在Application.Resources中循环引用的IValueConverter [英] Circular referenced IValueConverter in Application.Resources
问题描述
我遇到了错误:
StaticResource not found for key maxLength
设置如下:
app.xaml中的转换器设置，其中还包含一个数据模板
Converter setup in app.xaml, which also contains a datatemplate
<Application.Resources>
<ResourceDictionary>
<ext:MaxLengthStringConverter x:Key="maxLength"/>
....
<DataTemplate x:Key="HotelViewModel">
<tripSegmentPartViews:HotelView
Padding="0"
HeightRequest="60"
BorderWidth="1"
BorderColor="{ext:ColourResource Divider}"
BordersToDraw="{x:Static controls:Borders.Top}"
BackgroundColor="Transparent"/>
</DataTemplate>
....
数据模板中HotelView.xaml中的
视图使用转换器
view in the HotelView.xaml which is in the datatemplate, uses the converter
....
<Label Text="{Binding HotelName, Converter={StaticResource maxLength}, ConverterParameter=10}"
AbsoluteLayout.LayoutFlags="All"
AbsoluteLayout.LayoutBounds="1,0.2,0.62,0.5"
VerticalOptions="End"
HorizontalOptions="Start"
FontSize="20"
/>
....
如果我将转换器移至HotelView.xaml资源字典，它将起作用
If I move the converter to HotelView.xaml resource dictionary it works
如果我更改对DynamicResource的引用，则不会使用
If I change the reference to a DynamicResource it is not used
很明显，它具有最大长度的基本要求(它会缩短字符串并在所需的长度范围内添加'...')，我希望能够在整个应用程序中使用它，而不必在其中引用它多个资源词典.
Obviously with something as basic as max length (which shortens the string and adds '...' if its over the required length) I want to be able to use it through out the application, and not have to reference it in multiple resource dictionaries.
这是一个错误吗?
-----------------编辑------------------
----------------- edit ------------------
好吧，我用一个包含以下内容的最小应用程序重现了此错误消息:
OK I have reproduced this errror with a minimum app consisting of:
App1.xaml
App1.xaml
<?xml version="1.0" encoding="utf-8" ?>
<Application xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="App1.App"
xmlns:ext="clr-namespace:App1.Extensions;assembly=App1"
xmlns:local="clr-namespace:App1;assembly=App1">
<Application.Resources>
<ResourceDictionary>
<ext:MyConverter x:Key="conv"></ext:MyConverter>
<DataTemplate x:Key="dt">
<local:View1></local:View1>
</DataTemplate>
</ResourceDictionary>
</Application.Resources>
</Application>
Page1.xaml
Page1.xaml
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="App1.Page1"
xmlns:local="clr-namespace:App1;assembly=App1">
<StackLayout>
<Label Text="Page1" VerticalOptions="Center" HorizontalOptions="Center" TextColor="White" />
<ListView ItemTemplate="{StaticResource dt}" ItemsSource="List">
</ListView>
</StackLayout>
</ContentPage>
View1.xaml
View1.xaml
<?xml version="1.0" encoding="utf-8" ?>
<ContentView xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="App1.View1"
xmlns:ext="App1.Extensions">
<Label Text="{Binding MainText, Converter={StaticResource conv}" VerticalOptions="Center" HorizontalOptions="Center" TextColor="White"/>
</ContentView>
App1.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Xamarin.Forms;
namespace App1
{
public partial class App : Application
{
public App()
{
InitializeComponent();
// The root page of your application
Page1 p = new Page1();
p.BindingContext = new {
MainText = "test",
List = new List<string>() { "test"}
};
var navContainer = new NavigationPage(p);
navContainer.BarBackgroundColor = Color.Red;
navContainer.BarTextColor = Color.White;
MainPage = navContainer;
}
}
}
使用VS仿真器在运行时引发错误
The error is thrown at runtime using the VS emulator
推荐答案
根据您的描述，您希望能够一次定义一个IValueConverter，并在任何Xamarin.Forms ContentPage中使用它，而无需使用以便继续在本地XAML页中指定转换器.
