18.5.1. Base Event Loop
***********************

**Source code:** Lib/asyncio/events.py

The event loop is the central execution device provided by "asyncio".
It provides multiple facilities, including:

* Registering, executing and cancelling delayed calls (timeouts).

* Creating client and server transports for various kinds of
  communication.

* Launching subprocesses and the associated transports for
  communication with an external program.

* Delegating costly function calls to a pool of threads.

class asyncio.BaseEventLoop

   This class is an implementation detail.  It is a subclass of
   "AbstractEventLoop" and may be a base class of concrete event loop
   implementations found in "asyncio".  It should not be used
   directly; use "AbstractEventLoop" instead. "BaseEventLoop" should
   not be subclassed by third-party code; the internal interface is
   not stable.

class asyncio.AbstractEventLoop

   Abstract base class of event loops.

   This class is not thread safe.


18.5.1.1. Run an event loop
===========================

AbstractEventLoop.run_forever()

   Run until "stop()" is called.  If "stop()" is called before
   "run_forever()" is called, this polls the I/O selector once with a
   timeout of zero, runs all callbacks scheduled in response to I/O
   events (and those that were already scheduled), and then exits. If
   "stop()" is called while "run_forever()" is running, this will run
   the current batch of callbacks and then exit.  Note that callbacks
   scheduled by callbacks will not run in that case; they will run the
   next time "run_forever()" is called.

   在 3.5.1 版更改.

AbstractEventLoop.run_until_complete(future)

   Run until the "Future" is done.

   If the argument is a coroutine object, it is wrapped by
   "ensure_future()".

   Return the Future's result, or raise its exception.

AbstractEventLoop.is_running()

   Returns running status of event loop.

AbstractEventLoop.stop()

   Stop running the event loop.

   This causes "run_forever()" to exit at the next suitable
   opportunity (see there for more details).

   在 3.5.1 版更改.

AbstractEventLoop.is_closed()

   Returns "True" if the event loop was closed.

   3.4.2 新版功能.

AbstractEventLoop.close()

   Close the event loop. The loop must not be running.  Pending
   callbacks will be lost.

   This clears the queues and shuts down the executor, but does not
   wait for the executor to finish.

   This is idempotent and irreversible. No other methods should be
   called after this one.

coroutine AbstractEventLoop.shutdown_asyncgens()

   Schedule all currently open *asynchronous generator* objects to
   close with an "aclose()" call.  After calling this method, the
   event loop will issue a warning whenever a new asynchronous
   generator is iterated.  Should be used to finalize all scheduled
   asynchronous generators reliably.  Example:

      try:
          loop.run_forever()
      finally:
          loop.run_until_complete(loop.shutdown_asyncgens())
          loop.close()

   3.6 新版功能.


18.5.1.2. Calls
===============

Most "asyncio" functions don't accept keywords. If you want to pass
keywords to your callback, use "functools.partial()". For example,
"loop.call_soon(functools.partial(print, "Hello", flush=True))" will
call "print("Hello", flush=True)".

注解: "functools.partial()" is better than "lambda" functions,
  because "asyncio" can inspect "functools.partial()" object to
  display parameters in debug mode, whereas "lambda" functions have a
  poor representation.

AbstractEventLoop.call_soon(callback, *args)

   Arrange for a callback to be called as soon as possible.  The
   callback is called after "call_soon()" returns, when control
   returns to the event loop.

   This operates as a FIFO (first-in, first-out) queue, callbacks are
   called in the order in which they are registered.  Each callback
   will be called exactly once.

   Any positional arguments after the callback will be passed to the
   callback when it is called.

   An instance of "asyncio.Handle" is returned, which can be used to
   cancel the callback.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.call_soon_threadsafe(callback, *args)

   Like "call_soon()", but thread safe.

   查看 并发和多线程 章节的文档。


18.5.1.3. Delayed calls
=======================

The event loop has its own internal clock for computing timeouts.
Which clock is used depends on the (platform-specific) event loop
implementation; ideally it is a monotonic clock.  This will generally
be a different clock than "time.time()".

