WPF TreeView IsExpanded 繫結不上的問題

Johar發表於2018-10-31
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最近專案上需要通過MVVM來控制TreeView，其中需要需要控制通過搜尋來定位某個節點，正常邏輯下，首先通過需要在樹上面找到該節點，然後選中該節點，並將該節點的父節點展開，這個時候需要通過MVVM來控制，需要繫結起來，只是一直沒有binding上，程式碼如下：

MVVM示例程式碼：

  1 using System;
  2 using System.Collections.Generic;
  3 using System.Collections.ObjectModel;
  4 using System.Linq;
  5 using System.Text;
  6 using System.Threading.Tasks;
  7 
  8 namespace DragDrop
  9 {
 10     class DataItem : NotifyPropertyBase, ICloneable
 11     {
 12         public DataItem(string header, int deepth = 1)
 13         {
 14             Header = header;
 15             Deepth = deepth;
 16         }
 17 
 18         public object Clone()
 19         {
 20             DataItem dataItem = new DataItem(Header, Deepth);
 21             dataItem.IsExpanded = IsExpanded;
 22             dataItem.IsSelected = IsSelected;
 23             dataItem.Deepth = Deepth;
 24             foreach (DataItem item in Items)
 25                 dataItem.Items.Add((DataItem)item.Clone());
 26             return dataItem;
 27         }
 28 
 29         private string header;
 30         public string Header
 31         {
 32             get { return header; }
 33             set
 34             {
 35                 SetProperty(ref header, value);
 36             }
 37         }
 38 
 39         private bool isSelected;
 40         public bool IsSelected
 41         {
 42             get { return isSelected; }
 43             set { SetProperty(ref isSelected, value); }
 44         }
 45 
 46         private bool isExpanded;
 47         public bool IsExpanded
 48         {
 49             get { return isExpanded; }
 50             set
 51             {
 52                 SetProperty(ref isExpanded, value);
 53                 Console.WriteLine("{0}--{1}", Header, IsExpanded);
 54             }
 55         }
 56 
 57         private int deepth;
 58         public int Deepth
 59         {
 60             get { return deepth; }
 61             set
 62             {
 63                 if (deepth != value)
 64                 {
 65                     deepth = value;
 66                     SetProperty(ref deepth, value);
 67                 }
 68             }
 69         }
 70 
 71         private ObservableCollection<DataItem> mItems = null;
 72         public ObservableCollection<DataItem> Items
 73         {
 74             get
 75             {
 76                 if (mItems == null)
 77                     mItems = new ObservableCollection<DataItem>();
 78                 return mItems;
 79             }
 80         }       
 81         
 82     }
 83 
 84     class Data
 85     {
 86         private static Data mInstance = new Data();
 87 
 88         public static Data Instance
 89         {
 90             get { return mInstance; }
 91         }
 92 
 93         private ObservableCollection<DataItem> GenerateTreeViewItems()
 94         {
 95             ObservableCollection<DataItem> items = new ObservableCollection<DataItem>();
 96 
 97             DataItem item1 = new DataItem("TreeViewItem1");
 98             item1.Items.Add(new DataItem("SubItem1", item1.Deepth));
 99             item1.Items.Add(new DataItem("SubItem2", item1.Deepth));
100             item1.Items.Add(new DataItem("SubItem3", item1.Deepth));
101             item1.Items.Add(new DataItem("SubItem4", item1.Deepth));
102             items.Add(item1);
103 
104             DataItem item2 = new DataItem("TreeViewItem2");
105             item2.Items.Add(new DataItem("SubItem1", item2.Deepth));
106             item2.Items.Add(new DataItem("SubItem2", item2.Deepth));
107             items.Add(item2);
108 
109             DataItem item3 = new DataItem("TreeViewItem3");
110             item3.Items.Add(new DataItem("SubItem1", item3.Deepth));
111             item3.Items.Add(new DataItem("SubItem2", item3.Deepth));
112             item3.Items.Add(new DataItem("SubItem3", item3.Deepth));
113             item3.Items.Add(new DataItem("SubItem4", item3.Deepth));
114             item3.Items.Add(new DataItem("SubItem5", item3.Deepth));
115             item3.Items.Add(new DataItem("SubItem6", item3.Deepth));
116             item3.Items.Add(new DataItem("SubItem7", item3.Deepth));
117             item3.Items.Add(new DataItem("SubItem8", item3.Deepth));
118             items.Add(item3);
119 
120             return items;
121         }
122 
123         private ObservableCollection<DataItem> GenerateListItems()
124         {
125             ObservableCollection<DataItem> items = new ObservableCollection<DataItem>();
126             items.Add(new DataItem("ListBoxItem1"));
127             items.Add(new DataItem("ListBoxItem2"));
128             items.Add(new DataItem("ListBoxItem3"));
129             items.Add(new DataItem("ListBoxItem4"));
130             items.Add(new DataItem("ListBoxItem5"));
131             return items;
132         }
133 
134         public ObservableCollection<DataItem> TreeViewItems
135         {
136             get
137             {
138                 if (mTreeViewItems == null)
139                     mTreeViewItems = GenerateTreeViewItems();
140                 return mTreeViewItems;
141             }
142         }
143 
144         public ObservableCollection<DataItem> ListBoxItems
145         {
146             get
147             {
148                 if (mListBoxItems == null)
149                     mListBoxItems = GenerateListItems();
150                 return mListBoxItems;
151             }
152         }
153 
154         private ObservableCollection<DataItem> mTreeViewItems = null;
155         private ObservableCollection<DataItem> mListBoxItems = null;
156     }
157 }

