1. 启动admin
    首先需要将django.contrib.admin添加到INSTALL_APP中
    当加载django中app的时候，执行每一个app下的admin.py
2. 注册模型
    每个模型都可以指定一个ModelAdmin
    封装了该模型特定的管理功能
    实例化一个AdminSite,并告诉它模型类和ModelAmin
    admin.site --> 实例化一个AdminSite
    admin.site.register(Book) 注册Book表
3.  设计URL并将AdminSite实例挂钩到您的URLconfig中
    url(r‘admin/‘,admin.site.urls)

#首先admin.site = AdminSite()

    class AdminSite(object):
        ....

        def __init__(self, name=‘admin‘):
            self._registry = {}   #  AdminSite初始化函数定义了一个字典
            self.name = name
            self._actions = {‘delete_selected‘: actions.delete_selected}
            self._global_actions = self._actions.copy()
            all_sites.add(self)


         def register(self, model_or_iterable, admin_class=None, **options):
            # 判断是否自定义了ModelAdmin
            if not admin_class:
                admin_class = ModelAdmin

            if isinstance(model_or_iterable, ModelBase):
                model_or_iterable = [model_or_iterable]
            for model in model_or_iterable:
                if model._meta.abstract:
                    raise ImproperlyConfigured(
                        ‘The model %s is abstract, so it cannot be registered with admin.‘ % model.__name__
                    )

                if model in self._registry:
                    raise AlreadyRegistered(‘The model %s is already registered‘ % model.__name__)

                if not model._meta.swapped:

                    if options:
                        # For reasons I don‘t quite understand, without a __module__
                        # the created class appears to "live" in the wrong place,
                        # which causes issues later on.
                        options[‘__module__‘] = __name__
                        admin_class = type("%sAdmin" % model.__name__, (admin_class,), options)

                    # 以传入的model类作为字典的key,ModelAdmin作为字典value
                    self._registry[model] = admin_class(model, self)

 #使用装饰器，将urls()装饰为属性，urls返回的是一个元组，元组的第一个参数是一个列表，由get_urls()返回
    @property
    def urls(self):
        return self.get_urls(), ‘admin‘, self.name


    def get_urls(self):
        from django.conf.urls import url, include
        from django.contrib.contenttypes import views as contenttype_views

        def wrap(view, cacheable=False):
            .....
            return update_wrapper(wrapper, view)

        # Admin-site-wide views.
        urlpatterns = [
            url(r‘^$‘, wrap(self.index), name=‘index‘),
            url(r‘^login/$‘, self.login, name=‘login‘),
            url(r‘^logout/$‘, wrap(self.logout), name=‘logout‘),
            url(r‘^password_change/$‘, wrap(self.password_change, cacheable=True), name=‘password_change‘),
            url(r‘^password_change/done/$‘, wrap(self.password_change_done, cacheable=True),
                name=‘password_change_done‘),
            url(r‘^jsi18n/$‘, wrap(self.i18n_javascript, cacheable=True), name=‘jsi18n‘),
            url(r‘^r/(?P<content_type_id>\d+)/(?P<object_id>.+)/$‘, wrap(contenttype_views.shortcut),
                name=‘view_on_site‘),
        ]

        valid_app_labels = []
        #   模型类,ModelAdmin     self._registry字典
        for model, model_admin in self._registry.items():
            urlpatterns += [
                # url(r‘^%s/%s/‘ % (模型所在的app名, 模型名), include(每个模型对应的ModelAdmin中urls)),
                url(r‘^%s/%s/‘ % (model._meta.app_label, model._meta.model_name), include(model_admin.urls)),
            ]
            if model._meta.app_label not in valid_app_labels:
                valid_app_labels.append(model._meta.app_label)
            .....

        #返回了n个URL
        return urlpatterns

admin.site.register(models.UserInfo)　# 注册的时候没有传入admin_class,默认使用的是ModelAdmin　

for model, model_admin in self._registry.items():
    urlpatterns += [
        url(r‘^%s/%s/‘ % (model._meta.app_label, model._meta.model_name), include(model_admin.urls)),
    ]

    def get_urls(self):
        from django.conf.urls import url

        def wrap(view):
            def wrapper(*args, **kwargs):
                return self.admin_site.admin_view(view)(*args, **kwargs)
            wrapper.model_admin = self
            return update_wrapper(wrapper, view)

        info = self.model._meta.app_label, self.model._meta.model_name

        urlpatterns = [
            url(r‘^$‘, wrap(self.changelist_view), name=‘%s_%s_changelist‘ % info),
            url(r‘^add/$‘, wrap(self.add_view), name=‘%s_%s_add‘ % info),
            url(r‘^(.+)/history/$‘, wrap(self.history_view), name=‘%s_%s_history‘ % info),
            url(r‘^(.+)/delete/$‘, wrap(self.delete_view), name=‘%s_%s_delete‘ % info),
            url(r‘^(.+)/change/$‘, wrap(self.change_view), name=‘%s_%s_change‘ % info),
            # For backwards compatibility (was the change url before 1.9)
            url(r‘^(.+)/$‘, wrap(RedirectView.as_view(
                pattern_name=‘%s:%s_%s_change‘ % ((self.admin_site.name,) + info)
            ))),
        ]
        return urlpatterns

    @property
    def urls(self):
        return self.get_urls()