如何在Swift中将数据从父视图控制器多次传递到Container VC Child？

Question

I have a button on ViewControllerA (Parent) that I want to update a variable inside of ViewControllerB (Child). ViewControllerB is a container view inside of ViewControllerA.

This is the variable in ViewControllerB I want to update multiple times from the Parent ViewControllerA button press:

@IBOutlet weak var childViewHeight: NSLayoutConstraint!

Because the child view, ViewControllerB, is connected by an embed segue, it seems I can only pass data from ViewControllerA to ViewControllerB once via the prepareForSegue method. The performSegue method causes the program to crash with a SIGABRT Error.

I know it is generally considered bad practice to try to update an IBOutlet from a separate class or view controller, but I need a way for a button press on ViewControllerA to change the height constraint for ViewControllerA and ViewControllerB simultaneously.

If this is impossible in my current approach, please give me another suggestion of how to redesign my app to make this possible.

Update - This is the code that causes the crash:

@IBAction func button(_ sender: AnyObject) {
    performSegue(withIdentifier: "seg", sender: self)
}

Update - Here is the result when I type in "bt" in the debug console:

* thread #1: tid = 0x1fcdd, 0x000000010d9b1f06 libsystem_kernel.dylib`__pthread_kill + 10, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
frame #0: 0x000000010d9b1f06 libsystem_kernel.dylib`__pthread_kill + 10
frame #1: 0x000000010dad24ec libsystem_pthread.dylib`pthread_kill + 90
frame #2: 0x000000010d7040b3 libsystem_c.dylib`abort + 129
frame #3: 0x000000010d9d043a libc++abi.dylib`abort_message + 266
frame #4: 0x000000010d9f4a9f libc++abi.dylib`default_terminate_handler() + 267
frame #5: 0x000000010c7b559f libobjc.A.dylib`_objc_terminate() + 103
frame #6: 0x000000010d9f1c09 libc++abi.dylib`std::__terminate(void (*)()) + 8
frame #7: 0x000000010d9f1894 libc++abi.dylib`__cxa_rethrow + 99
frame #8: 0x000000010c7b54b7 libobjc.A.dylib`objc_exception_rethrow + 40
frame #9: 0x000000010a2eebf1 CoreFoundation`CFRunLoopRunSpecific + 433
frame #10: 0x000000010f6d7a48 GraphicsServices`GSEventRunModal + 161
frame #11: 0x000000010ad27e8b UIKit`UIApplicationMain + 159
* frame #12: 0x000000010a1c60cf ContainerVC2`main + 111 at AppDelegate.swift:12
frame #13: 0x000000010d6586bd libdyld.dylib`start + 1

Update - Here is the "bt" console output with the exception breakpoint in place:

