总结下我的懂得：
strong weak是ARC惹人的，strong相当于retain，然则对于有的类型，例如NSString，则应用strong相当于应用copy
如许直接用strong就可以直接处理retain和strong的情况了。
weak相当于assign，然则比后者多一点：对象被干掉落时会将weak引用设为nil，而对nil发送消息都不会导致崩溃
    NSString *strongStr;
不然weak引用为野指针，会出现问题
@interface AppDelegate : UIResponder <UIApplicationDelegate>
{
    NSString *copStr;
}
@property (retain, nonatomic) NSString *retainStr;
@property (strong, nonatomic) NSString *strongStr;
@property (copy, nonatomic) NSString *copStr;
@end
    NSMutableString *newStr = [NSMutableString stringWithFormat:@"abcd"];
    self.retainStr = newStr;
    self.strongStr = newStr;
    self.copStr = newStr;
    
    [newStr appendString:@"123"];
    
    NSLog(@"strong string:%@", self.strongStr);
    NSLog(@"retain sting:%@", self.retainStr);
    NSLog(@"copy string:%@", self.copStr);
输出结不雅：
2014-05-02 13:31:17.660 TestMultipleThreadDownload[552:60b] retain sting:abcd123
这里说strong 有copy的功能是缺点的哦。实验例子
2014-05-02 13:31:17.662 TestMultipleThreadDownload[552:60b] strong string:abcd123
    NSString *retainStr;
2014-05-02 13:31:17.663 TestMultipleThreadDownload[552:60b] copy string:abcd