Swift之优化UITableViewCell高度计算

"最近在想着怎么提高自己的iOS技能，不断的阅读别人的blog。发现读完了不练手很快又忘掉了，练手吧又不知道从何处下手。所以就想着看看别人写的库是怎么实现的，体量比较大的看起来很费劲，影响士气。那就从体量比较小的看起。"

编辑：Bison

投稿： 大石头布

有些人会问，别人已经有了为啥还要再实现一遍，OC和Swift不是可以桥接么直接用就行了。我想说用swift实现主要是想理解作者的每行代码的含义，作者的思想，以及对应swift中的实现方式。（OC和swift还是有挺多区别的），理解了库的每行代码，自己使用也放心，自己实现了一个以后有什么别的需求自己直接可以改自己这版代码，掌握了他的思想，可以类比写出别的比较好用的库。（一举多得 还不好好看源码?）

没有缓存拖动过程截图

缓存后拖动过程截图

看下作者的实现思路：（从上往下看）

1、 创建了一个_FDTemplateLayoutCellHeightCache类，就是管理Cache的一个类，里面有两个属性四个方法。

_FDTemplateLayoutCellHeightCacheAbsentValue 这个是一个静态常量，就是用来标记没有缓存高度row 。 <br> 

buildHeightCachesAtIndexPathsIfNeeded:indexPaths 

class ZZTemplateLayoutCellHeightCache:NSObject,NSCopying{      var sections:[[CGFloat]] = []      func copyWithZone(zone: NSZone) -> AnyObject {         let theCopy = ZZTemplateLayoutCellHeightCache()         theCopy.sections = self.sections         return theCopy     }      // 标记没有被cache的值     static let ZZTemplateLayoutCellHeightCacheAbsentValue:CGFloat = -1      /**      初始化Height缓存数组 对每个元素做标记       - parameter indexPaths: indexPaths数组      */     func buildHeightCachesAtIndexPathsIfNeeded(indexPaths:[NSIndexPath]){         guard indexPaths.count > 0 else { return }         indexPaths.forEach { (indexPath) -> () in             //如果section 数组里面还没有 创建一个对象的空数组             if indexPath.section >= self.sections.count{                 self.sections.insert([], atIndex: indexPath.section)             }              var rows = self.sections[indexPath.section]             //如果对应的row不存在 创建一个并标记为没有被cache             if indexPath.row >= rows.count {                 rows.insert(ZZTemplateLayoutCellHeightCache.ZZTemplateLayoutCellHeightCacheAbsentValue, atIndex: indexPath.row)             }             self.sections[indexPath.section] = rows         }      }      /**      indexPath是否已经被缓存       - parameter indexPath: indexPath       - returns: Bool      */     func hasCachedHeightAtIndexPath(indexPath:NSIndexPath)->Bool{          self.buildHeightCachesAtIndexPathsIfNeeded([indexPath])         return self.sections[indexPath.section][indexPath.row] != ZZTemplateLayoutCellHeightCache.ZZTemplateLayoutCellHeightCacheAbsentValue      }      /**      缓存高度       - parameter height:    高度      - parameter indexPath: indexPath索引      */     func cacheHeight(height:CGFloat,byIndexPath indexPath:NSIndexPath){          self.buildHeightCachesAtIndexPathsIfNeeded([indexPath])         self.sections[indexPath.section][indexPath.row] = height      }      /**      获取已经缓存的height       - parameter indexPath: indexPath索引       - returns: height      */     func cachedHeightAtIndexPath(indexPath:NSIndexPath)->CGFloat{         self.buildHeightCachesAtIndexPathsIfNeeded([indexPath])         return self.sections[indexPath.section][indexPath.row]     } } 

NSMutableDictionary *templateCellsByIdentifiers = objc_getAssociatedObject(self, _cmd); 

extension UITableViewCell{     //在私有嵌套 struct 中使用 static var，这样会生成我们所需的关联对象键，但不会污染整个命名空间。     private struct AssociatedKeys{         static var zz_isTemplateLayoutCell = "zz_isTemplateLayoutCell"     }      ///扩展 添加属性 是否为临时布局的cell     var zz_isTemplateLayoutCell:Bool? {         set{             objc_setAssociatedObject(self, &AssociatedKeys.zz_isTemplateLayoutCell,newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_COPY_NONATOMIC)         }         get{             return objc_getAssociatedObject(self, &AssociatedKeys.zz_isTemplateLayoutCell) as? Bool         }     } } 

