You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: test_region_change_observer.rs
    path of file: ./repos/tikv/tests/integrations/raftstore
    
    the code of the file until where you have to start completion: // Copyright 2018 TiKV Project Authors. Licensed under Apache-2.0.

use std::{
    mem,
    sync::{
        mpsc::{channel, sync_channel, Receiver, SyncSender},
        Arc,
    },
    time::Duration,
};

use engine_rocks::RocksEngine;
use kvproto::metapb::Region;
use raft::StateRole;
use raftstore::coprocessor::{
    BoxRegionChangeObserver, Coprocessor, ObserverContext, RegionChangeEvent, RegionChangeObserver,
    RegionChangeReason,
};
use test_raftstore::{new_node_cluster, Cluster, NodeCluster};
use tikv_util::{
    store::{find_peer, new_peer},
    HandyRwLock,
};

#[derive(Clone)]
struct TestObserver {
    sender: SyncSender<(Region, RegionChangeEvent)>,
}

impl Coprocessor for TestObserver {}

impl RegionChangeObserver for TestObserver {
    fn on_region_changed(
        &self,
        ctx: &mut ObserverContext<'_>,
        event: RegionChangeEvent,
        _: StateRole,
    ) {
        self.sender.send((ctx.region().clone(), event)).unwrap();
    }
}

fn test_region_change_observer_impl(mut cluster: Cluster<RocksEngine, NodeCluster<RocksEngine>>) {
    let pd_client = Arc::clone(&cluster.pd_client);
    pd_client.disable_default_operator();

    let receiver: Receiver<(Region, RegionChangeEvent)>;
    let r1;
    {
        let (tx, rx) = channel();

        cluster
            .sim
            .wl()
            .post_create_coprocessor_host(Box::new(move |id, host| {
                if id == 1 {
                    let (sender, receiver) = sync_channel(10);
                    host.registry.register_region_change_observer(
                        1,
                        BoxRegionChangeObserver::new(TestObserver { sender }),
                    );
                    tx.send(receiver).unwrap();
                }
            }));
        r1 = cluster.run_conf_change();

        // Only one node has node_id = 1
        receiver = rx.recv().unwrap();
        rx.try_recv().unwrap_err();
    }

    // Init regions
    let init_region_event = receiver.recv().unwrap();
    receiver.try_recv().unwrap_err();
    assert_eq!(init_region_event.1, RegionChangeEvent::Create);
    assert_eq!(init_region_event.0.get_id(), r1);
    assert_eq!(init_region_event.0.get_peers().len(), 1);

    // Change peer
    pd_client.must_add_peer(r1, new_peer(2, 10));
    let add_peer_event = receiver.recv().unwrap();
    receiver.try_recv().unwrap_err();
    assert_eq!(
        add_peer_event.1,
        RegionChangeEvent::Update(RegionChangeReason::ChangePeer)
    );
    assert_eq!(add_peer_event.0.get_id(), r1);
    assert_eq!(add_peer_event.0.get_peers().len(), 2);
    assert_ne!(
        add_peer_event.0.get_region_epoch(),
        init_region_event.0.get_region_epoch()
    );

    // Split
    cluster.must_put(b"k1", b"v1");
    cluster.must_put(b"k2", b"v2");
    cluster.must_put(b"k3", b"v3");
    cluster.must_split(&add_peer_event.0, b"k2");
    let mut split_update = receiver.recv().unwrap();
    let mut split_create = receiver.recv().unwrap();
    // We should receive an `Update` and a `Create`. The order of them is not
    // important.
    if split_update.1 != RegionChangeEvent::Update(RegionChangeReason::Split) {
        mem::swap(&mut sp
    
fn test_region_change_observer_impl(mut cluster: Cluster<RocksEngine, NodeCluster<RocksEngine>>) {
    let pd_client = Arc::clone(&cluster.pd_client);
    pd_client.disable_default_operator();

    let receiver: Receiver<(Region, RegionChangeEvent)>;
    let r1;
    {
        let (tx, rx) = channel();

        cluster
            .sim
            .wl()
            .post_create_coprocessor_host(Box::new(move |id, host| {
                if id == 1 {
                    let (sender, receiver) = sync_channel(10);
                    host.registry.register_region_change_observer(
                        1,
                        BoxRegionChangeObserver::new(TestObserver { sender }),
                    );
                    tx.send(receiver).unwrap();
                }
            }));
        r1 = cluster.run_conf_change();

        // Only one node has node_id = 1
        receiver = rx.recv().unwrap();
        rx.try_recv().unwrap_err();
    }

    // Init regions
    let init_region_event = receiver.recv().unwrap();
    receiver.try_recv().unwrap_err();
    assert_eq!(init_region_event.1, RegionChangeEvent::Create);
    assert_eq!(init_region_event.0.get_id(), r1);
    assert_eq!(init_region_event.0.get_peers().len(), 1);

