今天的这个bug，真的出现在我的项目里面了。。。知道原因时差点破腹自尽

我自闭了

事情是这样的，原本有个控制器是用Nav push 来显示的，现在需要改成用modal present 来显示。。还有比这个更容易的需求吗？？？？

但是bug出现了。。。

写个简易代码演示下：

// 控制器B的viewDidLoad()

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    UIView *view = [[UIView alloc] init];
    view.backgroundColor = [UIColor redColor];
    view.frame = CGRectMake(0, 0, 100, 100);
    view.center = self.view.center;
    [self.view addSubview:view];
}

这代码够简单吧，加了全局断点，然后如下：

全局断点

好吧，我去掉断点

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '<NewViewController: 0x7fa6d6f0ace0> returned nil from -traitCollection, which is not allowed.'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007fff23b98bde __exceptionPreprocess + 350
    1   libobjc.A.dylib                     0x00007fff503b5b20 objc_exception_throw + 48
    2   CoreFoundation                      0x00007fff23b98958 +[NSException raise:format:arguments:] + 88
    3   Foundation                          0x00007fff255eb7be -[NSAssertionHandler handleFailureInFunction:file:lineNumber:description:] + 166
    4   UIKitCore                           0x00007fff46dd08bd _UIGetCurrentFallbackTraitCollection + 962
    5   UIKitCore                           0x00007fff476029fa UIViewCommonInitWithFrame + 617
    6   UIKitCore                           0x00007fff47602753 -[UIView initWithFrame:] + 98
    7   UIKitCore                           0x00007fff47601fa3 -[UIView init] + 44
    8   PresentDemo                         0x000000010ab18f05 -[NewViewController viewDidLoad] + 85
    9   UIKitCore                           0x00007fff46af87c2 -[UIViewController _sendViewDidLoadWithAppearanceProxyObjectTaggingEnabled] + 83
    10  UIKitCore                           0x00007fff46afd6d3 -[UIViewController loadViewIfRequired] + 1084
    11  UIKitCore                           0x00007fff46afdaf0 -[UIViewController view] + 27
    12  UIKitCore                           0x00007fff46a1776d -[_UIFullscreenPresentationController _setPresentedViewController:] + 84
    13  UIKitCore                           0x00007fff46a0b105 -[UIPresentationController initWithPresentedViewController:presentingViewController:] + 184
    14  UIKitCore                           0x00007fff46b0ec8e -[UIViewController _presentViewController:withAnimationController:completion:] + 3553
    15  UIKitCore                           0x00007fff46b1180b __63-[UIViewController _presentViewController:animated:completion:]_block_invoke + 98
    16  UIKitCore                           0x00007fff46b11d23 -[UIViewController _performCoordinatedPresentOrDismiss:animated:] + 511
    17  UIKitCore                           0x00007fff46b11769 -[UIViewController _presentViewController:animated:completion:] + 187
    18  UIKitCore                           0x00007fff46b119d0 -[UIViewController presentViewController:animated:completion:] + 150
    19  PresentDemo                         0x000000010ab18c82 -[ViewController viewDidAppear:] + 146
    20  UIKitCore                           0x00007fff46b036a2 -[UIViewController _setViewAppearState:isAnimating:] + 934
    21  UIKitCore                           0x00007fff46a57e2d -[UINavigationController viewDidAppear:] + 173
    22  UIKitCore                           0x00007fff46b036a2 -[UIViewController _setViewAppearState:isAnimating:] + 934
    23  UIKitCore                           0x00007fff46b040a6 -[UIViewController _endAppearanceTransition:] + 196
    24  UIKitCore                           0x00007fff46a0bf2b __48-[UIPresentationController transitionDidFinish:]_block_invoke + 138
    25  UIKitCore                           0x00007fff4716b85a _runAfterCACommitDeferredBlocks + 352
    26  UIKitCore                           0x00007fff4715c63c _cleanUpAfterCAFlushAndRunDeferredBlocks + 248
    27  UIKitCore                           0x00007fff4718bc6e _afterCACommitHandler + 85
    28  CoreFoundation                      0x00007fff23afaeb7 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
    29  CoreFoundation                      0x00007fff23af594e __CFRunLoopDoObservers + 430
    30  CoreFoundation                      0x00007fff23af5fca __CFRunLoopRun + 1514
    31  CoreFoundation                      0x00007fff23af56b6 CFRunLoopRunSpecific + 438
    32  GraphicsServices                    0x00007fff3815cbb0 GSEventRunModal + 65
    33  UIKitCore                           0x00007fff47162a67 UIApplicationMain + 1621
    34  PresentDemo                         0x000000010ab191a4 main + 116
    35  libdyld.dylib                       0x00007fff5123bcf5 start + 1
    36  ???                                 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '<NewViewController: 0x7fa6d6f0ace0> returned nil from -traitCollection, which is not allowed.'
然后自闭了。。。。

直接push代码如下：

[self.navigationController pushViewController:[NewViewController alloc] animated:YES];

一点毛病没有，啥都正常。。。
所以我改的modal代码：

[self presentViewController:[NewViewController alloc] animated:YES completion:nil];

真的是疯了，主要是注意力全在 NewViewController 中。。。最后才看到，没有 init

 [self presentViewController:[[NewViewController alloc] init] animated:YES completion:nil];

。。。。。。。。。。
心中一千匹草泥马奔过。。。。
所以大家要细心啊。。。

alloc只是开辟了内存空间，但是没有进行初始化。为啥push可以。。。求大佬解释