* thread #1: tid = 0x219bd, 0x000000010afca2ee libobjc.A.dylib`objc_exception_throw, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
frame #0: 0x000000010afca2ee libobjc.A.dylib`objc_exception_throw
frame #1: 0x0000000108b7dec2 CoreFoundation`+[NSException raise:format:arguments:] + 98
frame #2: 0x0000000109079455 Foundation`-[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 195
frame #3: 0x0000000109f65309 UIKit`__67-[UIStoryboardEmbedSegueTemplate newDefaultPerformHandlerForSegue:]_block_invoke + 438
frame #4: 0x0000000109ce05e4 UIKit`-[UIStoryboardSegueTemplate _performWithDestinationViewController:sender:] + 453
frame #5: 0x0000000109ce03ee UIKit`-[UIStoryboardSegueTemplate _perform:] + 82
frame #6: 0x00000001096dc45b UIKit`-[UIViewController performSegueWithIdentifier:sender:] + 99
* frame #7: 0x00000001089d99b3 ContainerVC2`ViewController1.button(sender=0x00007fff57224658, self=0x00007fcddb707cb0) -> () + 131 at ViewController.swift:9
frame #8: 0x00000001089d9a26 ContainerVC2`@objc ViewController1.button(AnyObject) -> () + 54 at ViewController.swift:0
frame #9: 0x000000010953eb6f UIKit`-[UIApplication sendAction:to:from:forEvent:] + 83
frame #10: 0x00000001096bf927 UIKit`-[UIControl sendAction:to:forEvent:] + 67
frame #11: 0x00000001096bfc08 UIKit`-[UIControl _sendActionsForEvents:withEvent:] + 388
frame #12: 0x00000001096be6aa UIKit`-[UIControl touchesBegan:withEvent:] + 414
frame #13: 0x00000001095aabbd UIKit`-[UIWindow _sendTouchesForEvent:] + 1188
frame #14: 0x00000001095ac8d6 UIKit`-[UIWindow sendEvent:] + 3984
frame #15: 0x000000010955a1e1 UIKit`-[UIApplication sendEvent:] + 281
frame #16: 0x0000000109d1502f UIKit`__dispatchPreprocessedEventFromEventQueue + 3314
frame #17: 0x0000000109d0dc4e UIKit`__handleEventQueue + 4879
frame #18: 0x0000000108b1fcb1 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
frame #19: 0x0000000108b04c6c CoreFoundation`__CFRunLoopDoSources0 + 556
frame #20: 0x0000000108b04156 CoreFoundation`__CFRunLoopRun + 918
frame #21: 0x0000000108b03b5d CoreFoundation`CFRunLoopRunSpecific + 285
frame #22: 0x000000010deeca48 GraphicsServices`GSEventRunModal + 161
frame #23: 0x000000010953ce8b UIKit`UIApplicationMain + 159
frame #24: 0x00000001089db0cf ContainerVC2`main + 111 at AppDelegate.swift:12
frame #25: 0x000000010be6d6bd libdyld.dylib`start + 1

and here is the "bt" output after I hit the "continue program execution" button once:

* thread #1: tid = 0x219bd, 0x000000010c206607 libc++abi.dylib`__cxa_throw, queue = 'com.apple.main-thread', stop reason = breakpoint 1.2
frame #0: 0x000000010c206607 libc++abi.dylib`__cxa_throw
frame #1: 0x000000010afca443 libobjc.A.dylib`objc_exception_throw + 341
frame #2: 0x0000000108b7dec2 CoreFoundation`+[NSException raise:format:arguments:] + 98
frame #3: 0x0000000109079455 Foundation`-[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 195
frame #4: 0x0000000109f65309 UIKit`__67-[UIStoryboardEmbedSegueTemplate newDefaultPerformHandlerForSegue:]_block_invoke + 438
frame #5: 0x0000000109ce05e4 UIKit`-[UIStoryboardSegueTemplate _performWithDestinationViewController:sender:] + 453
frame #6: 0x0000000109ce03ee UIKit`-[UIStoryboardSegueTemplate _perform:] + 82
frame #7: 0x00000001096dc45b UIKit`-[UIViewController performSegueWithIdentifier:sender:] + 99
* frame #8: 0x00000001089d99b3 ContainerVC2`ViewController1.button(sender=0x00007fff57224658, self=0x00007fcddb707cb0) -> () + 131 at ViewController.swift:9
frame #9: 0x00000001089d9a26 ContainerVC2`@objc ViewController1.button(AnyObject) -> () + 54 at ViewController.swift:0
frame #10: 0x000000010953eb6f UIKit`-[UIApplication sendAction:to:from:forEvent:] + 83
frame #11: 0x00000001096bf927 UIKit`-[UIControl sendAction:to:forEvent:] + 67
frame #12: 0x00000001096bfc08 UIKit`-[UIControl _sendActionsForEvents:withEvent:] + 388
frame #13: 0x00000001096be6aa UIKit`-[UIControl touchesBegan:withEvent:] + 414
frame #14: 0x00000001095aabbd UIKit`-[UIWindow _sendTouchesForEvent:] + 1188
frame #15: 0x00000001095ac8d6 UIKit`-[UIWindow sendEvent:] + 3984
frame #16: 0x000000010955a1e1 UIKit`-[UIApplication sendEvent:] + 281
frame #17: 0x0000000109d1502f UIKit`__dispatchPreprocessedEventFromEventQueue + 3314
frame #18: 0x0000000109d0dc4e UIKit`__handleEventQueue + 4879
frame #19: 0x0000000108b1fcb1 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
frame #20: 0x0000000108b04c6c CoreFoundation`__CFRunLoopDoSources0 + 556
frame #21: 0x0000000108b04156 CoreFoundation`__CFRunLoopRun + 918
frame #22: 0x0000000108b03b5d CoreFoundation`CFRunLoopRunSpecific + 285
frame #23: 0x000000010deeca48 GraphicsServices`GSEventRunModal + 161
frame #24: 0x000000010953ce8b UIKit`UIApplicationMain + 159
frame #25: 0x00000001089db0cf ContainerVC2`main + 111 at AppDelegate.swift:12
frame #26: 0x000000010be6d6bd libdyld.dylib`start + 1

