What are the reasons why an IBOutlet (connected) could be nil?
I have one in may application which is always nil, even if I recreate everything from scratch (declaration and control).
It could be that your nib is messed up, but I find a common reason is having two instances where you think you only have one, and the one you're using in your code is not the one you connected.
If you've also defined a loadView method that creates the view, it is possible based on how you initialize it. If you initialize it using alloc-init and the nib name is not the same as class name, then you can have a case where the outlet is nil. But Chuck's answer seems more reasonable to assume.
One reason I just got stung by: If the nib file is not included in the target resource files for some reason (like you had the targets unchecked when you added it to the project), Xcode doesn't throw an error but all the outlets from that nib are going to be null...
One possibility: Suppose the IBOutlet container is a singleton object with a function like:
+ (singletonObject*) sharedInstance {
if(!gGlobalSingletonPointer) {
gGlobalSingletonPointer = [[singletonObject alloc] init];
}
return gGlobalSingletonPointer;
}
You create the singleton object "on demand" if it doesn't already exist. You save a global pointer to it, as you create it, in that function.
If you also instantiate such an object in InterfaceBuilder, and connect its outlets, this object will be created without sharedInstance being called. If you subsequently call sharedInstance, a new object is created (sans IBOutlet connections).
The solution is to update the global pointer in singletonObject's init or awakeFromNib function.
Are you using a UINavigationController?
If so, open your MainWindow.xib in IB and make sure that your root controller's nib name is set correctly in the Attributes Inspector.
Why would this not be set correctly? One reason is the 'rename' refactoring doesn't update this, and then the internals won't find the nib with which to wire your UI. Or you renamed the nib yourself, and didn't update this field.
Are you doing something unusual with File's Owner? If you're not in one of the situations where the nib is loaded automatically (main nib loaded by application or nib loaded by view controller, document, or window controller), then you have to load the nib programmatically.
来源：https://stackoverflow.com/questions/6223457/reasons-for-an-iboutlet-to-be-nil