Django——CBV源码解析

以下是views模块调用as_view()方法的代码示例

# urls.py
from django.contrib import admin
from django.urls import path
import app.viewsurlpatterns = [path('admin/', admin.site.urls),path('app/', app.views.task.as_view()),
]

# views.py
class task(View):# 根据id获取def get(self, request, u_id):response = {'code': '200', 'msg': "查询成功"}return JsonResponse(json.dumps(response), safe=False)

该功能实现的是当有get请求发送到app这个接口时会返回一个response字典

首先引入问题：为什么浏览器向后端发送get请求时会被该get方法精准接受？

其实是因为在注册url时app.views调用的as_view()方法帮我们做好了大部分规划

Ctrl+左键进入as_view()源码

class View:http_method_names = ["get","post","put","patch","delete","head","options","trace",]def __init__(self, **kwargs):for key, value in kwargs.items():setattr(self, key, value)@classonlymethoddef 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("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)view.view_class = clsview.view_initkwargs = initkwargs# __name__ and __qualname__ are intentionally left unchanged as# view_class should be used to robustly determine the name of the view# instead.view.__doc__ = cls.__doc__view.__module__ = cls.__module__view.__annotations__ = cls.dispatch.__annotations__# Copy possible attributes set by decorators, e.g. @csrf_exempt, from# the dispatch method.view.__dict__.update(cls.dispatch.__dict__)# Mark the callback if the view class is async.if cls.view_is_async:markcoroutinefunction(view)return viewdef 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_allowedreturn handler(request, *args, **kwargs)def http_method_not_allowed(self, request, *args, **kwargs):logger.warning("Method Not Allowed (%s): %s",request.method,request.path,extra={"status_code": 405, "request": request},)response = HttpResponseNotAllowed(self._allowed_methods())if self.view_is_async:async def func():return responsereturn func()else:return response

@classonlymethod表示只能用类调用此方法，这也是为什么我们只能用as_views()而不是as_views
这个时候我们来到了task(View)继承的View类下的as_view()方法
中间的步骤先不管直接看return view

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)

这个时候可以看出其实我们就是在调用父类的view方法
这里的request参数就是我们的浏览器接受的request请求，如果没填request则会弹出一个error
重点是最后调用了实例中的dispatch方法
既然我们的task类调用了dispatch方法那么就应该在task类下搜寻这个方法，但是很明显我们没有写过这方法，因此又回到父类View中的dispatch方法(这俩方法挨得很近，往下翻翻就找到了)

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_allowedreturn handler(request, *args, **kwargs)

if request.method.lower() in self.http_method_names:当我们的request请求类型存在于http_method_names时
那么先看看这个http_method_names是什么东西

http_method_names = ["get","post","put","patch","delete","head","options","trace",
]

其实就是个定义好的字符串列表
再接着看dispatch

handler = getattr(self, request.method.lower(), self.http_method_not_allowed)

其实就是从我们task实例中获取相应的HTTP请求方法，如果不存在就用它默认的
最后返回handler，再解释一下gatter的用法

class Test(object):x = 1a = Test()
print(getattr(a, 'x'))  # 获取属性 x 值
# 结果：1
print(getattr(a, 'y', 'None'))  # 获取属性 y 值不存在，但设置了默认值
# 结果：None
print(a.x)  # 效果等同于上面
# 结果：1