DWR设置同步：DWREngine.setAsync(false);
  
DWR设置异步：DWREngine.setAsync(true);   其中async 是异步的意思。
 
DWR默认是异步，所以当需要设置DWR同步时，只需DWREngine.setAsync(false);  但是当处理完成之后

需要将DWR设置回默认的异步状态，所以需再加上DWREngine.setAsync(true); 
 
代码示例：
Js代码 

function getCity() {  
     var province = theForm.elements("province").value;  
   
DWREngine.setAsync(false);  
     aaaService.queryCityList(province , {callback:function(data){  
          DWRUtil.removeAllOptions("city");  
          DWRUtil.addOptions("city", [{dm:"", mc:"---请选择---"}], "dm", "mc");  
         DWRUtil.addOptions("city", data, "value", "label");  
     }});  
DWREngine.setAsync(true);  
   
     var city = theForm.elements("city").value;  
     getTown(city);  
}