c# - 使用 UINavigationController(Type navigationBarType, Type toolbarType) 构造函数指定自定义导航栏

Question

我正在尝试使用 UINavigationController(Type navigationBarType, Type toolbarType) 构造函数来指定自定义 UINavigationBar 派生类，在 C# 中定义。

但是，我的所有尝试都会导致空引用异常。我认为这与没有为我的托管类找到 native 类有关吗？

if (UIDevice.CurrentDevice.CheckSystemVersion(7, 0))
    rootNavController = new UINavigationController(typeof(UnderlayNavigationBar), null);
else
    rootNavController = new UINavigationController();

传递默认的 typeof(UIToolbar) 而不是第二个 null 参数也无济于事。

System.NullReferenceException: Object reference not set to an instance of an object
  at MonoTouch.ObjCRuntime.Runtime.ConstructNSObject[NSObject] (IntPtr ptr, System.Type type, MissingCtorResolution missingCtorResolution) [0x00037] in /Developer/MonoTouch/Source/monotouch/src/ObjCRuntime/Runtime.cs:365
  at MonoTouch.ObjCRuntime.Runtime.ConstructNSObject (IntPtr ptr, IntPtr klass, MissingCtorResolution missingCtorResolution) [0x00013] in /Developer/MonoTouch/Source/monotouch/src/ObjCRuntime/Runtime.cs:348
  at MonoTouch.ObjCRuntime.Runtime.GetNSObject (IntPtr ptr, MissingCtorResolution missingCtorResolution) [0x00021] in /Developer/MonoTouch/Source/monotouch/src/ObjCRuntime/Runtime.cs:430
  at MonoTouch.ObjCRuntime.Runtime.TryGetOrConstructNSObjectWrapped (IntPtr ptr) [0x00000] in /Developer/MonoTouch/Source/monotouch/src/ObjCRuntime/Runtime.cs:658
  at at (wrapper native-to-managed) MonoTouch.ObjCRuntime.Runtime:TryGetOrConstructNSObjectWrapped (intptr)
  at at (wrapper managed-to-native) MonoTouch.ObjCRuntime.Messaging:void_objc_msgSend_IntPtr (intptr,intptr,intptr)
  at MonoTouch.UIKit.UINavigationController.set_ViewControllers (MonoTouch.UIKit.UIViewController[] value) [0x00028] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UINavigationController.g.cs:345

如果有人对我为什么使用这个构造函数感兴趣，我正在移植这个指南:http://b2cloud.com.au/how-to-guides/custom-uinavigationbar-colors-in-ios7

Answer 1

我应该更详细地阅读调用堆栈，尤其是 GetNSObject (IntPtr ptr, MissingCtorResolution missingCtorResolution)。

现在完全有道理。该实例是在 native 代码中构造的，因此需要采用 native 实例的 IntPtr 的托管构造函数重载。将该构造函数添加到我的派生类中解决了这个问题。

public UnderlayNavigationBar(IntPtr handle) : base(handle)
{
}