代码解释：
声明两个变量
@property (retain,nonatomic) NSString *rStr;
@property (copy, nonatomic) NSString *cStr;
在viewDidLoad中添加如下代码：
NSMutableString *mStr = [NSMutableString stringWithFormat:@"abc"];
self.rStr = mStr;
self.cStr = mStr;
NSLog(@"mStr:%p,%p", mStr,&mStr);
NSLog(@"retainStr:%p,%p", _rStr, &_rStr);
NSLog(@"copyStr:%p,%p", _cStr, &_cStr);
[mStr appendString:@"de"];
NSLog(@"retainStr:%@", _rStr);
NSLog(@"copyStr:%@", _cStr);
NSString *mStr1 = [NSString stringWithFormat:@"abc"];
self.rStr = mStr1;
self.cStr = mStr1;
NSLog(@"mStr:%p,%p", mStr1,&mStr1);
NSLog(@"retainStr:%p,%p", _rStr, &_rStr);
NSLog(@"copyStr:%p,%p", _cStr, &_cStr);
[mStr1 stringByAppendingString:@"de"];
NSLog(@"retainStr:%@", _rStr);
NSLog(@"copyStr:%@", _cStr);
NSLog(@"mStr:%p,%p", mStr1,&mStr1);
NSLog(@"retainStr:%p,%p", _rStr, &_rStr);
NSLog(@"copyStr:%p,%p", _cStr, &_cStr);
输出结果：
mStr:0x7f935ac05980,0x7fff533609f8
retainStr:0x7f935ac05980,0x7f935aeb6390
copyStr:0xa000000006362613,0x7f935aeb6398
retainStr:abcde
copyStr:abc
mStr:0xa000000006362613,0x7fff533609f0
retainStr:0xa000000006362613,0x7f935aeb6390
copyStr:0xa000000006362613,0x7f935aeb6398
retainStr:abc
copyStr:abc
mStr:0xa000000006362613,0x7fff533609f0
retainStr:0xa000000006362613,0x7f935aeb6390
copyStr:0xa000000006362613,0x7f935aeb6398
总结： 把一个对象赋值给一个属性变量，当这个对象变化了，如果希望属性变量变化就使用strong属性，如果希望属性变量不跟着变化，就是用copy属性。当把NSMutableString赋值给NSString的时候，才会有不同，如果是赋值是NSString对象，那么使用copy还是strong，结果都是一样的，因为NSString对象根本就不能改变自身的值，它是不可变的.