深入解析当下大热的前后端分离组件django-rest_framework系列二

视图三部曲

一部曲 · 使用混合（mixins）

上一节的视图部分：

from rest_framework.views import APIView
from rest_framework.response import Response
from .models import *
from django.shortcuts import HttpResponse
from django.core import serializers

from rest_framework import serializers

class BookSerializers(serializers.ModelSerializer):
      class Meta:
          model=Book
          fields="__all__"
          #depth=1

class PublshSerializers(serializers.ModelSerializer):

      class Meta:
          model=Publish
          fields="__all__"
          depth=1

class BookViewSet(APIView):

    def get(self,request,*args,**kwargs):
        book_list=Book.objects.all()
        bs=BookSerializers(book_list,many=True,context={'request': request})
        return Response(bs.data)

    def post(self,request,*args,**kwargs):
        print(request.data)
        bs=BookSerializers(data=request.data,many=False)
        if bs.is_valid():
            print(bs.validated_data)
            bs.save()
            return Response(bs.data)
        else:
            return HttpResponse(bs.errors)

class BookDetailViewSet(APIView):

    def get(self,request,pk):
        book_obj=Book.objects.filter(pk=pk).first()
        bs=BookSerializers(book_obj,context={'request': request})
        return Response(bs.data)

    def put(self,request,pk):
        book_obj=Book.objects.filter(pk=pk).first()
        bs=BookSerializers(book_obj,data=request.data,context={'request': request})
        if bs.is_valid():
            bs.save()
            return Response(bs.data)
        else:
            return HttpResponse(bs.errors)

class PublishViewSet(APIView):

    def get(self,request,*args,**kwargs):
        publish_list=Publish.objects.all()
        bs=PublshSerializers(publish_list,many=True,context={'request': request})
        return Response(bs.data)

    def post(self,request,*args,**kwargs):

        bs=PublshSerializers(data=request.data,many=False)
        if bs.is_valid():
            # print(bs.validated_data)
            bs.save()
            return Response(bs.data)
        else:
            return HttpResponse(bs.errors)

class PublishDetailViewSet(APIView):

    def get(self,request,pk):

        publish_obj=Publish.objects.filter(pk=pk).first()
        bs=PublshSerializers(publish_obj,context={'request': request})
        return Response(bs.data)

    def put(self,request,pk):
        publish_obj=Publish.objects.filter(pk=pk).first()
        bs=PublshSerializers(publish_obj,data=request.data,context={'request': request})
        if bs.is_valid():
            bs.save()
            return Response(bs.data)
        else:
            return HttpResponse(bs.errors)

我们使用这一套逻辑，意味着有一张模型表，就要将上面的代码写一遍，代码的复用性很差，所有，我们要对复用的部分进行封装，我们通过三步，一步一步的来实现代码的复用和封装。

第一步：大体思路是使用混合类（多继承的形式），借助封装好的mixins类

from rest_framework import mixins

　　我们可以发现，这些复用的代码中，有两个变量是必须要知道的，一个是模型表中的数据，一个是该模型表对应的serializers对象，所以我们可以将这两个变量单独给提出来放到类的静态属性中，这样整个类中都可以调用。

使用方法：

将查看所有数据的方法封装到一个类中：

class ListModelMixin(object):
    """
    List a queryset.
    """
    def list(self, request, *args, **kwargs):
        queryset = self.filter_queryset(self.get_queryset())

        page = self.paginate_queryset(queryset)
        if page is not None:
            serializer = self.get_serializer(page, many=True)
            return self.get_paginated_response(serializer.data)

        serializer = self.get_serializer(queryset, many=True)
        return Response(serializer.data)

所以，我们处理查看所有数据的视图时，直接将这个类继承，同时，在get方法中，将list方法返回即可。

同样的方式，将添加数据，查看某一条数据，编辑某一条数据，删除某一条数据也分别封装到一个个类下，只需要在对应的请求方法中，返回继承类中对应的方法即可。

添加数据封装到类：

class CreateModelMixin(object):
    """
    Create a model instance.
    """
    def create(self, request, *args, **kwargs):
        serializer = self.get_serializer(data=request.data)
        serializer.is_valid(raise_exception=True)
        self.perform_create(serializer)
        headers = self.get_success_headers(serializer.data)
        return Response(serializer.data, status=status.HTTP_201_CREATED, headers=headers)

    def perform_create(self, serializer):
        serializer.save()

    def get_success_headers(self, data):
        try:
            return {'Location': str(data[api_settings.URL_FIELD_NAME])}
        except (TypeError, KeyError):
            return {}

