iOS - segue and tableView

Question

I am new to iOS programming. I am following a book and some tutorials to learn it. I need some help in understanding these methods

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"PushAppDetails"])
    {
        AppDetailsViewController *appDetailsViewController = segue.destinationViewController;
        UITableViewCell *cell = sender;
        appDetailsViewController.appDetails =
        [[AppDetails alloc] initWithName:cell.textLabel.text
                             description:cell.detailTextLabel.text];
    }
}



{
    //Set the CellIdentifier that you set in the storyboard
    static NSString *CellIdentifier = @"AppCell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    switch (indexPath.row)
    {
        case 0:
            cell.textLabel.text = @"Awesome App";
            cell.detailTextLabel.text = @"Long description of the awesome app...";
            break;
        case 1:
            cell.textLabel.text = @"Even More Awesome App";
            cell.detailTextLabel.text = @"Long description of the even more awesome app...";
            break;
        case 2:
            cell.textLabel.text = @"The Most Awesome App Ever";
            cell.detailTextLabel.text =
            @"Long description of the most awesome app ever seen...";
            break;

        default:
            cell.textLabel.text = @"Unkown";
            cell.detailTextLabel.text = @"Unknown";
            break;
    }

    return cell;
}

What I am not understanding here is these lines

 UITableViewCell *cell = sender;
            appDetailsViewController.appDetails =
            [[AppDetails alloc] initWithName:cell.textLabel.text
                                 description:cell.detailTextLabel.text];

I got this that I am identifying the segue from this line[segue.identifier isEqualToString:@"PushAppDetails"] and then I created the object of AppdetailsViewController class but i didn't get what this line is doing

 UITableViewCell *cell = sender; 

and how this line is calling the bottom table function where the switch function is and description of each cell and neither this line

appDetailsViewController.appDetails =
        [[AppDetails alloc] initWithName:cell.textLabel.text
                             description:cell.detailTextLabel.text];

i have a method in my appDetails Class .. why not simply i can do this if i have to access that method

 AppDetails *app = new [AppDetails alloc]init
  [app initWithName:cell.textLabel.text
                             description:cell.detailTextLabel.text];

I actually come from java so i feel a little difficult to understand this

Answer 1

UITableViewCell *cell = sender; basically casts sender via a variable to an instance of UITableViewCell. This is used so you can access cell.textLabel and cell.detailTextLabel later. Since sender is of type id you can't write sender.textLabel. But you could cast in place if you like:

appDetailsViewController.appDetails =
    [[AppDetails alloc] initWithName:((UITableViewCell *)sender).textLabel.text
                         description:((UITableViewCell *)sender).detailTextLabel.text];

If you like that more feel free to use it. I personally prefer the assignment to the specific class, because it's more obvious.

and this line does not call tableView:cellForRowAtIndexPath:, it takes the NSStrings that are already set on the UITableViewCell. Apples code that calls prepareForSegue:sender: will pass the touched cell as parameter sender.

---

AppDetails *app = new [AppDetails alloc]init
[app initWithName:cell.textLabel.text description:cell.detailTextLabel.text];

First of all that's not valid Objective-C. If you write code in Objective-C you should not expect that you can use Java syntax.

And in Objective-c you should not call init more than once, depending on the implementation of init this can have many weird effects.

For example take this code:

- (id)init {
    if (self = [super init]) {
        self.label = [[UILabel alloc] init];
        [self.view addSubview:self.label];
    }
    return self;
}

- (id)initWithName:(NSString *)name {
    if (self = [self init]) {   // calls init
        self.label.text = name;
    }
    return self;
}

If you call init it will add a new UILabel, and if you call initWithName: later, it will add another UILabel, because initWithName: will call init itself.
So if you call init first and then initWithName: you end up with two UILabels.
Since you don't know the implementation details of most init methods you are calling you should never call init more than once in Objective-C.

init... should always be a part of [[Object alloc] init....