在后面的代码中设置 DataContext 的顺序

Question

在后面的代码中设置DataContext的正确方法是：

public ViewConstructor()
{
    InitializeComponent();
    DataContext = new MyViewModel();
}

或

public ViewConstructor()
{
    DataContext = new MyViewModel();
    InitializeComponent();
}

?

Answer 1

回答是：视情况而定。

如果您的屏幕包含大量数据或复杂的数据驱动的可视化，您可能希望延迟或错开其加载，这可能会使第一个选项更好。如果它是一个简单的数据绑定表单，则第二个选项可能更可取。或者它甚至可以忽略不计。与往常一样，测量是在您的特定条件下回答问题的最佳方式。

让我们看看在绑定上启用跟踪会发生什么。

<Window x:Class="WpfApplication9.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Grid IsEnabled="{Binding IsEnabled, PresentationTraceSources.TraceLevel=High}" />
</Window>

第一个选项 - InitializeComponent first

public MainWindow()
{
    Debug.WriteLine("Initializing");
    InitializeComponent();
    Debug.WriteLine("Initialized");
    Debug.WriteLine("Setting DataContext");
    DataContext = new ViewModel();
    Debug.WriteLine("DataContext Set");
}

在这里我们看到在InitializeComponent（加载XAML 时）期间绑定尝试解析，但看到DataContext 为空，因此它们的评估是延迟。

调试输出：

Initializing
System.Windows.Data Warning: 56 : Created BindingExpression (hash=55924514) for Binding (hash=26055869)
System.Windows.Data Warning: 58 :   Path: 'IsEnabled'
System.Windows.Data Warning: 60 : BindingExpression (hash=55924514): Default mode resolved to OneWay
System.Windows.Data Warning: 61 : BindingExpression (hash=55924514): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=55924514): Attach to System.Windows.Controls.Grid.IsEnabled (hash=21411931)
System.Windows.Data Warning: 67 : BindingExpression (hash=55924514): Resolving source 
System.Windows.Data Warning: 70 : BindingExpression (hash=55924514): Found data context element: Grid (hash=21411931) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=55924514): DataContext is null
System.Windows.Data Warning: 65 : BindingExpression (hash=55924514): Resolve source deferred
Initialized
Setting DataContext
DataContext Set
System.Windows.Data Warning: 67 : BindingExpression (hash=55924514): Resolving source 
System.Windows.Data Warning: 70 : BindingExpression (hash=55924514): Found data context element: Grid (hash=21411931) (OK)
System.Windows.Data Warning: 78 : BindingExpression (hash=55924514): Activate with root item ViewModel (hash=45063479)
System.Windows.Data Warning: 108 : BindingExpression (hash=55924514):   At level 0 - for ViewModel.IsEnabled found accessor ReflectPropertyDescriptor(IsEnabled)
System.Windows.Data Warning: 104 : BindingExpression (hash=55924514): Replace item at level 0 with ViewModel (hash=45063479), using accessor ReflectPropertyDescriptor(IsEnabled)
System.Windows.Data Warning: 101 : BindingExpression (hash=55924514): GetValue at level 0 from ViewModel (hash=45063479) using ReflectPropertyDescriptor(IsEnabled): 'False'
System.Windows.Data Warning: 80 : BindingExpression (hash=55924514): TransferValue - got raw value 'False'
System.Windows.Data Warning: 89 : BindingExpression (hash=55924514): TransferValue - using final value 'False'

第二个选项 - 首先设置DataContext

public MainWindow()
{
    Debug.WriteLine("Setting DataContext");
    DataContext = new ViewModel();
    Debug.WriteLine("DataContext Set");
    Debug.WriteLine("Initializing");
    InitializeComponent();
    Debug.WriteLine("Initialized");
}

这里的绑定在初始化期间立即评估。

调试输出：

Setting DataContext
DataContext Set
Initializing
System.Windows.Data Warning: 56 : Created BindingExpression (hash=27331439) for Binding (hash=41386841)
System.Windows.Data Warning: 58 :   Path: 'IsEnabled'
System.Windows.Data Warning: 60 : BindingExpression (hash=27331439): Default mode resolved to OneWay
System.Windows.Data Warning: 61 : BindingExpression (hash=27331439): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=27331439): Attach to System.Windows.Controls.Grid.IsEnabled (hash=16919637)
System.Windows.Data Warning: 67 : BindingExpression (hash=27331439): Resolving source 
System.Windows.Data Warning: 70 : BindingExpression (hash=27331439): Found data context element: Grid (hash=16919637) (OK)
System.Windows.Data Warning: 78 : BindingExpression (hash=27331439): Activate with root item ViewModel (hash=25445597)
System.Windows.Data Warning: 108 : BindingExpression (hash=27331439):   At level 0 - for ViewModel.IsEnabled found accessor ReflectPropertyDescriptor(IsEnabled)
System.Windows.Data Warning: 104 : BindingExpression (hash=27331439): Replace item at level 0 with ViewModel (hash=25445597), using accessor ReflectPropertyDescriptor(IsEnabled)
System.Windows.Data Warning: 101 : BindingExpression (hash=27331439): GetValue at level 0 from ViewModel (hash=25445597) using ReflectPropertyDescriptor(IsEnabled): 'False'
System.Windows.Data Warning: 80 : BindingExpression (hash=27331439): TransferValue - got raw value 'False'
System.Windows.Data Warning: 89 : BindingExpression (hash=27331439): TransferValue - using final value 'False'
Initialized