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)
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
I have a working demo at:
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.
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