ios - 旋转时多次调用方法 collectionview(cellForItemAtIndexPath)

Question

我正在学习 Swift。我有 2 个问题。
完整来源是 here .

1.为什么collectionview(cellForItemAtIndexPath)会这样调用很多次？
编辑:我想知道的是为什么每次旋转事件都会多次调用 collectionview(cellForItemAtIndexPath)。

1)旋转法

override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {

    print("4 self.collectionView.frame=\(self.collectionView.frame)")
    print("4 size=\(size)")

    self.collectionView.frame.size = size

    let currentSize = self.collectionView.bounds.size
    let offset = CGFloat(self.indexPathRow) * size.height
    print("offset=\(offset)")
    self.collectionView.setContentOffset(CGPointMake(0, offset), animated: true)
    print("self.collectionView.contentOffset=\(self.collectionView.contentOffset)")

    // Suppress the layout errors by invalidating the layout
    self.collectionView.collectionViewLayout.invalidateLayout();


    // Where is the best location of super? Top of this method or bottom?
    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
}

2) Collection View 方法

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

    currentOffset = self.collectionView.contentOffset
    print("currentOffset=\(currentOffset)")

    //print("indexPath=\(indexPath.row)")
    let frame = self.collectionView.frame

    let cell = collectionView.dequeueReusableCellWithReuseIdentifier(reuseCellIdentifier, forIndexPath: indexPath) as! MainCollectionViewCell

    let imageName = dataArray[indexPath.row]
    let image = UIImage(named: imageName)
    cell.imageView.image = image
    if (frame.size.width >= frame.size.height) {
        cell.imageView.frame = frame
    }
    else {
        cell.imageView.frame = CGRectMake(0, 0, frame.width*2, frame.height)
    }
    print("cell.imageView.frame=\(frame)")
    print("cell.imageView.image=\(cell.imageView.image!.size)")

    if (indexPath.row % 2 == 0) {
        cell.backgroundColor = UIColor.orangeColor()
    }
    else {
        cell.backgroundColor = UIColor.yellowColor()
    }

    currentIndex = Int(currentOffset.y / frame.size.height)
    print("currentIndex=\(currentIndex)")
    indexPathRow = indexPath.row

    return cell
}

1. 当我从横向旋转到纵向时遇到警告消息。我不想看到这个警告，但我不能。以下是我旋转时的日志消息。这条消息显示了多个被调用的方法。 (你可以遇到这个日志 - 通过横向第 12 个单元格然后旋转到纵向。)

    4 self.collectionView.frame=(0.0, 0.0, 667.0, 375.0)
    4 size=(375.0, 667.0)
    2016-06-10 14:22:05.750 CollectionViewWithoutStoryboard[49615:2682527] the behavior of the UICollectionViewFlowLayout is not defined because:
    2016-06-10 14:22:05.750 CollectionViewWithoutStoryboard[49615:2682527] the item width must be less than the width of the UICollectionView minus the section insets left and right values, minus the content insets left and right values.
    2016-06-10 14:22:05.751 CollectionViewWithoutStoryboard[49615:2682527] The relevant UICollectionViewFlowLayout instance is , and it is attached to ; layer = ; contentOffset: {0, 4875}; contentSize: {667, 7500}> collection view layout: .
    2016-06-10 14:22:05.751 CollectionViewWithoutStoryboard[49615:2682527] Make a symbolic breakpoint at UICollectionViewFlowLayoutBreakForInvalidSizes to catch this in the debugger.
    offset=8671.0
    self.collectionView.contentOffset=(0.0, 4729.0)
    self.collectionView.frame=(0.0, 0.0, 375.0, 667.0)
    currentOffset=(0.0, 4729.0)
    cell.imageView.frame=(0.0, 0.0, 375.0, 667.0)
    cell.imageView.image=(1920.0, 804.0)
    currentIndex=7
    currentOffset=(0.0, 4729.0)
    cell.imageView.frame=(0.0, 0.0, 375.0, 667.0)
    cell.imageView.image=(1920.0, 804.0)
    currentIndex=7
    self.collectionView.frame=(0.0, 0.0, 375.0, 667.0)
    currentOffset=(0.0, 5449.0)
    cell.imageView.frame=(0.0, 0.0, 375.0, 667.0)
    cell.imageView.image=(1920.0, 804.0)
    currentIndex=8
    currentOffset=(0.0, 6029.5)
    cell.imageView.frame=(0.0, 0.0, 375.0, 667.0)
    cell.imageView.image=(1920.0, 804.0)
    currentIndex=9
    currentOffset=(0.0, 6717.0)
    cell.imageView.frame=(0.0, 0.0, 375.0, 667.0)
    cell.imageView.image=(1920.0, 804.0)
    currentIndex=10
    currentOffset=(0.0, 7393.0)
    cell.imageView.frame=(0.0, 0.0, 375.0, 667.0)
    cell.imageView.image=(1920.0, 804.0)
    currentIndex=11
    currentOffset=(0.0, 8218.5)
    cell.imageView.frame=(0.0, 0.0, 375.0, 667.0)
    cell.imageView.image=(1920.0, 804.0)
    currentIndex=12

Answer 1

检查这个问题

Answer detailing how cellForRowAtIndexPath works

上面的答案详细说明了 cellForRowAtIndexPath 如何在 UITableView 中工作

UICollectionView 中的

cellForItemAtIndexPath 是围绕相同的概念构建的。您可以将这两个控件视为 UIScrollView 的优化版本。由于 TableView 或 CollectionView 可能包含多个项目，因此控件会重复使用 View ，并且在需要绘制特定单元格时调用此方法。 因此，不要将这些方法视为初始化程序，而是将其视为需要指定单元格外观的绘图方法。

我希望这个解释对您有所帮助!