在 golang 中等待函数完成

func A(){
  go print("hello")
}

func main() {
  A()
  // here I want to wait for the print to happen
  B()
}

var l sync.mutex

func a() {
    go func() {
        print("hello")
        l.unlock()
    }()
}

func b() {
    print("world")
}

func testlock(t *testing.t) {
    l.lock()
    a()
    l.lock()
    // here i want to wait for the print to happen
    b()
    l.unlock()
}

var wg sync.waitgroup

func a() {
    go func() {
        print("hello")
        wg.done()
    }()
}

func b() {
    print("world")
}

func testlock(t *testing.t) {
    wg.add(1)
    a()
    wg.wait()
    // here i want to wait for the print to happen
    b()
}

func A() chan struct{} {
    c := make(chan struct{})
    go func() {     
        print("hello")
        c <- struct{}{}
    }()
    return c
}

func B() {
    print("world")
}

func TestLock(t *testing.T) {
    c := A()
    // here I want to wait for the print to happen
    <-c
    B()
}