要实现在点击新单元格图像时从前景中移除前一个图像的所需行为，您可以跟踪当前显示的图像视图，并在添加新图像之前将其移除。

var currentImageView: UIImageView?

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: FriendCollectionViewCell.cellId, for: indexPath) as! FriendCollectionViewCell
    
    let tapGestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(cellTappedMethod(_:)))
    cell.profileImageView?.gestureRecognizers?.removeAll()
    cell.profileImageView.isUserInteractionEnabled = true
    cell.profileImageView.tag = indexPath.row
    cell.profileImageView.addGestureRecognizer(tapGestureRecognizer)
    
    return cell
}

@objc func cellTappedMethod(_ sender: UITapGestureRecognizer) {
    let imageView = sender.view as! UIImageView
    
    currentImageView?.removeFromSuperview()
    
    let newImageView = UIImageView(image: imageView.image)
    newImageView.frame = CGRect(x: 0, y: 0, width: self.view.frame.width * 0.7, height: 300.0)
    newImageView.center = CGPoint(x: UIScreen.main.bounds.center.x - 15, y: UIScreen.main.bounds.center.y - 200)
    newImageView.backgroundColor = UIColor.clear
    newImageView.contentMode = .scaleAspectFit
    newImageView.isUserInteractionEnabled = true
    
    let tap = UITapGestureRecognizer(target: self, action: #selector(dismissFullscreenImage))
    newImageView.addGestureRecognizer(tap)
    self.view.addSubview(newImageView)
    currentImageView = newImageView
}

@objc func dismissFullscreenImage(sender: UITapGestureRecognizer) {
    sender.view?.removeFromSuperview()
    currentImageView = nil
}

在这段代码中，currentImageView变量是在类级别声明的，以跟踪当前显示在屏幕上的图像视图。点击单元格图像时，将从超级视图中删除前一个图像视图（如果有），然后再添加新视图。currentImageView变量将使用新的图像视图进行更新，并且在关闭全屏图像时，将清除引用以指示当前未显示任何图像视图。
至于在newImageView外部点击时没有调用touchesBegan的问题，这是因为newImageView位于集合视图的顶部并拦截了触摸事件。一种解决方案是子类化UICollectionView并覆盖子类中的touchesBegan方法。

class MyCollectionView: UICollectionView {
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        super.touchesBegan(touches, with: event)
    }
}