Set target's Build Settings -> "Swift Compiller - Code Generation" -> "Objective-C Bridging Header" option in:
$(PROJECT_DIR)/../lib/backendless/include/Backendless-Bridging-Header.h

class Weather : BackendlessEntity {

    var temperature = 24.7
    var condition = "Very Nice"

    // description func
    func description() -> NSString {
        return "<Weather> " + condition + ":" + temperature.description
    }

}
    
// create Backendless instance
var backendless = Backendless.sharedInstance();

// - sync method with class instance/fault as return
var result : AnyObject = backendless.persistenceService.save(Weather())
if (result is Weather) {
    var obj : AnyObject = backendless.persistenceService.findById("Weather", sid:(result as Weather).objectId)
}
if (result is Fault) {
    var fault : Fault = result as Fault
    NSLog("FAULT")
}

// - sync method with fault as reference
var fault : Fault?
var weather : Weather = backendless.persistenceService.save(Weather(), error: &fault) as Weather
if (fault == nil) {
    var obj : AnyObject = backendless.persistenceService.findById("Weather", sid: weather.objectId)
}
else {
    NSLog("FAULT")
}

// - async method with block-based callbacks
backendless.persistenceService.save(
    Weather(),
    response: { (var result : AnyObject!) -> () in
        var obj : AnyObject = self.backendless.persistenceService.findById("Weather", sid: (result as Weather).objectId)
    },
    error: { (var fault : Fault!) -> () in
        NSLog("FAULT")
    }
)

// - save object as dictionary of properties
var os = ["iOS":"Apple", "android":"Google"]
var fault : Fault?
var result = backendless.persistenceService.save("MobileOS", entity:os, error: &fault)