参考这篇文章 Swift & the Objective-C Runtime

更多Runtime知识扩展阅读： Objective-C Runtime

extension UITableView{      private struct AssociatedKeys{         static var zz_debugLogEnabled = "zz_debugLogEnabled"         static var zz_chc = "zz_chc"         static var zz_tmpCellForReuseId = "zz_tmpCellForReuseId"         static var zz_autoCacheInvalidationEnabled = "zz_autoCacheInvalidationEnabled"         static var zz_precacheEnabled = "zz_precacheEnabled"     }      /// 扩展 添加属性 是否在debug的情况下打印     var zz_debugLogEnabled:Bool? {         set{             objc_setAssociatedObject(self, &AssociatedKeys.zz_debugLogEnabled,newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_COPY_NONATOMIC)         }         get{             return objc_getAssociatedObject(self, &AssociatedKeys.zz_debugLogEnabled) as? Bool         }     }       private var zz_chc:ZZTemplateLayoutCellHeightCache?{         set{             objc_setAssociatedObject(self, &AssociatedKeys.zz_chc,newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_COPY_NONATOMIC)         }         get{             return objc_getAssociatedObject(self, &AssociatedKeys.zz_chc) as? ZZTemplateLayoutCellHeightCache         }     }      private var zz_tmpCellForReuseId:[String:UITableViewCell]?{         set{             objc_setAssociatedObject(self, &AssociatedKeys.zz_tmpCellForReuseId,newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_COPY_NONATOMIC)         }         get{             return objc_getAssociatedObject(self, &AssociatedKeys.zz_tmpCellForReuseId) as? [String:UITableViewCell]         }     }      var zz_autoCacheInvalidationEnabled:Bool?{         set{             objc_setAssociatedObject(self, &AssociatedKeys.zz_autoCacheInvalidationEnabled,newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_COPY_NONATOMIC)         }         get{             return objc_getAssociatedObject(self, &AssociatedKeys.zz_autoCacheInvalidationEnabled) as? Bool         }     }      var zz_precacheEnabled:Bool?{         set{             objc_setAssociatedObject(self, &AssociatedKeys.zz_precacheEnabled,newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_COPY_NONATOMIC)         }         get{             return objc_getAssociatedObject(self, &AssociatedKeys.zz_precacheEnabled) as? Bool         }     } } 

 /**      根据identifier获取UITableViewCell       - parameter identifier: identifier       - returns: UITableViewCell      */     func zz_templateCellForReuseIdentifier(identifier:String)->UITableViewCell{         assert(identifier.characters.count>0, "需要一个正确的identifier - /(identifier)")          if zz_tmpCellForReuseId == nil {             zz_tmpCellForReuseId = [:]         }         var templateCell = zz_tmpCellForReuseId![identifier]         if templateCell == nil{              templateCell = self.dequeueReusableCellWithIdentifier(identifier)             assert(templateCell != nil, "cell必须已经用identifier注册过: - /(identifier)")             templateCell!.zz_isTemplateLayoutCell = true             zz_tmpCellForReuseId![identifier] = templateCell             self.zz_debugLog("cell布局已经创建: - /(identifier)")         }          return templateCell!     } 

/**      返回ZZTemplateLayoutCellHeightCache的一个实例对象 如果已经存在直接获取       - returns: ZZTemplateLayoutCellHeightCache对象      */     func zz_cellHeightCache()->ZZTemplateLayoutCellHeightCache{         if zz_chc == nil{             zz_chc = ZZTemplateLayoutCellHeightCache()         }         return self.zz_chc!     } 

- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(id)arg waitUntilDone:(BOOL)wait modes:(NSArray *)array; 

