OC setter方法内部实现

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];

}