条件视图处理

条件装饰器

Sometimes (in fact, quite often) you can create functions to rapidly compute the ETag value or the last-modified time for a resource, without needing to do all the computations needed to construct the full view. Django can then use these functions to provide an "early bailout" option for the view processing. Telling the client that the content has not been modified since the last request, perhaps.

这两个函数被当做参数传递到 django.views.decorators.http.condition 装饰器。这个装饰器使用两个函数（你只需要支持其中一个，如果你不能很快计算这两个数量）来判断 HTTP 请求的 headers 和这些资源是否匹配。如果它们没有匹配，会计算一份资源的副本，并调用视图。

条件装饰器如下：

condition(etag_func=None, last_modified_func=None)

The two functions, to compute the ETag and the last modified time, will be passed the incoming request object and the same parameters, in the same order, as the view function they are helping to wrap. The function passed last_modified_func should return a standard datetime value specifying the last time the resource was modified, or None if the resource doesn't exist. The function passed to the etag decorator should return a string representing the ETag for the resource, or None if it doesn't exist.

如果它们没有通过视图设置并且请求的方法是安全的(GET 和 HEAD)，那么装饰器就会在请求上设置 ETag 和 Last-Modified headers 。

用一个例子来解释如何有效地使用这个功能。假设你已经有了这些模型，代表一个博客系统：

import datetime
from django.db import models

class Blog(models.Model):
    ...

class Entry(models.Model):
    blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
    published = models.DateTimeField(default=datetime.datetime.now)
    ...

如果在首页正在显示最新的博客文章，只会在有新博客文章时改变，你可以很快地计算最后修改时间。你需要与该博客关联的每篇文章的最新发布时间。一种方法是这样做的：

def latest_entry(request, blog_id):
    return Entry.objects.filter(blog=blog_id).latest("published").published

然后你可以使用这个函数预先为首页视图提供未变动页面的检测：

from django.views.decorators.http import condition

@condition(last_modified_func=latest_entry)
def front_page(request, blog_id):
    ...