Lazy loading Properties in swift

Question

I'm attempting to wrap my head around the Swift language. A common pattern when building views in code with Objective-C is to override UI properties and lazy load them like so:

@property(nonatomic, strong) UILabel *myLabel;  - (UILabel *)myLabel {      if (!_myLabel) {          _myLabel = [[UILabel alloc] initWithFrame:CGRectMake(20.0f, 75.0f, 320.0f, 20.0f)];         [_myLabel setFont:[UIFont subHeadlineFont]];         [_myLabel setTextColor:[UIColor subHeadlineColor]];         [_myLabel setText:@"Hello World"];      }      return _myLabel; }  - (void)viewDidLoad {     [super viewDidLoad];     [self.view addSubview:self.myLabel]; } 

This allows for the configuration of UIElements to be self contained within their setup but doesn't result in reconfiguring them every time.

It seems we don't have access to the backing store in Swift and the @lazy keyword doesn't really have the same semantics.

I'm curious if anyone has identified a similar pattern in Swift that allows one to keep the configuration of variables and constants together with their declaration in a neat syntactic way that doesn't result in reconfiguration every time?

Answer 1

I think a lazy property initialized with a closure would work:

lazy var myLabel: UILabel = {     var temporaryLabel: UILabel = UILabel()     ...     return temporaryLabel }() 

As I read “The Swift Programming Language.” (Checkerboard example) the closure is only evaluated once).

Answer 2

class Thingy {     init(){         println("making a new Thingy")     } }  var thingy = {     Thingy(); }()  println("\(self.thingy)") println("\(self.thingy)") 

The log message "making a new Thingy" appears just once, proving that only one Thingy was created - the closure was called only once, namely to initialize this property. This is effectively what you are describing. All you have to do is add more to the closure so as to configure it the returned object.

If you make the var @lazy and comment out the println statements, no Thingy is created, proving that the laziness does what it is intended to do; you could omit this, however, since you know that the label will in fact always be needed early on. The point of @lazy is to prevent the closure from ever being called unless the getter is called, but you are always going to call the getter so that is pointless in your situation.