UPDATE - prepareforsegue code:

override func prepare(for segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "seg" {
        var vcB: ViewControllerB?
        vcB = segue.desinationViewController as? ViewControllerB
}

Answer 1

You don't need to call performSegue for an embed segue. The embed segue is fired automatically when the containing view controller is loaded from the storyboard.

You can use prepareForSegue in the containing view controller to get a reference to the contained view controller (it will be the destinationViewController in the segue). Once you have the reference you can store it in a property and use that to interact with it. Rather than updating the constraint directly, I would suggest that you invoke a function on the view controller that updates its constraint:

class ViewControllerA: UIViewController {

    var viewControllerB: ViewControllerB?

    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        if segue.identifier == "seg" {
            self.viewControllerB = segue.destinationViewController as? ViewControllerB
        }
     }

     @IBAction func button(_ sender: AnyObject) {
         self.viewControllerB?.doSomethingWithHeight(newHeight)
     }
}



class ViewControllerB: UIViewController {

    @IBOutlet weak var childViewHeight: NSLayoutConstraint!

    func doSomethingWithHeight(newHeight: CGFloat) {
        self.childViewHeight.constant = newHeight
    }
}

Answer 2

At the point that prepare for segue is called, ViewController B's outlets have not been set yet. They are all nil and so trying to access one will cause a crash.

For what you are trying to do, the dirtiest approach is to change the layoutContraint's constant from inside view controller A by accessing it through the childViewControllers array of view controller A. You will have to cast the first element in the array (assuming you only have one child) into an object of the view controller B class.

The above is a very dirty way to do it, but it will get the job done until you can learn some more.

@IBAction buttonAction(sender: UIButton) {
    if let viewControllerB = childViewControllers[0] as? ViewControllerB {
        viewControllerB.childViewHeight.constant = 25.0
    }
}

Again, this is a very dirty way to do it, but it will get the job done while you are learning the right way.

---

I've been asked why the above is such a bad idea...

Imagine that you didn't write the app in question, and you are tasked with fixing a bug with the constraint's constant. The first thing you would need to do to fix the bug would be to find all the places that mutate that constant.

In an ideal world, that constant would only be mutated in one place (I have a single method in my ViewController class called updateUI that handles all the IBOutlet mutation.) If that is the case, then fixing the bug will just be a matter of understanding the one function where the constant is getting changed.

The next best situation is that there is only one class that changes the constraint's constant. Then all you have to do to fix the bug is to understand how the class works. A larger task that understanding a single function, but doable.

However, using the code above means that there are multiple classes that adjust the constraint's constant, and if there are two classes doing it, who's to say there aren't more? In this case, fixing the bug requires an understanding of the entire app.

---

A better way to do it...

Presumably, you have multiple view controllers on the screen that are interconnected in some way through your Model code. The model represents your source of truth. Your two view controllers should be getting notified when the model changes and update themselves and their views accordingly. If you don't have a model representing the abstraction that is affecting these two views, then you should make one.