Django中CBV及其源码解释

FBV（function base views） 就是在视图里使用函数处理请求。

 

CBV（class base views） 就是在视图里使用类处理请求。

Python是一个面向对象的编程语言，如果只用函数来开发，有很多面向对象的优点就错失了（继承、封装、多态）。所以Django在后来加入了Class-Based-View。可以让我们用类写View。这样做的优点主要下面两种：

1. 提高了代码的复用性，可以使用面向对象的技术，比如Mixin（多继承）
2. 可以用不同的函数针对不同的HTTP方法处理，而不是通过很多if判断，提高代码可读性

CBV源码解析:

首先,在urls中,我们会有这样一个访问函数:

path('login/',views.LoginView.as_view() )

　　当访问login时,django会调用  views.LoginView.as_view() 中 as_view() 这个函数,那么必须先要去看看LoginView中该函数构成:

class LoginView(View):
    def get(self,request):

        return render(request,"login.html")
    def post(self,request):

        return HttpResponse("Hello")

　　LoginView 是我们在视图中创建的类,继承View,LoginView 没有as_view这个方法,于是我们来到继承的父类View,并找到as_view

 

 @classonlymethod
    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)
            if hasattr(self, 'get') and not hasattr(self, 'head'):
                self.head = self.get
            self.request = request
            self.args = args
            self.kwargs = kwargs
            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

从源码中可以看到as_view最后返回了一个view

 此时

path('login/',views.LoginView.as_view() )    ------->>>>   path('login/',View.view ) 


当有用户登录Login页面时,dajango会自动调用 View.view

又找到view的方法,源码如下:

 def view(request, *args, **kwargs):
            self = cls(**initkwargs)
            if hasattr(self, 'get') and not hasattr(self, 'head'):
                self.head = self.get
            self.request = request
            self.args = args
            self.kwargs = kwargs
            return self.dispatch(request, *args, **kwargs)

解读一下view方法:

self = cls(**initkwargs)  cls为调用as_view时传入的类,为调用该类的对象 LoginView ,这里self为 LoginView 的实例对象.

return self.dispatch(request, *args, **kwargs)  最后返回的时候,先调用了self.dispatch(request, *args, **kwargs)这个方法.而cls就是LoginView ,一步步找到

dispatch这个方法

dispatch源码(分发):

    def dispatch(self, request, *args, **kwargs):
        # Try to dispatch to the right method; if a method doesn't exist,
        # defer to the error handler. Also defer to the error handler if the
        # request method isn't on the approved list.
        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)

if request.method.lower() in self.http_method_names:
　　 handler = getattr(self, request.method.lower(), self.http_method_not_allowed)

其中

http_method_names = ['get', 'post', 'put', 'patch', 'delete', 'head', 'options', 'trace']
获取反射获取到对应的方法('get', 'post', 'put'....)
最后返回:handler(request, *args, **kwargs) 的调用结果,完成分发

 self.http_method_not_allowed 为错误或找不到值时报错调用的方法

 

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