循环引用 && weak strong

_weak typeof(self) weakSelf = self;
self.block = ^{
dispatch_async(dispatch_get_global_queue(0,0),^{
[NSThread sleepForTimeInterval:1];
NSLog(@"%@",weakSelf);
});
return self;
};

class Parent{
var name:String
var child: Child?
init(name: String){
self.name = name
}
}

class Child{
var name: String
weak var parent: Parent!
init(name:String, parent: Parent){
self.name = name
self.parent = parent
}
}    

_weak typeof(self) weakSelf = self;
self.block = ^{
[weakSelf description];
};