Django Rest Framework（第三天）-CBV源码分析

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2.1 CBV源码分析

FBV : function base views ，就是在视图里使用函数处理请求，这个是最基本的django视图方法，在这里就不再赘述。

CBV : class base views，就是在视图里使用类处理请求。

使用CBV，优点主要有两种： 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。

这里我们直接使用上面的代码来看下如何使用CBV模式：

1、 首先从quickstart/urls.py入手

urlpatterns = [
    url(r'^index/$',views.IndexView.as_view()),
]

2、quickstart/views.py

from django.views import  View
class IndexView(View):
    def get(self,request, *args, **kwargs):
        print("get")
        return HttpResponse("ok")

    def dispatch(self, request, *args, **kwargs):
        print("dispatch")
        ret = super(IndexView,self).dispatch(request, *args, **kwargs)
        print("ret",ret)

        return  HttpResponse(ret)

运行，输入:http://127.0.0.1:8018/index ,可以看到输出结果如下：

dispatch
get
ret <HttpResponse status_code=200, "text/html; charset=utf-8">
[07/Oct/2018 15:52:08] "GET /index/ HTTP/1.1" 200 2

我们分析源码从路由配置开始，url(r'^index/$',views.IndexView.as_view()),我们进入as_view()，这里我们直接贴出class View的源码：

class View:
    """
    Intentionally simple parent class for all views. Only implements
    dispatch-by-method and simple sanity checking.
    """

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

    def __init__(self, **kwargs):
        """
        Constructor. Called in the URLconf; can contain helpful extra
        keyword arguments, and other things.
        """
        # Go through keyword arguments, and either save their values to our
        # instance, or raise an error.
        for key, value in kwargs.items():
            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))

        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

    def dispatch(self, request, *args, **kwargs):
        ...

    def http_method_not_allowed(self, request, *args, **kwargs):
        ...

    def options(self, request, *args, **kwargs):
        ...

    def _allowed_methods(self):
        ...

由于我们的配置中views.IndexView.as_view()参数为null,所以在for key in initkwargs会直接跳过，如果不为null,就去判断执行接下来的代码，大概意思就是，if key in cls.http_method_names:和if not hasattr(cls, key):，如果传递过来的字典中某键包含在 http_method_names 列表中和本类中没有该键属性都会跑出异常，http_method_names列表如下

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

接着执行一下代码片段：

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

view.view_class = cls和view.view_initkwargs = initkwargs就是为那个view函数添加了属性，函数也可以当做对象来使用，这在django框架中非常常见，最后通过update_wrapper(view, cls, updated=())和update_wrapper(view, cls.dispatch, assigned=())进行了再次对view函数的属性添加 比如：

urlpatterns = [
url(r'^index/$',views.IndexView.as_view(name="safly"))
]

def update_wrapper(wrapper,
                   wrapped,
                   assigned = WRAPPER_ASSIGNMENTS,
                   updated = WRAPPER_UPDATES):
    """Update a wrapper function to look like the wrapped function

       wrapper is the function to be updated
       wrapped is the original function
       assigned is a tuple naming the attributes assigned directly
       from the wrapped function to the wrapper function (defaults to
       functools.WRAPPER_ASSIGNMENTS)
       updated is a tuple naming the attributes of the wrapper that
       are updated with the corresponding attribute from the wrapped
       function (defaults to functools.WRAPPER_UPDATES)
    """
    for attr in assigned:
        try:
            value = getattr(wrapped, attr)
        except AttributeError:
            pass
        else:
            setattr(wrapper, attr, value)
    for attr in updated:
        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
    # Issue #17482: set __wrapped__ last so we don't inadvertently copy it
    # from the wrapped function when updating __dict__
    wrapper.__wrapped__ = wrapped
    # Return the wrapper so this can be used as a decorator via partial()
    return wrapper

个人认为就是为view函数，添加了一些属性，用来更新某些操作，以及可以获取某些数据操作。

最后return view，就是调用as_view函数下的内置函数view():

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)

通过源码可以看出，为本类实例对象赋值：request、args、kwargs，最后执行return self.dispatch(request, args, *kwargs)

所以说，views.IndexView.as_view()中的url配置最后会返回view()方法。对本类对象进行了一些封装： self.request = request self.args = args self.kwargs = kwargs 最后再调用dispatch函数：

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)

上述代码通过判断request的请求方式，通过反射的方式，判断是否在http_method_names列表中，没有的话进行相关处理，赋值操作handler，handler = self.http_method_not_allowed 我们索性来看下handler的输出，它是一个函数

handler <bound method IndexView.get of <app01.views.IndexView object at 0x0661BE10>>
# 如果没有找到，就会构造一个默认的，代码如下：

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}
        )
        return http.HttpResponseNotAllowed(self._allowed_methods())

class HttpResponseNotAllowed(HttpResponse):
    status_code = 405

    def __init__(self, permitted_methods, *args, **kwargs):
        super(HttpResponseNotAllowed, self).__init__(*args, **kwargs)
        self['Allow'] = ', '.join(permitted_methods)

    def __repr__(self):
        return '<%(cls)s [%(methods)s] status_code=%(status_code)d%(content_type)s>' % {
            'cls': self.__class__.__name__,
            'status_code': self.status_code,
            'content_type': self._content_type_for_repr,
            'methods': self['Allow'],
        }

所以handler是一个绑定了IndexView中get函数的一个函数，代码的最后handler(request, args, *kwargs)方法执行，其实就是执行我们代码中CBV类IndexView的get方法:

def get(self,request, *args, **kwargs):
        return HttpResponse("ok")

最后返回return http.HttpResponseNotAllowed(self._allowed_methods())

<HttpResponse status_code=200, "text/html; charset=utf-8">

它是一个对象是继承自HttpResponse的对象,也就是在我们的代码中返回即可。

所以最终的结果是：

dispatch
get
ret <HttpResponse status_code=200, "text/html; charset=utf-8">
[07/Oct/2018 15:52:08] "GET /index/ HTTP/1.1" 200 2

整个流程：

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