查看某一条数据封装的类：

class RetrieveModelMixin(object):
    """
    Retrieve a model instance.
    """
    def retrieve(self, request, *args, **kwargs):
        instance = self.get_object()
        serializer = self.get_serializer(instance)
        return Response(serializer.data)

编辑数据的类：

class UpdateModelMixin(object):
    """
    Update a model instance.
    """
    def update(self, request, *args, **kwargs):
        partial = kwargs.pop('partial', False)
        instance = self.get_object()
        serializer = self.get_serializer(instance, data=request.data, partial=partial)
        serializer.is_valid(raise_exception=True)
        self.perform_update(serializer)

        if getattr(instance, '_prefetched_objects_cache', None):
            # If 'prefetch_related' has been applied to a queryset, we need to
            # forcibly invalidate the prefetch cache on the instance.
            instance._prefetched_objects_cache = {}

        return Response(serializer.data)

    def perform_update(self, serializer):
        serializer.save()

    def partial_update(self, request, *args, **kwargs):
        kwargs['partial'] = True
        return self.update(request, *args, **kwargs)

删除数据的类：

class DestroyModelMixin(object):
    """
    Destroy a model instance.
    """
    def destroy(self, request, *args, **kwargs):
        instance = self.get_object()
        self.perform_destroy(instance)
        return Response(status=status.HTTP_204_NO_CONTENT)

    def perform_destroy(self, instance):
        instance.delete()

通过上面的方式，我们会有一个疑问，我们传入的pk值，是怎么处理的。

我们在处理类的视图函数时，必须要继承一个APIView的类，同样的，也将这个类进行了封装：放到了rest_framework.generics下面的GenericAPIView中；

from rest_framework import views

class GenericAPIView(views.APIView):
    pass

这个类继承了APIView，并进行了相应的扩展。我们传的pk值，就是在这里被处理的。

处理单条数据时，我们肯定要先将对应的数据取出来，再做对应的处理，同样的，封装到类中肯定也做了类似的处理，我们观察处理单条数据的类，会发现这样一行代码：instance = self.get_object()  很明显，肯定是取数据去了，怎么取？不知道，看看，先找到这个方法。我们从本类和继承的类中依次去找，最后在GenericAPIView中找到了这个方法：

    def get_object(self):
        """
        Returns the object the view is displaying.

        You may want to override this if you need to provide non-standard
        queryset lookups.  Eg if objects are referenced using multiple
        keyword arguments in the url conf.
        """
        queryset = self.filter_queryset(self.get_queryset())

        # Perform the lookup filtering.
        lookup_url_kwarg = self.lookup_url_kwarg or self.lookup_field

        assert lookup_url_kwarg in self.kwargs, (
            'Expected view %s to be called with a URL keyword argument '
            'named "%s". Fix your URL conf, or set the `.lookup_field` '
            'attribute on the view correctly.' %
            (self.__class__.__name__, lookup_url_kwarg)
        )

        filter_kwargs = {self.lookup_field: self.kwargs[lookup_url_kwarg]}
        obj = get_object_or_404(queryset, **filter_kwargs)

        # May raise a permission denied
        self.check_object_permissions(self.request, obj)

        return obj

这个方法最终将obj返回了，那就要好奇了，怎么一下子将obj取出来的，继续查看  obj=get_object_or_404(queryset,**fi..)

def get_object_or_404(queryset, *filter_args, **filter_kwargs):
    """
    Same as Django's standard shortcut, but make sure to also raise 404
    if the filter_kwargs don't match the required types.
    """
    try:
        return _get_object_or_404(queryset, *filter_args, **filter_kwargs)
    except (TypeError, ValueError, ValidationError):
        raise Http404

进行了一个异常处理，还是没有我们要的内容，继续找  _get_object_or_404(queryset,*...,**...)

def get_object_or_404(klass, *args, **kwargs):
    """
    Uses get() to return an object, or raises a Http404 exception if the object
    does not exist.

    klass may be a Model, Manager, or QuerySet object. All other passed
    arguments and keyword arguments are used in the get() query.

    Note: Like with get(), an MultipleObjectsReturned will be raised if more than one
    object is found.
    """
    queryset = _get_queryset(klass)
    try:
        return queryset.get(*args, **kwargs)
    except AttributeError:
        klass__name = klass.__name__ if isinstance(klass, type) else klass.__class__.__name__
        raise ValueError(
            "First argument to get_object_or_404() must be a Model, Manager, "
            "or QuerySet, not '%s'." % klass__name
        )
    except queryset.model.DoesNotExist:
        raise Http404('No %s matches the given query.' % queryset.model._meta.object_name)

