ios - 使用键检索 PFObject

Question

我在使用 key 成功检索 PFObject 时遇到问题。对象最后为零:

    var object = PFObject(className:"Topic")
    var query = PFQuery(className:"Topic")
    query.whereKey("title", equalTo:"Default")
    query.getFirstObjectInBackgroundWithBlock {
        (object: PFObject!, error: NSError!) -> Void in
        if object != nil {
            NSLog("The getFirstObject request failed.")
        } else {
            // The find succeeded.
            NSLog("Successfully retrieved the object.")
        }
    }

用它来创建关注关系

user.signUpInBackgroundWithBlock {
            (succeeded: Bool!, error: NSError!) -> Void in
            if error == nil {
                NetworkManager.sharedInstance.follow(object, completionHandler: {
                    (error) -> () in
                    if error == nil {
                    }
                    else {
                        println("follow topic failure")
                    }
                })
            }
            else {
                println("signup failure")
            }
        }
    }

测试对象的检索是否成功:

    println("this is the title of the default topic")
    println(object["title"] as String)

控制台打印:

this is the title of the default topic
fatal error: unexpectedly found nil while unwrapping an Optional value

Answer 1

如果您尝试在 getFirstObjectInBackgroundWithBlock block 之外打印 object["title"]，您当前的代码将会导致一些问题。

(1) 我认为您正试图从您在代码顶部创建的 object PFObject 打印，即 var object = PFObject(className:"Topic")，而不是您的 getFirstObjectInBackgroundWithBlock 创建的用于其范围内的 object；但是从未检索到 getFirstObjectInBackgroundWithBlock 之外的 PFObject 对象。

(2) getFirstObjectInBackgroundWithBlock block 是异步运行的，这意味着它是在后台获取的，可能不会立即可用；因此，即使 getFirstObjectInBackgroundWithBlock 之外的 PFObject 对象 被设置为包含 getFirstObjectInBackgroundWithBlock 返回的 PFObject 对象，您也可以'保证对象将准备好在 block 外使用，因为当您尝试在 block 外打印 object["title"] 时 getFirstObjectInBackgroundWithBlock 仍然可以获取对象 block 。

(3) 即使在您的 block 中，您似乎也不理解您的条件，因为 if object != nil 您正在打印请求已失败，但实际上已成功。

以下是我建议的打印对象并为 signUpInBackgroundWithBlock 使用相同的 object 的方法:

// fetchedObject is a global variable created to store
// the retrieved object, but make sure the 
// getFirstObjectInBackgroundWithBlock has had time to
// fetch the object before trying to use fetchedObject;
// also check to make sure it's non-nil
var fetchedObject:PFObject!

func signUp () {

    var query = PFQuery(className:"Topic")
    query.getFirstObjectInBackgroundWithBlock {
        (object: PFObject!, error: NSError!) -> Void in
        if object != nil {

            // Save the object as a global object for use
            // outside of this method
            self.fetchedObject = object

            // The find succeeded.
            println("Successfully retrieved the object.")
            println("This is the title of the default topic:")
            if let defaultTopic:String = object["title"] as? String {
                println(defaultTopic)
            }

            user.signUpInBackgroundWithBlock {
                (succeeded: Bool!, error: NSError!) -> Void in
                if error == nil {
                    NetworkManager.sharedInstance.follow(object, completionHandler: {
                        (error) -> () in
                        if error == nil {
                        }
                        else {
                            println("follow topic failure")
                        }
                    })
                }
                else {
                    println("signup failure")
                }
            }
        } else {
            println("The getFirstObject request failed with error: \(error)")
        }
    }
}