHasChanged || (CurrentPosition != oldCurrentPosition); currentItemHasChanged = currentItemHasChanged || (CurrentItem != oldCurrentItem); } RaiseCurrencyChanges(false, currentItemHasChanged, currentPositionHasChanged, beforeFirstHasChanged, afterLastHasChanged);}
4. CollectionViewsError
CollectionViewsError 类中主要定义了 DataGrid 控件数据，就是 CollectionView 中的错误，我们来看一下都定义了哪些错误：
- EnumeratorVersionChanged - InvalidOperationException，“Collection was modified; enumeration operation cannot execute.”
- MemberNotAllowedDuringAddOrEdit - InvalidOperationException，"'{0}' is not allowed during an AddNew or EditItem transaction."
- NoAccessWhileChangesAreDeferred - InvalidOperationException，"This value cannot be accessed while changes are deferred."
- ItemNotAtIndex - InvalidOperationException，"The {0} item is not in the collection."
- RemovedItemNotFound - InvalidOperationException，"The removed item is not found in the source collection."
- CollectionChangedOutOfRange - InvalidOperationException，"The collection change is out of bounds of the original collection."
- AddedItemNotInCollection - InvalidOperationException，"The added item is not in the collection."
- CancelEditNotSupported - InvalidOperationException，"CancelEdit is not supported for the current edit item."
- MemberNotAllowedDuringTransaction - InvalidOperationException，"'{0}' is not allowed during a transaction started by '{1}'."
- MemberNotAllowedForView - InvalidOperationException，"'{0}' is not allowed for this view."
总结
这里我们把 DataGrid 的 CollectionView 相关类介绍完成了，作为 DataGrid 相关分享的第一篇，后面我们会继续分享 Utilities 和最重要的 DataGrid 的相关重点。
最后，再跟大家安利一下 WindowsCommunityToolkit 的官方微博：, 大家可以通过微博关注最新动态。
衷心感谢 WindowsCommunityToolkit 的作者们杰出的工作，感谢每一位贡献者，Thank you so much, ALL WindowsCommunityToolkit AUTHORS !!!