Cache类增加has方法 get方法支持默认值

library/think/cache/driver/Memcache.php

@@ -11,7 +11,6 @@
 
 namespace think\cache\driver;
 
-use think\Cache;
 use think\Exception;
 
 class Memcache
@@ -56,6 +55,18 @@ class Memcache
         }
     }
 
+    /**
+     * 判断缓存
+     * @access public
+     * @param string $name 缓存变量名
+     * @return bool
+     */
+    public function has($name)
+    {
+        $name = $this->options['prefix'] . $name;
+        return $this->handler->get($name) ? true : false;
+    }
+
     /**
      * 读取缓存
      * @access public