从另一个班级的一个班级快速更改属性（特别是UILabel）

Question

I'm trying to change the text of a UILabel in a UITableView (a property of one class,) from inside another class, but I'm having trouble. The code looks like this (The problem is at the end, in the didSelectRowAtIndexPath method of myDataSource)

The View Controller

class ViewController: UIViewController
{
    //MARK: Properties
    @IBOutlet weak var myTableView: UITableView!
    @IBOutlet weak var myLabel: UILabel!

    //needed so data from myDataSource is retained by ViewController and not just thrown away
    let importedDataSource = myDataSource()


    override func viewDidLoad()
    {
        super.viewDidLoad()

        myTableView.dataSource = myDataSource()
        myTableView.delegate = myDataSource()

    }

   override func didReceiveMemoryWarning()
       {super.didReceiveMemoryWarning()}

}

The UITableViewDataSource and Delegate

class myDataSource: NSObject, UITableViewDataSource, UITableViewDelegate
{
    let cellIdentifier = "myTableViewCell"
    let myArray = ["Label one", "Label two", "Label three"]

    //MARK: TableViewDataSource
    func numberOfSectionsInTableView(tableView: UITableView) -> Int
     {return 1}

    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int
    {return myArray.count}

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell
    {
        let cell = tableView.dequeueReusableCellWithIdentifier(cellIdentifier, forIndexPath: indexPath) as! myTableViewCell

        cell.myCellLabel.text = myArray[indexPath.row]

        return cell
    }
    //MARK:TableView Delegate
   func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath)
     {  
          //This Line Here returns the error
          ViewController().myLabel.text = "test"
          //I want the end product to be ViewController().myLabel.text = myArray[indexPath.row], but left it as "test" just to simplify and isolate the error
     }

}

(there's also a UITableViewCell class called myTableViewCell, but I left it out to be shorter)

Running returns "fatal error: unexpectedly found nil while unwrapping an Optional value" upon selecting one of the rows. How should I call myLabel so as to avoid this problem? I've tried hooking it up as an @IBOutlet inside myDataSource by ctrl-dragging it from the storyboard, but as I expected, it only lets me connect it to the view controller.

sorry, the code is a bit confusing with the two similarly named labels. I'm trying to call the myLabel var created in the ViewController (first piece of code), not the myCellLabel created in myTableViewCell (not shown)

Answer 1

You need to get the actual table view cell that was tapped in order to access it's variables. Use cellForRowAtIndexPath . It'll look something like this:

func tableView(tableView: UITableView, didDeselectRowAtIndexPath indexPath: NSIndexPath) {
    if let myCell = tableView.cellForRowAtIndexPath(indexPath) as? myTableViewCell {
        myCell.myLabel.text = "test"
    }
}

Answer 2

The reason you are seeing the message " fatal error: unexpectedly found nil while unwrapping an Optional value " is because you are trying to assign a value to a property of nil .

In the implementation of your UITableViewDelegate method tableView(_:didSelectRowAtIndexPath:) the statement:

ViewController().myLabel.text = "test"

1. creates a new instance of your ViewController class (that is what ViewController() does)
2. accesses the myLabel property of the new instance (recall that in your declaration of the class you defined this property to be of type UILabel! , an implicitly-unwrapped optional UILabel , and implicitly-unwrapped optionals begin life as nil until they are assigned a value)
3. tries to assign "test" to the text property of myLabel - but as we just noted, myLabel is nil and this is why you are receiving the error message

EDIT

In an effort to assist you in your stated goal of " trying to change the text of a UILabel " (that label being the property myLabel of your ViewController instance), the route I would recommend is:

• have your ViewController class adopt the UITableViewDelegate protocol
• implement your UITableViewDelegate protocol methods inside of your ViewController class, where they will have direct access to the myLabel property

once you've done this, your implementation of that delegate method (now residing inside of your ViewController class) would look like:

func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {  
  myLabel.text = myTableView.dataSource.myArray[indexPath.row]
}

...and while this will work, even this can be improved - I recommend you check out Apple's Table View Programming Guide

furthermore, unless there is a compelling reason not to, I also recommend that you have your ViewController class act as your UITableViewDataSource - from the Table View Programming Guide: " [t]he class creating the table view typically makes itself the data source and delegate by adopting the UITableViewDataSource and UITableViewDelegate protocols " (see this section )