设置wpf绑定的跟踪级别为high，output中观察wpf的处理流程：
- 对于一个无效的绑定，wpf尝试了5次，最后一次设置成了默认值。
- 所以在程序中要尽量避免无效绑定
<TextBlock Text="{Binding Title, PresentationTraceSources.TraceLevel=High}" />
-------------------------------------------------------------------------------------------------------
System.Windows.Data Warning: 56 : Created BindingExpression (hash=66394201) for Binding (hash=49086616)
System.Windows.Data Warning: 58 : Path: 'Title'
System.Windows.Data Warning: 60 : BindingExpression (hash=66394201): Default mode resolved to OneWay
System.Windows.Data Warning: 61 : BindingExpression (hash=66394201): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=66394201): Attach to System.Windows.Controls.TextBlock.Text (hash=41050810)
// 1
System.Windows.Data Warning: 67 : BindingExpression (hash=66394201): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=66394201): Found data context element: TextBlock (hash=41050810) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=66394201): DataContext is null
// 延迟重试
System.Windows.Data Warning: 65 : BindingExpression (hash=66394201): Resolve source deferred
// 2
System.Windows.Data Warning: 67 : BindingExpression (hash=66394201): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=66394201): Found data context element: TextBlock (hash=41050810) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=66394201): DataContext is null
// 3
System.Windows.Data Warning: 67 : BindingExpression (hash=66394201): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=66394201): Found data context element: TextBlock (hash=41050810) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=66394201): DataContext is null
// 4
System.Windows.Data Warning: 67 : BindingExpression (hash=66394201): Resolving source
System.Windows.Data Warning: 70 : BindingExpression (hash=66394201): Found data context element: TextBlock (hash=41050810) (OK)
System.Windows.Data Warning: 71 : BindingExpression (hash=66394201): DataContext is null
// 5 - last chance
System.Windows.Data Warning: 67 : BindingExpression (hash=66394201): Resolving source (last chance)
System.Windows.Data Warning: 70 : BindingExpression (hash=66394201): Found data context element: TextBlock (hash=41050810) (OK)
System.Windows.Data Warning: 78 : BindingExpression (hash=66394201): Activate with root item <null>
System.Windows.Data Warning: 106 : BindingExpression (hash=66394201): Item at level 0 is null - no accessor
System.Windows.Data Warning: 80 : BindingExpression (hash=66394201): TransferValue - got raw value {DependencyProperty.UnsetValue}
System.Windows.Data Warning: 88 : BindingExpression (hash=66394201): TransferValue - using fallback/default value ''
System.Windows.Data Warning: 89 : BindingExpression (hash=66394201): TransferValue - using final value ''
