快照如何干扰父/子关系？

Question

摘要

1. 添加childVC到parentVC
2. 快照 childVC.view
3. 它错了.

为什么？

情况

我刚刚遇到一个奇怪的行为,我想知道它是否正常或是否是一个bug.

我有一个视图控制器childVC,它是一个孩子parentVC.创建父/子关系时,我的代码是

    [parentVC addChildViewController:childVC] ;
    [parentVC.view addSubview:childVC.view] ;
    [childVC didMoveToParentViewController: parentVC] ;

几行,我想创建一个快照childVC.view.我的代码是

UIView * view = childVC.view ;

UIGraphicsBeginImageContextWithOptions(view.contentSize, NO, 0);
{
    [...]

    [view drawViewHierarchyInRect:view.bounds
               afterScreenUpdates:YES];

    image = UIGraphicsGetImageFromCurrentImageContext();

    [...]
}
UIGraphicsEndImageContext();

窃听器

然后我有错误:

*由于未捕获的异常'UIViewControllerHierarchyInconsistency'终止应用程序,原因:'子视图控制器:应该有父视图控制器:( null)但实际的父级是:'*第一次抛出调用堆栈:(0 CoreFoundation 0x0260b466 __exceptionPreprocess + 182 1 libobjc.A .dylib
0x02290a97 objc_exception_throw + 44 2 CoreFoundation
0x0260b38d + [NSException raise:format:] + 141 3 UIKit
0x01136710 - [UIView(Hierarchy)_associatedViewControllerForwardsAppearanceCallbacks:performHierarchyCheck:isRoot:] + 352 4 UIKit 0x01136b13 - [UIView(Hierarchy)_willMoveToWindow:withAncestorView: ] + 285 5 UIKit 0x0114330a - [UIView(内部)_addSubview:定位:relativeTo:] + 511 6 UIKit 0x01136252 - [UIView(Hierarchy)addSubview:] + 56 7 UIKit
0x0114ab0e + [_ UIReplicantView _pendingSnapshotOfTarget:snapshotBlock:] + 584 8 UIKit 0x011312fe - [UIView drawViewHierarchyInRect:afterScreenUpdates:] + 287

题

为什么会这样？我可以解决这个问题吗？

更多细节

实际上,childVC.view是一个UIScrollView和快照的代码是

UIScrollView * scrollView = (UIScrollView *)childVC.view;

UIGraphicsBeginImageContextWithOptions(scrollView.contentSize, NO, 0);
{
    CGPoint savedContentOffset = scrollView.contentOffset;
    CGRect savedFrame = scrollView.frame;

    scrollView.contentOffset = CGPointZero;
    scrollView.frame = CGRectMake(0,
                            0,
                            scrollView.contentSize.width,
                            scrollView.contentSize.height);

    [scrollView drawViewHierarchyInRect:scrollView.bounds
               afterScreenUpdates:YES];

    image = UIGraphicsGetImageFromCurrentImageContext();

    scrollView.contentOffset = savedContentOffset;
    scrollView.frame = savedFrame;
}
UIGraphicsEndImageContext();

也许这会产生很大的不同.

Answer 1

也许您的父视图控制器正在被释放，因此会(null)出现异常消息？