[WPF] 如何调试Data Binding
前言
在WPF开发中,将ViewModel中对象绑定到UI上时,会出现明明已经将数据对象Binding到UI,但是UI上就是不显示等等的问题。这篇博客将介绍WPF Data Binding调试相关的内容。
场景一(Binding的属性不存在)
ViewModel:
public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); this.DataContext = new ViewModel() { Id = 100, Name = "Tom", Age = 24}; } } public class ViewModel { public int Id { get; set; } public string Name { get; set; } public int Age { get; set; } }
XAML:
<StackPanel Margin="10"> <TextBlock Text="{Binding ID}" /> <TextBlock Text="{Binding Name}" Margin="0,10" /> <TextBlock Text="{Binding Age}" /> </StackPanel>
运行结果:
UI中Binding的ID值没有显示出来。请注意加粗的代码,在UI代码中,由于拼写错误,将Id写成了ID。但是这段代码在编译时不会报错,在VS Output窗口中也不会有提示/警告信息。在程序运行时,仔细查看VS Output窗口,此时会有如下信息 (对信息进行了精简)
System.Windows.Data Error: 40 : BindingExpression path error: 'ID' property not found on 'object' ''ViewModel' (HashCode=20915929)'. BindingExpression:Path=ID; DataItem='ViewModel' (HashCode=20915929); target element is 'TextBlock' (Name=''); target property is 'Text' (type 'String')
这段信息告诉提示说在ViewModel对象上没有找到ID属性,此时我们再去检查一下ViewModel发现,原来是将Id错误的写成了ID。一般这种错误的提示开头为:System.Windows.Data Error:
场景二(使用System.Diagnostics来追踪)
XAML:
<Window x:Class="WpfBindingDebug.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d" Title="MainWindow" Height="350" Width="525"> <StackPanel> <TextBlock Text="{Binding Title}" /> </StackPanel> </Window>
将Title属性Binding到TextBlock的Text属性上面,XAML和C#代码中均未指定DataContext属性。编译项目并运行程序,在VS Output中没有任何提示/警告信息。此时应该如何调试呢?可以通过设置PresentationTraceSources对象的TraceLevel来强制WPF输出所有的Binding方面的信息。
更多PresentationTraceSources信息可以参考:
对XAML代码进行如下修改(注意加粗的代码行):
<Window x:Class="WpfBindingDebug.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase" mc:Ignorable="d" Title="MainWindow" Height="350" Width="525"> <StackPanel> <TextBlock Text="{Binding Title, diag:PresentationTraceSources.TraceLevel=High}" /> </StackPanel> </Window>
编译并运行程序,在VS Output窗口中可以找到关于Binding的信息(对信息进行了精简):
System.Windows.Data Warning: 56 : Created BindingExpression (hash=39201736) for Binding (hash=44325851)
System.Windows.Data Warning: 58 : Path: 'Title'
System.Windows.Data Warning: 60 : BindingExpression (hash=39201736): Default mode resolved to OneWay
System.Windows.Data Warning: 61 : BindingExpression (hash=39201736): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=39201736): Attach to System.Windows.Controls.TextBlock.Text (hash=17911681)
System.Windows.Data Warning: 67 : BindingExpression (hash=39201736): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=39201736): Found data context element: TextBlock (hash=17911681) (OK)
....
System.Windows.Data Warning: 67 : BindingExpression (hash=39201736): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=39201736): Found data context element: TextBlock (hash=17911681) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=39201736): DataContext is null
System.Windows.Data Warning: 67 : BindingExpression (hash=39201736): Resolving source (last chance)
System.Windows.Data Warning: 70 : BindingExpression (hash=39201736): Found data context element: TextBlock (hash=17911681) (OK)
System.Windows.Data Warning: 78 : BindingExpression (hash=39201736): Activate with root item <null>
System.Windows.Data Warning: 106 : BindingExpression (hash=39201736): Item at level 0 is null - no accessor
System.Windows.Data Warning: 80 : BindingExpression (hash=39201736): TransferValue - got raw value {DependencyProperty.UnsetValue}
System.Windows.Data Warning: 88 : BindingExpression (hash=39201736): TransferValue - using fallback/default value ''
System.Windows.Data Warning: 89 : BindingExpression (hash=39201736): TransferValue - using final value ''
注意:在Visual Studio 2010中需要进行如下设置才能看到上面的提示信息:因为VS 2010默认将下面的设为Off。
Tools -> Options -> Debugging -> Output Window -> WPF Trace Settings -> Data Binding -> set to Warning
程序一直在尝试寻找Visual Tree上的可以Binding的Title值,最终找到一个合适的,DependencyProperty.UnsetValue。
上述方法对查找单个页面Binding很有用,当然我们也可以全局的来收集这些Binding信息。在App.xaml.cs中添加:
public partial class App : Application { protected override void OnStartup(StartupEventArgs e) { PresentationTraceSources.Refresh(); PresentationTraceSources.DataBindingSource.Listeners.Add(new ConsoleTraceListener()); PresentationTraceSources.DataBindingSource.Listeners.Add(new DebugTraceListener()); PresentationTraceSources.DataBindingSource.Switch.Level = SourceLevels.Warning | SourceLevels.Error; base.OnStartup(e); } } public class DebugTraceListener : TraceListener { public override void Write(string message) { // Write your log here. } public override void WriteLine(string message) { // Write your log here. } }
场景三(使用ValueConverter来调试)
场景一和场景二中的方法解决因拼写错误或者无明确DataContext时非常有效。不过有时候真正的通过VS进行调试一下,更加直观,便捷。可以使用实现一个简单的调试使用的Converter,然后将其Binding到目标上,
public class DebugConverter : IValueConverter { public object Convert(object value, Type targetType, object parameter, CultureInfo culture) { Debugger.Break(); return value; } public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) { Debugger.Break(); return value; } }
Debugger.Break()的效果和VS中F9设置断点是一样的。
XAML
<Window x:Class="WpfBindingDebug.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:local="clr-namespace:WpfBindingDebug" mc:Ignorable="d" Title="MainWindow" Height="350" Width="525"> <Window.Resources> <local:DebugConverter x:Key="DebugConverter" /> </Window.Resources> <StackPanel> <TextBlock Text="{Binding Id, Converter={StaticResource DebugConverter}}" /> </StackPanel> </Window>
感谢您的阅读~如果您有其他关于Data Binding的调试方式,欢迎在评论区指出~