I have a calculator class, a first ViewController to insert the values and a second ViewController to show the result of the calculation. Unfortunately I get a error called "Can't unwrap Optional.None" if I click the button. I know it's something wrong with the syntax, but I don't know how to improve it.
The button in the first Viewcontroller is set to "Segue: Show (e.g. Push)" in the storyboard to switch to the secondViewController if he gets tapped.
the calculator class is something like:
class Calculator: NSObject {
func calculate (a:Int,b:Int) -> (Int) {
var result = a * b
return (result)
}
}
The Viewcontroller calls the function, inserts a/b and wants to change the label which is located in the secondviewcontroller:
class ViewController: UIViewController {
#IBAction func myButtonPressed(sender : AnyObject) {
showResult()
}
var numberOne = 4
var numberTwo = 7
var myCalc = Calculator()
func showResult () {
var myResult = myCalc.calculate(numberOne, b: numberTwo)
println("myResult is \(String(myResult))")
var myVC = secondViewController()
myVC.setResultLabel(myResult)
}
And here is the code of the secondViewController
class secondViewController: UIViewController {
#IBOutlet var myResultLabel : UILabel = nil
func setResultLabel (resultValue:Int) {
myResultLabel.text = String(resultValue)
}
init(coder aDecoder: NSCoder!)
{
super.init(coder: aDecoder)
}
In Swift, everything is public by default.
Define your variables outside the classes:
import UIKit
var placesArray: NSMutableArray!
class FirstViewController: UIViewController {
//
..
//
}
and then access it
import UIKit
class TableViewController: UITableViewController {
//
placesArray = [1, 2, 3]
//
}
The problem here is that the FirstViewController has no reference to the instance of SecondViewController. Because of this, this line:
secondViewController.setResultLabel(myResult)
does nothing (except probably causing the Can't unwrap Optional.None error). There are a few ways to solve this problem. If you are using storyboard segues you can use the -prepareForSegue method of UIViewController. Here is an example:
In FirstViewController:
override func prepareForSegue(segue: UIStoryboardSegue!,sender: AnyObject!){
//make sure that the segue is going to secondViewController
if segue.destinationViewController is secondViewController{
// now set a var that points to that new viewcontroller so you can call the method correctly
let nextController = (segue.destinationViewController as! secondViewController)
nextController.setResultLabel((String(myResult)))
}
}
Note: this code will not run as is because the function has no access to the result variable. you'll have to figure that out yourself :)
I think the issue here is, you are trying to set the UI component (here, its the label : myResultLabel)
When segue is fired from first view controller, the second view has not yet been initialized. In other words, the UI object "myResultLabel" is still nil.
To solve this, you will need to create a local string variable in second controller. Now, set that string to what you are trying to display, and finally, set the actual label in "viewDidLoad()" of the second controller.
Best Regards,
Gopal Nair.
Related
I am new to Swift and Xcode. I am building an Financial Expense ios app.
In my first view controller, I created a referencing outlet for a label called expenseNum.
In my second view controller, I have a function for a button called Add Expense. When it is clicked, I need it to update the expenseNum variable with the amount of the expense.
What is the best way to go about this? I had created an object of the first view controller class and accessed it like "firstviewcontroller.expenseNum" but this will create a new instance of the class and I need it to be all the same instance so it can continuously add to the same variable. Thanks for the help!
You need a delegate
protocol SendManager {
func send(str:String)
}
In first
class FirstVc:UIViewcontroller , SendManager {
func send(str:string) {
self.expenseNum.text = str
}
}
when you present SecondVc
let sec = SecondVc()
sec.delegate = self
// present
In second
class SecondVc:UIViewcontroller {
var delegate:SendManager?
#IBAction func btnClicked(_ sender:UIButton) {
delegate?.send(str:"value")
}
}
// setting delegate
in viewDidLoad of SecondVc
if let first = self.tabBarController.viewControllers[0] as? FirstVc {
self.delegate = first
}
There are several ways you can pass data from ViewController2 to another ViewController1
The best way here is Protocol Delegates
Please follow below steps to pass data
In Your SecondViewController from where you want to send data back declare a protocol at the top of class declaration
protocol SendDataBack: class {
func sendDataFromSecondVCtoFirstVC(myValue: String)
}
Now in the class , declare a object of your protocol in same ViewController
weak var myDelegateObj: SendDataBack?
