如下定义Student，在构造函数传入闭包。假设这个类是已经存在的，调用方无法修改；那调用时应该注意避免循环引用
class Student{
private var doWork:(()->())
init(delegate: @escaping (()->())){
self.doWork = delegate
}
deinit {
print("deinit student")
}
}
Teacher内部持有Student，如果Student的构造函数需要的闭包直接指向Teacher的成员方法，会发生循环引用
不应该写为
myStudent = Student(delegate:classBegin)
需要把classBegin放到一个weak self的闭包中被呼叫才能避免循环引用
class Teacher{
private func classBegin() {
print("hi, classmates!")
}
private var myStudent : Student?
func addStudent() {
//这样写无法释放，会内存泄露
//myStudent = Student(delegate:classBegin)
myStudent = Student(delegate: {
[weak self] in
self?.classBegin()
})
}
deinit {
print("deinit teacher")
}
}
