Swift closure crashes when called as Objective-C block

Question

In my project, I have both Objective-C and Swift code. I have some objects that have properties containing blocks to clean up some UITableView configuration. Using it works in Objective-C, but crashes when using Swift.

I have reduced the problem to be as minimal as possible, while still being reproducible.

// in Objective-C
@interface MyClass : NSObject
@property (copy, nonatomic) NSString* (^block)();
- (NSString *)callTheBlock;
@end

@implementation MyClass
- (NSString *)callTheBlock {
    if (self.block) {
        return self.block();
    } else {
        return @"There is no spoon";
    }
}
@end

// In Swift
// I actually have this in my AppDelegate to run when the app starts, but that shouldn't matter

class AppDelegate: UIResponder, UIApplicationDelegate {
    var myClass: MyClass?

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: NSDictionary?) -> Bool {
    self.myClass = MyClass()
    self.myClass?.block = { () -> String in 
        NSLog("In Closure")
        var string = "String From Closure" // <-- "po string" correctly prints the string
        return string // <-- This is where it crashes
    }

    let output = self.myClass?.callTheBlock()
    NSLog("\(output)")
...

}

I end up getting EXC_BAD_ACCESS, so I'm guessing it has something to do with something being released when it shouldn't, but I can't figure out what I'm doing wrong.

The stack trace is quite minimal:

* thread #1: tid = 0x4a8b6a, 0xbff2d9c8, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0xbff2d9c8)
  * frame #0: 0xbff2d9c8
    frame #1: 0xbff2d9e0

However, if I change the block to want a string as a parameter:

// MyClass 
@property (copy, nonatomic) NSString* (^block)(NSString *string);

// Using it in Swift
self.myClass?.block = { (inString: String!) -> String in 
    NSLog("In Closure")
    var string = "String From Closure"
    return string
}

I get a much more telling stack trace:

* thread #1: tid = 0x4a9a38, 0x01b07e63 libobjc.A.dylib`objc_release + 19, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x40000013)
    frame #0: 0x01b07e63 libobjc.A.dylib`objc_release + 19
  * frame #1: 0x0005e442 MyApp`MyApp.AppDelegate.(inString=Some) -> (ObjectiveC.UIApplication, didFinishLaunchingWithOptions : Swift.Optional<ObjectiveC.NSDictionary>) -> Swift.Bool).(closure #1) + 594 at AppDelegate.swift:22
    frame #2: 0x0005e4ad MyApp`reabstraction thunk helper from @callee_owned (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) to @callee_owned (@in Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@out Swift.ImplicitlyUnwrappedOptional<Swift.String>) + 77 at AppDelegate.swift:19
    frame #3: 0x0005cbaa MyApp`partial apply forwarder for reabstraction thunk helper from @callee_owned (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) to @callee_owned (@in Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@out Swift.ImplicitlyUnwrappedOptional<Swift.String>) + 90 at AppDelegate.swift:0
    frame #4: 0x0005e5e7 MyApp`reabstraction thunk helper from @callee_owned (@in Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@out Swift.ImplicitlyUnwrappedOptional<Swift.String>) to @callee_owned (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) + 71 at AppDelegate.swift:19
    frame #5: 0x0005ce64 MyApp`partial apply forwarder for reabstraction thunk helper from @callee_owned (@in Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@out Swift.ImplicitlyUnwrappedOptional<Swift.String>) to @callee_owned (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) + 132 at AppDelegate.swift:0
    frame #6: 0x0005e7ee MyApp`reabstraction thunk helper from @callee_owned (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> (@owned Swift.ImplicitlyUnwrappedOptional<Swift.String>) to @callee_unowned @objc_block (@unowned Swift.ImplicitlyUnwrappedOptional<ObjectiveC.NSString>) -> (@autoreleased Swift.ImplicitlyUnwrappedOptional<ObjectiveC.NSString>) + 478 at AppDelegate.swift:19
    frame #7: 0x0008fa3f MyApp`-[MyClass callTheBlock](self=0x7beadc50, _cmd=0x000edee6) + 143 at MyClass.m:38
    frame #8: 0x0005bcf8 MyApp`MyApp.AppDelegate.application (application=0x7c160390, launchOptions=None, self=0x7c162d50)(ObjectiveC.UIApplication, didFinishLaunchingWithOptions : Swift.Optional<ObjectiveC.NSDictionary>) -> Swift.Bool + 1544 at AppDelegate.swift:24
    frame #9: 0x0005cf55 MyApp`@objc MyApp.AppDelegate.application (MyApp.AppDelegate)(ObjectiveC.UIApplication, didFinishLaunchingWithOptions : Swift.Optional<ObjectiveC.NSDictionary>) -> Swift.Bool + 101 at AppDelegate.swift:0
    frame #10: 0x0090d14f UIKit`-[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 309
    frame #11: 0x0090daa1 UIKit`-[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 1810
    frame #12: 0x00912667 UIKit`-[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 824
    frame #13: 0x00926f92 UIKit`-[UIApplication handleEvent:withNewEvent:] + 3517
    frame #14: 0x00927555 UIKit`-[UIApplication sendEvent:] + 85
    frame #15: 0x00914250 UIKit`_UIApplicationHandleEvent + 683
    frame #16: 0x03b9df02 GraphicsServices`_PurpleEventCallback + 776
    frame #17: 0x03b9da0d GraphicsServices`PurpleEventCallback + 46
    frame #18: 0x01cf6ca5 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 53
    frame #19: 0x01cf69db CoreFoundation`__CFRunLoopDoSource1 + 523
    frame #20: 0x01d2168c CoreFoundation`__CFRunLoopRun + 2156
    frame #21: 0x01d209d3 CoreFoundation`CFRunLoopRunSpecific + 467
    frame #22: 0x01d207eb CoreFoundation`CFRunLoopRunInMode + 123
    frame #23: 0x00911d9c UIKit`-[UIApplication _run] + 840
    frame #24: 0x00913f9b UIKit`UIApplicationMain + 1225
    frame #25: 0x0005dbae MyApp`top_level_code + 78 at AppDelegate.swift:12
    frame #26: 0x0005dbeb MyApp`main + 43 at AppDelegate.swift:0
    frame #27: 0x024e3701 libdyld.dylib`start + 1

edit: I originally thought it had something to do with being called asynchronously, but it was reproducible calling it directly afterwards like this, so I fixed the code and stack trace to reflect that.

Answer 1

I figured it out with the help of the pop-up documentation! When dealing with Objective-C, most things will require a !. I'm pretty sure this is because Objective-C has no concept of Optionals like in Swift.

Instead of defining my closure as:

... self.myClass?.block = { (inString: String!) -> String in ...

It works when changing it to:

... self.myClass?.block = { (inString: String!) -> String! in ...

Which makes sense, because that is what is recommended in the pop-up documentation.