forceTouchCapability返回nil

我试图把一些3D触摸到应用程序中,我已经在运行到一个奇怪的问题forceTouchCapability检查返回nil上viewDidLoad而不是在viewWillAppear/viewDidAppear.

我知道这只适用于iOS 9+,因此我添加了检查以验证traitCollection视图控制器上的属性是否响应,forceTouchCapability如下所示:

- (void)loadView {

   self.view = [[MyView alloc] init];
}

- (void)viewDidLoad {

   [super viewDidLoad];

   // Checking the force touch availability here
   if ([self.traitCollection respondsToSelector:@selector(forceTouchCapability)] &&
        self.traitCollection.forceTouchCapability == UIForceTouchCapabilityAvailable) {

       // This won't get called because forceTouchCapability is returning nil 
       // which corresponds to UIForceTouchCapabilityUnknown
       [self registerForPreviewingWithDelegate:self sourceView:self.view];
   }
}

在LLDB中,在if语句中有一个断点,输入对应的po [self.traitCollection forceTouchCapability]返回值.但是,本身并非如此.nilUIForceTouchCapabilityUnknowntraitCollectionnil

根据UIForceTouchCapabilityUnknown的文档:

UIForceTouchCapabilityUnknown: The availability of 3D Touch is unknown. For example, if you create a view but have not yet added it to your app’s view hierarchy, the view’s trait collection has this value.

此时是否未将视图添加到层​​次结构中？

我很好奇是否有人遇到过这个问题以及如何解决这个问题？我想避免添加这个,viewDidAppear因为这可以被调用相当多.

如果它有帮助,我在iOS 9.1上用Xcode 7.2在6S上运行它

视图尚未添加到视图层次结构中.您可以通过在调试控制台中检查superview来轻松查看

(lldb) po self.view.superview
nil

如果这就是您所看到的,那么该视图尚未添加到层次结构中:所以您必须将检查放在其他位置.

这有点令人困惑,因为在Apple的ViewControllerPreview示例应用程序中它位于viewDidLoad中.但它确实应该在traitCollectionDidChange:中,因为那时你确定该视图已被添加到应用程序的层次结构中.

这是我使用的代码(适用于iOS 8,如果你不需要支持那么随意移动外部条件).

- (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection {
   [super traitCollectionDidChange:previousTraitCollection];

   if ([self.traitCollection respondsToSelector:@selector(forceTouchCapability)]) {
       if (self.traitCollection.forceTouchCapability == UIForceTouchCapabilityAvailable) {
           // retain the context to avoid registering more than once
           if (!self.previewingContext) {
               self.previewingContext = [self registerForPreviewingWithDelegate:self sourceView:self.view];
           }
       } else {
           [self unregisterForPreviewingWithContext:self.previewingContext];
           self.previewingContext = nil;
       }
   }
}

这样做的另一个好处是,如果用户在应用程序运行时更改其3D Touch设置,您的视图将被注册/取消注册.