Setting UITableView datasource programmatically makes it reload table asynchronously

Question

I want to call tableView.reloadData() and be sure that after that table rows are rendered. The problem is that not happening when I am setting datasource programmatically before that. Example:

class ViewController: UIViewController, UITableViewDataSource {
    @IBOutlet weak var tableView: UITableView!
    override func viewDidLoad() {
        super.viewDidLoad()
        tableView.dataSource = self
        tableView.reloadData()
        print("after reloadData")
    }
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 1
    }
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        print("cellForRowAt")
        return UITableViewCell()
    }
}

Output is:

print("after reloadData")
print("cellForRowAt")

I expect it to print “cellForRowAt” first. Why is that happening?

Update: It's silly but I thought reloadData() was synchronous. What did the trick for me is: self.tableView.layoutIfNeezded() Thank you and thx this guy.

You are reloading the table before it is on screen, so it doesn't need to perform a rendering pass. Once the tableview is visible (After `viewDidLayoutSubviews`) it will need cells — Paulw11, Sep 25 '19 at 11:58
Well actually the call to the method tableView.reloadData() is asynchronous . Or more acurately methods like numberOfRows ,heightForRow gets calledUp first ,but CellForRowAtIndexPath doesnot. This is the reason why you see this behaviour. — Friend, Sep 25 '19 at 12:09

score 0 · Accepted Answer · answered Sep 25 '19 at 12:14

0

The function tableView.reloadData() is asynchronous it contains a patch of calls to dataSource methods in order until that happens you see the print first

answered Sep 25 '19 at 12:14

Shehata Gamal

98,760
8
65
87

Setting UITableView datasource programmatically makes it reload table asynchronously

1 Answers1