Semaphore to sync request Swift4 stopped forever - ios

Question

I need sync request from web service, so I use sempahore:
class func syncProducts() {
print("syncProducts() 1")
let idsLocal = getProductsIds()
let semaphore = DispatchSemaphore(value: 0)
var idsCloud : [Int] = []
print("Getting cloud ids... 1")
OdooService.getProductsIds { (params: [Int]) in
print("SuccessBlock size ids: \(params.count) 1")
idsCloud = params
semaphore.signal()
}
semaphore.wait()
print("Depois do GetproductsIds: 1")
}
but in this example, the app keep locks forever! The request never ends. This is my function to request data from webserver and returns to success block if is case.
static func getProductsIds(successBlock: #escaping (_ params: [Int]) -> Void) {
// check odoo auth
let dispatch = DispatchGroup()
if( OdooAuth.uid == 0 ) {
print("Odoo offline, tentando reconectar...")
dispatch.enter()
OdooAuth.reconnect(successBlock: { params in
print("Reconectado com sucesso...")
dispatch.leave()
}, failureBlock: { params in
print("Falha no ReAuth")
return
})
}
print("Start request from Odoo...")
let fieldsProducts = ["id"]
let options = [ "fields": fieldsProducts] as [String : Any]
var idsList : [Int] = []
let params = [OdooAuth.db, OdooAuth.uid, OdooAuth.password,"product.template","search_read",[],options] as [Any]
AlamofireXMLRPC.request(OdooAuth.host2, methodName: "execute_kw", parameters: params).responseXMLRPC {
(response: DataResponse<XMLRPCNode>) -> Void in
switch response.result {
case .success( _):
print("Success to get Ids")
let str = String(data: response.data!, encoding: String.Encoding.utf8) as String!
let options = AEXMLOptions()
let xmlDoc = try? AEXMLDocument(xml: (str?.data(using: .utf8))!,options: options)
//print(xmlDoc!.xml)
for child in (xmlDoc?.root["params"]["param"]["value"]["array"]["data"].children)! {
for childValue in child["struct"].children {
let id = childValue["value"]["int"].value!
idsList.append(Int(id)!)
//print("Id: \(id)")
}
}
successBlock(idsList)
break
case .failure(let error):
print("Error to get Ids: \(error.localizedDescription)")
break
} // fim switch
} // fim request
} // fim getProductsIds
I don't know if semaphores is the best way to do this, but I need sync the requests! I tried use DispatchGroup() like in reauth, but not works too.

Answer 1

I would expect that the deadlock is a result of getProductsIds callback being called on main thread, which is blocked by the semaphore. As far as I know, by default Alamofire dispatches the callback on the main thread, which I would expect is the case of AlamofireXMLRPC, since it is a wrapper around Alamofire.
I would strongly recommend to not block main thread during an async operation.
If, however, for any really really good reason you cannot do otherwise, you will need to make sure that the callback won't get dispatched on the main dispatch queue (since that one is blocked waiting for the signal). Alamofire itself has a response overload that allows to specify the DispatchQueue object on which to run the callback. It seems that AlamofireXMLRPC has one too, so I would try to utilize that and change
AlamofireXMLRPC.request(OdooAuth.host2, methodName: "execute_kw", parameters: params)
.responseXMLRPC {
// process result
}
to:
AlamofireXMLRPC.request(OdooAuth.host2, methodName: "execute_kw", parameters: params)
.responseXMLRPC(queue: DispatchQueue.global(qos: .background)) {
// process result
}
I based it on the github source code of AlamofireXMLRPC, but have not worked with it before, so maybe there will be some syntactic errors. But it should point you to right direction. Still, I would recommend you NOT to block the thread (I am repeating myself, but this is really very important point).