注解: Timeouts (relative *delay* or absolute *when*) should not
  exceed one day.

AbstractEventLoop.call_later(delay, callback, *args)

   Arrange for the *callback* to be called after the given *delay*
   seconds (either an int or float).

   An instance of "asyncio.Handle" is returned, which can be used to
   cancel the callback.

   *callback* will be called exactly once per call to "call_later()".
   If two callbacks are scheduled for exactly the same time, it is
   undefined which will be called first.

   The optional positional *args* will be passed to the callback when
   it is called. If you want the callback to be called with some named
   arguments, use a closure or "functools.partial()".

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.call_at(when, callback, *args)

   Arrange for the *callback* to be called at the given absolute
   timestamp *when* (an int or float), using the same time reference
   as "AbstractEventLoop.time()".

   这个函数的行为与 "call_later()" 相同。

   An instance of "asyncio.Handle" is returned, which can be used to
   cancel the callback.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.time()

   Return the current time, as a "float" value, according to the event
   loop's internal clock.

参见: "asyncio.sleep()" 函数


18.5.1.4. Futures
=================

AbstractEventLoop.create_future()

   Create an "asyncio.Future" object attached to the loop.

   This is a preferred way to create futures in asyncio, as event loop
   implementations can provide alternative implementations of the
   Future class (with better performance or instrumentation).

   3.5.2 新版功能.


18.5.1.5. Tasks
===============

AbstractEventLoop.create_task(coro)

   Schedule the execution of a coroutine object: wrap it in a future.
   Return a "Task" object.

   三方的事件循环可以使用它们自己定义的 "Task" 类的子类来实现互操作性
   。这个例子里，返回值的类型是 "Task" 的子类。

   This method was added in Python 3.4.2. Use the "async()" function
   to support also older Python versions.

   3.4.2 新版功能.

AbstractEventLoop.set_task_factory(factory)

   Set a task factory that will be used by
   "AbstractEventLoop.create_task()".

   If *factory* is "None" the default task factory will be set.

   If *factory* is a *callable*, it should have a signature matching
   "(loop, coro)", where *loop* will be a reference to the active
   event loop, *coro* will be a coroutine object.  The callable must
   return an "asyncio.Future" compatible object.

   3.4.4 新版功能.

AbstractEventLoop.get_task_factory()

   Return a task factory, or "None" if the default one is in use.

   3.4.4 新版功能.


18.5.1.6. Creating connections
==============================

coroutine AbstractEventLoop.create_connection(protocol_factory, host=None, port=None, *, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None)

   Create a streaming transport connection to a given Internet *host*
   and *port*: socket family "AF_INET" or "AF_INET6" depending on
   *host* (or *family* if specified), socket type "SOCK_STREAM".
   *protocol_factory* must be a callable returning a protocol
   instance.

   This method is a coroutine which will try to establish the
   connection in the background.  When successful, the coroutine
   returns a "(transport, protocol)" pair.

   基本操作的时间顺序如下：

   1. The connection is established, and a transport is created to
      represent it.

   2. *protocol_factory* is called without arguments and must
      return a protocol instance.

   3. The protocol instance is tied to the transport, and its
      "connection_made()" method is called.

   4. The coroutine returns successfully with the "(transport,
      protocol)" pair.

   创建的transport是一个实现相关的双向流。

   注解: *protocol_factory* can be any kind of callable, not
     necessarily a class.  For example, if you want to use a pre-
     created protocol instance, you can pass "lambda: my_protocol".

   Options that change how the connection is created:

   * *ssl*: if given and not false, a SSL/TLS transport is created
     (by default a plain TCP transport is created).  If *ssl* is a
     "ssl.SSLContext" object, this context is used to create the
     transport; if *ssl* is "True", a context with some unspecified
     default settings is used.

