Rust : actor模式 与 Actix库

一直想了解rust中actor并发模式，Actix库是rust中知名的库。看看Actix库的说明，走进actor。
这个库的重要几个概念：
1、actor
任何实现Actor trait的类型，就是一个actor.actor有生命周期，几个状态：

（1）Started

（2） Running

（3）Stopping

（4）Stopped

我们来看一下Actor trait:
里面有start()、start_default()等不带参数的函数，大都返回的是Addr < Self >。我们看到了一个Addr类型。因为所有的Actors有一个邮箱Addr，而Actors之间的通信是通过messages来实现的，Actors之间只知道messages地址，而不能侵入到对方内部。

pub trait Actor: Sized + 'static {

    type Context: ActorContext;

    fn started(&mut self, ctx: &mut Self::Context) {}

    fn stopping(&mut self, ctx: &mut Self::Context) -> Running {
        Running::Stop
    }
    fn start(self) -> Addr
    where
        Self: Actor>,
    {
        Context::new().run(self)
    }

    fn start_default() -> Addr
    where
        Self: Actor> + Default,
    {
        Self::default().start()
    }

    /// Start new actor in arbiter's thread.

    fn start_in_arbiter(arb: &Arbiter, f: F) -> Addr
    where
        Self: Actor>,
        F: FnOnce(&mut Context) -> Self + Send + 'static,
    {
        let (tx, rx) = channel::channel(DEFAULT_CAPACITY);
        // create actor
        arb.exec_fn(move || {
            let mut ctx = Context::with_receiver(rx);
            let act = f(&mut ctx);
            let fut = ctx.into_future(act);
            actix_rt::spawn(fut);
        });
        Addr::new(tx)
   }

    fn create(f: F) -> Addr
    where
        Self: Actor>,
        F: FnOnce(&mut Context) -> Self + 'static,
    {
        let mut ctx = Context::new();
        let act = f(&mut ctx);
        ctx.run(act)
    }
}

自定义类，实现Actor：

use actix::prelude::*;

struct MyActor {
    count: usize,
}

impl Actor for MyActor {
    type Context = Context;
    // 启动的时侯，进行一些个性化设置，比如
     fn started(&mut self, ctx: &mut Self::Context) {
        ctx.set_mailbox_capacity(1);
    }
}
let addr = MyActor.start();

2、message 、handler、 address、Recipient

（1）任何实现Message trait类型，是一个message.

pub trait Message {
    /// The type of value that this message will resolved with if it is
    /// successful.
    type Result: 'static;
}

（2）所有的Actors之间的通信是通过messages来实现的。通信的message发往目标邮箱，Actors调用message handlers（句柄），执行上下文（context）.

（3）handler是啥？

pub trait Handler
where
    Self: Actor,
    M: Message,
{
    /// The type of value that this handler will return.
    type Result: MessageResponse;

    /// This method is called for every message received by this actor.
    fn handle(&mut self, msg: M, ctx: &mut Self::Context) -> Self::Result;
}

实现handler:

struct Ping(usize);
impl Handler for MyActor {
    type Result = usize;

    fn handle(&mut self, msg: Ping, ctx: &mut Context) -> Self::Result {
        self.count += msg.0;

        self.count
    }
}

(4) 自定义类，实现Message：

use actix::prelude::*;

struct Ping(usize);

impl Message for Ping {
    type Result = usize;
}

（5）如何发送message?
首先要用到Addr object。具体地说，有几种方法在actors之间发送message:

Addr::do_send(M) - 忽视返回任何错误的方式，因为邮箱可能满了，也可能关闭。
Addr::try_send(M) - 如果错误 ，会返回 SendError。
Addr::send(M) - 会返回future object ，带有处理过程的结果信息。

（6）Recipient -收件人
收件人是一个Actor发给另外一个不同类型Actor时的地址。比如，订阅者与发送者。

3、context （上下文）、Mailbox

（1）Context字面上是上下文。具体有什么东东？看看源码：

pub struct Context
where
    A: Actor>,
{
    parts: ContextParts,
    mb: Option>,
}

