dispatch 方法可以直接在用户请求之前在一些事情，例如计算请求耗时，判断客户端请求头

    1. class LoginView(View):
    2.     def dispatch(self, request, *args, **kwargs):
    3.         print('开始请求---->')
    4.         ret = super().dispatch(request,*args,**kwargs)
    5.         print('请求结束----->')
    6.         return  ret
    8.     def get(self,request):
    9.         print('get')
    10.         return  render(request,'login.html')
    12.     def post(self,request):
    13.         print('post')
    14.         username = request.POST.get('username')
    15.         password = request.POST.get('password')
    16.         return  HttpResponse(username+password)

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