How to purge tableview cells before cellforRowAt is called? - swift

Question

I have the following code
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell{
//creating a cell using the custom class
if (indexPath.row == 0 && indexPath.section == 0) {
var cell:UITableViewCell;
cell = tableView.dequeueReusableCell(withIdentifier: "book", for: indexPath)
cell.selectionStyle = UITableViewCell.SelectionStyle.none;
self.ref.observe(DataEventType.value, with: { (snapshot) in
self.ref.child("featured").observeSingleEvent(of: .value, with: { (snapshot) in
// Get user value
let value = snapshot.value as? NSDictionary
let title = value?["title"] as? String ?? ""
self.featured = value?["url"] as? String ?? ""
let label = UILabel(frame: CGRect(x: 0, y: 0, width: cell.frame.size.width, height: 24))
label.numberOfLines = 0
label.font = UIFont(name: "CeraPro-Medium", size: 20);
label.textColor = UIColor.black
label.text = NSLocalizedString(" Free Daily Pick", comment: "featured")
label.adjustsFontForContentSizeCategory = true
cell.addSubview(label)
if let image = value?["image"] as? String, image.isEmpty {
print("String is nil or empty.")
} else {
if let image = value?["image"] {
let imageView = UIImageView();
imageView.frame = CGRect(x: 0,
y: 75,
width: cell.frame.size.width,
height: 150)
imageView.contentMode = .scaleAspectFill
imageView.isUserInteractionEnabled = false
imageView.sd_setImage(with: URL(string: image as! String), placeholderImage: nil)
cell.addSubview(imageView)
let label = UILabel(frame: CGRect(x: 10,
y: cell.frame.size.height-60,
width: cell.frame.size.width,
height: 50));
label.textColor = UIColor.black
label.backgroundColor = UIColor.init(red: 1, green: 1, blue: 1, alpha: 0.5)
label.font = UIFont(name: "CeraPro-Bold", size: 16)
label.text = " \(title)"
let view = UIView()
view.frame = CGRect(x: 0, y: 0, width: cell.frame.size.width, height: 150);
view.tag = 123
view.addSubview(imageView)
view.addSubview(label)
cell.contentView.addSubview(view)
}
}
// ...
}) { (error) in
print(error.localizedDescription)
}
})
return cell
} else {
let cell = tableView.dequeueReusableCell(withIdentifier: "book", for: indexPath) as! BookTableViewCell
cell.cellIndex = indexPath
cell.dataSource = self
cell.delegate = self
cell.backgroundColor = UIColor.white
var books = [Book]();
let count = self.enterpriseBooks.count;
if count > 0 && indexPath.section <= self.enterpriseBooks_2.count {
books = self.enterpriseBooks_2[indexPath.section - 1]!;
}
if (indexPath.section == (count + 1)) {
books = nytbooks;
} else if (indexPath.section == (count + 2)) {
books = trendingbooks;
} else if (indexPath.section == (count + 3)) {
books = newbooks
}
if (books.count > 0) {
if (cell.collectionView === nil) {
cell.addCollectionView();
cell.collectionView.reloadData()
}
}
return cell
}
}
Here for section 0, row 0 - I show a featured image
For section 1, 2, 3 - I show a horizontal collection view of book images.
What's happening is by using dequeue function to get the table view cell, the table view is caching some table view cell content which overlaps each other. Is there a way to purge table view cells before cellForRowAt is called?

Answer 1

Programmatically adding subviews to a cell seems a terrible idea to me. Try to use auto layout and xib files whenever possible.
Anyway, you theoretically could remove all the subviews from a dequeued cell and then adding them again.
for view in cell.subviews {
view.removeFromSuperview()
}
That said, there are better approaches to use a special cell in the first position, like using different cell types.

Answer 2

well, to handle caching data you have to handle all else cases to assign appropriate values to UI Elements in case of nil/ failure scenarios. For an instance you have to assign nil to UIImageView's image property if image url is nil/ empty like this
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell{
//creating a cell using the custom class
if (indexPath.row == 0 && indexPath.section == 0) {
var cell:UITableViewCell;
cell = tableView.dequeueReusableCell(withIdentifier: "book", for: indexPath)
cell.selectionStyle = UITableViewCell.SelectionStyle.none;
self.ref.observe(DataEventType.value, with: { (snapshot) in
self.ref.child("featured").observeSingleEvent(of: .value, with: { (snapshot) in
// Get user value
let value = snapshot.value as? NSDictionary
let title = value?["title"] as? String ?? ""
self.featured = value?["url"] as? String ?? ""
let label = UILabel(frame: CGRect(x: 0, y: 0, width: cell.frame.size.width, height: 24))
label.numberOfLines = 0
label.font = UIFont(name: "CeraPro-Medium", size: 20);
label.textColor = UIColor.black
label.text = NSLocalizedString(" Free Daily Pick", comment: "featured")
label.adjustsFontForContentSizeCategory = true
cell.addSubview(label)
let imageView = UIImageView();
if let image = value?["image"] as? String, image.isEmpty {
print("String is nil or empty.")
imageView.image = nil
// same goes for the label
} else {
if let image = value?["image"] {
imageView.frame = CGRect(x: 0,
y: 75,
width: cell.frame.size.width,
height: 150)
imageView.contentMode = .scaleAspectFill
imageView.isUserInteractionEnabled = false
imageView.sd_setImage(with: URL(string: image as! String), placeholderImage: nil)
cell.addSubview(imageView)
let label = UILabel(frame: CGRect(x: 10,
y: cell.frame.size.height-60,
width: cell.frame.size.width,
height: 50));
label.textColor = UIColor.black
label.backgroundColor = UIColor.init(red: 1, green: 1, blue: 1, alpha: 0.5)
label.font = UIFont(name: "CeraPro-Bold", size: 16)
label.text = " \(title)"
let view = UIView()
view.frame = CGRect(x: 0, y: 0, width: cell.frame.size.width, height: 150);
view.tag = 123
view.addSubview(imageView)
view.addSubview(label)
cell.contentView.addSubview(view)
}
}
// ...
}) { (error) in
print(error.localizedDescription)
}
})
return cell
} else {
let cell = tableView.dequeueReusableCell(withIdentifier: "book", for: indexPath) as! BookTableViewCell
cell.cellIndex = indexPath
cell.dataSource = self
cell.delegate = self
cell.backgroundColor = UIColor.white
var books = [Book]();
let count = self.enterpriseBooks.count;
if count > 0 && indexPath.section <= self.enterpriseBooks_2.count {
books = self.enterpriseBooks_2[indexPath.section - 1]!;
}
if (indexPath.section == (count + 1)) {
books = nytbooks;
} else if (indexPath.section == (count + 2)) {
books = trendingbooks;
} else if (indexPath.section == (count + 3)) {
books = newbooks
}
if (books.count > 0) {
if (cell.collectionView === nil) {
cell.addCollectionView();
cell.collectionView.reloadData()
}
}
return cell
}
}
In all the nil cases you are doing nothing with related UI element and that allows dequeueReusability to retain old content. Just assign nil or default value to UI elements in else / nil case, it will resolve your issue