Go 语言中如何利用协程实现等待机制？

package main

import (
    "fmt"
    "time"
)

func main() {
    ch := make(chan int, 10)

    for i := 0; i < 10; i++ {
        go func(ch chan int, key int) {
            fmt.println("goroutine start: ", key)
            time.sleep(time.duration(key) * time.second)
            fmt.println("goroutine end: ", key)
            ch <- key
        }(ch, i)
    }

    fmt.println("main start")
    for i := 0; i < 10; i++ {
        key := <-ch
        fmt.println("main get: ", key)
    }
    fmt.println("main end")
}

package main

import (
    "context"
    "fmt"
    "time"
)

func main() {
    ctx, cancel := context.WithCancel(context.Background())

    for i := 0; i < 10; i++ {
        go func(ctx context.Context, key int) {
            fmt.Println("goroutine start: ", key)
            select {
            case <-ctx.Done():
                fmt.Println("goroutine canceled: ", key)
            case <-time.After(time.Duration(key) * time.Second):
                fmt.Println("goroutine end: ", key)
            }
        }(ctx, i)
    }

    fmt.Println("main start")
    time.Sleep(time.Duration(3) * time.Second)
    cancel()
    fmt.Println("main end")
}