介面程式碼：

 1 <Window x:Class="DragDrop.MainWindow"
 2         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 3         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 4         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
 5         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 6         xmlns:local="clr-namespace:DragDrop"
 7         mc:Ignorable="d"
 8         Title="MainWindow" Height="350" Width="525">
 9     <Window.Resources>
10         <HierarchicalDataTemplate x:Key="treeViewTemplate" DataType="{x:Type local:DataItem}" ItemsSource="{Binding Items}">
11             <TextBlock Text="{Binding Header}"/>
12         </HierarchicalDataTemplate>
13 
14         <Style x:Key="TreeViewStyle" TargetType="{x:Type TreeViewItem}">
15             <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
16             <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
17             <Style.Triggers>
18                 <DataTrigger Binding="{Binding Deepth}" Value="1">
19                     <Setter Property="IsExpanded" Value="True"/>
20                 </DataTrigger>
21             </Style.Triggers>
22         </Style>
23     </Window.Resources>
24     <Grid x:Name="mTopLevelGrid">
25         <TreeView x:Name="mTreeView" Grid.Column="0"
26                   ItemsSource="{Binding Source={x:Static local:Data.Instance}, Path=TreeViewItems}"
27                   ItemTemplate="{StaticResource treeViewTemplate}"
28                   ItemContainerStyle="{StaticResource TreeViewStyle}"/>
29     </Grid>
30 </Window>

資料的繫結沒有問題，介面沒有問題，奇怪的是IsSelected可以正常繫結，但是IsExpanded就是不行，後來發現這兩個屬性唯一的區別就是在TreeView的式樣中

1 <Style.Triggers>
2      <DataTrigger Binding="{Binding Deepth}" Value="1">
3            <Setter Property="IsExpanded" Value="True"/>
4       </DataTrigger>
5 </Style.Triggers>

後面把這個DataTrigger這段程式碼註釋掉，編譯一下，重新執行，IsExpanded就可以正常繫結。後面自己思考了一下，MVVM模式實現將ViewModel與介面上面的顯示繫結起來，實際上也就是使用了觀察者模式，而Trigger的原理應該和資料binding是一樣的，而且這個Trigger寫在後面，可能直接把前面的繫結取代了，但是試了一下，將DataTrigger放到前面，也還是繫結不上，這種可能性排除；個人猜測可能是Trigger的優先順序比較高，從而造成IsExpanded一直Binding不上去。

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