Cannot access property on Swift type from Objective-C

问题

I am trying to access a Swift class's Double? property from Objective-C.

class BusinessDetailViewController: UIViewController {

    var lat : Double?
    var lon : Double?

    // Other elements...
}

In another view controller, I am trying to access lat like following:

#import "i5km-Swift.h"
@interface ViewController ()

@property (strong, nonatomic) BusinessDetailViewController *businessDetailViewController;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.businessDetailViewController = [[BusinessDetailViewController alloc] initWithNibName:@"BusinessDetailViewController" bundle:nil];
    self.businessDetailViewController.lat = businessArray[1]; /* THIS GIVES ME AN ERROR */
}

and I am getting

Property 'lat' not found on object of type 'BusinessDetailViewController *'

Why can't I access this property? What am I missing?

回答1:

Optional values of non-Objective-C types aren't bridged into Objective-C. That is, the first three properties of TestClass below would be available in Objective-C, but the fourth wouldn't:

class TestClass: NSObject {
    var nsNumberVar: NSNumber = 0      // obj-c type, ok
    var nsNumberOpt: NSNumber?         // optional obj-c type, ok
    var doubleVar: Double = 0          // bridged Swift-native type, ok
    var doubleOpt: Double?             // not bridged, inaccessible
}

In your Objective-C code, you'd access those first three properties like this:

TestClass *optTest = [[TestClass alloc] init];
optTest.nsNumberOpt = @1.0;
optTest.nsNumberVar = @2.0;
optTest.doubleVar = 3.0;

In your case, you can either convert lat and long to be non-Optional or switch them to be instances of NSNumber.

---

Note that you need to be careful about your Objective-C code if you take the second approach (switching lat and lon to non-optional properties of type NSNumber) -- while the Swift compiler will prevent you from assigning nil to non-optional properties, the Objective-C compiler has no qualms about allowing it, letting nil values sneak into your Swift code with no chance of catching them at runtime. Consider this method on TestClass:

extension TestClass {
    func badIdea() {
        // print the string value if it exists, or 'nil' otherwise
        println(nsNumberOpt?.stringValue ?? "nil")

        // non-optional: must have a value, right?
        println(nsNumberVar.stringValue)
    }
}

This works fine if invoked with values in both of the properties, but if nsNumberVar is set to nil from the Objective-C code, this will crash at runtime. Note that there is no way to check whether or not nsNumberVar is nil before using it!

TestClass *optTest = [[TestClass alloc] init];
optTest.nsNumberOpt = @1.0;
optTest.nsNumberVar = @2.0;
[optTest badIdea];
// prints 1, 2

optTest.nsNumberOpt = nil;
optTest.nsNumberVar = nil;
[optTest badIdea];
// prints nil, then crashes with an EXC_BAD_ACCESS exception

回答2:

If your property is a Swift protocol type, just add @objc in front of it.

Example:

class Foo: UIViewController {
   var delegate: FooDelegate?
   ...
}

@objc protocol FooDelegate {
   func bar()
}

回答3:

Optionals is a swift specific feature, not available in obj-c. Optional class instances work because a nil optional can be mapped to a nil value, but value types (int, floats, etc.) are not reference types, hence variable of those types don't store a reference, but the value itself.

I don't know if there's a solution - a possible workaround is creating non optional properties mapping the nil value to an unused data type value (such as -1 when representing an index, or 999999 for a coordinate):

class Test {
    var lat : Double? {
        didSet {
            self._lat = self.lat != nil ? self.lat! : 999999
        }
    }
    var lon : Double? {
        didSet {
            self._lon = self.lon != nil ? self.lon! : 999999
        }
    }

    var _lat: Double = 99999999
    var _lon: Double = 99999999
}

That should expose the _lat and _lon properties to obj-c.

Note that I have never tried that, so please let us know if it works.

回答4:

[UInt? Int? or Double? properties] cannot be marked @objc because its type cannot be represented in Objective-C.

It is, however, possible to "wrap" them in a NSNumber like so :

class Foo {
    var bar:Double?
}

// MARK: Objective-C Support
extension Foo {
    /// bar is `Double?` in Swift and `(NSNumber * _Nullable)` in Objective-C
    @objc(bar)
    var z_objc_bar:NSNumber? {
        get {
            return bar as NSNumber?
        }
        set(value) {
            bar = value?.doubleValue ?? nil
        }
    }
}

来源：https://stackoverflow.com/questions/26366082/cannot-access-property-on-swift-type-from-objective-c