ios 协议（delegate）使用过程中遇到assign attribute must be unsafeunretained

今天在应用协定的进程中，偶然发明这样应用

@interface AppDelegate (){
    id<ChatDelegate>  testdelegate;
}
@property (nonatomic , assign) id<ChatDelegate> testdelegate;

@end

@implementation AppDelegate
@synthesize testdelegate;
@interface AppDelegate (){