UiCollectionView必须使用非nil布局参数初始化Swift 3

Question

I have searched StackOverflow for answers to this and have tried different ways of implementing this. I have a collectionView that has a button that, when pressed, attempts to initialize another collectionView . Here's the stack trace:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'UICollectionView must be initialized with a non-nil layout parameter'
*** First throw call stack:
(
    0   CoreFoundation                      0x000000010adb3d4b __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x00000001079b321e objc_exception_throw + 48
    2   CoreFoundation                      0x000000010ae1d2b5 +[NSException raise:format:] + 197
    3   UIKit                               0x0000000108eca775 -[UICollectionView initWithFrame:collectionViewLayout:] + 76
    4   UIKit                               0x0000000108f13abb -[UICollectionViewController _newCollectionViewWithFrame:collectionViewLayout:] + 108
    5   UIKit                               0x0000000108f12c94 -[UICollectionViewController loadView] + 678
    6   UIKit                               0x000000010873761c -[UIViewController loadViewIfRequired] + 201
    7   UIKit                               0x0000000108737e70 -[UIViewController view] + 27
    8   UIKit                               0x0000000108ff86a4 -[_UIFullscreenPresentationController _setPresentedViewController:] + 87
    9   UIKit                               0x0000000108712702 -[UIPresentationController initWithPresentedViewController:presentingViewController:] + 141
    10  UIKit                               0x000000010874ae97 -[UIViewController _presentViewController:withAnimationController:completion:] + 3956
    11  UIKit                               0x000000010874e26b -[UIViewController _performCoordinatedPresentOrDismiss:animated:] + 530
    12  UIKit                               0x000000010874dd51 -[UIViewController presentViewController:animated:completion:] + 179
    13  TheAppProject                   0x0000000107385842 _TFC17TheAppProject15DailyController12publishedTapfT6senderCSo8UIButton_T_ + 754
    14  TheAppProject                   0x00000001073859fa _TToFC17TheAppProject15DailyController12publishedTapfT6senderCSo8UIButton_T_ + 58
    15  UIKit                               0x00000001085978bc -[UIApplication sendAction:to:from:forEvent:] + 83
    16  UIKit                               0x000000010871dc38 -[UIControl sendAction:to:forEvent:] + 67
    17  UIKit                               0x000000010871df51 -[UIControl _sendActionsForEvents:withEvent:] + 444
    18  UIKit                               0x000000010871ce4d -[UIControl touchesEnded:withEvent:] + 668
    19  UIKit                               0x0000000108605545 -[UIWindow _sendTouchesForEvent:] + 2747
    20  UIKit                               0x0000000108606c33 -[UIWindow sendEvent:] + 4011
    21  UIKit                               0x00000001085b39ab -[UIApplication sendEvent:] + 371
    22  UIKit                               0x0000000108da072d __dispatchPreprocessedEventFromEventQueue + 3248
    23  UIKit                               0x0000000108d99463 __handleEventQueue + 4879
    24  CoreFoundation                      0x000000010ad58761 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
    25  CoreFoundation                      0x000000010ad3d98c __CFRunLoopDoSources0 + 556
    26  CoreFoundation                      0x000000010ad3ce76 __CFRunLoopRun + 918
    27  CoreFoundation                      0x000000010ad3c884 CFRunLoopRunSpecific + 420
    28  GraphicsServices                    0x000000010dc1ba6f GSEventRunModal + 161
    29  UIKit                               0x0000000108595c68 UIApplicationMain + 159
    30  TheAppProject                   0x0000000107366d8f main + 111
    31  libdyld.dylib                       0x000000010bd6368d start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException

Here's my code:

private let reuseIdentifier = "cell"

class MyCollectionViewController: UICollectionViewController,  UICollectionViewDelegateFlowLayout {

let myCell = MyCell()

override func viewDidLoad() {
    super.viewDidLoad()

    let layout = UICollectionViewFlowLayout()
    let collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)

    collectionView.backgroundColor = UIColor.black
    collectionView.alwaysBounceVertical = true
    collectionView.delegate = self
    collectionView.dataSource = self

    self.collectionView!.register(MyCell.self, forCellWithReuseIdentifier: reuseIdentifier)
}

// MARK: - Layout

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

    return CGSize(width: 325, height: 630)
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {

    return 60.0
  }
}

Answer 1

Your MyCollectionViewController class is a subclass of UICollectionViewController which has a property of collectionView which is the actual UICollectionView itself. So the separate UICollectionView you create in this line:

let collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)

is not the same as self.collectionView which is the one provided by the UICollectionViewController itself.

So this is not the source of the error you are getting.

In fact the error you are seeing is relayed to how you are creating and presenting the MyCollectionViewController itself. You don't show the code for how that is created but you are not passing any kind of layout to it. For example to create it in code you would do this:

let layout = UICollectionViewFlowLayout()
let myCollectionVC = MyCollectionViewController(collectionViewLayout: layout)