Swift blocks not working

Question

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)

Seems to be the same problem as this one: http://stackoverflow.com/questions/24586293/cast-closures-blocks. — Martin R, Jul 06 '14 at 12:20
It's similar but different. That question is about dealing with blocks that you receive as return values *from* Objc methods. This one is about passing Swift closures as blocks *to* Objc methods. — Rob Keniger, Aug 27 '14 at 01:29

score 29 · Accepted Answer · edited Nov 21 '15 at 02:21

29

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

edited Nov 21 '15 at 02:21

Sam Corder

5,374
3
25
30

answered Jul 29 '14 at 11:05

Bryan Chen

45,816
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That worked like a charm! Thank you so much. Only thing, apparently `ctx["test"] =` doesn't work. `ctx.setObject(obj, forKeyedSubscript:"test")` does however. Once again, Thank you, and here is your bounty in 22 hours when I'm able to. – hannesr Jul 29 '14 at 12:23
On Beta6 `reinterpretCast(block) as AnyObject` has been deprecated in favor of `unsafeBitCast(block, AnyObject)` – Víctor B. Aug 24 '14 at 19:15
4

Actually, that would be `unsafeBitCast(block, AnyObject.self)` – Rob Keniger Aug 27 '14 at 01:23
This line `var ctx["test"] = reinterpretCast(block) as AnyObject` should be `ctx.setObject(unsafeBitCast(block, AnyObject.self), forKeyedSubscript: "test")`, tested on Xcode Version 6.1.1 (6A2008a). – paulvs Jan 25 '15 at 15:18
1

With the latest Swift 2 syntax, the block should be defined as `@convention(block) (String) -> Void` – Zhao Sep 22 '15 at 20:20

score 1 · Answer 2 · answered Jul 29 '14 at 22:15

I have a working demo at:

https://github.com/dankogai/swift-jsdemo

And here is the part that implements block registration:

typealias ID = AnyObject!
extension JSContext {
    func fetch(key:NSString)->JSValue {
        return getJSVinJSC(self, key)
    }
    func store(key:NSString, _ val:ID) {
        setJSVinJSC(self, key, val)
    }
    func store(key:NSString, _ blk:()->ID) {
        setB0JSVinJSC(self, key, blk)
    }
    func store(key:NSString, _ blk:(ID)->ID) {
        setB1JSVinJSC(self, key, blk)
    }
    func store(key:NSString, _ blk:(ID,ID)->ID) {
        setB2JSVinJSC(self, key, blk)
    }
}

You need a very small objc code and bridging header to make it work. See the repository for details.

Swift blocks not working

2 Answers2

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