drf源码剖析----as_view()

path('example/', views.ExampleView.as_view())
# as_view()作为入口函数，先去类ExampleView()中找as_view()函数

# 类ExampleView()中没有找到，去父类APIView()中找as_view()函数
class ExampleView(APIView):

    def get(self, request):
        return Response('get')

    def post(self, request):
        return Response('post')

    def put(self, request):
        return Response('put')

class APIView(View):
    @classmethod
    def as_view(cls, **initkwargs):
        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

        view = super().as_view(**initkwargs)  
        # 将父类View中()的as_view()函数返回给view
        view.cls = cls
        view.initkwargs = initkwargs

        return csrf_exempt(view)
        # 返回免除csrf认证的view()函数

class View:
    @classonlymethod
    def as_view(cls, **initkwargs):
        for key in initkwargs:
            if key in cls.http_method_names:
                raise TypeError(
                    "The method name %s is not accepted as a keyword argument "
                    "to %s()." % (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)
            self.setup(request, *args, **kwargs)
            if not hasattr(self, "request"):
                raise AttributeError(
                    "%s instance has no 'request' attribute. Did you override "
                    "setup() and forget to call super()?" % cls.__name__
                )
            return self.dispatch(request, *args, **kwargs)

        return view

    def dispatch(self, request, *args, **kwargs):
        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
        return handler(request, *args, **kwargs)
        # 用反射得到请求函数并返回