     参见: SSL/TLS安全事项

   * *server_hostname*, is only for use together with *ssl*, and
     sets or overrides the hostname that the target server's
     certificate will be matched against.  By default the value of the
     *host* argument is used.  If *host* is empty, there is no default
     and you must pass a value for *server_hostname*.  If
     *server_hostname* is an empty string, hostname matching is
     disabled (which is a serious security risk, allowing for man-in-
     the-middle-attacks).

   * *family*，*proto*，*flags* 是可选的地址族，协议和标志，通过传递
     给 getaddrinfo() 来解析 *host*。如果给出，这些应该都是来自
     "socket" 模块相应的常量的整数。

   * *sock*, if given, should be an existing, already connected
     "socket.socket" object to be used by the transport. If *sock* is
     given, none of *host*, *port*, *family*, *proto*, *flags* and
     *local_addr* should be specified.

   * *local_addr*, if given, is a "(local_host, local_port)" tuple
     used to bind the socket to locally.  The *local_host* and
     *local_port* are looked up using getaddrinfo(), similarly to
     *host* and *port*.

   在 3.5 版更改: On Windows with "ProactorEventLoop", SSL/TLS is now
   supported.

   参见: The "open_connection()" function can be used to get a pair
     of ("StreamReader", "StreamWriter") instead of a protocol.

coroutine AbstractEventLoop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, *, family=0, proto=0, flags=0, reuse_address=None, reuse_port=None, allow_broadcast=None, sock=None)

   Create datagram connection: socket family "AF_INET" or "AF_INET6"
   depending on *host* (or *family* if specified), socket type
   "SOCK_DGRAM". *protocol_factory* must be a callable returning a
   protocol instance.

   This method is a coroutine which will try to establish the
   connection in the background.  When successful, the coroutine
   returns a "(transport, protocol)" pair.

   Options changing how the connection is created:

   * *local_addr*，如果指定的话，就是一个 "(local_host, local_port)"
     元组，用于在本地绑定套接字。 *local_host* 和 *local_port* 是使用
     "getaddrinfo()" 来查找的。

   * *remote_addr*，如果指定的话，就是一个 "(remote_host,
     remote_port)" 元组，用于同一个远程地址连接。*remote_host* 和
     *remote_port* 是使用 "getaddrinfo()" 来查找的。

   * *family*, *proto*, *flags* 是可选的地址族，协议和标志，其会被传
     递 给 "getaddrinfo()" 来完成 *host* 的解析。如果要指定的话，这些
     都应 该是来自于 "socket" 模块的对应常量。

   * *reuse_address* tells the kernel to reuse a local socket in
     TIME_WAIT state, without waiting for its natural timeout to
     expire. If not specified will automatically be set to "True" on
     UNIX.

   * *reuse_port* tells the kernel to allow this endpoint to be
     bound to the same port as other existing endpoints are bound to,
     so long as they all set this flag when being created. This option
     is not supported on Windows and some UNIX's. If the
     "SO_REUSEPORT" constant is not defined then this capability is
     unsupported.

   * *allow_broadcast* 告知内核允许此端点向广播地址发送消息。

   * *sock* 可选择通过指定此值用于使用一个预先存在的，已经处于连接状
     态 的 "socket.socket" 对象，并将其提供给此传输对象使用。如果指定
     了这 个值， *local_addr* 和 *remote_addr* 就应该被忽略 (必须为
     "None") 。

   On Windows with "ProactorEventLoop", this method is not supported.

   参见 UDP echo 客户端协议  和 UDP echo 服务端协议 的例子。

   在 3.4.4 版更改: 添加了 *family*, *proto*, *flags*,
   *reuse_address*, *reuse_port*, *allow_broadcast* 和 *sock* 等参数。

coroutine AbstractEventLoop.create_unix_connection(protocol_factory, path, *, ssl=None, sock=None, server_hostname=None)

   Create UNIX connection: socket family "AF_UNIX", socket type
   "SOCK_STREAM". The "AF_UNIX" socket family is used to communicate
   between processes on the same machine efficiently.

