Question

我正在使用parse，我正在尝试返回表的row.count并将其输出到label.text。

func numberOfPostedJobsCount() -> Int{
    var postsCount: Int?
    var query = PFQuery(className: "Post")
    if let user = PFUser.currentUser() {
        query.whereKey("user", equalTo: user)
    }
    query.findObjectsInBackgroundWithBlock { (objects:[AnyObject]?, error) -> Void in
        if error == nil {
            //query succeeded
            print("\(objects?.count)")
        } else {
            print("\(error)")
        }
    }

    return postsCount!
}

viewDidLoad（）中的代码是

numberOfPostedJobs.text = String(numberOfPostedJobsCount())

错误：致命错误：在解包可选值时意外发现nil

编辑1：我尝试添加公共或私有变量，但它没有用。

func numberOfPostedJobsCount() -> (Int) {
    var num1: Int = 0
    var query = PFQuery(className: "Post")
    if let user = PFUser.currentUser() {
        query.whereKey("user", equalTo: user)
    }
    query.findObjectsInBackgroundWithBlock { (objects:[AnyObject]?, error) -> Void in

        if error == nil {
            //query succeeded
            print("\(objects?.count)")
            num1 = (objects?.count)!
            print(num1)
        } else {
            print("\(error)")
        }
    }

    return num1
}

Answer 1

原因是postsCount已声明，但从未初始化，因此当你解开它时它是nil。

请注意函数findObjectsInBackgroundWithBlock异步工作，这意味着在返回语句之后始终调用该块。在此块中，您必须更新数据源和/或UI。

修改

从您的函数返回Void并使用完成处理程序，例如

func numberOfPostedJobsCount(completion: (Int) -> ()) { var query = PFQuery(className: "Post") if let user = PFUser.currentUser() { query.whereKey("user", equalTo: user) } query.findObjectsInBackgroundWithBlock { (objects:[AnyObject]?, error) -> Void in if error == nil { //query succeeded print("\(objects?.count)") let num1 = (objects?.count)! completion(num1) } else { print("\(error)") completion(0) } } }

你以这种方式调用函数

numberOfPostedJobsCount() { (postsCount) -> () in numberOfPostedJobs.text = String(postsCount) }

Answer 2

您可以使用信号量使异步方法同步工作：

问题陈述的示例方法

func test()->Int {
    var postsCount: Int?
    let sema:dispatch_semaphore_t = dispatch_semaphore_create(0)

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)) { () -> Void in
        postsCount = 100
        dispatch_semaphore_signal(sema)
    }

    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)
    return postsCount!
}

以上代码：

func numberOfPostedJobsCount() -> Int{
var postsCount: Int?
var query = PFQuery(className: "Post")
let sema:dispatch_semaphore_t = dispatch_semaphore_create(0)
if let user = PFUser.currentUser() {
    query.whereKey("user", equalTo: user)
}
query.findObjectsInBackgroundWithBlock { (objects:[AnyObject]?, error) -> Void in
    if error == nil {
        //query succeeded
        postsCount = objects?.count
        dispatch_semaphore_signal(sema)
        print("\(objects?.count)")
    } else {
        postsCount = 0
        dispatch_semaphore_signal(sema)
        print("\(error)")
    }
}
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)
return postsCount!
}

在代码块中获取值以在swift中返回函数

2 个答案: