源码解析Django CBV的本质

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