这么一堆中，就  queryset.get(*args,**kwargs)  是我们要的内容，我们通过url传入pk=val，被**kwargs捕获，传入这个代码中，等同于models.模型类.objects.all().get(**{"pk":val})，这种形式很眼熟，这就是我们取值的操作。

mixin类编写视图

from rest_framework import mixins
from rest_framework import generics

class BookViewSet(mixins.ListModelMixin,
                  mixins.CreateModelMixin,
                  generics.GenericAPIView):

    queryset = Book.objects.all()
    serializer_class = BookSerializers

    def get(self, request, *args, **kwargs):
        return self.list(request, *args, **kwargs)

    def post(self, request, *args, **kwargs):
        return self.create(request, *args, **kwargs)

class BookDetailViewSet(mixins.RetrieveModelMixin,
                    mixins.UpdateModelMixin,
                    mixins.DestroyModelMixin,
                    generics.GenericAPIView):
    queryset = Book.objects.all()
    serializer_class = BookSerializers

    def get(self, request, *args, **kwargs):
        return self.retrieve(request, *args, **kwargs)

    def put(self, request, *args, **kwargs):
        return self.update(request, *args, **kwargs)

    def delete(self, request, *args, **kwargs):
        return self.destroy(request, *args, **kwargs)

特别需要注意的是，使用这种方式，设计url时，需要传参的url，必须使用分组命名匹配的形式，而且，命名必须是pk，否则报错

urlpatterns = [

　　# 错误写法
    url(r'^books/$',views.Book.as_view()),
    url(r'^books/(\d+)/$',views.BookDeail.as_view()),

　　# 正确写法
    url(r'^publish/$',views.Publish.as_view()),
    url(r'^publish/(?P<pk>\d+)/$',views.PublishDeail.as_view()), #必须是pk
]

采用这种方式匹配url，跟封装的源码有关；

class GenericAPIView(views.APIView):
    """
    Base class for all other generic views.
    """
    # You'll need to either set these attributes,
    # or override `get_queryset()`/`get_serializer_class()`.
    # If you are overriding a view method, it is important that you call
    # `get_queryset()` instead of accessing the `queryset` property directly,
    # as `queryset` will get evaluated only once, and those results are cached
    # for all subsequent requests.
    queryset = None
    serializer_class = None

    # If you want to use object lookups other than pk, set 'lookup_field'.
    # For more complex lookup requirements override `get_object()`.
    lookup_field = 'pk'
    

上面的代码虽然一定程度上减少了代码的复用性，但是，我们对于每一个模型表任然要进行大量的复用代码。

备注：在继承类中配置了lookup_field属性，If you want to use object lookups other than pk, set 'lookup_field'

第二步：将这些封装的类进一步封装。

二部曲 · 使用通用的基于类的视图

通过使用mixin类，我们使用更少的代码重写了这些视图，但我们还可以再进一步。REST框架提供了一组已经混合好（mixed-in）的通用视图，我们可以使用它来简化我们的views.py模块。

将我们不需要接收传参的视图函数整体封装到一个类（查看所有数据，添加数据）

class ListCreateAPIView(mixins.ListModelMixin,
                        mixins.CreateModelMixin,
                        GenericAPIView):
    """
    Concrete view for listing a queryset or creating a model instance.
    """
    def get(self, request, *args, **kwargs):
        return self.list(request, *args, **kwargs)

    def post(self, request, *args, **kwargs):
        return self.create(request, *args, **kwargs)

同样的，需要接收一个pk值的视图封装：

class RetrieveUpdateDestroyAPIView(mixins.RetrieveModelMixin,
                                   mixins.UpdateModelMixin,
                                   mixins.DestroyModelMixin,
                                   GenericAPIView):
    """
    Concrete view for retrieving, updating or deleting a model instance.
    """
    def get(self, request, *args, **kwargs):
        return self.retrieve(request, *args, **kwargs)

    def put(self, request, *args, **kwargs):
        return self.update(request, *args, **kwargs)

    def patch(self, request, *args, **kwargs):
        return self.partial_update(request, *args, **kwargs)

    def delete(self, request, *args, **kwargs):
        return self.destroy(request, *args, **kwargs)

借助这个封装，进一步的简化代码量

from rest_framework import mixins
from rest_framework import generics

class BookViewSet(generics.ListCreateAPIView):

    queryset = Book.objects.all()
    serializer_class = BookSerializers

class BookDetailViewSet(generics.RetrieveUpdateDestroyAPIView):
    queryset = Book.objects.all()
    serializer_class = BookSerializers

class PublishViewSet(generics.ListCreateAPIView):

    queryset = Publish.objects.all()
    serializer_class = PublshSerializers

class PublishDetailViewSet(generics.RetrieveUpdateDestroyAPIView):
    queryset = Publish.objects.all()
    serializer_class = PublshSerializers