   This method is a coroutine which will try to establish the
   connection in the background.  When successful, the coroutine
   returns a "(transport, protocol)" pair.

   *path* is the name of a UNIX domain socket, and is required unless
   a *sock* parameter is specified.  Abstract UNIX sockets, "str", and
   "bytes" paths are supported.

   See the "AbstractEventLoop.create_connection()" method for
   parameters.

   Availability: UNIX.


18.5.1.7. Creating listening connections
========================================

coroutine AbstractEventLoop.create_server(protocol_factory, host=None, port=None, *, family=socket.AF_UNSPEC, flags=socket.AI_PASSIVE, sock=None, backlog=100, ssl=None, reuse_address=None, reuse_port=None)

   Create a TCP server (socket type "SOCK_STREAM") bound to *host* and
   *port*.

   Return a "Server" object, its "sockets" attribute contains created
   sockets. Use the "Server.close()" method to stop the server: close
   listening sockets.

   参数：

   * The *host* parameter can be a string, in that case the TCP
     server is bound to *host* and *port*. The *host* parameter can
     also be a sequence of strings and in that case the TCP server is
     bound to all hosts of the sequence. If *host* is an empty string
     or "None", all interfaces are assumed and a list of multiple
     sockets will be returned (most likely one for IPv4 and another
     one for IPv6).

   * *family* can be set to either "socket.AF_INET" or "AF_INET6" to
     force the socket to use IPv4 or IPv6. If not set it will be
     determined from host (defaults to "socket.AF_UNSPEC").

   * *flags* 是用于 "getaddrinfo()" 的位掩码。

   * *sock* can optionally be specified in order to use a
     preexisting socket object. If specified, *host* and *port* should
     be omitted (must be "None").

   * *backlog* 是传递给 "listen()" 的最大排队连接的数量（默认为100）
     。

   * *ssl* 可被设置为一个 "SSLContext" 以在接受的连接上启用 SSL。

   * *reuse_address* tells the kernel to reuse a local socket in
     TIME_WAIT state, without waiting for its natural timeout to
     expire. If not specified will automatically be set to "True" on
     UNIX.

   * *reuse_port* 告知内核，只要在创建的时候都设置了这个标志，就允许
     此 端点绑定到其它端点列表所绑定的同样的端口上。这个选项在 Windows
     上 是不支持的。

   This method is a coroutine.

   在 3.5 版更改: On Windows with "ProactorEventLoop", SSL/TLS is now
   supported.

   参见: The function "start_server()" creates a ("StreamReader",
     "StreamWriter") pair and calls back a function with this pair.

   在 3.5.1 版更改: The *host* parameter can now be a sequence of
   strings.

coroutine AbstractEventLoop.create_unix_server(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None)

   Similar to "AbstractEventLoop.create_server()", but specific to the
   socket family "AF_UNIX".

   This method is a coroutine.

   Availability: UNIX.

coroutine BaseEventLoop.connect_accepted_socket(protocol_factory, sock, *, ssl=None)

   Handle an accepted connection.

   This is used by servers that accept connections outside of asyncio
   but that use asyncio to handle them.

   参数：

   * *sock* is a preexisting socket object returned from an "accept"
     call.

   * *ssl* 可被设置为一个 "SSLContext" 以在接受的连接上启用 SSL。

   This method is a coroutine.  When completed, the coroutine returns
   a "(transport, protocol)" pair.

   3.5.3 新版功能.


18.5.1.8. Watch file descriptors
================================

On Windows with "SelectorEventLoop", only socket handles are supported
(ex: pipe file descriptors are not supported).

On Windows with "ProactorEventLoop", these methods are not supported.

AbstractEventLoop.add_reader(fd, callback, *args)

   Start watching the file descriptor for read availability and then
   call the *callback* with specified arguments.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_reader(fd)

   Stop watching the file descriptor for read availability.

AbstractEventLoop.add_writer(fd, callback, *args)

   Start watching the file descriptor for write availability and then
   call the *callback* with specified arguments.