And now in your Add Expense button action just call the delegate method
myDelegateObj?.sendDataFromSecondVCtoFirstVC(myValue: yourValue)
Now go to your first ViewController
the place from where you have pushed/present to SecondViewController you must have taken the object of SecondVC to push to push from first
if let secondVC = (UIStoryboard.init(name: "Main", bundle: nil)).instantiateViewController(withIdentifier: "secondVCID") as? SecondViewController {
vc?.myDelegateObj = self
self.navigationController?.pushViewController(secondVC, animated: true)
**OR**
self.present(secondVC, animated: true, completion: nil)
}
now in your FirstViewController make an extension of FirstViewVC
extension FirstViewController: SendDataBack {
func sendDataFromSecondVCtoFirstVC(myValue: String) {
}
}
I think you can make a variable in your properties in second ViewController (before viewDidLoad method)
var delegate: FirstViewController? = nil
and use from the properties of the first view controller anywhere of the second view controller.
delegate!.mainTableView.alpha=1.0
//for example access to a tableView in first view controller
The simplest way to achieve this is to use a public var. Add a new Swift file to your project, call it Globals. Declare the public variable in Globals.swift like so:
public var theValue: Int = 0
Set its required value in the first ViewController, and you'll find you can read it in the second with ease.
I know this question has been ask a lot on stack overflow. So I have a TabBarController that has 2 NavigationController, which both NavigationController have a TableViewController. I am using firebase to get a user, saving the user into a variable called currentUser. Now my problem starts here, I want to set the 2nd Navigation/Tableview controller title to the user's name. I know how to pass data using the prepare for segue, however there is no segue in TabBarController.
I've found a solution, not sure if its good or bad. What I did was make the first controller to be the delegate of the tab bar. Then I added tabBarController did select method. Here is the code.
class FirstTableVC: UITableViewController, UITabBarControllerDelegate {
var currentUser: User?
override func viewDidLoad() {
super.viewDidLoad()
tabBarController?.delegate = self
}
//Code that saves user
func code() {
...
...
...
}
func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
if viewController == tabBarController.viewControllers![1] {
let navController = tabBarController.viewControllers![1] as? UINavigationController
let secondTableVC = navController?.topViewController as! SecondTableVC
secondTableVC.currentUser = currentUser
}
}
}
class SecondTableVC: UITableViewController {
var currentUser: User?
override func viewDidLoad() {
super.viewDidLoad()
navigationItem.title =currentUser?.name
}
}
This works but not sure if this is a good way to do. I was wondering if there is a better way or a more efficient way. Thanks :)
Added:
Okay read this article about passing data using tabController. The author says that we should pass data using the state of the app. I am not really sure what he means by this. This is what I though he meant.
Example code:
class Person {
var name: String
var email: String
static var currentPerson: Person?
init(name: String, email: String) {
self.name = name
self.email = email
}
}
Can some please help me clarify . Thanks.
There's nothing wrong with this solution. Another way would be to have an abstracted class responsible for the user login object (instead of FirstTableVC having responsibility) that is accessible from both FirstTableVC and SecondTableVC
Add this in your FirstTableVC
func code() {
...
...
...
self.changeTabbarTitle()
}
func changeTabbarTitle() {
if let items = self.tabBarController?.tabBar.items {
items[1].title = currentUser?.name
}
}
You can use with delegate protocol
create NavigationTitle Protocol in FirstViewController
protocol NavigationTitle{
func setTitle(name:String)
}
class FirstViewController: UITableViewController,UITabBarControllerDelegate{
var delegate: NavigationTitle?
func setCurrentUser(){
delegate.setTitle(name:self.currentUser?.name)
}
func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
if viewController == tabBarController.viewControllers![1] {
let navController = tabBarController.viewControllers![1] as? UINavigationController
let secondTableVC = navController?.topViewController as! SecondTableVC
self.delegate = secondTableVC.self
}
}
}
implement protocol in SecondVC
class SecondVC: UITableViewController,NavigationTitle{
func setTitle(name:String){
navigationItem.title = name
}
}
I have a view controller (containing my menu) presented on top of another view controller (my application).
