1.setter方法内部实现：
- (void)setCount:(NSNumber *)newCount {
[newCount retain];
[_count release];
// Make the new assignment.
_count = newCount;
}
2.赋值
- (void)reset {
NSNumber *zero = [[NSNumber alloc] initWithInteger:0];
[self setCount:zero];
[zero release];
}
