表格视图单元格说明

Question

Im following a course online to learn iOS. Im using Swift 4.2.

My question is about this method:

// This function is defining each cell and adding contenet to it.
    internal func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

        let cell = UITableViewCell(style: .default, reuseIdentifier: "Cell")

        cell.textLabel?.text = cellContent[indexPath.row]

        return cell

    }

---

Exactly how the above method works in the following code? I know that the above method describes each cell of the Table View but does it gets called by the table view for each row?

What does indexpath.row means exactly? Im confused with this one.

Please help me. Thanks.

import UIKit

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {

    var cellContent:Array<String> = ["Amir", "Akbar", "Anthony"]


    // This function is setting the total number of cells in the table view
    internal func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {

        return cellContent.count

    }

    // This function is defining each cell and adding contenet to it.
    internal func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

        let cell = UITableViewCell(style: .default, reuseIdentifier: "Cell")

        cell.textLabel?.text = cellContent[indexPath.row]

        return cell

    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }


}

Answer 1

Apple's documentation on IndexPath says: Each index in an index path represents the index into an array of children from one node in the tree to another, deeper, node.

In plain English it basically means IndexPath s are a way to access a two dimensional array, which is what the dataSource of a tableView is. The tableView needs to know how many sections it has, and how many rows there are in each section.

In your case there is only one section so you don't need to worry about indexPath.section because the section is always 0. There is only one array (your cellContent array) in the tableView 's multidimensional dataSource so you can access elements using indexPath.row . If you had more than one cellsContent Array you would have to use indexPath.section to access the right one before you could use indexPath.row

You've omitted the numberOfSections method of UITableViewDatasource which returns 1 by default.

Answer 2

In addition to Tom Pearson answer on IndexPath,

internal func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell 

Yes, this method is called for every visible cells in the tableView.

If you use below method in cellForRowAt method, cells will be reused without instantiating more cell objects.

let cell = tableView.dequeueReusableCell(withIdentifier: action,
                                                   for: indexPath as IndexPath)

Once the cell goes out of visibility (lets say it is scrolled), the cell object is reused for newly visible row and it is not newly created for each and every cells. There lies the power of this method.

Usually code will be like this.

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

    guard let cell = tableView.dequeueReusableCell(withIdentifier: "someIdentifier",
                                                   for: indexPath as IndexPath) as? SomeCell else {
        fatalError("Cell is not of type SomeCell")
    }

    cell.title.font = somefont
    cell.title?.textColor = somecolor
    cell.backgroundColor = UIColor.clear

    return cell
}