通过上面的代码，我们已经减少了大量的代码，但是，好像我们还在复用一些代码，queryset和serializer_class 这两个参数。所以我们继续封装。

第三步的思路：将接收pk的url和不接受pk的url合为一个视图，会有一个问题，那就是，get请求，我们不管查看所有数据还是查看某一条数据，都会走同一个视图，怎么可以解决这个问题？问题的根源是出在分发上，我们可以重写。

　　我们将视图合二为一，就需要将继承的所有的类进一步封装。

class ModelViewSet(mixins.CreateModelMixin,
                   mixins.RetrieveModelMixin,
                   mixins.UpdateModelMixin,
                   mixins.DestroyModelMixin,
                   mixins.ListModelMixin,
                   GenericViewSet):
    """
    A viewset that provides default `create()`, `retrieve()`, `update()`,
    `partial_update()`, `destroy()` and `list()` actions.
    """
    pass

导入：

from rest_framework.viewsets import ModelViewSet

三部曲 · viewsets.ModelViewSet

urls.py:

    url(r'^books/$', views.BookViewSet.as_view({"get":"list","post":"create"}),name="book_list"),
    url(r'^books/(?P<pk>\d+)$', views.BookViewSet.as_view({
                'get': 'retrieve',
                'put': 'update',
                'patch': 'partial_update',
                'delete': 'destroy'
            }),name="book_detail"),

views.py:

class BookViewSet(viewsets.ModelViewSet):
    queryset = Book.objects.all()
    serializer_class = BookSerializers

使用这种方式，url就必须在as_view()方法里，传一个字典参数。而之前是不需要传的，显然，重写了as_view()方法，那我们就去找这个重写as_view的类。ModelViewSet中前五个是我们第一步封装的，唯有最后一个GenericViewSet，是重写的

class GenericViewSet(ViewSetMixin, generics.GenericAPIView):
    """
    The GenericViewSet class does not provide any actions by default,
    but does include the base set of generic view behavior, such as
    the `get_object` and `get_queryset` methods.
    """
    pass

这个类中通过混合类的方式进行继承，后面的是我们之前用过的，唯有前面的ViewSetMixin是扩展的，很显然，是这个类重写了as_view()

class ViewSetMixin(object):
    """
    This is the magic.

    Overrides `.as_view()` so that it takes an `actions` keyword that performs
    the binding of HTTP methods to actions on the Resource.

    For example, to create a concrete view binding the 'GET' and 'POST' methods
    to the 'list' and 'create' actions...

    view = MyViewSet.as_view({'get': 'list', 'post': 'create'})
    """

    @classonlymethod
    def as_view(cls, actions=None, **initkwargs):
        """
        Because of the way class based views create a closure around the
        instantiated view, we need to totally reimplement `.as_view`,
        and slightly modify the view function that is created and returned.
        """
      　　# 此处省略部分源码...

        def view(request, *args, **kwargs):
            self = cls(**initkwargs)
            # We also store the mapping of request methods to actions,
            # so that we can later set the action attribute.
            # eg. `self.action = 'list'` on an incoming GET request.
            self.action_map = actions

            # Bind methods to actions
            # This is the bit that's different to a standard view
            for method, action in actions.items():
                handler = getattr(self, action)
                setattr(self, method, handler)

            if hasattr(self, 'get') and not hasattr(self, 'head'):
                self.head = self.get

            self.request = request
            self.args = args
            self.kwargs = kwargs

            # And continue as usual
            return self.dispatch(request, *args, **kwargs)

        # 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=())

        # We need to set these on the view function, so that breadcrumb
        # generation can pick out these bits of information from a
        # resolved URL.
        view.cls = cls
        view.initkwargs = initkwargs
        view.suffix = initkwargs.get('suffix', None)
        view.actions = actions
        return csrf_exempt(view)

　　一样的，这个as_view()最终返回一个view函数，跟之前是一样的，那么接收的这个参数有什么用呢？肯定在view函数中，调用了这个参数，并实现了某些东西。（actions接收了传的字典参数）

    for method, action in actions.items():
        handler = getattr(self, action)
        setattr(self, method, handler)

这三行代码是进行了相应的处理，首先，循环这个参数，通过反射得到value对应的函数名（list、create、retrieve、update、destory），最后通过setattr实现了我们调用哪个key(method)就会执行对应的value，从而完美的解决了分发的问题。

　　通过这三次分发，实现了代码的复用。

总结

　　通过三层封装，使得我们可以非常快速的通过几行代码实现一个模型类的增删改查，大大的提高了开发效率。有利就有弊，封装的越完善，就意味着不够灵活。下个系列中，带来restframework中的非常有用的三个套件。