DART異步/等待不等待

Question

所以我一直在試驗DART（我的核心語言是C++和嵌入式C派生）。因此，我的代碼可能並不漂亮，因爲我更像是程序員程序員，但我正在學習和學習......我一直在等待同步的期貨問題上掙扎，基本上，我根本無法獲得DART等待。以下代碼建立與小型嵌入式設備的套接字連接並提取信息。這一切都有效，但注意到操作順序應該是main（）從控制檯獲取一些信息，然後調用方法cardStatus運行，並通過套接字連接從嵌入式設備獲取信息。這是等待發生的地方。當Future返回時，它應該轉到printstuff（）方法。我補充說，應該去爲了讀打印語句：

• 這應該打印第一
• 這應該打印第二
• 這應該打印3

而是因爲等待是不發生在cardstatus呼叫（這很費時），我得到：

• 這應該打印第1
• 這應該打印3
• 這應該打印第二

我跟着就利用異步的另一個線程，而且似乎用這個Other thread的一個堅實的方式至少以下（我嘗試了。然後用一個類似的結果完成，所以有一些核心，我覺得我失蹤）..但我一直堅持這一個星期。

下面的代碼以及控制檯輸出。

import 'dart:io'; 
import 'dart:async' show Future; 

const String STATUS = "#111111;"; 

String defaultIP = "10.1.14.202"; 
int defaultConfigPort = 5111; 
int defaultControlPort = 6722; 

var card = new Map(); 

getInput(String defaults) { 
    String takenin = stdin.readLineSync(); 
    if (takenin == '') takenin = defaults; 
    return takenin; 
} 

Future main() async { 
    stdout.write('What is the IP address of the card ($defaultIP): '); 
    String ipaddress = getInput(defaultIP); 
    defaultIP = ipaddress; 
    print ("This should print 1st"); 
    stdout.writeln("Connecting to $defaultIP"); 
    await cardStatus(defaultIP, defaultConfigPort, STATUS, card); 
    printstuff(); 
} 

printstuff() { 
    stdout.writeln(card['subnet']); 
    print ("This should print 3rd"); 
} 
Future cardStatus(String ip, int port, String message, Map card) { 
    return new Future.delayed(Duration.ZERO,() { 
    Socket.connect(ip, port).then((socket) { 
     print('Connected to: ' 
      '${socket.remoteAddress.address}:${socket.remotePort}'); 

     socket.listen((data) { 
     print(new String.fromCharCodes(data).trim()); 
     List str1 = (new String.fromCharCodes(data).trim().split(',')); 
     print(str1); 
     print ("This should print 2nd"); 
     //var card = new Map(); 
     card['ip'] = str1[0]; 
     card['subnet'] = str1[1]; 
     card['gateway'] = str1[2]; 
     card['unknown'] = str1[3]; 
     card['persist'] = str1[4] == 'true'; 
     card['build'] = str1[5]; 
     card['serial'] = str1[6].substring(0, 14); 
     card['cloudpassword'] = str1[6].substring(14, 20); 
     card['DNS'] = str1[7]; 
     card['cloudhost'] = str1[8]; 
     card['cloudenabled'] = str1[9] == 'true'; 
     print(card['ip']); 
     }, 
      onDone:() { 
      print("Done"); 
      socket.destroy(); 
      }); 

//Send the request 
     socket.write(message); 
    }); 
    }); 
} 

並且這是當前的控制檯輸出。注意，如果cardStatus已經完成，null將不會爲null，它將被打印爲str1。

What is the IP address of the card (10.1.14.202): 
This should print 1st 
Connecting to 10.1.14.202 
null 
This should print 3rd 
Connected to: 10.1.14.202:5111 
>10.1.14.202,255.255.255.0,10.1.14.1,,0,435,F44900A60040F8000000,192.168.1.1,connect.tutuuu.com,0; 
[>10.1.14.202, 255.255.255.0, 10.1.14.1, , 0, 435, F44900A60040F8000000, 192.168.1.1, connect.tutuuu.com, 0;] 
This should print 2nd 
10.1.14.202 
Done 

Process finished with exit code 0 

感謝您的幫助！

Answer 1

您在Socket.connect之前缺少return。就目前而言，您的代碼只是開始連接，但從未等到未來。我強烈建議儘可能使用新的await/async語法。

這裏是一個正在運行的例子，的確讓谷歌主頁：

import 'dart:io'; 
import 'dart:async' show Future; 

Future main() async { 
    print("This should print 1st"); 
    await cardStatus('www.google.com', 80, 'GET /\nHTTP 1.1\n\n'); 
    printstuff(); 
} 

printstuff() { 
    print("This should print 3rd"); 
} 

Future cardStatus(String ip, int port, String message) { 
    return new Future.delayed(Duration.ZERO,() { 
    return Socket.connect(ip, port).then((socket) { 
     print('Connected to: ' 
      '${socket.remoteAddress.address}:${socket.remotePort}'); 

     socket.listen((data) { 
     List str1 = (new String.fromCharCodes(data).trim().split(',')); 
     print(str1.first); 
     print("This should print 2nd"); 
     }, onDone:() { 
     print("Done"); 
     socket.destroy(); 
     }, onError: (e) { 
     print("Error while listening: $e"); 
     }); 
     socket.write(message); 
    }); 
    }); 
} 

下面使用等待着輕微刪節版，和try/catch來處理錯誤：

import 'dart:io'; 
import 'dart:async' show Future; 

Future main() async { 
    print("This should print 1st"); 
    await cardStatus('www.google.com', 80, 'GET /\nHTTP 1.1\n\n'); 
    print("This should print 3rd"); 
} 

Future<String> cardStatus(String ip, int port, String message) async { 
    var socket = await Socket.connect(ip, port); 
    print('Connected to: ' 
     '${socket.remoteAddress.address}:${socket.remotePort}'); 
    socket.write(message); 
    print("Sent request"); 
    try { 
    var response = await socket.fold(
     '', 
     (String acc, List<int> data) => 
      acc + new String.fromCharCodes(data).trim()); 
    print("Received response: ${response.substring(0, 10)}"); 
    return response; 
    } finally { 
    socket.close(); 
    } 
}