秒如果語句在第一個語句在一個函數中完成之前被調用

Question

我有一個函數，其中有2個if語句。代碼：

func pictureFromFirebase(loginMethod: Int) 
    { 
    if loginMethod == 0 //FB 
    { 
     var profilePic = FBSDKGraphRequest(graphPath: "me/picture", parameters: ["height":300, "width":300, "redirect":false], httpMethod: "GET") 
     let profilePicRef = storageRef.child((user?.uid)!+"/profile_pic.jpg") 
     profilePicRef.data(withMaxSize: 1 * 1024 * 1024) { data, error in 
      if let error = error { 
       // Uh-oh, an error occurred! 
      } else { 
       if (data != nil) 
       { 
       print("no need to download image from facebook") 
       self.profileImage.image = UIImage (data: data!) 
       } 
      } 
     } 
     if profileImage.image == nil 
     { 
     print("downloading image from facebook") 
     profilePic?.start(completionHandler: {(_ connection, _ result, _ error) -> Void in 
      if (error == nil) 
      { 
       if let dictionary = result as? [String:Any], let data = dictionary["data"] as? [String:Any], 
        let urlPic = data["url"] as? String { 
        if let imageData = NSData(contentsOf: NSURL(string: urlPic)! as URL) 
        { 
         let uploadTask = profilePicRef.put(imageData as Data, metadata: nil){ 
         metadata, error in 
          if (error == nil) 
          { 
           let downloadUrl = metadata!.downloadURL 
          } 
          else 
          { 
           print("error in downloading image") 
          } 
         } 
         self.profileImage.image = UIImage(data: imageData as Data) 
        } 
       } 

      } 
     }) 
    } 
    } 
    } 

當這個功能被觸發它只有叫：

FIRAuth.auth()?.addStateDidChangeListener() { (auth, user) in 
      print("called") 
      if let user = user { 
       if(activeUser != user){ 
       let name = user.displayName 
       self.profileName.text = name 
       self.pictureFromFirebase(loginMethod: 0) 
       } 
      } 

這是從調試，當我已經設定下載來自Facebook的圖像的圖像輸出正常不需要：

-called 從Facebook -downloading圖像-called 從Facebook -downloading形象 - 沒有需要從Facebook下載圖片- 沒有需要從Facebook

下載圖像這是從調試輸出，當我刪除整個第二if語句（如果profileImage.image ==無）：

-called -called -no需要從Facebook下載圖片 -no需要從Facebook下載圖片

從我可以看到，除了函數被調用兩次，這也是錯誤的，第二個語句被調用之前執行第一個語句。由於第一條語句在設置profileImage之前需要一段時間，因此當第二條if語句正在檢查此值時，它爲零。我怎樣才能確保這不會再發生？謝謝。

Answer 1

異步方法！在進入下一步之前，您必須等待完成。

把第二塊第一完成處理程序中 - 這樣

func pictureFromFirebase(loginMethod: Int) 
{ 
    if loginMethod == 0 //FB 
    { 
     var profilePic = FBSDKGraphRequest(graphPath: "me/picture", parameters: ["height":300, "width":300, "redirect":false], httpMethod: "GET") 
     let profilePicRef = storageRef.child((user?.uid)!+"/profile_pic.jpg") 
     profilePicRef.data(withMaxSize: 1 * 1024 * 1024) { data, error in 
      if let error = error { 
       // Uh-oh, an error occurred! 
       // but we don't need to do anything yet. Try to download the profile pic 
      } 
      if (data != nil) 
      { 
       print("no need to download image from facebook") 
       self.profileImage.image = UIImage (data: data!) 
      } 
      else 
      { 
       // THIS IS THE BLOCK THAT HAS BEEN MOVED 
       // WHICH WILL NOW BE EXECUTED IN TWO CONDITIONS - 
       // 1. AN ERROR IN THE DOWNLOAD 
       // 2. NO PROFILE PIC AVAILABLE 
       print("downloading image from facebook") 
       profilePic?.start(completionHandler: {(_ connection, _ result, _ error) -> Void in 
        if (error == nil) 
        { 
         if let dictionary = result as? [String:Any], let data = dictionary["data"] as? [String:Any], 
          let urlPic = data["url"] as? String { 
          if let imageData = NSData(contentsOf: NSURL(string: urlPic)! as URL) 
          { 
           let uploadTask = profilePicRef.put(imageData as Data, metadata: nil){ 
            metadata, error in 
            if (error == nil) 
            { 
             let downloadUrl = metadata!.downloadURL 
            } 
            else 
            { 
             print("error in downloading image") 
            } 
           } 
           self.profileImage.image = UIImage(data: imageData as Data) 
          } 
         } 

        } 
       }) 
      } 

     } 
    } 
}