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_writer(fd)

   Stop watching the file descriptor for write availability.

The watch a file descriptor for read events example uses the low-level
"AbstractEventLoop.add_reader()" method to register the file
descriptor of a socket.


18.5.1.9. Low-level socket operations
=====================================

coroutine AbstractEventLoop.sock_recv(sock, nbytes)

   Receive data from the socket.  Modeled after blocking
   "socket.socket.recv()" method.

   The return value is a bytes object representing the data received.
   The maximum amount of data to be received at once is specified by
   *nbytes*.

   With "SelectorEventLoop" event loop, the socket *sock* must be non-
   blocking.

   This method is a coroutine.

coroutine AbstractEventLoop.sock_sendall(sock, data)

   Send data to the socket.  Modeled after blocking
   "socket.socket.sendall()" method.

   The socket must be connected to a remote socket. This method
   continues to send data from *data* until either all data has been
   sent or an error occurs.  "None" is returned on success.  On error,
   an exception is raised, and there is no way to determine how much
   data, if any, was successfully processed by the receiving end of
   the connection.

   With "SelectorEventLoop" event loop, the socket *sock* must be non-
   blocking.

   This method is a coroutine.

coroutine AbstractEventLoop.sock_connect(sock, address)

   Connect to a remote socket at *address*.  Modeled after blocking
   "socket.socket.connect()" method.

   With "SelectorEventLoop" event loop, the socket *sock* must be non-
   blocking.

   This method is a coroutine.

   在 3.5.2 版更改: "address" no longer needs to be resolved.
   "sock_connect" will try to check if the *address* is already
   resolved by calling "socket.inet_pton()".  If not,
   "AbstractEventLoop.getaddrinfo()" will be used to resolve the
   *address*.

   参见: "AbstractEventLoop.create_connection()" and
     "asyncio.open_connection()".

coroutine AbstractEventLoop.sock_accept(sock)

   Accept a connection.  Modeled after blocking
   "socket.socket.accept()".

   此 *scoket* 必须绑定到一个地址上并且监听连接。返回值是一个 "(conn,
   address)" 对，其中 *conn* 是一个 *新*的套接字对象，用于在此连接上收
   发数据，*address* 是连接的另一端的套接字所绑定的地址。

   The socket *sock* must be non-blocking.

   This method is a coroutine.

   参见: "AbstractEventLoop.create_server()" and "start_server()".


18.5.1.10. Resolve host name
============================

coroutine AbstractEventLoop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)

   This method is a coroutine, similar to "socket.getaddrinfo()"
   function but non-blocking.

coroutine AbstractEventLoop.getnameinfo(sockaddr, flags=0)

   This method is a coroutine, similar to "socket.getnameinfo()"
   function but non-blocking.


18.5.1.11. Connect pipes
========================

On Windows with "SelectorEventLoop", these methods are not supported.
Use "ProactorEventLoop" to support pipes on Windows.

coroutine AbstractEventLoop.connect_read_pipe(protocol_factory, pipe)

   Register read pipe in eventloop.

   *protocol_factory* should instantiate object with "Protocol"
   interface.  *pipe* is a *file-like object*. Return pair
   "(transport, protocol)", where *transport* supports the
   "ReadTransport" interface.

   使用 "SelectorEventLoop" 事件循环， *pipe* 被设置为非阻塞模式。

   This method is a coroutine.

coroutine AbstractEventLoop.connect_write_pipe(protocol_factory, pipe)

   Register write pipe in eventloop.

   *protocol_factory* should instantiate object with "BaseProtocol"
   interface. *pipe* is *file-like object*. Return pair "(transport,
   protocol)", where *transport* supports "WriteTransport" interface.

   使用 "SelectorEventLoop" 事件循环， *pipe* 被设置为非阻塞模式。

   This method is a coroutine.

参见: The "AbstractEventLoop.subprocess_exec()" and
  "AbstractEventLoop.subprocess_shell()" methods.


