添加约束使应用程序崩溃

Question

我有一个用于设计 UITableViewCell 的自定义 UIView，每件事都能完美运行，只是没有正确调整大小。当我尝试添加约束时它崩溃了。我使用了下面的代码。

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"identifier"];
    if(!cell) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"identifier"];
        UIView *vBackView = [[UIView alloc] initWithFrame:aFrame];
        [[cell contentView] addSubview:vBackView];

        [vBackView addConstraint:[NSLayoutConstraint constraintWithItem:[cell contentView]
                                                              attribute:NSLayoutAttributeTop
                                                              relatedBy:NSLayoutRelationEqual
                                                                 toItem:vBackView
                                                              attribute:NSLayoutAttributeTop
                                                             multiplier:1.0
                                                               constant:0.0]];

        [vBackView addConstraint:[NSLayoutConstraint constraintWithItem:[cell contentView]
                                                              attribute:NSLayoutAttributeLeading
                                                              relatedBy:NSLayoutRelationEqual
                                                                 toItem:vBackView
                                                              attribute:NSLayoutAttributeLeading
                                                             multiplier:1.0
                                                               constant:0.0]];

        [vBackView addConstraint:[NSLayoutConstraint constraintWithItem:[cell contentView]
                                                              attribute:NSLayoutAttributeBottom
                                                              relatedBy:NSLayoutRelationEqual
                                                                 toItem:vBackView
                                                              attribute:NSLayoutAttributeBottom
                                                             multiplier:1.0
                                                               constant:0.0]];

        [vBackView addConstraint:[NSLayoutConstraint constraintWithItem:[cell contentView]
                                                              attribute:NSLayoutAttributeTrailing
                                                              relatedBy:NSLayoutRelationEqual
                                                                 toItem:vBackView
                                                              attribute:NSLayoutAttributeTrailing
                                                             multiplier:1.0
                                                               constant:0.0]];
    }

    // Doing other stuff

    return cell;
}

在日志中显示

2015-06-13 12:51:57.482 DigitalBoard[5314:70921] The view hierarchy is not prepared for the constraint: <NSLayoutConstraint:0x7fb508dc0ae0 UITableViewCell:0x7fb508dbb0a0'cellIdentifier'.top == UIView:0x7fb508dc16f0.top>
    When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView _viewHierarchyUnpreparedForConstraint:] to debug.

2015-06-13 12:51:57.485 DigitalBoard[5314:70921] View hierarchy unprepared for constraint.
    Constraint: <NSLayoutConstraint:0x7fb508dc0ae0 UITableViewCell:0x7fb508dbb0a0'cellIdentifier'.top == UIView:0x7fb508dc16f0.top>
    Container hierarchy: 
<UIView: 0x7fb508dc16f0; frame = (0 0; 400 44); autoresize = RM+BM; tag = 1; layer = <CALayer: 0x7fb508dc0720>>
   | <UILabel: 0x7fb508dc5310; frame = (15 0; 105 44); text = 'Meeting #'; opaque = NO; autoresize = RM+BM; userInteractionEnabled = NO; tag = 2; layer = <_UILabelLayer: 0x7fb508dc54c0>>
   | <UIButton: 0x7fb508dc5880; frame = (130 7; 80 30); opaque = NO; autoresize = RM+BM; tag = 3; layer = <CALayer: 0x7fb508da95d0>>
   | <UIButton: 0x7fb508dc5fa0; frame = (220 7; 80 30); opaque = NO; autoresize = RM+BM; tag = 4; layer = <CALayer: 0x7fb508dc5c60>>
   | <UIButton: 0x7fb508dc61c0; frame = (310 7; 80 30); opaque = NO; autoresize = RM+BM; tag = 5; layer = <CALayer: 0x7fb508dc5e90>>
   | <UIImageView: 0x7fb508dc6540; frame = (0 42; 400 2); autoresize = RM+BM; userInteractionEnabled = NO; layer = <CALayer: 0x7fb508dc6680>>
    View not found in container hierarchy: <UITableViewCell: 0x7fb508dbb0a0; frame = (0 0; 320 44); layer = <CALayer: 0x7fb508da98e0>>
    That view's superview: NO SUPERVIEW

2015-06-13 12:51:57.498 DigitalBoard[5314:70921] *** Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to install constraint on view.  Does the constraint reference something from outside the subtree of the view?  That's illegal. constraint:<NSLayoutConstraint:0x7fb508dc0ae0 UITableViewCell:0x7fb508dbb0a0'cellIdentifier'.top == UIView:0x7fb508dc16f0.top> view:<UIView: 0x7fb508dc16f0; frame = (0 0; 400 44); autoresize = RM+BM; tag = 1; layer = <CALayer: 0x7fb508dc0720>>'
*** First throw call stack:
(
etc....

任何人都可以帮助了解究竟是什么问题。

Answer 1

您不应将约束添加到vBackView。我假设vBackView 是cell.contentView 的子视图，在这种情况下，您应该将约束添加到cell.contentView。

崩溃日志告诉你原因：

当添加到视图时，约束的项必须是该视图（或视图本身）的后代。

将每一行代码更改为开始：[cell.contentView addConstraint:...。