[Mod]Close #540，实现LogEngine的单例模式

vnpy/trader/vtEngine.py

@@ -518,6 +518,16 @@ class LogEngine(object):
     LEVEL_WARN = logging.WARN
     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):