From what you mention you want to be able to define a IValueConverter the once, and use it from any Xamarin.Forms ContentPage, without the need to keep specifying the converter in the local XAML page.
这可以通过执行以下操作来实现:-
This can be achieved by doing the following:-
在您的PCL中，通常有App.cs.
您将需要删除它，并添加一个名为App.cs的新Forms Xaml Page.
You will need to delete this, and add a new Forms Xaml Page called App.cs.
这将同时生成App.xaml和相关的App.cs文件.
This will generate both the App.xaml and related App.cs files.
在此问题中，(如何对图像进行数据绑定?)，有一个名为MyByteToImageSourceConverter的转换器.
In this question, (How can I databind an image?), there is a converter called MyByteToImageSourceConverter.
我将举例说明:-
App.xaml
<?xml version="1.0" encoding="utf-8" ?>
<Application xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="SampleApp.App"
xmlns:local="clr-namespace:{namespace reference goes here to the converter}"
>
<Application.Resources>
<ResourceDictionary>
<local:MyByteToImageSourceConverter x:Key="kyByteToImageSourceConverter"/>
</ResourceDictionary>
</Application.Resources>
</Application>
因此，在上面我们用键定义了转换器，然后我们就可以从 all 其他Xamarin.Forms ContentPage引用.
So in the above we have defined our converter with a key, that we will then be able to reference from all other Xamarin.Forms ContentPage's.
App.cs
namespace SampleApp
{
public partial class App
: Xamarin.Forms.Application
{
public App()
{
InitializeComponent();
//
this.MainPage = new ByteToImageExample2();
}
}
}
在code-behind中，我们需要从ContentPage更改默认继承并指定Xamarin.Forms.Application.
In the code-behind we need to change the default inheritance from ContentPage and specify Xamarin.Forms.Application.
我们还通过this.MainPage = ...
ByteToImageExample2.xaml :-
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="SampleApp.ByteToImageExample2"
>
<StackLayout>
<Image Source="{Binding MyImageAsBytes, Converter={StaticResource kyByteToImageSourceConverter}}"/>
</StackLayout>
</ContentPage>
在我们的ContentPage中，我们可以在上面看到通过StaticResource kyByteToImageSourceConverter引用在App.xaml中指定的转换器.
In our ContentPage we can see above we are referencing the converter that we specified in App.xaml via the StaticResource kyByteToImageSourceConverter.
正如在App.xaml中定义的那样，我们可以在所有页面中重复使用此功能，而无需在本地指定转换器的位置.
As that was definied in App.xaml, we can re-use this in all our pages without the need to specify the location of the converter locally.
为完整起见，其背后的代码是:-
For completeness the code-behind is:-
ByteToImageExample2.cs :-
public partial class ByteToImageExample2 : ContentPage
{
public ByteToImageExample2()
{
InitializeComponent();
//
byte[] bytImage = { your image as a byte collection }
//
this.BindingContext = new MyImageViewModel()
{
MyImageAsBytes = bytImage
};
}
}
更新1:-
您可以在App.xaml中添加以下内容:-
You can have the following in your App.xaml:-
<DataTemplate x:Key="kyByteToImage3ExampleDataTemplate2">
<ViewCell>
<local2:MyCustomView1/>
</ViewCell>
</DataTemplate>
，其中local2:MyCustomView1引用您的自定义视图，在此示例中，该视图定义为:-
with local2:MyCustomView1 referencing your custom view, which for this example is defined as:-
<ContentView xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="SampleApp.MyCustomView1">
<Image Source="{Binding MyImage, Converter={StaticResource kyByteToImageSourceConverter}}" Aspect="AspectFit" />
</ContentView>
此自定义视图仍使用App.xaml中指定的Converter，并且仍然呈现而无需在ContentView类中本地指定Converter的需要.
This custom view still uses a Converter as specified from the App.xaml and does still render, without the need to specify the Converter locally within the ContentView class.
这篇关于在Application.Resources中循环引用的IValueConverter的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