impl Context
where
    A: Actor,
{
    #[inline]
    pub(crate) fn new() -> Self {
        let mb = Mailbox::default();
        Self {
            parts: ContextParts::new(mb.sender_producer()),
            mb: Some(mb),
        }
    }
    #[inline]
    pub fn with_receiver(rx: AddressReceiver) -> Self {
        let mb = Mailbox::new(rx);
        Self {
            parts: ContextParts::new(mb.sender_producer()),
            mb: Some(mb),
        }
    }
    #[inline]
    pub fn run(self, act: A) -> Addr {
        let fut = self.into_future(act);
        let addr = fut.address();
        actix_rt::spawn(fut);
        addr
    }
    pub fn into_future(mut self, act: A) -> ContextFut {
        let mb = self.mb.take().unwrap();
        ContextFut::new(self, act, mb)
    }
    pub fn handle(&self) -> SpawnHandle {
        self.parts.curr_handle()
    }
    pub fn set_mailbox_capacity(&mut self, cap: usize) {
        self.parts.set_mailbox_capacity(cap)
    }
}

impl AsyncContextParts for Context
where
    A: Actor,
{
    fn parts(&mut self) -> &mut ContextParts {
        &mut self.parts
    }
}
pub trait ContextFutureSpawner
where
    A: Actor,
    A::Context: AsyncContext,
{
    fn spawn(self, ctx: &mut A::Context);
    fn wait(self, ctx: &mut A::Context);
}

impl ContextFutureSpawner for T
where
    A: Actor,
    A::Context: AsyncContext,
    T: ActorFuture + 'static,
{
    #[inline]
    fn spawn(self, ctx: &mut A::Context) {
        let _ = ctx.spawn(self);
    }
    #[inline]
    fn wait(self, ctx: &mut A::Context) {
        ctx.wait(self);
    }
}

里面有 ContextParts和Option两部分内容构成。
（2）找到ContextParts的源码，我们来看看：

pub struct ContextParts
where
    A: Actor,
    A::Context: AsyncContext,
{
    addr: AddressSenderProducer,
    flags: ContextFlags,
    wait: SmallVec<[ActorWaitItem; 2]>,
    items: SmallVec<[Item; 3]>,
    handles: SmallVec<[SpawnHandle; 2]>,
}

（3）我们来看一下Mailbox中的设计：

pub struct Mailbox
where
    A: Actor,
    A::Context: AsyncContext,
{
    msgs: AddressReceiver,
}

impl Mailbox
where
    A: Actor,
    A::Context: AsyncContext,
{
    #[inline]
    pub fn new(msgs: AddressReceiver) -> Self {
        Self { msgs }
    }
    pub fn capacity(&self) -> usize {
        self.msgs.capacity()
    }
    pub fn set_capacity(&mut self, cap: usize) {
        self.msgs.set_capacity(cap);
    }
    #[inline]
    pub fn connected(&self) -> bool {
        self.msgs.connected()
    }
    pub fn address(&self) -> Addr {
        Addr::new(self.msgs.sender())
    }
    pub fn sender_producer(&self) -> AddressSenderProducer {
        self.msgs.sender_producer()
    }
    pub fn poll(&mut self, act: &mut A, ctx: &mut A::Context) {
        #[cfg(feature = "mailbox_assert")]
        let mut n_polls = 0u16;
        loop {
            let mut not_ready = true;
            // sync messages
            loop {
                if ctx.waiting() {
                    return;
                }
                match self.msgs.poll() {
                    Ok(Async::Ready(Some(mut msg))) => {
                        not_ready = false;
                        msg.handle(act, ctx);
                    }
                    Ok(Async::Ready(None)) | Ok(Async::NotReady) | Err(_) => break,
                }
                #[cfg(feature = "mailbox_assert")]
                {
                    n_polls += 1;
                    assert!(n_polls < MAX_SYNC_POLLS, "Too many messages are being processed. Use Self::Context::notify() instead of direct use of address");
                }
            }
            if not_ready {
                return;
            }
        }
    }
}

4、Arbiter 、SyncArbiter

为Actors提供了异步执行的上下文环境，当一个actor运行时，Arbiters控制着actor包含特定执行状况的上下文环境。 Arbiters需要运行许多函数，包括起系统线程的函数、进行事件轮询、异步分发事件轮询任务、对异步任务进行支持。
当起一个actor时，是在一个系统的线程中运行的，这样效率比较高。也就是说，一个线程可能会针对N个actor.
事件轮询
在事件轮询时，对应的 Arbiter会控制事件轮询事件池的线程。 Arbiter会对任务队列进行排队，往往的情况是，你可以把Arbiter看成"single-threaded event loop".

5、future
从Future库可以看出：

    pub trait Future {
        /// The type of value produced on completion.
        #[stable(feature = "futures_api", since = "1.36.0")]
        type Output;
        #[stable(feature = "futures_api", since = "1.36.0")]
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll;
    }