18.5.1.12. UNIX signals
=======================

Availability: UNIX only.

AbstractEventLoop.add_signal_handler(signum, callback, *args)

   Add a handler for a signal.

   如果信号数字非法或者不可捕获，就抛出一个 "ValueError" 。如果建立处
   理器的过程中出现问题，会抛出一个 "RuntimeError" 。

   Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_signal_handler(sig)

   Remove a handler for a signal.

   Return "True" if a signal handler was removed, "False" if not.

参见: "signal" 模块。


18.5.1.13. Executor
===================

Call a function in an "Executor" (pool of threads or pool of
processes). By default, an event loop uses a thread pool executor
("ThreadPoolExecutor").

coroutine AbstractEventLoop.run_in_executor(executor, func, *args)

   Arrange for a *func* to be called in the specified executor.

   The *executor* argument should be an "Executor" instance. The
   default executor is used if *executor* is "None".

   Use functools.partial to pass keywords to the *func*.

   This method is a coroutine.

   在 3.5.3 版更改: "BaseEventLoop.run_in_executor()" no longer
   configures the "max_workers" of the thread pool executor it
   creates, instead leaving it up to the thread pool executor
   ("ThreadPoolExecutor") to set the default.

AbstractEventLoop.set_default_executor(executor)

   Set the default executor used by "run_in_executor()".


18.5.1.14. 错误处理API
======================

允许自定义事件循环中如何去处理异常。

AbstractEventLoop.set_exception_handler(handler)

   将 *handler* 设置为新的事件循环异常处理器。

   If *handler* is "None", the default exception handler will be set.

   If *handler* is a callable object, it should have a matching
   signature to "(loop, context)", where "loop" will be a reference to
   the active event loop, "context" will be a "dict" object (see
   "call_exception_handler()" documentation for details about
   context).

AbstractEventLoop.get_exception_handler()

   Return the exception handler, or "None" if the default one is in
   use.

   3.5.2 新版功能.

AbstractEventLoop.default_exception_handler(context)

   默认的异常处理器。

   This is called when an exception occurs and no exception handler is
   set, and can be called by a custom exception handler that wants to
   defer to the default behavior.

   *context* 参数和 "call_exception_handler()" 中的同名参数完全相同。

AbstractEventLoop.call_exception_handler(context)

   调用当前事件循环异常处理器。

   *context* is a "dict" object containing the following keys (new
   keys may be introduced later):

   * 'message': 错误消息；

   * 'exception' （可选）: 异常对象；

   * 'future' （可选）:  "asyncio.Future" 实例。

   * 'handle' （可选）: "asyncio.Handle" 实例；

   * 'protocol' （可选）: Protocol 实例；

   * 'transport' （可选）: Transport 实例；

   * 'socket' （可选）： "socket.socket" 实例。

   注解: Note: this method should not be overloaded in subclassed
     event loops.  For any custom exception handling, use
     "set_exception_handler()" method.


18.5.1.15. Debug mode
=====================

AbstractEventLoop.get_debug()

   获取事件循环调试模式状态（"bool"）。

   如果环境变量 "PYTHONASYNCIODEBUG" 是一个非空字符串，就返回 "True"
   ，否则就返回 "False" 。

   3.4.2 新版功能.

AbstractEventLoop.set_debug(enabled: bool)

   设置事件循环的调试模式。

   3.4.2 新版功能.

参见: debug mode of asyncio.


18.5.1.16. Server
=================

class asyncio.Server

   Server listening on sockets.

   Object created by the "AbstractEventLoop.create_server()" method
   and the "start_server()" function. Don't instantiate the class
   directly.

   close()

      停止服务：关闭监听的套接字并且设置 "sockets" 属性为 "None" 。

      用于表示已经连进来的客户端连接会保持打开的状态。

      服务器是被异步关闭的，使用 "wait_closed()" 协程来等待服务器关闭
      。

   coroutine wait_closed()

      等待 "close()" 方法执行完毕。

      This method is a coroutine.

   sockets

      List of "socket.socket" objects the server is listening to, or
      "None" if the server is closed.


