详解Python的Flask框架中的signals信号机制
Flask 提供了信号(Signals)功能，是一种消息分发机制。类似于钩子(Hooks)。使用信号功能可以降低程序的耦合，分解复杂的业务模型。例如在更新了产品数据后，可以发送一个信号。当有需要对产品数据进行处理的功能时，就可以捕获信号进行处理。比如要建立产品缓存，或是更新搜索索引等。
定义信号
Flask 信号功能使用了 Blinker 模块，所以需要先安装 Blinker 模块
pip install blinker
定义一个信号:
from blinker import Namespace product_saved = Namespace()
也可以使用 Flask 包装的 singles 对象:
from flask.singles import Namespace
发送信号
发送信号需要带上 app 实例方法，示例如下：
product_saved.send(app, product=product)
app 后面可以添加要传递的参数，但必须以 name=value 的格式，不支持使用单个变量名的方式。
收接信号
接收信号可以使用 connect_via 装饰器函数:
@product_saved.connect_via(app) def updateCache(app, product): print(product)
Flask 中有以下核心信号:
1.flask.template_rendered
这个信号发送于一个模板被渲染成功后。信号传递的template是模板的实例，context是环境对象是一个字典。
订阅示例:
def log_template_renders(sender, template, context, **extra):
sender.logger.debug('Rendering template "%s" with context %s',
template.name or 'string template',
context)
from flask import template_rendered
template_rendered.connect(log_template_renders, app)
2.flask.request_started
这个信号发送于请求开始之前，且请求环境设置完成之后。因为请求环境已经绑定， 所以订阅者可以用标准的全局代理，如 request 来操作请求。
订阅示例:
def log_request(sender, **extra):
sender.logger.debug('Request context is set up')
from flask import request_started
request_started.connect(log_request, app)
flask.request_finished
这个信号发送于向客户端发送响应之前。信号传递的response为将要发送的响应。
订阅示例:
def log_response(sender, response, **extra):
sender.logger.debug('Request context is about to close down. '
'Response: %s', response)
from flask import request_finished
request_finished.connect(log_response, app)
flask.got_request_exception
这个信号发送于请求进行中发生异常的时候。它的发送 早于 标准异常处理介于。 在调试模式下，虽然没有异常处理，但发生异常时也发送这个信号。信号传递的exception是异常对象。
订阅示例:
def log_exception(sender, exception, **extra):
sender.logger.debug('Got exception during processing: %s', exception)
from flask import got_request_exception
got_request_exception.connect(log_exception, app)
flask.request_tearing_down
这个信号发送于请求崩溃的时候，不管是否引发异常。目前，侦听此信号的函数在一般 崩溃处理器后调用，但是没有什么东西可用。
订阅示例:
def close_db_connection(sender, **extra): session.close()from flask import appcontext_tearing_down request_tearing_down.connect(close_db_connection, app)
在 Flask 版本 0.9 中，这还会传递一个exc关键字参数，如果这个参数存在的话。 这个参数是引发崩溃的异常的引用。
3.flask.appcontext_tearing_down
当应用环境崩溃时发送这个信号。这个信号总是会发送，甚至是因为一个异常引发的 崩溃。侦听这个信号的函数会在常规崩溃处理器后被调用，但是你无法回馈这个信号。
订阅示例:
def close_db_connection(sender, **extra): session.close()from flask import request_tearing_down appcontext_tearing_down.connect(close_db_connection, app)
这还会传递一个exc关键字参数，如果这个参数存在的话。这个参数是引发崩溃的 异常的引用。
4.flask.appcontext_pushed
当一个应用的环境被压入时，应用会发送这个信号。这个信号通常用于在单元测试中 临时钩接信息。例如可以用于改变g对象中现存的资源。
用法示例:
from contextlib import contextmanagerfrom
flask import appcontext_pushed
@contextmanagerdef user_set(app, user):
def handler(sender, **kwargs):
g.user = user
with appcontext_pushed.connected_to(handler, app):
yield
在测试代码中这样写:
def test_user_me(self):
with user_set(app, 'john'):
c = app.test_client()
resp = c.get('/users/me')
assert resp.data == 'username=john'
New in version 0.10.
5.appcontext_popped
当一个应用的环境被弹出时，应用会发送这个信号。这个信号通常写成appcontext_tearing_down 信号。
6.flask.message_flashed
当应用闪现一个消息时会发出这个信号。message`参数是消息内容， `category参数是消息类别。
订阅示例:
recorded = []def record(sender, message, category, **extra): recorded.append((message, category)) from flask import message_flashed message_flashed.connect(record, app)
小结
信号可以让你在一瞬间安全地订阅它们。例如，这些临时的订阅对测试很有帮助。使用信号时，不要让信号订阅者（接收者）发生异常，因为异常会造成程序中断。
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