I would need to access the presenting view controller (below my menu) from the presented view controller (my menu), for example to access some variables or make the presenting view controller perform one of its segues.
However, I just can't figure out how to do it.
I'm aware of the "presentingViewController" and "presentedViewController" variables but I didn't manage to use them successfully.
Any Idea ?
Code (from the presented VC, which as a reference to the AppDelegate in which the window is referenced) :
if let presentingViewController = self.appDelegate.window?.rootViewController?.presentingViewController {
presentingViewController.performSegue(withIdentifier: "nameOfMySegue", sender: self)
}
Here is a use of the delegation Design pattern to talk back to the presenting view controller.
First Declare a protocol, that list out all the variables and methods a delegate is expected to respond to.
protocol SomeProtocol {
var someVariable : String {get set}
func doSomething()
}
Next : Make your presenting view controller conform to the protocol.
Set your presenting VC as the delegate
class MainVC: UIViewController, SomeProtocol {
var someVariable: String = ""
func doSomething() {
// Implementation of do
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
// Your code goes here.
if let destVC = segue.destination as? SubVC{
destVC.delegate = self
}
}
}
Finally, when you are ready to call a method on the presenting VC (Delegate).
class SubVC: UIViewController {
var delegate : SomeProtocol?
func whenSomeEventHappens() {
// For eg : When a menu item is selected
// Set some Variable
delegate?.someVariable = "Some Value"
// Call a method on the deleate
delegate?.doSomething()
}
}
Assuming that VCApplication is presenting VCMenu, in VCMenu you can access VCApplication with:
weak let vcApplication = self.presentingViewController as? VCApplicationType
Your example self.appDelegate.window?.rootViewController?.presentingViewController is looking for the ViewController that presented the rootViewController - it will be nil.
EDIT
Per TheAppMentor I've added weak so there are no retain cycles.
I have the following code in one of my classes.
override func tableView(tableView: UITableView!, didSelectRowAtIndexPath indexPath: NSIndexPath!) {
parkCode = tableView.cellForRowAtIndexPath(indexPath).text
RTATab.codeText.text = parkCode.substringToIndex(3)
RTATab.codeLetter.text = parkCode.substringFromIndex(3)
self.dismissModalViewControllerAnimated(true)
}
The RTATab referenced above is another class I have made (type UIViewController) and in that class I have declared it as a global class as show below as I need to access some of the textfields (codeText and codeLetter) in its view.
import UIKit
import messageUI
import CoreData
import QuartzCore
var RTATab : ViewController = ViewController()
class ViewController: UIViewController, MFMessageComposeViewControllerDelegate {
//some code
}
When I run this, I get a can't unwrap optional.none error on the line RTATab.codeText.text = parkCode.substringToIndex(3).
Can someone please help. Do I need to have an initialiser in viewController class?
Thanks
You are getting this error the text of the cell is nil. Before calling methods on parkCode, you must first check if it is nil:
let possibleParkCode = tableView.cellForRowAtIndexPath(indexPath).text
if let parkCode = possibleParkCode {
RTATab.codeText.text = parkCode.substringToIndex(3)
RTATab.codeLetter.text = parkCode.substringFromIndex(3)
}
You're getting the error because RTATab.codeText and RTATab.codeLetter are nil -- the way you're initializing RTATab doesn't actually link up its properties with the text fields in your storyboard. If you truly just need a global version of the view controller, you'd need to give it a storyboard ID and load it using something like:
var RTATab: ViewController = UIStoryboard(name: "Main", bundle: nil).instantiateViewControllerWithIdentifier("RTA") as ViewController
However, my guess is that it's a view controller you've already displayed elsewhere that you want to update, in which case you're better off just setting up a data structure that can hold the "parkCode" values and pulling them back into the correct view controller when it's time to display them.
