Xamarin Forms CustomLabel crash in iOS - xamarin.forms

Question

I have a custom label control in which the font alone is being changed to Bold.
Got the following crash log from Appstore which I am unable to reproduce.
The crash log points to navigating to .xaml with my custom control.
public class CustomLabelBold : Label
{
public CustomLabelBold()
{
FontFamily = "Avenir Next";
}
}
Following is the crash log:
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Triggered by Thread: 0
Thread 0 name:
Thread 0 Crashed:
0 libsystem_kernel.dylib 0x00000001814492ec __pthread_kill + 8
1 libsystem_pthread.dylib 0x00000001815ea288 pthread_kill$VARIANT$mp + 376 (pthread.c:1484)
2 libsystem_c.dylib 0x00000001813b7db0 __abort + 152 (abort.c:128)
3 libsystem_c.dylib 0x00000001813b7d18 abort + 152 (abort.c:99)
4 MyProject 0x00000001031046e0 print_callback(char const*, int) + 39061216 (runtime.m:1216)
5 MyProject 0x00000001030ff1a8 monoeg_g_log + 39039400 (goutput.c:125)
6 MyProject 0x00000001030ba098 major_alloc_object + 38756504 (sgen-marksweep.c:697)
7 MyProject 0x00000001030dbec4 copy_object_no_checks + 38895300 (sgen-copy-object.h:71)
8 MyProject 0x00000001030db470 simple_nursery_serial_scan_object + 38892656 (sgen-minor-copy-object.h:250)
9 MyProject 0x00000001030dbdf0 simple_nursery_serial_drain_gray_stack + 38895088 (sgen-gray.h:191)
10 MyProject 0x00000001030b390c finish_gray_stack + 38729996 (sgen-gc.c:1099)
11 MyProject 0x00000001030b29d0 collect_nursery + 38726096 (sgen-gc.c:1807)
12 MyProject 0x00000001030af388 sgen_perform_collection + 38712200 (sgen-gc.c:2534)
13 MyProject 0x00000001030a5f00 sgen_alloc_obj_nolock + 38674176 (sgen-alloc.c:257)
14 MyProject 0x00000001030a639c sgen_alloc_obj + 38675356 (sgen-alloc.c:423)
15 MyProject 0x000000010307f100 mono_gc_alloc_obj + 38514944 (sgen-mono.c:948)
16 MyProject 0x0000000100ce46a8 wrapper_managed_to_native_object___icall_wrapper_mono_gc_alloc_obj_intptr_intptr + 104
17 MyProject 0x0000000100cde564 wrapper_alloc_object_AllocSmall_intptr_intptr + 228
18 MyProject 0x0000000100fd71c4 Xamarin_Forms_Core_Xamarin_Forms_BindableObject__ctor + 4272580 (.D:\agent\_work\1\s\Xamarin.Forms.Core\BindableObject.cs:1)
19 MyProject 0x000000010103326c Xamarin_Forms_Core_Xamarin_Forms_VisualElement__ctor + 4649580 (.D:\agent\_work\1\s\Xamarin.Forms.Core\VisualElement.cs:122)
20 MyProject 0x0000000100fe3bc0 Xamarin_Forms_Core_Xamarin_Forms_View__ctor + 4324288 (.D:\agent\_work\1\s\Xamarin.Forms.Core\View.cs:24)
21 MyProject 0x000000010100fa9c Xamarin_Forms_Core_Xamarin_Forms_Label__ctor + 4504220 (.D:\agent\_work\1\s\Xamarin.Forms.Core\Label.cs:57)
22 MyProject 0x00000001018bd5e4 Core_MyProject_Core_CustomControls_CustomLabelBold__ctor + 20

Answer 1

Android requires you to specify the actual file name of the font you want to use.
If you just hand over the font name, this will cause a crash.
So make sure you set your FontFamily property to the correct file name (in your case the name of the font defining the bold version).
Also I would suggest adapting the code in your custom renderer as follows, in order to prevent your app from crashing when having specified the wrong font:
protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
{
base.OnElementChanged(e);
TextView label = (TextView)Control;
if (e.NewElement?.FontFamily != null)
{
Typeface font = null;
try
{
font = Typeface.CreateFromAsset(AndroidApp.Application.Context.Assets, e.NewElement.FontFamily);
}
catch (Exception)
{
font = Typeface.Default;
}
label.Typeface = font;
}
}
This will result in your label falling back to the default font, instead of your whole app going down. As a bonus effect, it will also enable you testing your app within the xamarin live player or inspecting your screen in the forms previewer, since native resources are not available when you use one of those execution environments.
For iOS, make sure that you have an entry in your info.plist which lists all the fonts you have added to your app.
<key>UIAppFonts</key>
<array>
<string>fontawesome.ttf</string>
<string>OpenSans-Light.ttf</string>
<string>OpenSans-Bold.ttf</string>
<string>OpenSans-LightItalic.ttf</string>
<string>OpenSans-Italic.ttf</string>
<string>OpenSans-Regular.ttf</string>
<string>Lato-Bold.ttf</string>
<string>Lato-Regular.ttf</string>
</array>
Also make sure that the font file is in your Resources Folder and its build action is set to "BundleResource". Strangely enough, iOS needs to have the FontFamily property set to the filename without the .ttf extension.
I tried recreating your crash, but to no success. The worst thing that happened was that the default font was being rendered.
Even with all fonts in the right place and the property set to the right name, without a custom renderer, the font didn't appear at all.
This is from the custom renderer i am using for iOS labels:
protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
{
base.OnElementChanged(e);
if (e.NewElement != null && e.NewElement.FontFamily != null)
{
e.NewElement.FontFamily = e.NewElement.FontFamily.Replace(".ttf", "");
}
}