Django CBV模式的源码解析
通常来说，http请求的本质就是基于Socket
Django的视图函数，可以基于FBV模式，也可以基于CBV模式。
基于FBV的模式就是在Django的路由映射表里进行url和视图函数的关联，而基于CBV的模式则是在views.py文件中定义视图类，在视图类中视图函数，如get,post,put,delete等
使用Django新建一个项目，新建一个路由映射
from django.conf.urls import url
from django.contrib import admin
from app01 import views
urlpatterns = [
url(r'^cbv/$',views.CBV.as_view())
]
对应的views.py文件内容：
from django.shortcuts import render,HttpResponse
from django.views import View
class CBV(View):
def get(self,request):
return HttpResponse("GET")
def post(self,request):
return HttpResponse(<span class="hljs-string">"<span class="hljs-keyword">POST</span>"</span>)</code></pre>
启动项目，使用浏览器请求URLhttp://127.0.0.1:8000/cbv/,浏览器显示结果为:
请求到达Django会先执行Django中间件里的方法，然后进行进行路由匹配。
在路由匹配完成后，会执行CBV类中的as_view方法。
CBV中并没有定义as_view方法，由于CBV继承自Django的View,所以会执行Django的View类中的as_view方法
Django的View类的as_view方法的部分源码
class View(object):
"""
Intentionally simple parent class for all views. Only implements
dispatch-by-method and simple sanity checking.
"""
http_method_names = [<span class="hljs-string">'get'</span>, <span class="hljs-string">'post'</span>, <span class="hljs-string">'put'</span>, <span class="hljs-string">'patch'</span>, <span class="hljs-string">'delete'</span>, <span class="hljs-string">'head'</span>, <span class="hljs-string">'options'</span>, <span class="hljs-string">'trace'</span>]
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">__init__</span><span class="hljs-params">(self, **kwargs)</span>:</span>
<span class="hljs-string">"""
Constructor. Called in the URLconf; can contain helpful extra
keyword arguments, and other things.
"""</span>
<span class="hljs-comment"># Go through keyword arguments, and either save their values to our</span>
<span class="hljs-comment"># instance, or raise an error.</span>
<span class="hljs-keyword">for</span> key, value <span class="hljs-keyword">in</span> six.iteritems(kwargs):
setattr(self, key, value)
@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))
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">view</span><span class="hljs-params">(request, *args, **kwargs)</span>:</span>
self = cls(**initkwargs)
<span class="hljs-keyword">if</span> hasattr(self, <span class="hljs-string">'get'</span>) <span class="hljs-keyword">and</span> <span class="hljs-keyword">not</span> hasattr(self, <span class="hljs-string">'head'</span>):
self.head = self.get
self.request = request
self.args = args
self.kwargs = kwargs
<span class="hljs-keyword">return</span> self.dispatch(request, *args, **kwargs)
view.view_class = cls
view.view_initkwargs = initkwargs
<span class="hljs-comment"># take name and docstring from class</span>
update_wrapper(view, cls, updated=())
<span class="hljs-comment"># and possible attributes set by decorators</span>
<span class="hljs-comment"># like csrf_exempt from dispatch</span>
update_wrapper(view, cls.dispatch, assigned=())
<span class="hljs-keyword">return</span> view</code></pre>
从View的源码可以看出，在View类中，先定义了http请求的八种方法
http_method_names = ['get', 'post', 'put', 'patch', 'delete', 'head', 'options', 'trace']
在as_view方法中进行判断，如果请求的方法没在http_method_names中，则会抛出异常，这里的cls实际上指的是自定义的CBV类
接着as_view方法中又定义view方法，在view方法中对CBV类进行实例化，得到self对象，然后在self对象中封装浏览器发送的request请求
self = cls(**initkwargs)
最后又调用了self对象中的dispatch方法并返回dispatch方法的值来对request进行处理
此时，由于self对象就是CBV实例化得到，所以会先执行自定义的CBV类中的dispatch方法。如果CBV类中没有定义dispatch方法则执行Django的View中的dispatch方法
Django的View中的dispatch方法源码
class View(object):
"""
中间省略
"""
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)
在dispatch方法中，把request.method转换为小写再判断是否在定义的http_method_names中，如果request.method存在于http_method_names中，则使用getattr反射的方式来得到handler
在这里的dispatch方法中，self指的是自定义的CBV类实例化得到的对象
从CBV类中获取request.method对应的方法，再执行CBV中的方法并返回
由此，可以知道如果在Django项目中使用CBV的模式，实际上调用了getattr的方式来执行获取类中的请求方法对应的函数
结论：
CBV基于反射实现根据请求方式不同，执行不同的方法
自定义dispatch方法
如果想在基于CBV模式的项目中在请求某个url时执行一些操作，则可以在url对应的类中定义dispatch方法
修改views.py文件
class CBV(View):
def dispatch(self, request, *args, **kwargs):
func = getattr(self,request.method.lower())
return func(request,*args,**kwargs)
def get(self,request):
return HttpResponse(<span class="hljs-string">"<span class="hljs-keyword">GET</span>"</span>)
def post(self,request):
return HttpResponse(<span class="hljs-string">"<span class="hljs-keyword">POST</span>"</span>)</code></pre>
也可以使用继承的方式重写dispatch方法：
class CBV(View):
def dispatch(self, request, *args, **kwargs):
print("before")
res = super(CBV, self).dispatch(request, *args, **kwargs)
print("after")
return res
def get(self,request):
return HttpResponse(<span class="hljs-string">"<span class="hljs-keyword">GET</span>"</span>)
def post(self,request):
return HttpResponse(<span class="hljs-string">"<span class="hljs-keyword">POST</span>"</span>)</code></pre>
刷新浏览器，Django后台打印结果如下：
浏览器页面结果
同理，如果有基于CBV的多个类，并且有多个类共用的功能，为了避免重复，可以单独定义一个类，在这个类中重写dispatch方法，然后让url对应的视图类继承这个类
修改urls.py文件
from django.conf.urls import url
from django.contrib import admin
from app01 import views
urlpatterns = [
url(r'^cbv1/$',views.CBV1.as_view()),
url(r'^cbv2/$',views.CBV2.as_view()),
]
views.py文件内容
from django.shortcuts import render,HttpResponse
from django.views import View
class BaseView(object):
def dispatch(self, request, *args, **kwargs):
func = getattr(self, request.method.lower())
return func(request, *args, **kwargs)
class CBV1(BaseView,View):
def get(self,request):
return HttpResponse("CBV1 GET")
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">post</span><span class="hljs-params">(self,request)</span>:</span>
<span class="hljs-keyword">return</span> HttpResponse(<span class="hljs-string">"CBV1 POST"</span>)
class CBV2(BaseView,View):
def get(self,request):
return HttpResponse("CBV2 GET")
<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">post</span><span class="hljs-params">(self,request)</span>:</span>
<span class="hljs-keyword">return</span> HttpResponse(<span class="hljs-string">"CBV2 POST"</span>)</code></pre>
通过python的面向对象可以知道，请求到达视图类时，会先执行CBV1和CBV2类中的dispatch方法，然而CBV1和CBV2类中并没有dispatch方法，则会按照顺序在父类中查找dispatch方法，此时就会执行BaseView类中的dispatch方法了
用浏览器请求urlhttp://127.0.0.1:8000/cbv1/，浏览器页面显示
用浏览器请求urlhttp://127.0.0.1:8000/cbv2/，浏览器页面显示
