如何在Objective-C中创建对象？

Question

I am new in iOS development. Currently I am reading this tutorial http://www.appcoda.com/how-to-handle-row-selection-in-uitableview/ . I am facing problem when I am reading this line

UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
cell.accessoryType = UITableViewCellAccessoryCheckmark;

I know object in objective-c is created by following way

 classname *objecname = [[classname alloc]init];

My confusion point is here UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];

How cell object is created here? Please tell details.

Answer 1

Based on the tutorial you linked to, I'm assuming you are dealing with selecting a row. When you select a row, you have access to the NSIndexPath of that row, which contains two parts:

1. The section of the cell
2. The row of the cell

With that information, let's break down the confusing code: UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];

UITableViewCell *cell

This part declares your variable. You've chosen to name it cell . You could just as easily have named it theCell , like so:

UITableViewCell *theCell

The * means that you're declaring a pointer, which is just a reference to an actual object, or the table view cell.

[tableView cellForRowAtIndexPath:indexPath]

The tableView refers to the UITableView that was just selected. UITableView has a method called cellForRowAtIndexPath , and what that method does is retrieve the cell at the specified section and row of the UITableView . In your case, it retrieves the row that you've just selected and stores the reference to it in your cell variable.

When declaring a new object, yes, it would take on the following syntax:

classname *objecname = [[classname alloc] init];

The key word here is new . When dealing with selecting rows in a UITableView , you don't want to create a new cell because you can't select a cell that doesn't exist. You want to get the cell that the user has just selected.

Answer 2

I assume you have this code in your didSelectRowAtIndexPath: method, is that right?

If so, you're tapping on a cell that is on the screen, so the cell object already exists. Where and how was it created? Inside cellForRowAtIndexPath: , which you have already written.

So when you make this call

UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];

it just gives you the cell object to work with, but it doesn't create a new one.

Answer 3

The cell is not created with that line. That line gets a reference to a cell that is already in the table view at that indexPath.