UITableView自上而下布局指南以编程方式插入内容

由于我试图正确实现iAds，即共享ADBannerView的单个实例，因此我正在UIViewController中以编程方式创建UITableView，并将其添加到视图中。下面是我的UIViewController子类的几个片段：

从视图下载

iAds的实现增加了尝试和证明我的UITableView实现的正确性或错误性

如您所见，注释掉了3个约束。每种方法似乎都有不同的结果。除非被要求，否则我不会发布它们的截图，但我会描述它们

NSLayoutConstraintitem:self.tableView，属性：.Top，relatedBy:.Equal，toItem:self.view，属性：.Top，乘数：1，常数：0

iOS 7：表格顶部，表格内容位于屏幕顶部。内容在导航栏后面

iOS 8：表格顶部和表格内容位于导航栏下方。内容在导航栏下方是否正确

NSLayoutConstraintitem:self.tableView，属性：.Top，relatedBy:.Equal，toItem:self.view，属性：.Top，乘数：1，常数：self.topLayoutGuide.length

iOS 7：表格顶部，表格内容位于屏幕顶部。内容在导航栏后面

iOS 8：表格顶部和表格内容位于导航栏下方。内容在导航栏下方是否正确

NSLayoutConstraintitem:self.tableView，属性：.Top，relatedBy:.Equal，toItem:self.topLayoutGuide，属性：.Bottom，乘数：1，常数：0

iOS 7：表格顶部和表格内容位于导航栏下方。内容在导航栏下方是否正确

iOS 8：表的顶部位于导航栏的底部正确，但表的内容位于导航栏的下方，加上看起来像偏移的高度，再次不正确

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我知道我可以做一个if-iOS7{…}else{…}，但是感觉很脏，我感觉是我缺乏理解导致了这个问题，所以我想知道如何在iOS 7和8上进行这项工作，而不必进行版本检查，如果可能的话。

我最终找到了一种方法。我不确定它到底有多黑客，但到目前为止，它在iOS 7.0-8.2上运行良好

override func viewDidLayoutSubviews() {
        if self.tableView != nil {
            // Setting both of these to 0 seems to fix some auto layout issues that crop up in iOS 7/8 depending on
            // which item the layout is to, e.g., in iOS 8, having the UITableView's top be to the topLayoutGuide's bottom will
            // cause a gap at the top of the UITableView, but this removes that gap and doesn't seem to affect iOS 7
            self.tableView.contentInset = UIEdgeInsetsZero
            self.tableView.scrollIndicatorInsets = UIEdgeInsetsZero

            if self.showingiAd {
                let delegate = UIApplication.sharedApplication().delegate as AppDelegate
                if let bannerView = delegate.bannerView {
                    let bannerViewHeight = bannerView.frame.size.height
                    self.tableViewBottomLayoutConstraint.constant = -bannerViewHeight
                }
            }
        }
    }

func showiAds(animated: Bool) {
    println("Show iAd")
    if !self.showingiAd {
        println("Showing iAd")
        self.showingiAd = true

        // Add the banner view below the content before it's then animated in to view
        let delegate = UIApplication.sharedApplication().delegate as AppDelegate
        let bannerView = delegate.bannerView
        self.bannerBottomConstraint = NSLayoutConstraint(item: bannerView, attribute: .Bottom, relatedBy: .Equal, toItem: self.bottomLayoutGuide, attribute: .Top, multiplier: 1, constant: bannerView.frame.size.height)

        if (bannerView.superview != self.view) {
            bannerView.removeFromSuperview()
        }
        self.view.addSubview(bannerView)
        self.view.addConstraints([
            self.bannerBottomConstraint,
            NSLayoutConstraint(item: bannerView, attribute: .Left, relatedBy: .Equal, toItem: self.view, attribute: .Left, multiplier: 1, constant: 0),
            NSLayoutConstraint(item: bannerView, attribute: .Right, relatedBy: .Equal, toItem: self.view, attribute: .Right, multiplier: 1, constant: 0),
            ])
        self.view.layoutIfNeeded()


        // Only the changing of the value of the top of the banner is animated so it "slides in" from the bottom
        self.bannerBottomConstraint.constant = 0
        self.view.setNeedsUpdateConstraints()
        UIView.animateWithDuration(animated ? 0.5 : 0, animations: { () -> Void in
            // Calling layoutIfNeeded here will animate the layout constraint cosntant change made above
            self.view.layoutIfNeeded()
            }, completion: { (completed) -> Void in
                if completed {
                    println("Completed animation")
                }
        })
    }
}

func hideiAds() {
    println("Hide iAd")
    if self.self.showingiAd {
        self.showingiAd = false
        println("Hiding iAd")
        let delegate = UIApplication.sharedApplication().delegate as AppDelegate
        let bannerView = delegate.bannerView
        if bannerView.superview == self.view {
            bannerView.removeFromSuperview()
        }
        self.view.removeConstraint(self.tableViewBottomLayoutConstraint)
        self.tableViewBottomLayoutConstraint = NSLayoutConstraint(item: self.tableView, attribute: .Bottom, relatedBy: .Equal, toItem: self.bottomLayoutGuide, attribute: .Top, multiplier: 1, constant: 0)
        self.view.addConstraint(self.tableViewBottomLayoutConstraint)
    }
}

override func viewDidLayoutSubviews() {
        if self.tableView != nil {
            // Setting both of these to 0 seems to fix some auto layout issues that crop up in iOS 7/8 depending on
            // which item the layout is to, e.g., in iOS 8, having the UITableView's top be to the topLayoutGuide's bottom will
            // cause a gap at the top of the UITableView, but this removes that gap and doesn't seem to affect iOS 7
            self.tableView.contentInset = UIEdgeInsetsZero
            self.tableView.scrollIndicatorInsets = UIEdgeInsetsZero

            if self.showingiAd {
                let delegate = UIApplication.sharedApplication().delegate as AppDelegate
                if let bannerView = delegate.bannerView {
                    let bannerViewHeight = bannerView.frame.size.height
                    self.tableViewBottomLayoutConstraint.constant = -bannerViewHeight
                }
            }
        }
    }