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UICollectionView scrollToItem会中断约束,并且不会滚动

interface-builder uicollectionview swift ios

MusicDev · 技术社区 · 7 年前

首先,这是我收到的错误消息:

2018-10-08 01:10:04.886509-0600 Nebula Messenger[1542:262927] [CollectionView] An attempt to prepare a layout while a prepareLayout call was already in progress (i.e. reentrant call) has been ignored. Please file a bug. UICollectionView instance is (&lt;UICollectionView: 0x10703e600; frame = (0 92; 375 491); clipsToBounds = YES; autoresize = RM+BM; gestureRecognizers = &lt;NSArray: 0x281a741e0&gt;; layer = &lt;CALayer: 0x281431da0&gt;; contentOffset: {0, -8}; contentSize: {375, 0}; adjustedContentInset: {8, 0, 8, 0}&gt; collection view layout: &lt;UICollectionViewFlowLayout: 0x10605fe10&gt;)
2018-10-08 01:10:04.891365-0600 Nebula Messenger[1542:262927] [LayoutConstraints] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "&lt;NSLayoutConstraint:0x283726670 UIView:0x106065480.width == 200   (active)&gt;",
    "&lt;NSLayoutConstraint:0x283726760 UILabel:0x106065190.left == UIView:0x106065480.left + 8   (active)&gt;",
    "&lt;NSLayoutConstraint:0x2837267b0 UILabel:0x106065190.right == UIView:0x106065480.right - 8   (active)&gt;",
    "&lt;NSLayoutConstraint:0x283726850 UILabel:0x106065190.width == 200   (active)&gt;"
)
Will attempt to recover by breaking constraint 
&lt;NSLayoutConstraint:0x283726850 UILabel:0x106065190.width == 200   (active)&gt;
Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in &lt;UIKitCore/UIView.h&gt; may also be helpful.
2018-10-08 01:10:04.896386-0600 Nebula Messenger[1542:262927] [LayoutConstraints] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "&lt;NSLayoutConstraint:0x28372bac0 UIView:0x1061705c0.width == 200   (active)&gt;",
    "&lt;NSLayoutConstraint:0x28372bbb0 UILabel:0x1061702d0.left == UIView:0x1061705c0.left + 8   (active)&gt;",
    "&lt;NSLayoutConstraint:0x28372bc00 UILabel:0x1061702d0.right == UIView:0x1061705c0.right - 8   (active)&gt;",
    "&lt;NSLayoutConstraint:0x28372bca0 UILabel:0x1061702d0.width == 200   (active)&gt;"
)

这是我用来滚动的:

func scrollToBottom() {
    DispatchQueue.main.async {
        self.messagesCollection.layoutIfNeeded()
        self.messagesCollection?.scrollToItem(at: IndexPath(item: 
            self.msgList.count-1, section: 0), at: 
            UICollectionView.ScrollPosition.bottom, animated: true)
    }
}

当视图加载时,我称之为:

override func viewDidLayoutSubviews(){
        super.viewDidLayoutSubviews()
        self.messagesCollection.layoutIfNeeded()
        self.scrollToBottom()
}

我的第一个问题是它加载时不会向右滚动,我必须先手动滚动它,然后它会向上滚动到底部。

也就是说,除非CollectionView中有很多单元格,在这种情况下,它将继续尝试向上滚动我,直到我与它抗争到可以让它向相反方向滚动为止。

第二个问题(这似乎是主要问题)是错误消息。我得到了很多,基本上它只是告诉我,为了滚动,必须打破限制,但这对我没有任何意义。滚动如何打破约束?

这个 scrollToBottom 函数在放置时不起作用 viewDidLoad 或 viewDidAppear .

我并不反对使用不同的方法来滚动CollectionView,因为当前的方法已经感觉有点老套了。

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