swiftblockjavascriptcore

Swift blocks not working


I've been trying to figure out how to use JavaScriptCore in swift. I'm running into problems however when I have to deal with blocks as arguments, seems like the block is run immediately and the arguments gets the return value of the block. What am I doing wrong?

Working Objective C code:

JSContext* context = [[JSContext alloc] initWithVirtualMachine:[[JSVirtualMachine alloc] init]];
context[@"test"] = ^(NSString *string) {
    //code
};

What I've tried:

1:

var ctx = JSContext(virtualMachine:JSVirtualMachine())
var ctx["test"] = {(string:NSString)->() in /*code*/ }

//Gives me "'JSContext' does not have a member named 'subscript'"

2:

var ctx = JSContext(virtualMachine:JSVirtualMachine())
let n: (string: String)->() = {string in /*code*/}

ctx.setObject(n, forKeyedSubscript:"test")

//Gives me "Type '(x: String) -> () does not conform to protocol 'AnyObject'"

3:

var ctx = JSContext(virtualMachine:JSVirtualMachine())
let n: (string: String)->() = {string in /*code*/}

ctx.setObject(n as AnyObject, forKeyedSubscript:"test")

//Gives me "Cannot downcast from '(string: String) -> () to non-@objc protocol type 'AnyObject'"

Am I missing something here, or is this just a bug in Swift?

Edit:

I've now also tried suggestions from Cast closures/blocks

class Block<T> {
    let f : T
    init (_ f: T) { self.f = f }
}

and then

ctx.setObject(Block<()->Void> {
        /*code*/
    }, forKeyedSubscript: "test")

This solution lets me compile but I get a runtime error:

Thread 1: EXC_BREAKPOINT (code=EXC_I386_BPT, subcode=0x0)

Solution

  • It has something to do with how Swift implement closure. You need to use @convention(block) to annotate that the closure is ObjC block. Use unsafeBitCast to force cast it

    var block : @convention(block) (NSString!) -> Void = {
       (string : NSString!) -> Void in 
        println("test")
    }
    
    ctx.setObject(unsafeBitCast(block, AnyObject.self), forKeyedSubscript: "test")
    

    from REPL

    swift
    Welcome to Swift!  Type :help for assistance.
      1> import Foundation
      2> var block : @convention(block)(NSString!) -> Void = {(string : NSString!) -> Void in println("test")}
    block: @convention(block)(NSString!) -> Void =
      3> var obj: AnyObject = reinterpretCast(block) as AnyObject
    obj: __NSMallocBlock__ = {} // familiar block type