Either parkCode or RTATab.codeText don't exist (are nil). You need to check for their existence prior to dereferencing either of them.
override func tableView (tableView: ...) {
if let theParkCode = tableView.cellForRowAtIndexPath(indexPath).text {
parkCode = theParkCode;
if let theCodeText = RTATab.codeText {
theCodeText.text = parkCode?.substringToIndex(3)
}
if let theCodeLetter ... {
// ...
}
}
}
Note: the above code depends on how your ViewController (the class of RTATab) declares its instance variables for codeText and codeLetter - I've assumed as optionals.
I know that this has to be a simple fix, but can't seem to understand why my code is not working. Basically I am trying to send a value from a text field in 1 view to a 2nd view's label.
ViewController.swift
#IBOutlet var Text1st: UITextField
#IBAction func Goto2ndView(sender: AnyObject) {
let view2 = self.storyboard.instantiateViewControllerWithIdentifier("view2") as MyView2
//view2.Label2nd.text=text;
self.navigationController.pushViewController(view2, animated: true)
}
MyView2.swift
#IBOutlet var Label2nd: UILabel
override func viewDidLoad() {
super.viewDidLoad()
var VC = ViewController()
var string = (VC.Text1st.text) //it doesn't like this, I get a 'Can't unwrap Option.. error'
println(string)
}
-------EDITED UPDATED CODE FROM (drewag)-------
ViewController.swift
let text = "text"
var sendString = Text1st.text
println(sendString) //successfully print it out.
let view2 = self.storyboard.instantiateViewControllerWithIdentifier("view2") as MyView2
view2.Label2nd.text=sendString;
self.navigationController.pushViewController(view2, animated: true)
MyView2.swift
#IBOutlet var Label2nd: UILabel
override func viewDidLoad() {
super.viewDidLoad()
var VC = ViewController()
var string = self.Label2nd.text
println(string) //still getting the error of an unwrap optional.none
}
var VC = ViewController() creates a new instance of ViewController. Unless there is a default value, you are not going to get any value out of VC.Text1st.text. You really should use a string variable on your second view controller to pass the data to it.
Also, a note on common formatting:
Class names should start with a capital letter (as you have)
Method / function names should start with a lower case letter
UIViewController subclasses should have "Controller" included in their name, otherwise, it looks like it is a subclass of UIView which is an entirely different level of Model View Controller (the architecture of all UIKit and Cocoa frameworks)
Edit:
Here is some example code:
class ViewController1 : UIViewController {
...
func goToSecondView() {
var viewController = ViewController2()
viewController.myString = "Some String"
self.navigationController.pushViewController(viewController, animated: true)
}
}
class ViewController2 : UIViewController {
var myString : String?
func methodToUseMyString() {
if let string = self.myString {
println(string)
}
}
...
}
Note, I am not creating ViewController2 using a storyboard. I personally prefer avoiding storyboards because they don't scale well and I find editing them to be very cumbersome. You can of course change it to create the view controller out of the storyboard if you prefer.
jatoben is correct that you want to use optional binding. IBOutlets are automatically optionals so you should check the textfield to see if it is nil.
if let textField = VC.Text1st {
println(textField.text)
}
This should prevent your app from crashing, but it will not print out anything because your text field has not yet been initialized.
Edit:
If you want to have a reference to your initial ViewController inside your second you're going to have to change a few things. First add a property on your second viewcontroller that will be for the first view controller:
#IBOutlet var Label2nd: UILabel //existing code
var firstVC: ViewController? //new
Then after you create view2, set it's firstVC as the ViewController you are currently in:
let view2 = self.storyboard.instantiateViewControllerWithIdentifier("view2") as MyView2 //already in your code
view2.firstVC = self //new
Finally in your viewDidLoad in your second view controller, use firstVC instead of the ViewController you recreated. It will look something like this:
override func viewDidLoad() {
super.viewDidLoad()
if let textField = firstVC?.Text2nd {
println(textField.text)
}
}
Use optional binding to unwrap the property:
if let string = VC.Text1st.text {
println(string)
}