当数据绑定(返回源)在Xamarin表单中发生时，如何更改

Question

我的页面中有多个Entry视图使用数据绑定连接到视图模型。除了我需要在用户输入时计算一个值之外，一切都按预期进行

public decimal DownPayment
{
get => this.loanValues.DownPayment;
set
{
this.loanValues.DownPayment = value;
this.CalculateValues();
}
}
public decimal TradeInPrice
{
get => this.loanValues.TradeInPrice;
set
{
this.loanValues.TradeInPrice = value;
this.CalculateValues();
}
}
// Other properties

现在的问题是，每当用户输入值时，都会设置该属性。是否要更改数据绑定发生的事件(即在Completed而不是TextChanged上(？

我看到XAML绑定中有UpdateSourceEventName属性，但我不知道为什么它不起作用：

<Entry Placeholder="Down Payment ($)" Text="{Binding DownPayment, UpdateSourceEventName=Completed}" />

更新：UpdateSourceEventName的文档称其供Xamarin.Forms平台内部使用

Answer 1

您可以使用EventToCommandBehavior将条目的事件完成转换为命令并处理ViewModel中的逻辑。

using System;
using System.Collections.Generic;
using System.Text;
using Xamarin.Forms;
namespace App18
{
public class BehaviorBase<T> : Behavior<T> where T : BindableObject
{
public T AssociatedObject { get; private set; }
protected override void OnAttachedTo(T bindable)
{
base.OnAttachedTo(bindable);
AssociatedObject = bindable;
if (bindable.BindingContext != null)
{
BindingContext = bindable.BindingContext;
}
bindable.BindingContextChanged += OnBindingContextChanged;
}
protected override void OnDetachingFrom(T bindable)
{
base.OnDetachingFrom(bindable);
bindable.BindingContextChanged -= OnBindingContextChanged;
AssociatedObject = null;
}
void OnBindingContextChanged(object sender, EventArgs e)
{
OnBindingContextChanged();
}
protected override void OnBindingContextChanged()
{
base.OnBindingContextChanged();
BindingContext = AssociatedObject.BindingContext;
}
}
}

using System;
using System.Collections.Generic;
using System.Reflection;
using System.Text;
using System.Windows.Input;
using Xamarin.Forms;
namespace App18
{
public class EventToCommandBehavior : BehaviorBase<View>
{
Delegate eventHandler;
public static readonly BindableProperty EventNameProperty = BindableProperty.Create("EventName", typeof(string), typeof(EventToCommandBehavior), null, propertyChanged: OnEventNameChanged);
public static readonly BindableProperty CommandProperty = BindableProperty.Create("Command", typeof(ICommand), typeof(EventToCommandBehavior), null);
public static readonly BindableProperty CommandParameterProperty = BindableProperty.Create("CommandParameter", typeof(object), typeof(EventToCommandBehavior), null);
public static readonly BindableProperty InputConverterProperty = BindableProperty.Create("Converter", typeof(IValueConverter), typeof(EventToCommandBehavior), null);
public string EventName
{
get { return (string)GetValue(EventNameProperty); }
set { SetValue(EventNameProperty, value); }
}
public ICommand Command
{
get { return (ICommand)GetValue(CommandProperty); }
set { SetValue(CommandProperty, value); }
}
public object CommandParameter
{
get { return GetValue(CommandParameterProperty); }
set { SetValue(CommandParameterProperty, value); }
}
public IValueConverter Converter
{
get { return (IValueConverter)GetValue(InputConverterProperty); }
set { SetValue(InputConverterProperty, value); }
}
protected override void OnAttachedTo(View bindable)
{
base.OnAttachedTo(bindable);
RegisterEvent(EventName);
}
protected override void OnDetachingFrom(View bindable)
{
DeregisterEvent(EventName);
base.OnDetachingFrom(bindable);
}
void RegisterEvent(string name)
{
if (string.IsNullOrWhiteSpace(name))
{
return;
}
EventInfo eventInfo = AssociatedObject.GetType().GetRuntimeEvent(name);
if (eventInfo == null)
{
throw new ArgumentException(string.Format("EventToCommandBehavior: Can't register the '{0}' event.", EventName));
}
MethodInfo methodInfo = typeof(EventToCommandBehavior).GetTypeInfo().GetDeclaredMethod("OnEvent");
eventHandler = methodInfo.CreateDelegate(eventInfo.EventHandlerType, this);
eventInfo.AddEventHandler(AssociatedObject, eventHandler);
}
void DeregisterEvent(string name)
{
if (string.IsNullOrWhiteSpace(name))
{
return;
}
if (eventHandler == null)
{
return;
}
EventInfo eventInfo = AssociatedObject.GetType().GetRuntimeEvent(name);
if (eventInfo == null)
{
throw new ArgumentException(string.Format("EventToCommandBehavior: Can't de-register the '{0}' event.", EventName));
}
eventInfo.RemoveEventHandler(AssociatedObject, eventHandler);
eventHandler = null;
}
void OnEvent(object sender, object eventArgs)
{
if (Command == null)
{
return;
}
object resolvedParameter;
if (CommandParameter != null)
{
resolvedParameter = CommandParameter;
}
else if (Converter != null)
{
resolvedParameter = Converter.Convert(eventArgs, typeof(object), null, null);
}
else
{
resolvedParameter = eventArgs;
}
if (Command.CanExecute(resolvedParameter))
{
Command.Execute(resolvedParameter);
}
}
static void OnEventNameChanged(BindableObject bindable, object oldValue, object newValue)
{
var behavior = (EventToCommandBehavior)bindable;
if (behavior.AssociatedObject == null)
{
return;
}
string oldEventName = (string)oldValue;
string newEventName = (string)newValue;
behavior.DeregisterEvent(oldEventName);
behavior.RegisterEvent(newEventName);
}
}
}

在xaml中

</StackLayout>
<Entry WidthRequest="100" Text="{Binding Value1,Mode=OneWayToSource}"  Keyboard="Numeric"  >
<Entry.Behaviors>
<local:EventToCommandBehavior EventName="Unfocused" Command="{Binding CompletedCommand}"  />
</Entry.Behaviors>
</Entry>

<Entry WidthRequest="100" Text="{Binding Value2,Mode=OneWayToSource}" Keyboard="Numeric">
<Entry.Behaviors>
<local:EventToCommandBehavior EventName="Unfocused" Command="{Binding CompletedCommand}"  />
</Entry.Behaviors>
</Entry>

<Label WidthRequest="100" Text="{Binding Sum}"  TextColor="Black" />
</StackLayout>

在ViewModel中

public class MyViewModel : INotifyPropertyChanged
{

public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
double value1;
public double Value1
{
get
{
return value1;
}
set
{
if (value1 != value)
{
value1 = value;
NotifyPropertyChanged("Value1");
}
}
}
double value2;
public double Value2
{
get
{
return value2;
}
set
{
if (value2 != value)
{
value2 = value;
NotifyPropertyChanged("Value2");
}
}
}


private string sum;
public string Sum
{
get { return sum; }
set
{
if (sum != value)
{
sum = value;
NotifyPropertyChanged("Sum");
}
}
}
public ICommand CompletedCommand { get; set; }
public MyViewModel()
{
CompletedCommand = new Command(()=> {
Sum = (Value1 + Value2).ToString();

});
}

}