[Add]新增用于实现单例模式的VtSingleton类，修改LogEngine的单例模式实现

examples/CtaTrading/VT_setting.json

@@ -9,7 +9,7 @@
 	"darkStyle": true,
 	"language": "chinese",
 
-	"logActive": false,
+	"logActive": true,
 	"logLevel": "debug",
 	"logConsole": true,
 	"logFile": true,

vnpy/trader/vtEngine.py

@@ -538,6 +538,9 @@ class DataEngine(object):
 class LogEngine(object):
     """日志引擎"""
     
+    # 单例模式
+    __metaclass__ = VtSingleton
+    
     # 日志级别
     LEVEL_DEBUG = logging.DEBUG
     LEVEL_INFO = logging.INFO
@@ -545,16 +548,6 @@ class LogEngine(object):
     LEVEL_ERROR = logging.ERROR
     LEVEL_CRITICAL = logging.CRITICAL
 
-    # 单例对象
-    instance = None
-    
-    #----------------------------------------------------------------------
-    def __new__(cls, *args, **kwargs):
-        """创建对象，保证单例"""
-        if not cls.instance:
-            cls.instance = super(LogEngine, cls).__new__(cls, *args, **kwargs)
-        return cls.instance
-
     #----------------------------------------------------------------------
     def __init__(self):
         """Constructor"""

vnpy/trader/vtObject.py

@@ -327,6 +327,21 @@ class VtCancelOrderReq(object):
         self.sessionID = EMPTY_STRING           # 会话号
 
 
+########################################################################
+class VtSingleton(type):
+    """
+    单例，应用方式:静态变量 __metaclass__ = Singleton
+    """
+    
+    _instances = {}
+
+    #----------------------------------------------------------------------
+    def __call__(cls, *args, **kwargs):
+        """调用"""
+        if cls not in cls._instances:
+            cls._instances[cls] = super(VtSingleton, cls).__call__(*args, **kwargs)
+            
+        return cls._instances[cls]