当链接器设置为 "Link All" 时，会发生 Xamarin 错误。无法使用依赖关系服务

Question

现在，由于UIWebView已弃用，我必须将链接器设置为"全部链接"才能提交到App Store。在执行此操作时，我不得不向我的数据存储添加[Preserve(AllMembers = true)]以使它们以这种方式工作，但后来我遇到了这个问题。

以下行将返回空值：

var dp = DependencyService.Get<ITextMeter>();

在研究解决方案后，似乎最好的答案是将以下行添加到应用程序委托中：

DependencyService.Register<ITextMeter, TextMeterImplementation>();

一旦我这样做了，我就开始收到这个异常：

DependencyService: System.MissingMethodException: Default constructor not found for [Interface]https://forums.xamarin.com/discussion/71072/dependencyservice-system-missingmethodexception-default-constructor-not-found-for-interface

我只想找到一个可行的解决方案，允许所有内容与设置为"全部链接"的链接器一起工作。提前谢谢。

ITextMeter：

using System;
using Xamarin.Forms.Internals;
namespace RedSwipe.Services
{
public interface ITextMeter
{
double MeasureTextSize(string text, double width, double fontSize, string fontName = null);
}
}

文本计量实现：

using System.Drawing;
using Foundation;
using RedSwipe.iOS.Services;
using RedSwipe.Services;
using UIKit;
[assembly: Xamarin.Forms.Dependency(typeof(TextMeterImplementation))]
namespace RedSwipe.iOS.Services
{
public class TextMeterImplementation : ITextMeter
{
public double MeasureTextSize(string text, double width, double fontSize, string fontName = null)
{
var nsText = new NSString(text);
var boundSize = new SizeF((float)width, float.MaxValue);
var options = NSStringDrawingOptions.UsesFontLeading | NSStringDrawingOptions.UsesLineFragmentOrigin;
if (fontName == null)
{
fontName = "HelveticaNeue";
}
var attributes = new UIStringAttributes
{
Font = UIFont.FromName(fontName, (float)fontSize)
};
var sizeF = nsText.GetBoundingRect(boundSize, options, attributes, null).Size;
//return new Xamarin.Forms.Size((double)sizeF.Width, (double)sizeF.Height);
return (double)sizeF.Height;
}
}
}

Answer 1

在类实现之前将此[Preserve (AllMembers = true)]添加到TextMeterImplementation中。

您的代码如下所示：

using System.Drawing;
using Foundation;
using RedSwipe.iOS.Services;
using RedSwipe.Services;
using UIKit;
using Xamarin.Forms.Internals; // Add This import
[assembly: Xamarin.Forms.Dependency(typeof(TextMeterImplementation))]
namespace RedSwipe.iOS.Services
{
[Preserve (AllMembers = true)]
public class TextMeterImplementation : ITextMeter
{
public double MeasureTextSize(string text, double width, double fontSize, string fontName = null)
{
var nsText = new NSString(text);
var boundSize = new SizeF((float)width, float.MaxValue);
var options = NSStringDrawingOptions.UsesFontLeading | NSStringDrawingOptions.UsesLineFragmentOrigin;
if (fontName == null)
{
fontName = "HelveticaNeue";
}
var attributes = new UIStringAttributes
{
Font = UIFont.FromName(fontName, (float)fontSize)
};
var sizeF = nsText.GetBoundingRect(boundSize, options, attributes, null).Size;
//return new Xamarin.Forms.Size((double)sizeF.Width, (double)sizeF.Height);
return (double)sizeF.Height;
}
}
}