// MARK: - cell的预缓存 extension UITableView{      private func zz_precacheIfNeeded(){          if let pe = self.zz_precacheEnabled where !pe {             return         }         guard let delegate = self.delegate else { return }          if !delegate.respondsToSelector(Selector("tableView:heightForRowAtIndexPath:")){             return         }          let runLoop = CFRunLoopGetCurrent()         //一个空闲的Runloop ，当页面滚动时会切换到"UITrackingRunLoopMode"         let runLoopMode = kCFRunLoopDefaultMode          // 收集所有被缓存过的index paths         var mutableIndexPathsToBePrecached = self.zz_allIndexPathsToBePrecached()          let observer = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, CFRunLoopActivity.BeforeWaiting.rawValue, true, 0) { (observer, _) -> Void in             // 所有预缓存任务完成的时候 移除Observer             if mutableIndexPathsToBePrecached.count == 0{                 CFRunLoopRemoveObserver(runLoop, observer, runLoopMode)                 return             }              // 取出第一个indexPath作为此Runloop迭代的任务             let indexPath = mutableIndexPathsToBePrecached.first             mutableIndexPathsToBePrecached.removeFirst() //            这个方法将创建一个 Source 0 任务，分发到指定线程的 RunLoop 中，在给定的 Mode 下执行，若指定的 RunLoop 处于休眠状态，则唤醒它处理事件             self.performSelector("zz_precacheIndexPathIfNeeded:", onThread: NSThread.mainThread(), withObject: indexPath, waitUntilDone: false, modes: [NSDefaultRunLoopMode])         }          CFRunLoopAddObserver(runLoop, observer, runLoopMode)      }       /**      如果没有缓存的话 就缓存高度       - parameter indexPath: indexpath      */     func zz_precacheIndexPathIfNeeded(indexPath:NSIndexPath){         guard let delegate = self.delegate else { return }         if !self.zz_cellHeightCache().hasCachedHeightAtIndexPath(indexPath){             let height = delegate.tableView!(self, heightForRowAtIndexPath: indexPath)             self.zz_cellHeightCache().cacheHeight(height, byIndexPath: indexPath)             self.zz_debugLog("Precached - /(indexPath.section) , /(indexPath.row) , /(height)")         }     }      /**      获取所有需要被缓存的indexPath       - returns: [indexpath]      */     func zz_allIndexPathsToBePrecached()->[NSIndexPath]{          var allIndexPaths:[NSIndexPath] = []         for section in 0..<self.numberOfSections{             for row in 0..<self.numberOfRowsInSection(section){                  print("section is :/(section) , row is :/(row) ")                 let indexPath = NSIndexPath(forRow: row, inSection: section)                 if !self.zz_cellHeightCache().hasCachedHeightAtIndexPath(indexPath){                     allIndexPaths.append(indexPath)                 }              }         }         return allIndexPaths     }  } 

    /**       相当于 oc中的load类方法 在方法加载的第一时间执行      */     public override class func initialize() {          // 所有有可能要修改height缓存的方法         let selectors = [                             Selector("reloadData"),                             Selector("insertSections:withRowAnimation:"),                             Selector("deleteSections:withRowAnimation:"),                             Selector("reloadSections:withRowAnimation:"),                             Selector("moveSection:toSection:"),                             Selector("insertRowsAtIndexPaths:withRowAnimation:"),                             Selector("deleteRowsAtIndexPaths:withRowAnimation:"),                             Selector("reloadRowsAtIndexPaths:withRowAnimation:"),                             Selector("moveRowAtIndexPath:toIndexPath:")                         ]          // 对所有method转换成我们自己写的method         for index in 0..<selectors.count{              let originalSelector = selectors[index]             let swizzledSelector = NSSelectorFromString("zz_/(NSStringFromSelector(originalSelector))")              let originalMethod = class_getInstanceMethod(self, originalSelector)             let swizzledMethod = class_getInstanceMethod(self, swizzledSelector)              let didAddMethod = class_addMethod(self, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod))              if didAddMethod {                 class_replaceMethod(self, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod))             } else {                 method_exchangeImplementations(originalMethod, swizzledMethod);             }          }      } 

    func zz_reloadData(){         if let zz_auto = self.zz_autoCacheInvalidationEnabled where zz_auto{             self.zz_cellHeightCache().sections.removeAll()         }         self.zz_reloadData()         self.zz_precacheIfNeeded()     }      func zz_insertSections(sections: NSIndexSet, withRowAnimation animation: UITableViewRowAnimation){         if let zz_auto = self.zz_autoCacheInvalidationEnabled where zz_auto {             sections.forEach({ (index) -> () in                 self.zz_cellHeightCache().sections.insert([], atIndex: index)             })         }          self.zz_insertSections(sections, withRowAnimation: animation)         self.zz_precacheIfNeeded()     }      func zz_deleteSections(sections: NSIndexSet, withRowAnimation animation: UITableViewRowAnimation){          if let zz_auto = self.zz_autoCacheInvalidationEnabled where zz_auto {             sections.forEach({ (index) -> () in                 self.zz_cellHeightCache().sections.removeAtIndex(index)             })         }         self.zz_deleteSections(sections, withRowAnimation: animation)     } 

// MARK: - [Public] 提供给外部的方法 extension UITableView{     /**      计算获得height       - parameter identifier:    id      - parameter configuration: configuration       - returns: height      */     func zz_heightForCellWithIdentifier(identifier:String,configuration:((cell:UITableViewCell)->())?)->CGFloat{         // 通过id获取一个cell         let cell = self.zz_templateCellForReuseIdentifier(identifier)         //手动调用已确保和屏幕上显示的保持一致         cell.prepareForReuse()         configuration?(cell: cell)          //手动添加一个约束 确保动态内容 如label         let tempWidthConstraint = NSLayoutConstraint(item: cell.contentView,             attribute: NSLayoutAttribute.Width,             relatedBy: NSLayoutRelation.Equal,             toItem: nil,             attribute: NSLayoutAttribute.NotAnAttribute,             multiplier: 1.0,             constant: CGRectGetWidth(self.frame))          cell.contentView.addConstraint(tempWidthConstraint)          // 算出size         let fittingSize = cell.contentView.systemLayoutSizeFittingSize(UILayoutFittingCompressedSize)         // 移除约束         cell.contentView.removeConstraint(tempWidthConstraint)          return fittingSize.height     }      /**      供外界使用的方法 提供了高度的缓存       - parameter identifier:    identifier      - parameter indexPath:     indexPath      - parameter configuration: 供外部使用的闭包      - returns: Height      */     func zz_heightForCellWithIdentifier(identifier:String,cacheByIndexPath indexPath:NSIndexPath,configuration:((cell:UITableViewCell)->())?)->CGFloat{          if self.zz_autoCacheInvalidationEnabled == nil{             self.zz_autoCacheInvalidationEnabled = true         }          if self.zz_precacheEnabled == nil{             self.zz_precacheEnabled = true         }         print("cache : /(indexPath.section) , /(indexPath.row)")         // 如果存在缓存 从缓存中返回         if self.zz_cellHeightCache().hasCachedHeightAtIndexPath(indexPath){             self.zz_debugLog("cache : /(indexPath.section) , /(indexPath.row) , /(self.zz_cellHeightCache().cachedHeightAtIndexPath(indexPath))")             return self.zz_cellHeightCache().cachedHeightAtIndexPath(indexPath)         }          //计算          let height = self.zz_heightForCellWithIdentifier(identifier, configuration: configuration)          self.zz_debugLog("计算出的cache cache : /(indexPath.section) , /(indexPath.row) , /(height)")          //缓存         self.zz_cellHeightCache().cacheHeight(height, byIndexPath: indexPath)          return height     }  } 

 func tableView(tableView: UITableView, heightForRowAtIndexPath indexPath: NSIndexPath) -> CGFloat {          return tableView.zz_heightForCellWithIdentifier(String(ZZFeedCell), cacheByIndexPath: indexPath, configuration: { (cell) -> () in             guard let cell = cell as? ZZFeedCell else { return }             cell.entity = self.feedEntitySections[indexPath.section][indexPath.row]         })      } 

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