Swift closure in array becomes null in Objective-c
I created an objective-c method that will call the method via NSInvocation:
typedef void (^ScriptingEmptyBlock)();
typedef void (^ScriptingErrorBlock)(NSError *error);
- (void)scripting_execute:(NSString *)operation withParams:(nullable NSArray *)args {
SEL selector = [self scripting_selectorForOperation:operation];
Class class = [self class];
NSMethodSignature *signature = [class instanceMethodSignatureForSelector:selector];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
[invocation setSelector:selector];
[invocation setTarget:self];
for (int idx = 0; idx < args.count; idx ++) {
id arg = args[idx];
[invocation setArgument:&arg atIndex:idx + 2];
}
ScriptingEmptyBlock success = args[1];
// Breakpoint added on next line to test for nil
success(); // this is nil and would crash!
// (lldb) po args.count
// 3
// (lldb) po success
// Printing description of success:
// (ScriptingEmptyBlock) success = 0x0000000000000000
// (lldb) po args[1]
// (Function)
//[invocation getArgument:&success atIndex:2]; // also tried this and got nil as well
[invocation invoke];
}
The method takes an "operation", which is translated into a selector by overriding scripting_selectorForOperation:
in subclasses and then invoked.
This all works, except when the called method has block arguments, they are zero, I added a test for nil, which I describe with comments, when trying to read the closure from the array it will be zero.
Called as:
let successClosure: ScriptingEmptyBlock = {
print("Renamed product")
}
let errorClosure: ScriptingErrorBlock = { error in
print("Failed to rename product: \(error)")
}
let params:[Any] = [ "testName", successClosure, errorClosure]
object.scripting_execute (ScriptOperation.updateProductName.rawValue, withParams: params)
Why does the close go to zero?
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success
not nil
(actually NSArray
cannot contain nil
s). If you type it as NSLog(@"%@", success);
, it will say (Function)
, not (null)
. And if you type your class like NSLog(@"%@", [success class]);
it will say _SwiftValue
. Basically, it's a Swift value that connects to Objective-C.
The problem is that the object is success
indicating that it is not an Objective-C block. This is a quick closure, and Swift closure is not the same as Objective-C. Trying to use it as if it were an Objective-C block caused undefined behavior. po
in the debugger it doesn't display it correctly because it prints it assuming it's a type ScriptingEmptyBlock
(block type). If you do po (id) success
it will print (Function)
.
As for how you can explicitly put an Objective-C block in an array from Swift, the only way I have decided to do it is something like this:
let params:[Any] = [ "testName",
successClosure as (@convention(block) () -> Void)!,
errorClosure as (@convention(block) (NSError) -> Void)!]
object.scripting_execute (ScriptOperation.updateProductName.rawValue,
withParams: params)
I'm not sure why the function type needs to be injected internally !
, but it doesn't seem to work. Maybe someone else can find a better way.
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I have to admit that I don't quite understand why this is happening, but as far as I can tell it has nothing to do with use NSInvocation
and will happen even if we just pass Swift's closure to Objective-C via a type parameter id
. Passing an Objective-C block using id
works great, not sure why: Swift closures must be compatible with Objective-C blocks. As you know, elements NSArray
are of type id
, so any Objective-C object can be an element of an array.
To work around this issue of Swift's traversal closure access id
in Objective-C, one can introduce a wrapper class.
// In a header:
@interface EmptyBlockWrapper : NSObject
@property EmptyBlock _blk;
@end
// In an implementation file (just an empty implementation):
@implementation EmptyBlockWrapper
@end
We can then use a wrapper instance instead of a block as an array element in Swift:
let myBlock : EmptyBlock = {
print("In Swift EmptyBlock...")
}
let myBlockWrapper = EmptyBlockWrapper()
myBlockWrapper._blk = myBlock
In Objective-C method, we can call it like this, assuming args
NSArray *
:
EmptyBlockWrapper * emptyBlockWrapper = args[1];
emptyBlockWrapper._blk();
Hope this helps. Of course, this is just a simplified example to give you an idea; it can become much more attractive.
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