WPF：如果 UI 元素不可见，则停止绑定

Posted 2023-02-21

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【中文标题】WPF：如果 UI 元素不可见，则停止绑定【英文标题】：WPF: Stop Binding if a UI element is not visible 【发布时间】：2011-01-15 01:49:15 【问题描述】：

如果元素当前不可见，我可以延迟绑定 ui 元素吗？有时我有一个包含一些隐藏/最小化元素的表单，如果它们不在屏幕上，我不想更新它们。我怀疑答案是否定的，但问也无妨？

【问题讨论】：

你为什么要这样做？为了表现？是的性能，它实际上是懒惰（懒惰是发明之母），因为如果它们不可见，我应该从树中删除它们，以获得我需要的性能。 【参考方案1】：

答案是否定的，因为绑定可能导致元素再次可见。因此，如果绑定在隐藏控件上不起作用，它将不允许绑定再次使其可见。

【讨论】：

【参考方案2】：

我知道这是一个老问题，但由于我未能找到已实现的类或其他东西，我自己按照@Nir 的回答做了。

这是一个标记扩展，它将普通绑定包装为仅在对象IsVisible 属性第一次变为真时才真正绑定：

using System;
using System.ComponentModel;
using System.Globalization;
using System.Windows;
using System.Windows.Data;
using System.Windows.Markup;

namespace MakupExtensions 
    [MarkupExtensionReturnType(typeof(object))]
    public class LazyBindingExtension : MarkupExtension 
        public LazyBindingExtension() 
        
        public LazyBindingExtension(PropertyPath path) : this() 
            Path = path;
        

        public IValueConverter Converter 
            get;
            set;
        
        [TypeConverter(typeof(CultureInfoIetfLanguageTagConverter))]
        public CultureInfo ConverterCulture 
            get;
            set;
        
        public object ConverterParamter 
            get;
            set;
        
        public string ElementName 
            get;
            set;
        
        [ConstructorArgument("path")]
        public PropertyPath Path 
            get;
            set;
        
        public RelativeSource RelativeSource 
            get;
            set;
        
        public object Source 
            get;
            set;
        
        public UpdateSourceTrigger UpdateSourceTrigger 
            get;
            set;
        
        public bool ValidatesOnDataErrors 
            get;
            set;
        
        public bool ValidatesOnExceptions 
            get;
            set;
        
        public bool ValidatesOnNotifyDataErrors 
            get;
            set;
        

        private Binding binding;
        private DependencyObject bindingTarget;
        private DependencyProperty bindingTargetProperty;

        public override object ProvideValue(IServiceProvider serviceProvider) 
            var valueProvider = serviceProvider.GetService(typeof(IProvideValueTarget)) as IProvideValueTarget;
            if (valueProvider != null) 
                bindingTarget = valueProvider.TargetObject as DependencyObject;
                bindingTargetProperty = valueProvider.TargetProperty as DependencyProperty;
                if (bindingTargetProperty == null || bindingTarget == null) 
                    throw new NotSupportedException($"The property 'valueProvider.TargetProperty' on target 'valueProvider.TargetObject' is not valid for a LazyBinding. The LazyBinding target must be a DependencyObject, and the target property must be a DependencyProperty.");
                
                binding = new Binding 
                    Path = Path,
                    Converter = Converter,
                    ConverterCulture = ConverterCulture,
                    ConverterParameter = ConverterParamter
                ;
                if (ElementName != null) 
                    binding.ElementName = ElementName;
                
                if (RelativeSource != null) 
                    binding.RelativeSource = RelativeSource;
                
                if (Source != null) 
                    binding.Source = Source;
                
                binding.UpdateSourceTrigger = UpdateSourceTrigger;
                binding.ValidatesOnDataErrors = ValidatesOnDataErrors;
                binding.ValidatesOnExceptions = ValidatesOnExceptions;
                binding.ValidatesOnNotifyDataErrors = ValidatesOnNotifyDataErrors;
                return SetBinding();
            
            return null;
        
        public object SetBinding() 
            var uiElement = bindingTarget as UIElement;
            if (uiElement != null && !uiElement.IsVisible) 
                uiElement.IsVisibleChanged += UiElement_IsVisibleChanged;
            
            else 
                ConsolidateBinding();
            
            return bindingTarget.GetValue(bindingTargetProperty);
        
        private void ConsolidateBinding() => BindingOperations.SetBinding(bindingTarget, bindingTargetProperty, binding);
        private void UiElement_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e) 
            var uiElement = sender as UIElement;
            if (uiElement != null && uiElement.IsVisible) 
                uiElement.IsVisibleChanged -= UiElement_IsVisibleChanged;
                ConsolidateBinding();

