using combination of dispatch group and operation queue to wait for URLSession

serialApiCallWithDispatchGroup.swift

// let's say the project operation needs to be executed first before the employee

let group = DispatchGroup()

// employee block operation
let employeeBlockOperation = BlockOperation()
        employeeBlockOperation.addExecutionBlock {

            let employeeDataResource = EmployeeDataResource()
            employeeDataResource.getEmployee { (employees) in
                //do something with the employee response
            }
        }

// project block operation
        let projectBlockOperation = BlockOperation()
        projectBlockOperation.addExecutionBlock {
        
          // enter the dispatch group
            group.enter()
            let projectResource = ProjectDataResource()
            projectResource.getProject { (projectData) in
                // do something with the project response
                // leaving the block
                group.leave()
            }
// we need to wait for an entire execution block to complete
            group.wait()
        }

// since we want the project operation to kick off first we add dependency
 employeeBlockOperation.addDependency(projectBlockOperation)

// adding operations to the operation queue
        let operationQueue = OperationQueue()
        operationQueue.addOperation(employeeBlockOperation)
        operationQueue.addOperation(projectBlockOperation)