viewDidLoad is called twice

问题:

My viewDidLoad in a view controller is called twice. Once by [UIViewController View] and a second time by [UINib instanciateWithOwner:Options]. Why is this happening? Can it be prevented?

回答1:

Any code you put inside of viewDidLoad should be able to run multiple times with out any issues. If you have code that only needs to run once for your controller use -awakeFromNib. The reason is because the view of the view controller can be unloaded and loaded multiple times. The code inside of viewDidLoad should only modify the UI to reflect the current state.

Now that I got that out of the way, your particular issue looks to be a bug. See Ned's answer.

回答2:

Is this the same problem?

Why is viewDidLoad called twice when the rootViewController property of UIWindow is set?

Looks like it might be a bug in XCode 4.

回答3:

You might have to check the object building mechanism. If there is only one nib file with reference to the controller, then this method should not be called multiple times. (unless if the object is getting rebuilt).

I think you might have to make your code within ViewDidLoad idemPotent. It is always better to make sure, that framework call back methods make this assumption.

回答4:

There are two possibilities, whereby this issue happened in my iOS device frequently.

Rule #1: Do not call any view related setup in [init] function, all view related setup must be done in viewDidLoad and viewWillAppear.

Rule #2: Check viewDidLoad and viewWillAppear, are they calling correct super function? For example viewDidLoad -> super viewDidLoad and so on.

Hope this helps.

回答5:

In my case, I used self.view (once) in viewDidLoad while calling viewDidLoad in my unit tests. This resulted in two calls. However, when I replaced [testedViewController viewDidLoad] with [testedViewController view], the double call problem was gone.