iOS 7.1 removeFromSuperview crash

Question 1

Your program is crashing because you are releasing something more than once. That part is obvious.

The first step in finding it is to enable zombie detection in the debugger. (Project->Schemes->Edit Scheme->Diagnostics->Enable Zombie Objects). The goal here is to make your program crash sooner. This will drop you into the debugger as soon as you try to access a deallocated instance. Sometimes this will point you in the right direction, sometimes not, but it's always better to detect it as close to where the problem is as possible.

The next step is to use the Zombies instrument. This tool will give you more information than the previous step, but it's more complex to use (which is why I made it step 2 instead of step 1). The Zombies tool will keep track of all your allocations and releases, and detect when you try to access a zombie object.

The last resort is to start commenting out code. First comment out everything your program does between the time you create the view controller (the one that crashes) and when you release it. Then run the program and do whatever you need to do to make it display the bad view controller. It won't do anything, obviously, because it's just an empty view controller now, but it should not crash). Then start uncommenting blocks of code, a little bit at a time, and keep running it in between each iteration. This is a repetitive process, and can be tedious if your view controller code is large and complex. But the idea is to keep adding your code back in little by little until you add something back and it crashes - then you know you've found the piece of code that's causing the problem. You have to be creative here and choose carefully how you put your code back in - if your program has a nice modular design, you should be able to do this without much trouble. Spaghetti code will be difficult to do this with, but it might give you a good opportunity to restructure your code while you're at it. By going through this process, you'll narrow down the problem and eventually find the bug by process of elimination.

Question 2

It's usually not a good idea to modify an array while you're fast enumerating it. You appear to be using fast enumeration on a a view's array of subviews, and to be modifying that array at the same time (by removing subviews as you go). You could try something like this:

NSArray *subviewsCopy = [[contentVc subviews] copy];
for (UIView *subview in subviewsCopy) {
    [subview removeFromSuperview];
}

However, as some others have mentioned, it's a little odd that you need to go to the trouble of removing these subviews manually. Under normal circumstances a view controller's view (and the view hierarchy under it) will be cleaned up automatically when the view controller itself is deallocated.

There are also some good tools available that can help you track down the source of the issue. In particular, you should profile your app (in Xcode, under the Product menu) and choose the Zombies tool when Instruments prompts you. With Zombies you can see the retain/release history of an object that was messaged after it was deallocated.

If you're attempting this manual cleanup of the view hierarchy because you suspect that your views will be leaked otherwise, I suggest that you also try the Leaks tool in Instruments and verify that when this code is disabled the relevant views are actually leaked.

Question 3

UPDATED

try to do this:

NSArray *subviews = [NSArray arrayWithArray:[contentVc subviews]];
for (UIView *subView in subviews)
     [subView removeFromSuperview];

I think that you got the crash beacuse you're trying to fast enumerate an array that has variable length (in fact when you remove a subview, it is removed also from subview array).

If you want to remove the viewcontroller, just call:

[contentVc.view removeFromSuperView];
[contentVc removeFromParentViewController];

Question 4

sometimes crash for:

[myactualviewcontroller.view removeFromSuperview];

You shouldn't add or remove a controllers' view from a view hierarchy manually but rather rely in UIWindow's rootViewController, push your controller to a UINavigationController, etc., to get the system to add the view to private underlying superviews. Unless your creating a Custom Container View Controller, which I guess you aren't.

If you just want to handle views manually don't use view controllers as they won't get retained by the system and they won't get any rotation messages, etc. So using a view controller is pointless in that case anyway.

As for subview memory handling, subviews are retained by their superview, so as long as you don't keep a strong reference, you don't need to release subviews, just remove a common superview.

Again, if you properly use view controllers just releasing the controller will get rid of all views.

Finally, you should start using ARC.

Question 5

1.According to the Apple's documentation, calling removeFromSuperview will remove that view from superview and release it automatically.

So if you are using removeFromSuperview, then you should not call the [removedView release], which will crash your App.

Refer this screenshot from Apple.

enter image description here

In your dealloc implementation, you are having like so

- (void) dealloc {

    // Removed from Parent view and released.
    [mainView removeFromSuperview];

    // no mainView exists in memory , so it crashed the App.
    [mainView release];// Comment this line to avoid the crash

    [super dealloc];
}

2.You should not mute the container that are being enumerated.

You are having like this,

for (UIView *subView in [contentVc subviews])
     [subView removeFromSuperview];

Instead you can implement the same effect by having this one line from Apple.

[[contentVc subviews] makeObjectsPerformSelector:@selector(removeFromSuperview)];

Question 6

Please be sure that all of possible delegates removed before views deletion (i.e. someScrollViewDelegate = nil; before [someScrollView removeFromSuperview];) an/or animations are fully completed (all of CATransaction, [UIViev beginAnimation...], [UIView animateWithDuration...] etc.).

Question 7

please do the following:

1- debug the for (UIView *subView in [contentVc subviews]) and check how many times it iterate.

if it doesn't crash in the first hit you can add this line before you remove the view if (subView.superView != nil)

else try to make sure that you are not releasing the views twice somewhere else as it's keep showing and will not crash till you remove it from it's superview.

UPDATE2: i will consider that you are will aware of memory leaks, and that you have good experience in it.

whenever you add a subview to a view, this will retain the object by 1 in addition to the original 1, that will equal 2, then directly after adding the subview you have to release it, which will decrement the retain count back to one. here is the trick: you don't have to release the subview or remove it from it's parent view to get rid of the remaining retain count. you can simply remove or release the parent view. get the NSMutableArray As an example.
remove the [mainView removeFromSuperview]; from the dealloc: method. you can add it else where like viewWillDisappear: method. dealloc method shouldn't contain anything other than the release calls.

Question 8

Instead of:

for (UIView *subView in subviews)
     [subView removeFromSuperview];

Try:

[subviews makeObjectsPerformSelector:@selector(@"removeFromSuperview");

Question 9

Try checking if the view is != nil first before removeFromSuperview example:
@IBOutlet weak var btnSNSApple: UIView! if self.btnSNSApple != nil { self.btnSNSApple.removeFromSuperview() }