UICollectionView自定义布局教程

protocol PinterestLayoutDelegate {
  // 1
  func collectionView(collectionView:UICollectionView, heightForPhotoAtIndexPath indexPath:NSIndexPath, 
      withWidth:CGFloat) -> CGFloat
  // 2
  func collectionView(collectionView: UICollectionView, 
      heightForAnnotationAtIndexPath indexPath: NSIndexPath, withWidth width: CGFloat) -> CGFloat
}

override func prepareLayout() {
    // 1. Only calculate once
    if cache.isEmpty {
      // 2. Pre-Calculates the X Offset for every column and adds an array to increment the currently max Y Offset for each column
      // 每列宽度
      let columnWidth = contentWidth / CGFloat(numberOfColumns)
      var xOffset = [CGFloat]()
      // 其实就是xOffset就是两个,都是固定的.
      for column in 0 ..< numberOfColumns {
        xOffset.append(CGFloat(column) * columnWidth )
      }
      var column = 0
      var yOffset = [CGFloat](count: numberOfColumns, repeatedValue: 0)
      // 3. Iterates through the list of items in the first section
      for item in 0 ..< collectionView!.numberOfItemsInSection(0) {
        let indexPath = NSIndexPath(forItem: item, inSection: 0)
        // 4. Asks the delegate for the height of the picture and the annotation and calculates the cell frame.
        // 这个width是为了计算comment的长度的.
        let width = columnWidth - cellPadding*2
        let photoHeight = delegate.collectionView(collectionView!, heightForPhotoAtIndexPath: indexPath , withWidth:width)
        let annotationHeight = delegate.collectionView(collectionView!, heightForAnnotationAtIndexPath: indexPath, withWidth: width)
        let height = cellPadding +  photoHeight + annotationHeight + cellPadding
        let frame = CGRect(x: xOffset[column], y: yOffset[column], width: columnWidth, height: height)
        let insetFrame = CGRectInset(frame, cellPadding, cellPadding)
        // 5. Creates an UICollectionViewLayoutItem with the frame and add it to the cache
        let attributes = PinterestLayoutAttributes(forCellWithIndexPath: indexPath)
        attributes.photoHeight = photoHeight
        attributes.frame = insetFrame
        cache.append(attributes)
        // 6. Updates the collection view content height
        contentHeight = max(contentHeight, CGRectGetMaxY(frame))
        yOffset[column] = yOffset[column] + height
        column = column >= (numberOfColumns - 1) ? 0 : ++column
      }
    }
  }

column = column >= (numberOfColumns - 1) ? 0 : ++column.

override func prepareLayout() {  // 1
  if cache.isEmpty {    // 2
    let columnWidth = contentWidth / CGFloat(numberOfColumns)    var xOffset = [CGFloat]()    for column in 0 ..< numberOfColumns {
      xOffset.append(CGFloat(column) * columnWidth )
    }    var column = 0
    var yOffset = [CGFloat](count: numberOfColumns, repeatedValue: 0)    // 3
    for item in 0 ..< collectionView!.numberOfItemsInSection(0) {      let indexPath = NSIndexPath(forItem: item, inSection: 0)      // 4
      let width = columnWidth - cellPadding * 2
      let photoHeight = delegate.collectionView(collectionView!, heightForPhotoAtIndexPath: indexPath, 
          withWidth:width)      let annotationHeight = delegate.collectionView(collectionView!,
          heightForAnnotationAtIndexPath: indexPath, withWidth: width)      let height = cellPadding +  photoHeight + annotationHeight + cellPadding      let frame = CGRect(x: xOffset[column], y: yOffset[column], width: columnWidth, height: height)      let insetFrame = CGRectInset(frame, cellPadding, cellPadding)      // 5
      let attributes = UICollectionViewLayoutAttributes(forCellWithIndexPath: indexPath)
      attributes.frame = insetFrame
      cache.append(attributes)      // 6
      contentHeight = max(contentHeight, CGRectGetMaxY(frame))
      yOffset[column] = yOffset[column] + height
 
      column = column >= (numberOfColumns - 1) ? 0 : ++column
    }
  }
}

column = column >= (numberOfColumns - 1) ? 0 : ++column

column(现在是0) >= (numberOfColumns(始终为2,因为是定死的只有两列) - 1) ? 0: ++column

contentHeight = max(contentHeight, CGRectGetMaxY(frame))

override func collectionViewContentSize() -> CGSize {
  return CGSize(width: contentWidth, height: contentHeight)
}

override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {
  var layoutAttributes = [UICollectionViewLayoutAttributes]()
  for attributes  in cache {
    if CGRectIntersectsRect(attributes.frame, rect) {
      layoutAttributes.append(attributes)
    }
  }
  return layoutAttributes
}

func collectionView(collectionView:UICollectionView, heightForPhotoAtIndexPath indexPath:NSIndexPath,
      withWidth width:CGFloat) -> CGFloat {
    let photo = photos[indexPath.item]
    let boundingRect =  CGRect(x: 0, y: 0, width: width, height: CGFloat(MAXFLOAT))
    let rect  = AVMakeRectWithAspectRatioInsideRect(photo.image.size, boundingRect)
    return rect.size.height
  }

override func applyLayoutAttributes(layoutAttributes: UICollectionViewLayoutAttributes!) {
  super.applyLayoutAttributes(layoutAttributes)
  if let attributes = layoutAttributes as? PinterestLayoutAttributes {
    imageViewHeightLayoutConstraint.constant = attributes.photoHeight
  }
}