    // Change peer
    pd_client.must_add_peer(r1, new_peer(2, 10));
    let add_peer_event = receiver.recv().unwrap();
    receiver.try_recv().unwrap_err();
    assert_eq!(
        add_peer_event.1,
        RegionChangeEvent::Update(RegionChangeReason::ChangePeer)
    );
    assert_eq!(add_peer_event.0.get_id(), r1);
    assert_eq!(add_peer_event.0.get_peers().len(), 2);
    assert_ne!(
        add_peer_event.0.get_region_epoch(),
        init_region_event.0.get_region_epoch()
    );

    // Split
    cluster.must_put(b"k1", b"v1");
    cluster.must_put(b"k2", b"v2");
    cluster.must_put(b"k3", b"v3");
    cluster.must_split(&add_peer_event.0, b"k2");
    let mut split_update = receiver.recv().unwrap();
    let mut split_create = receiver.recv().unwrap();
    // We should receive an `Update` and a `Create`. The order of them is not
    // important.
    if split_update.1 != RegionChangeEvent::Update(RegionChangeReason::Split) {
        mem::swap(&mut split_update, &mut split_create);
    }
    // No more events
    receiver.try_recv().unwrap_err();
    assert_eq!(
        split_update.1,
        RegionChangeEvent::Update(RegionChangeReason::Split)
    );
    assert_eq!(split_update.0.get_id(), r1);
    assert_ne!(
        split_update.0.get_region_epoch(),
        add_peer_event.0.get_region_epoch()
    );
    let r2 = split_create.0.get_id();
    assert_ne!(r2, r1);
    assert_eq!(split_create.1, RegionChangeEvent::Create);
    if split_update.0.get_start_key().is_empty() {
        assert_eq!(split_update.0.get_end_key(), b"k2");
        assert_eq!(split_create.0.get_start_key(), b"k2");
        assert!(split_create.0.get_end_key().is_empty());
    } else {
        assert_eq!(split_update.0.get_start_key(), b"k2");
        assert!(split_update.0.get_end_key().is_empty());
        assert!(split_create.0.get_start_key().is_empty());
        assert_eq!(split_create.0.get_end_key(), b"k2");
    }

    // Merge
    pd_client.must_merge(split_update.0.get_id(), split_create.0.get_id());
    // An `Update` produced by PrepareMerge. Ignore it.
    assert_eq!(
        receiver.recv().unwrap().1,
        RegionChangeEvent::Update(RegionChangeReason::PrepareMerge)
    );
    let mut merge_update = receiver.recv().unwrap();
    let mut merge_destroy = receiver.recv().unwrap();
    // We should receive an `Update` and a `Destroy`. The order of them is not
    // important.
    if merge_update.1 != RegionChangeEvent::Update(RegionChangeReason::CommitMerge) {
        mem::swap(&mut merge_update, &mut merge_destroy);
    }
    // No more events
    receiver.try_recv().unwrap_err();
    assert_eq!(
        merge_update.1,
        RegionChangeEvent::Update(RegionChangeReason::CommitMerge)
    );
    assert!(merge_update.0.get_start_key().is_empty());
    assert!(merge_update.0.get_end_key().is_empty());
    assert_eq!(merge_destroy.1, RegionChangeEvent::Destroy);
    if merge_update.0.get_id() == split_update.0.get_id() {
        assert_eq!(merge_destroy.0.get_id(), split_create.0.get_id());
        assert_ne!(
            merge_update.0.get_region_epoch(),
            split_update.0.get_region_epoch()
        );
    } else {
        assert_eq!(merge_update.0.get_id(), split_create.0.get_id());
        assert_eq!(merge_destroy.0.get_id(), split_update.0.get_id());
        assert_ne!(
            merge_update.0.get_region_epoch(),
            split_create.0.get_region_epoch()
        );
    }

    // Move out from this node
    // After last time calling "must_add_peer", this region must have two peers
    assert_eq!(merge_update.0.get_peers().len(), 2);
    let r = merge_update.0.get_id();

    pd_client.must_remove_peer(r, find_peer(&merge_update.0, 1).unwrap().clone());

    let remove_peer_update = receiver.recv().unwrap();
    // After being removed from the region's peers, an update is triggered at first.
    assert_eq!(
        remove_peer_update.1,
        RegionChangeEvent::Update(RegionChangeReason::ChangePeer)
    );
    assert!(find_peer(&remove_peer_update.0, 1).is_none());

    let remove_peer_destroy = receiver.recv().unwrap();
    receiver.try_recv().unwrap_err();
    assert_eq!(remove_peer_destroy.1, RegionChangeEvent::Destroy);
    assert_eq!(remove_peer_destroy.0.get_id(), r);

    pd_client.must_add_peer(r, new_peer(1, 2333));
    let add_peer_event = receiver.recv().unwrap();
    receiver.try_recv().unwrap_err();
    assert_eq!(add_peer_event.1, RegionChangeEvent::Create);
    assert_eq!(add_peer_event.0.get_id(), r);
    assert_eq!(find_peer(&add_peer_event.0, 1).unwrap().get_id(), 2333);

    // No more messages
    receiver.recv_timeout(Duration::from_secs(1)).unwrap_err();
}