CollectionView 居左对齐（搜索、历史记录）

之前做使用collection做历史记录，不管sollection的怎么设置，item之间能设置最小距离，却不能设置最大距离，所以总是达不到预期效果。后来查看资料重写UICollectionViewFlowLayout方法来达到预期效果。废话不多说，上代码。

未完成效果.png 预期效果.png

一、下面是继承UICollectionViewFlowLayout写的类

import UIKit

extension UICollectionViewLayoutAttributes {

    /** 每行第一个item左对齐 **/
    func leftAlignFrame(sectionInset:UIEdgeInsets) {
        var frame = self.frame
        frame.origin.x = sectionInset.left
        self.frame = frame
    }
}

class UICollectionViewLeftAlignedLayout: UICollectionViewFlowLayout {

    //MARK: - 重新UICollectionViewFlowLayout的方法

    /** Collection所有的UICollectionViewLayoutAttributes */
    override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let attributesToReturn = super.layoutAttributesForElementsInRect(rect)!
        for attributes in attributesToReturn {
            if nil == attributes.representedElementKind {
                let indexPath = attributes.indexPath
                attributes.frame = self.layoutAttributesForItemAtIndexPath(indexPath)!.frame
            }
        }
        return attributesToReturn
    }

    /** 每个item的UICollectionViewLayoutAttributes */
    override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes? {
        //现在item的UICollectionViewLayoutAttributes
        let currentItemAttributes = super.layoutAttributesForItemAtIndexPath(indexPath)!
        //现在section的sectionInset
        let sectionInset = self.evaluatedSectionInset(indexPath.section)
        //是否是section的第一个item
        let isFirstItemInSection = indexPath.item == 0
        //出去section偏移量的宽度
        let layoutWidth: CGFloat = CGRectGetWidth(self.collectionView!.frame) - sectionInset.left - sectionInset.right
        //是section的第一个item
        if isFirstItemInSection {
            //每行第一个item左对齐
            currentItemAttributes.leftAlignFrame(sectionInset)
            return currentItemAttributes
        }
    
        //前一个item的NSIndexPath
        let previousIndexPath = NSIndexPath(forItem: indexPath.item - 1, inSection: indexPath.section)
        //前一个item的frame
        let previousFrame = self.layoutAttributesForItemAtIndexPath(previousIndexPath)!.frame
        //为现在item计算新的left
        let previousFrameRightPoint: CGFloat = previousFrame.origin.x + previousFrame.size.width
        //现在item的frame
        let currentFrame = currentItemAttributes.frame
        //现在item所在一行的frame
        let strecthedCurrentFrame = CGRectMake(sectionInset.left, currentFrame.origin.y, layoutWidth, currentFrame.size.height)
    
        //previousFrame和strecthedCurrentFrame是否有交集，没有，说明这个item和前一个item在同一行，item是这行的第一个item
        let isFirstItemInRow = !CGRectIntersectsRect(previousFrame, strecthedCurrentFrame)
        //item是这行的第一个item
        if isFirstItemInRow {
            //每行第一个item左对齐
            currentItemAttributes.leftAlignFrame(sectionInset)
            return currentItemAttributes
        }
        //不是每行的第一个item
        var frame = currentItemAttributes.frame
        //为item计算新的left = previousFrameRightPoint + item之间的间距
        frame.origin.x = previousFrameRightPoint + self.evaluatedMinimumInteritemSpacing(indexPath.item)
        //为item的frame赋新值
        currentItemAttributes.frame = frame
        return currentItemAttributes
    }

    //MARK: - System

    /** item行间距 **/
    private func evaluatedMinimumInteritemSpacing(ItemAtIndex:Int) -> CGFloat     {
        if let delete = self.collectionView?.delegate {
            weak var delegate = (delete as! UICollectionViewDelegateFlowLayout)
            if delegate!.respondsToSelector(#selector(UICollectionViewDelegateFlowLayout.collectionView(_:layout:minimumInteritemSpacingForSectionAtIndex:))) {
                let mini =  delegate?.collectionView!(self.collectionView!, layout: self, minimumInteritemSpacingForSectionAtIndex: ItemAtIndex)
                if mini != nil {
                    return mini!
                }
            }
        }
        return self.minimumInteritemSpacing
    }

    /** section的偏移量 **/
    private func evaluatedSectionInset(itemAtIndex:Int) -> UIEdgeInsets {
        if let delete = self.collectionView?.delegate {
            weak var delegate = (delete as! UICollectionViewDelegateFlowLayout)
            if delegate!.respondsToSelector(#selector(UICollectionViewDelegateFlowLayout.collectionView(_:layout:insetForSectionAtIndex:))) {
                let sectionInset =  delegate?.collectionView!(self.collectionView!, layout: self, insetForSectionAtIndex: itemAtIndex)
                if sectionInset != nil {
                    return sectionInset!
                }
            }
        }
        return self.sectionInset
    }
}

二、�下面在collectionView中使用这个类，我的collectionView是使用xib创建的。

/// 搜索记录
@IBOutlet weak var collectionView: UICollectionView!
/// 历史搜索类型
var historyArray = [String]()

override func awakeFromNib() {
    super.awakeFromNib()

    historyArray = ["关键词","iOS学习基础","牛奶","跳绳","轮滑鞋","香蕉","面包","苹果梨","苹果"]

    let layout = UICollectionViewLeftAlignedLayout()
    //竖向cell之间的距离
    layout.minimumLineSpacing = 15
    //横向cell之间的距离，没有设置0.0
    layout.minimumInteritemSpacing = 10
    //section的header的大小
    layout.headerReferenceSize = CGSizeMake(ScreenWidth , 44);
    //section的偏移量
    layout.sectionInset = UIEdgeInsetsMake(0, 10, 0, 10)
    //collectionView的偏移量
    collectionView.contentInset = UIEdgeInsetsMake(0, 0, 5, 0)
    //SearchCell:xib中item的identifier
    collectionView.registerNib(UINib.init(nibName: "SearchCell", bundle: nil), forCellWithReuseIdentifier: "SearchCell")
    //SearchHeaderView中sender的header的identifier
    collectionView.registerNib(UINib.init(nibName: "SearchHeaderView", bundle: nil), forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, withReuseIdentifier: "SearchHeaderView")
    collectionView.collectionViewLayout = layout
}

//MARK: - UICollectionViewDataSource

/** Section数 */
func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int{
    if historyArray.count > 0 {
        return 1
    }
    return 0
}

/** Items数 */
func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return historyArray.count
}

/** Items创建 */
func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell{
    let cell = collectionView.dequeueReusableCellWithReuseIdentifier("SearchCell", forIndexPath: indexPath) as? SearchCell
    cell?.name.text = historyArray[indexPath.item]
    return cell!
}

/** collection的header创建 */
func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -> UICollectionReusableView {
    let view = collectionView.dequeueReusableSupplementaryViewOfKind(UICollectionElementKindSectionHeader, withReuseIdentifier: "SearchHeaderView", forIndexPath: indexPath) as? SearchHeaderView
    return view!
}

//MARK: - UICollectionViewDelegate

/** 历史记录关键词点击 */
func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
   
}

//MARK: - UICollectionViewDelegateFlowLayout

/** 设置关键词大小 */
func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
    let name = historyArray[indexPath.item]
    let size = self.boundingRectWithSize(CGSizeMake(ScreenWidth - 20, 30), options: .UsesLineFragmentOrigin, attributes: [NSFontAttributeName: UIFont.systemFontOfSize(12)], context: nil).size
    let with = size.width
    return CGSizeMake(with + 40, 30)
}

参考文档
http://code.cocoachina.com/view/126206

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