18.5.1.17. Handle
=================

class asyncio.Handle

   A callback wrapper object returned by
   "AbstractEventLoop.call_soon()",
   "AbstractEventLoop.call_soon_threadsafe()",
   "AbstractEventLoop.call_later()", and
   "AbstractEventLoop.call_at()".

   cancel()

      Cancel the call.  If the callback is already canceled or
      executed, this method has no effect.


18.5.1.18. Event loop examples
==============================


18.5.1.18.1. call_soon() 的 Hello World 示例。
----------------------------------------------

Example using the "AbstractEventLoop.call_soon()" method to schedule a
callback. The callback displays ""Hello World"" and then stops the
event loop:

   import asyncio

   def hello_world(loop):
       print('Hello World')
       loop.stop()

   loop = asyncio.get_event_loop()

   # Schedule a call to hello_world()
   loop.call_soon(hello_world, loop)

   # Blocking call interrupted by loop.stop()
   loop.run_forever()
   loop.close()

参见: The Hello World coroutine example uses a coroutine.


18.5.1.18.2. 使用 call_later() 来展示当前的日期
-----------------------------------------------

Example of callback displaying the current date every second. The
callback uses the "AbstractEventLoop.call_later()" method to
reschedule itself during 5 seconds, and then stops the event loop:

   import asyncio
   import datetime

   def display_date(end_time, loop):
       print(datetime.datetime.now())
       if (loop.time() + 1.0) < end_time:
           loop.call_later(1, display_date, end_time, loop)
       else:
           loop.stop()

   loop = asyncio.get_event_loop()

   # Schedule the first call to display_date()
   end_time = loop.time() + 5.0
   loop.call_soon(display_date, end_time, loop)

   # Blocking call interrupted by loop.stop()
   loop.run_forever()
   loop.close()

参见: The coroutine displaying the current date example uses a
  coroutine.


18.5.1.18.3. 监控一个文件描述符的读事件
---------------------------------------

Wait until a file descriptor received some data using the
"AbstractEventLoop.add_reader()" method and then close the event loop:

   import asyncio
   try:
       from socket import socketpair
   except ImportError:
       from asyncio.windows_utils import socketpair

   # Create a pair of connected file descriptors
   rsock, wsock = socketpair()
   loop = asyncio.get_event_loop()

   def reader():
       data = rsock.recv(100)
       print("Received:", data.decode())
       # We are done: unregister the file descriptor
       loop.remove_reader(rsock)
       # Stop the event loop
       loop.stop()

   # Register the file descriptor for read event
   loop.add_reader(rsock, reader)

   # Simulate the reception of data from the network
   loop.call_soon(wsock.send, 'abc'.encode())

   # Run the event loop
   loop.run_forever()

   # We are done, close sockets and the event loop
   rsock.close()
   wsock.close()
   loop.close()

参见: The register an open socket to wait for data using a protocol
  example uses a low-level protocol created by the
  "AbstractEventLoop.create_connection()" method.

  The register an open socket to wait for data using streams example
  uses high-level streams created by the "open_connection()" function
  in a coroutine.


18.5.1.18.4. 为SIGINT和SIGTERM设置信号处理器
--------------------------------------------

Register handlers for signals "SIGINT" and "SIGTERM" using the
"AbstractEventLoop.add_signal_handler()" method:

   import asyncio
   import functools
   import os
   import signal

   def ask_exit(signame):
       print("got signal %s: exit" % signame)
       loop.stop()

   loop = asyncio.get_event_loop()
   for signame in ('SIGINT', 'SIGTERM'):
       loop.add_signal_handler(getattr(signal, signame),
                               functools.partial(ask_exit, signame))

   print("Event loop running forever, press Ctrl+C to interrupt.")
   print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
   try:
       loop.run_forever()
   finally:
       loop.close()

This example only works on UNIX.
