Question

我正在尝试使用MvxValueConverter根据布尔值设置LinearLayout的背景颜色。转换器看起来像这样：

public class BackgroundColorValueConverter : MvxValueConverter<bool, MvxColor>
{
    private static readonly MvxColor TrueBGColor = new MvxColor(0xDB, 0xFF, 0xCE);
    private static readonly MvxColor FalseBGColor = new MvxColor(0xD6, 0xF6, 0xFF);

    protected override MvxColor Convert(bool value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value ? TrueBGColor : FalseBGColor;
    }
}

在我的AXML布局中，我有以下代码：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:local="http://schemas.android.com/apk/res-auto"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    local:MvxBind="BackgroundColor MyBooleanValue, Converter=BackgroundColor">
  <TextView
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:gravity="center"
      android:textSize="18dp"
      local:MvxBind="Text MyText" />
</LinearLayout>

我收到以下错误：

Failed to create target binding for binding BackgroundColor for MyBooleanValue

完整的错误跟踪如下：

MvxBind:Error:  8.58 Problem seen during binding execution for binding BackgroundColor for MyBooleanValue - problem InvalidCastException: Cannot cast from source type to destination type.
03-05 14:18:46.434 I/mono-stdout(16474): MvxBind:Error:  8.58 Problem seen during binding execution for binding BackgroundColor for MyBooleanValue - problem InvalidCastException: Cannot cast from source type to destination type.
03-05 14:18:46.434 I/mono-stdout(16474):      at Cirrious.MvvmCross.Plugins.Color.Droid.BindingTargets.MvxViewBackgroundColorBinding.SetValueImpl (System.Object target, System.Object value) [0x00000] in <filename unknown>:0 
      at Cirrious.MvvmCross.Plugins.Color.Droid.BindingTargets.MvxViewBackgroundColorBinding.SetValueImpl (System.Object target, System.Object value) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Binding.Bindings.Target.MvxConvertingTargetBinding.SetValue (System.Object value) [0x00000] in <filename unknown>:0 
03-05 14:18:46.434 I/mono-stdout(16474):   at Cirrious.MvvmCross.Binding.Bindings.Target.MvxConvertingTargetBinding.SetValue (System.Object value) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Binding.Bindings.MvxFullBinding.UpdateTargetFromSource (System.Object value) [0x00000] in <filename unknown>:0

所以，老实说我不知道从哪里开始。我正在尝试的是什么？我使用正确的MvvmCross转换器吗？任何指针都会非常感激。

更新

将转换器更改为：

public class BackgroundColorValueConverter : MvxColorValueConverter
{
    private static readonly MvxColor TrueBGColor = new MvxColor(0xDB, 0xFF, 0xCE);
    private static readonly MvxColor FalseBGColor = new MvxColor(0xD6, 0xF6, 0xFF);

    protected override MvxColor Convert(object value, object parameter, System.Globalization.CultureInfo culture)
    {
        return (bool)value ? TrueBGColor : FalseBGColor;
    }
}

...解决了我的问题。我的TextColor MyBooleanValue, Converter=TextColor上也有LinearLayout，其功能与BackgroundColorValueConverter类似，我收到的错误是无法创建目标绑定。

我将AXML更改为：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:local="http://schemas.android.com/apk/res-auto"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    local:MvxBind="BackgroundColor MyBooleanValue, Converter=BackgroundColor">
  <TextView
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:gravity="center"
      android:textSize="18dp"
      local:MvxBind="Text MyText; TextColor MyBooleanValue, Converter=TextColor" />
</LinearLayout>

......一切按预期工作。对于任何碰巧在将来遇到这种情况的人：不要试图在TextColor上绑定LinearLayout，因为它不能像那样工作！

Answer 1

https://github.com/MvvmCross/MvvmCross-Tutorials/blob/master/ValueConversion/ValueConversion.UI.Droid/Resources/Layout/View_Colors.axml

中有BackgroundColor绑定的工作示例

这使用https://github.com/MvvmCross/MvvmCross/blob/v3.1/Plugins/Cirrious/Color/Cirrious.MvvmCross.Plugins.Color.Droid/BindingTargets/MvxViewBackgroundColorBinding.cs

中的BackgroundColor绑定

此示例是否适合您？

如果是，您能否发现该样本与您正在使用的样本之间的差异？ Color插件有问题吗？（它是否在您的UI项目中加载？）LinearLayout与TextView的问题是什么？您可以提供更多错误跟踪吗？您提供的一行跟踪是在https://github.com/MvvmCross/MvvmCross/blob/1ec7bc5f0307595c7ae11f56727dd0e9d2a2262f/Cirrious/Cirrious.MvvmCross.Binding/Bindings/MvxFullBinding.cs#L139上创建的 - 但在该行之前通常还有其他跟踪。

如果不是，那就令人担心，因为这意味着它是一个普遍的错误......

更新：（提供更多信息后）

我认为问题在于你的ValueConverter - 要使用Android，你的ValueConverter必须以Native类型结束 - 而不是与平台无关的MvxColor。您看到的错误是无效的强制转换异常 - 因为绑定正试图将您的MvxColor强制转换为https://github.com/MvvmCross/MvvmCross/blob/v3.1/Plugins/Cirrious/Color/Cirrious.MvvmCross.Plugins.Color.Droid/BindingTargets/MvxViewBackgroundColorBinding.cs#L25中的Android.Graphics.Color

要转换为Native，您可以使用MvxColorValueConverter基类 - 请参阅https://github.com/MvvmCross/MvvmCross/blob/v3.1/Plugins/Cirrious/Color/Cirrious.MvvmCross.Plugins.Color/MvxColorValueConverter.cs

其中一个例子是https://github.com/MvvmCross/MvvmCross-Tutorials/blob/master/ValueConversion/ValueConversion.Core/Converters/Converters.cs#L35

public class ContrastColorConverter : MvxColorValueConverter
{
    protected override MvxColor Convert(object value, object parameter, CultureInfo culture)
    {
        var input = (MvxColor) value;
        var brightnessToUse = SimpleContrast(input.R, input.G, input.B);
        return new MvxColor(brightnessToUse, brightnessToUse, brightnessToUse);
    }

    private static int SimpleContrast(params int[] value)
    {
        // this is only a very simple contrast method
        // for more advanced methods you need to look at HSV-type approaches

        int max = 0;
        foreach (var v in value)
        {
            if (v > max)
                max = v;
        }

        return 255 - max;
    }
}

https://github.com/MvvmCross/MvvmCross/wiki/Value-Converters#wiki-the-mvx-color-valueconverters

中有一些关于颜色转换器的文档

Answer 2

你在你的方法上返回一个布尔值，它期待一个MvxColor对象。

    protected override MvxColor Convert(object value, object parameter, System.Globalization.CultureInfo culture)
{
    (bool)value ? return TrueBGColor : return FalseBGColor;
}

Answer 3

面临同样的错误＆＃34;无法创建目标绑定...＆＃34;，只需将转换器从core-PCL项目下的文件夹移动到Droid项目下的新文件夹就可以了解我:)

通过布尔值设置LinearLayout背景颜色

3 个答案: