UICollectionView CustomCell ReUse

Question

我正在提取用戶的Photo Asset並嘗試在我的UICollectionView中創建自定義UICollectionViewCell，其中第一個索引是Camera Image，其他單元格是相機圖像。但是，當我滾動，它像索引圖像重建，我發現在我的細胞一些圖片讓我在index[0]

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

    //Deque an GridViewCell 
    let cell: GridViewCell = (collectionView.dequeueReusableCellWithReuseIdentifier(CellReuseIdentifier, forIndexPath: indexPath) as? GridViewCell)! 
    if indexPath.item != 0 { 

     let asset = assetsFetchResults[indexPath.item - 1] as! PHAsset 
     cell.representedAssetIdentifier = asset.localIdentifier 

     imageManager.requestImageForAsset(asset, targetSize: AssetGridThumbnailSize, contentMode: PHImageContentMode.AspectFill, options: nil) { (result: UIImage?, info: [NSObject : AnyObject]?) -> Void in 
      //print(result) 
      if cell.representedAssetIdentifier.isEqualToString(asset.localIdentifier) { 
       if let imageResult = result { 
        cell.imageView.image = imageResult 
       } 
      } 
     } 
    } else { 
     let cameraButton = UIButton(frame: CGRectMake(0,0,80,80)) 
     cameraButton.setTitle("Camera", forState: .Normal) 
     cameraButton.frame = cell.imageView.bounds 
     cell.imageView.addSubview(cameraButton) 
    } 

    return cell 
} 

如下的添加視圖用於說明 ​​

Answer 1

可重複使用的圖像當它們在視圖外滾動時，單元格會被集合視圖重新使用，以防止使用大量內存。當某個單元格在視圖外滾動時，集合視圖會將它們標記爲可重用。可重複使用的單元格將被集合視圖重用，而不是創建新的單元格。這將顯着減少內存使用。如果同時只有20個單元適合屏幕，爲什麼要在內存中保留1000個單元。

由於單元格被重用，它們仍然會包含前一個單元格的內容，這意味着該單元格仍然具有cameraButton作爲子視圖。與圖像一樣的故事，您需要手動從單元格中移除它們，並確保舊的內容被替換或刪除。

當您執行將在一段時間後下載並設置圖像到單元格的方法時，該單元格仍可能包含前一個單元格的圖像。您應該在cellForRowAtIndexPath方法的開頭清除圖像以刪除舊的圖像/子視圖。

見下面的例子：

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

    let cell = collectionView.dequeueReusableCellWithReuseIdentifier("Cell", forIndexPath: indexPath) as! GridViewCell 


    if indexPath.row == 0 { 
     // Show the title becouse it's the first cell. 
     cell.titleLabel.hidden = false 

     // Title needs to be set in storyboard 
     // Default image for camera roll needs to be set in storyboard 

    } else { 
     // All the other cells. 
     cell.titleLabel.hidden = true 

     // Clear old content 
     cell.imageView.image = nil 

     // I don't exactly know what all this code does behind the screen, but I assume it's a method that downloads the image and add it to the imageView when it's done. 
     let asset = assetsFetchResults[indexPath.item - 1] as! PHAsset 
     cell.representedAssetIdentifier = asset.localIdentifier 

     imageManager.requestImageForAsset(asset, targetSize: AssetGridThumbnailSize, contentMode: PHImageContentMode.AspectFill, options: nil) { (result: UIImage?, info: [NSObject : AnyObject]?) -> Void in 
      //print(result) 
      if cell.representedAssetIdentifier.isEqualToString(asset.localIdentifier) { 
       if let imageResult = result { 
        cell.imageView.image = imageResult 
       } 
      } 
     } 
} 

func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) { 

    // Cell that needs to open the camera roll when tapped) 
    if indexPath.row == 0 { 

     // Do something that opens the camera roll 

    } 

} 

我沒有測試此代碼，但它應該是與此類似。