iOS UIFont.ByName异常(值不能为null.参数名称:value) [英] iOS UIFont.ByName Exception (Value cannot be null. Parameter name: value)

问题描述

在自定义Xamarin Forms iOS渲染器中，我正在从文件中加载字体，当我进入UIFont.FromName时，它将显示以下异常.字体var是有效的CGFont实例，字体大小是有效的nfloat.有什么想法吗?

In a custom Xamarin Forms iOS renderer I am loading fonts from files and when I get to UIFont.FromName it thorws the exception shown below. The font var is a valid CGFont instance and fontsize is a valid nfloat. Any ideas?

var font = CGFont.CreateFromProvider(fontDataProvider);
Control.Font = UIFont.FromName(font.FullName, (nfloat)e.NewElement.FontSize);

stacktrace:

stacktrace:

System.ArgumentNullException: Value cannot be null.
Parameter name: value
  at UIKit.UILabel.set_Font (UIKit.UIFont value) [0x00011] in /Users/builder/data/lanes/1381/3afb4af5/source/maccore/src/build/ios/native/UIKit/UILabel.g.cs:337 
  at DecryptRenderFont.iOS.DecryptRenderControlRenderer.OnElementChanged (Xamarin.Forms.Platform.iOS.ElementChangedEventArgs`1[TElement] e) [0x00215] 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].SetElement (TElement element) [0x00118] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].Xamarin.Forms.Platform.iOS.IVisualElementRenderer.SetElement (Xamarin.Forms.VisualElement element) [0x00000] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.Platform.CreateRenderer (Xamarin.Forms.VisualElement element) [0x0001b] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.OnChildAdded (Xamarin.Forms.VisualElement view) [0x00000] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.Load () [0x00023] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].SetElement (TElement element) [0x000cc] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].Xamarin.Forms.Platform.iOS.IVisualElementRenderer.SetElement (Xamarin.Forms.VisualElement element) [0x00000] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.Platform.CreateRenderer (Xamarin.Forms.VisualElement element) [0x0001b] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.OnChildAdded (Xamarin.Forms.VisualElement view) [0x00000] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.Load () [0x00023] in <filename unknown>:0 
  at Xamarin.Forms.Platform.iOS.PageRenderer.ViewDidLoad () [0x0007d] in <filename unknown>:0 
  at (wrapper managed-to-native) ObjCRuntime.Messaging:IntPtr_objc_msgSendSuper (intptr,intptr)
  at UIKit.UIViewController.get_View () [0x00030] in /Users/builder/data/lanes/1381/3afb4af5/source/maccore/src/build/ios/native/UIKit/UIViewController.g.cs:2667 

推荐答案

1)，我要假设，您看到的异常堆栈未显示实际的底层错误，因为它来自 Xamarin.iOS 生成的 UILabel.g.cs 文件(这是

1 ) I am going to assume the exception stack you are seeing is not showing the actual underlaying error as it is coming from the Xamarin.iOS generated UILabel.g.cs file (that is a auto-generated wrapper within https://github.com/xamarin/xamarin-macios) and I personally have had a few issues tracing certain low-level OS exceptions/throws and thus the shown exception is a little (a lot?) misleading.

2)再次，我将假设，因为您说您正在从文件中加载字体" ，即这些字体未注册.假设您是通过 CGFont.CreateFromProvider 从应用程序捆绑包(或动态下载)中的文件加载这些字体的，并且由于它们在 UIAppFonts 下的info.plist中不存在键，则它们不会自动注册到iOS.您需要自己执行此步骤:

2) Again I am going to assume since you said you are "loading fonts from files" that these fonts are not registered. Assuming you are loading these fonts via a CGFont.CreateFromProvider from files in your app bundle (or dynimically downloaded) and since they do not exist in the info.plist under the UIAppFonts key, they are not auto-registered to iOS. You need to do that step yourself:

NSError error;
if (!CTFontManager.RegisterGraphicsFont(customFont, out error))
{
   // error occurred, checkout the contents of the NSError var
}

---

在使用iOS字体管理器( CTFontManager )注册了CGFont之后，您实际上可以在 UIKit 中使用它们，否则内部在iOS代码中会出现异常...需要编写一些ObjC/Swift在Xcode中进行检查，以查看实际抛出的错误是什么...

---

After registering the CGFont with the iOS font manager (CTFontManager) you can actually use them within UIKit, otherwise internally there is an exception within iOS code... Would need to write some ObjC/Swift to check it out in Xcode to see what the actual error being thrown is...

在异常之前但在 CreateFromProvider 之后的某个位置，您可以执行以下操作:

Somewhere before the exception but after your CreateFromProvider, you can do the following:

foreach (var familyNames in UIFont.FamilyNames.OrderBy(c => c).ToList())
{
    D.WriteLine(" * " + familyNames);
    foreach (var familyName in UIFont.FontNamesForFamilyName(familyNames).OrderBy(c => c).ToList())
    {
        D.WriteLine(" *-- " + familyName);
    }
}

注意: D 只是使用D = System.Diagnostics.Debug;

我敢打赌，您的字体不在列表中，但是在调用 CTFontManager.RegisterGraphicsFont 后，它将在该列表中，并且可以被 UIKit 使用.

I would bet that your font is not in the list, but after a call to CTFontManager.RegisterGraphicsFont it will be in that list and valid to be consumed by UIKit.

有趣的注意事项是，使用基于 Swift的应用程序而不注册字体不会导致应用程序崩溃.当然不会显示该字体，但是通过 UIFont(name:"XXXXX"，size:35)引用它只是一个无声的失败.

Interesting note in that using a Swift-based app and not registering the font does not cause a app crash. The font is not shown of course, but referencing it via UIFont(name: "XXXXX", size: 35) is just a silent failure.

因此，我建议您在使用自定义字体时使用一些防御性的 try {} catch {} ，以确保安全.

So I'd just recommend some defensive try{} catch{} around any use of your custom font to be safe.

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