admin源码流程
a. 运行程序，找到每一个app中的 admin.py 文件，并加载

app01.admin.py

创建admin.site中的对象

执行对象的 register方法，目的：讲注册类添加到 _registry中
_registry = {
models.Role: ModelAdmin(models.Role,admin.site),
models.UserInfo: ModelAdmin(models.UserInfo,admin.site)
models.UserType: ModelAdmin(models.UserType,admin.site)
}

      - app02.admin.py
          - 用app01.admin中创建那个admin.site对象
          - 执行对象的 register方法，目的：讲注册类添加到 _registry中 
              _registry = {
                  models.Role: ModelAdmin(models.Role,admin.site),
                  models.UserInfo: ModelAdmin(models.UserInfo,admin.site)
                  models.UserType: ModelAdmin(models.UserType,admin.site)
                  models.Article: ModelAdmin(models.Article,admin.site)
              }
  
      admin.site是一个对象（单例模式创建），其中封装了： 
          _registry = {
              models.Role: ModelAdmin(models.Role,admin.site),
              models.UserInfo: ModelAdmin(models.UserInfo,admin.site)
              models.UserType: ModelAdmin(models.UserType,admin.site)
              models.Article: ModelAdmin(models.Article,admin.site)
          }
  b. urls.py 
      再次调用 admin.site 对象的 urls属性：
          urlpatterns = [
              url(r'^admin/', admin.site.urls),
          ]
      
      class ModelAdmin(object):
          def __init__(self,model_class,site):
              self.model_class = model_class
              self.site = site 
              
          def changelist_view(self,request):
              data_list = self.model_class.objects.all()
              return HttpResponse('列表页面')

          def add_view(self,request):
              return HttpResponse('添加页面')


          def delete_view(self,request,nid):
              return HttpResponse('删除页面')

          def change_view(self,request,nid):
              return HttpResponse('修改页面')
          
          def get_urls(self):
               urlpatterns = [
                  url(r'^$', self.changelist_view),
                  url(r'^add/$', self.add_view),
                  url(r'^(.+)/delete/$', self.delete_view),
                  url(r'^(.+)/change/$', self.change_view),
              ]
              return urlpatterns
          
          @property 
          def urls(self):
              return self.get_urls()
          
      
      class AdminSite(object):
          def __init__(self):
              self._registry = {}
              
          def register(self,model_class,model_admin):
              self._registry[model_class] = model_admin(model_class,self)
          
          def get_urls(self):
              """
              models.Role: ModelAdmin(models.Role,admin.site),
              models.UserInfo: ModelAdmin(models.UserInfo,admin.site)
              models.UserType: ModelAdmin(models.UserType,admin.site)
              models.Article: ModelAdmin(models.Article,admin.site)
              """
              url_list = []
              for model_class,model_admin in self._registry.items():
                  app_name = model_class._meta.app_label
                  model_name = model_class._meta.model_name 
                  url_list += [
                      url('%s/%s' %(app_name,model_name,), include(model_admin.urls))
                  ]
                  
              return url_list
                  
          
          @property
          def urls(self):
              return (self.get_urls(), None,None )