Django（47）drf请求生命周期分析

前言

  一般我们写完序列化以后，我们就会开始写视图了，drf中我们一般使用CBV的方式，也就是类视图的方式，最基础的我们会使用from rest_framework.views import APIView，APIView继承自View，关于视图的详解，我们后续再细讲。本章介绍drf的请求生命周期
 

前置准备工作

我们先写一个视图类TestView，代码如下：

from rest_framework.views import APIView
from rest_framework.response import Response
class TestView(APIView):
    def get(self, request, *args, **kwargs):
        return Response("drf get ok")
    def post(self, request, *args, **kwargs):
        return Response("drf post ok")

注意：这里的Response必须是drf下的Response，不能是Django原生的HttpResponse或者是JsonResponse，否则会出错

接着，在urls.py中配置路由，如下

urlpatterns = [
    path('test/', views.TestView.as_view(), name="Test"),
]

然后我们访问http://127.0.0.1:8000/drf/test/，会出现下图样式，代表请求成功

接着我们在接口工具中使用POST请求方式访问，返回结果如下:

"drf post ok"

以上2种访问方式都成功了，接下来我们分析其中的请求过程以及原理
 

请求生命周期分析

首先我们先从路由配置中看到views.TestView.as_view()，调用的是TestView类视图下的as_view方法，但是我们上面定义该方法的时候，没有重写as_view()方法，所以会调用父类APIView中的as_view方法，源码如下：

@classmethod
def as_view(cls, **initkwargs):
    """
    Store the original class on the view function.

    This allows us to discover information about the view when we do URL
    reverse lookups.  Used for breadcrumb generation.
    """

    # 判断queryset是否是QuerySet对象
    if isinstance(getattr(cls, 'queryset', None), models.query.QuerySet):
        def force_evaluation():
            raise RuntimeError(
                'Do not evaluate the `.queryset` attribute directly, '
                'as the result will be cached and reused between requests. '
                'Use `.all()` or call `.get_queryset()` instead.'
            )
        cls.queryset._fetch_all = force_evaluation

    # 调用父类的as_view方法
    view = super().as_view(**initkwargs)
    view.cls = cls
    view.initkwargs = initkwargs

    # Note: session based authentication is explicitly CSRF validated,
    # all other authentication is CSRF exempt.
    # 禁用了csrf认证
    return csrf_exempt(view)

通过这行代码view = super().as_view(**initkwargs)，可以知道APIView的as_view方法也调用了父类View的as_view方法，源码如下:

def as_view(cls, **initkwargs):
    """Main entry point for a request-response process."""
    for key in initkwargs:
        if key in cls.http_method_names:
            raise TypeError("You tried to pass in the %s method name as a "
                            "keyword argument to %s(). Don't do that."
                            % (key, cls.__name__))
        if not hasattr(cls, key):
            raise TypeError("%s() received an invalid keyword %r. as_view "
                            "only accepts arguments that are already "
                            "attributes of the class." % (cls.__name__, key))

    def view(request, *args, **kwargs):
        self = cls(**initkwargs)
        # 如果有get属性，没有head属性，那么head就是get
        if hasattr(self, 'get') and not hasattr(self, 'head'):
            self.head = self.get

        # 初始化所有视图方法共享的属性
        self.setup(request, *args, **kwargs)

        # 如果没有request属性，报异常
        if not hasattr(self, 'request'):
            raise AttributeError(
                "%s instance has no 'request' attribute. Did you override "
                "setup() and forget to call super()?" % cls.__name__
            )

        # 返回一个`dispatch`方法
        return self.dispatch(request, *args, **kwargs)
    view.view_class = cls
    view.view_initkwargs = initkwargs

    # take name and docstring from class
    update_wrapper(view, cls, updated=())

    # and possible attributes set by decorators
    # like csrf_exempt from dispatch
    update_wrapper(view, cls.dispatch, assigned=())
    return view

as_view方法返回的是view，view返回的是dispatch方法，dispatch方法也是调用的APIView下的dispatch方法，源码如下：

def dispatch(self, request, *args, **kwargs):
    """
    `.dispatch()` is pretty much the same as Django's regular dispatch,
    but with extra hooks for startup, finalize, and exception handling.
    """
    self.args = args
    self.kwargs = kwargs
    # 初始化请求，返回的是Request对象
    request = self.initialize_request(request, *args, **kwargs)
    self.request = request
    self.headers = self.default_response_headers  # deprecate?

    try:
        # 在调用方法处理程序之前运行任何需要发生的操作
        self.initial(request, *args, **kwargs)

        # Get the appropriate handler method
        # 获取request的请求方法
        if request.method.lower() in self.http_method_names:
            handler = getattr(self, request.method.lower(),
                              self.http_method_not_allowed)
        else:
            handler = self.http_method_not_allowed
        
        response = handler(request, *args, **kwargs)

    except Exception as exc:
        # 在调用方法处理程序之前出现异常，则跑出异常
        response = self.handle_exception(exc)
    
    # 返回一个response响应对象
    self.response = self.finalize_response(request, response, *args, **kwargs)
    return self.response

dispatch返回一个response响应对象，得到请求的响应结果，返回给前台
 

总结

1. url请求走的是APIView的as_view函数
2. 在APIView的as_view调用父类(django原生)的as_view，还禁用了csrf认证
3. 在父类的as_view中的dispatch方法请求走的又是APIView的dispatch
4. 完成任务方法交给视图类函数处理，得到请求的响应结果，返回给前台