使用方法：

<ItemsControl ItemsSource="mx:LazyBinding Documents"/>

在这个例子中，它只会在 ItemsControl IsVisible 第一次变为真时绑定。

IsVisible 再次变为 false 时不会解除绑定，但我认为有人可以根据需要更改它。

【讨论】：

【参考方案3】：

没有内置的方法可以做到这一点 - 但您可以自己编写。

诀窍是将绑定包装在您自己的标记扩展中，该扩展使用原始绑定但在其周围添加新行为（例如，当您不希望绑定起作用时，通过将 UpdateSourceTrigger 设置为 Explicit。

这是一个示例（延迟绑定的数据传输）：

http://www.paulstovell.com/wpf-delaybinding

现在，有很多可能的边缘条件禁用不可见控件的绑定，尤其是在显示和隐藏控件方面，所以我不会为此编写通用扩展 - 但也许在您的特定应用程序中这可能很有用。

【讨论】：

这很奇怪，我写了一个类似的东西 - codeproject.com/KB/WPF/DelayedBindingTextBox.aspx【参考方案4】：

对于一种解决方法，我绑定了对象的可见性，当对象设置为可见时，该属性会触发其后面的元素的构造，该元素通过ContentPresenter 进行绑定。

【讨论】：

【参考方案5】：

改进的 MarkupExtension 将普通绑定包装到自动绑定/取消绑定数据模型（如果可见已更改）。参见之前的版本here。

using System;
using System.ComponentModel;
using System.Globalization;
using System.Windows;
using System.Windows.Data;
using System.Windows.Markup;

namespace UtilsWPF

    [MarkupExtensionReturnType(typeof(object))]
    public class LazyBindingExtension : MarkupExtension
    
        public LazyBindingExtension()
         

        public LazyBindingExtension(PropertyPath path) : this()
        
            Path = path;
        

        #region Properties

        public IValueConverter Converter  get; set; 
        [TypeConverter(typeof(CultureInfoIetfLanguageTagConverter))]
        public CultureInfo ConverterCulture  get; set; 
        public object ConverterParamter  get; set; 
        public string ElementName  get; set; 
        [ConstructorArgument("path")]
        public PropertyPath Path  get; set; 
        public RelativeSource RelativeSource  get; set; 
        public object Source  get; set; 
        public UpdateSourceTrigger UpdateSourceTrigger  get; set; 
        public bool ValidatesOnDataErrors  get; set; 
        public bool ValidatesOnExceptions  get; set; 
        public bool ValidatesOnNotifyDataErrors  get; set; 

        private Binding binding;
        private UIElement bindingTarget;
        private DependencyProperty bindingTargetProperty;

        #endregion

        #region Init

        public override object ProvideValue(IServiceProvider serviceProvider)
        
            var valueProvider = serviceProvider.GetService(typeof(IProvideValueTarget)) as IProvideValueTarget;
            if (valueProvider != null)
            
                bindingTarget = valueProvider.TargetObject as UIElement;

                if (bindingTarget == null)
                
                    throw new NotSupportedException($"Target 'valueProvider.TargetObject' is not valid for a LazyBinding. The LazyBinding target must be a UIElement.");
                

                bindingTargetProperty = valueProvider.TargetProperty as DependencyProperty;

                if (bindingTargetProperty == null)
                
                    throw new NotSupportedException($"The property 'valueProvider.TargetProperty' is not valid for a LazyBinding. The LazyBinding target property must be a DependencyProperty.");
                

                binding = new Binding
                
                    Path = Path,
                    Converter = Converter,
                    ConverterCulture = ConverterCulture,
                    ConverterParameter = ConverterParamter
                ;

                if (ElementName != null)
                
                    binding.ElementName = ElementName;
                

                if (RelativeSource != null)
                
                    binding.RelativeSource = RelativeSource;
                

                if (Source != null)
                
                    binding.Source = Source;
                

                binding.UpdateSourceTrigger = UpdateSourceTrigger;
                binding.ValidatesOnDataErrors = ValidatesOnDataErrors;
                binding.ValidatesOnExceptions = ValidatesOnExceptions;
                binding.ValidatesOnNotifyDataErrors = ValidatesOnNotifyDataErrors;

                return SetBinding();
            

            return null;
        

        public object SetBinding()
        
            bindingTarget.IsVisibleChanged += UiElement_IsVisibleChanged;

            updateBinding();

            return bindingTarget.GetValue(bindingTargetProperty);
        

        #endregion

        #region Event Handlers

        private void UiElement_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
        
            updateBinding();
        

        #endregion

        #region Update Binding

        private void updateBinding()
        
            if (bindingTarget.IsVisible)
            
                ConsolidateBinding();
            
            else
            
                ClearBinding();
            
        

        private bool _isBind;

        private void ConsolidateBinding()
        
            if (_isBind)
            
                return;
            

            _isBind = true;

            BindingOperations.SetBinding(bindingTarget, bindingTargetProperty, binding);
        

        private void ClearBinding()
        
            if (!_isBind)
            
                return;
            

            BindingOperations.ClearBinding(bindingTarget, bindingTargetProperty);

            _isBind = false;
        

        #endregion

使用方法：

<ItemsControl ItemsSource="utils:LazyBinding Documents"/>

【讨论】：

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