我是Objective-C中iOS开发和编程的新手.我一直在app dev库上做练习.
这是我想要了解的当前练习.
3.测试如果将可变字符串设置为该人的名字会发生什么,然后在调用修改后的sayHello方法之前改变该字符串.通过添加copy属性更改NSString属性声明并再次测试.
我尝试这样做,但是我修改的NSString实际上改变了,尽管使用了copy属性.
这是我的声明和实现以及我的测试代码.
XYZPerson.h
#import <Foundation/Foundation.h>
@interface XYZPerson : NSObject
@property (copy) NSString *firstName;
@property NSString *lastName;
@property NSDate *dob;
- (void)sayHello;
- (void)saySomething:(NSString *)greeting;
+ (id)init;
+ (id)personWithFirstName:(NSString *)firstName lastName:(NSString *)lastName dob:(NSDate *)dateOfBirth;
@end
//XYZPerson.m
#import "XYZPerson.h"
@implementation XYZPerson
@synthesize firstName = _firstName;
@synthesize lastName = _lastName;
@synthesize dob = _dob;
- (void)sayHello {
[self saySomething:@"Hello World!"];
NSLog(@"This is %@ %@",self.firstName,self.lastName);
}
- (void)saySomething:(NSString *)greeting {
NSLog(@"%@",greeting);
}
+ (id)init {
return [self personWithFirstName:@"Yorick" lastName:@"Robinson" dob:8/23/1990];
}
+ (id)personWithFirstName:(NSString *)firstName lastName:(NSString *)lastName dob:(NSDate *)dateOfBirth{
XYZPerson *person = [[self alloc] init];
person.firstName = firstName;
person.lastName = lastName;
person.dob = dateOfBirth;
return person;
}
@end
//Test code
#import <UIKit/UIKit.h>
#import "AppDelegate.h"
#import "XYZPerson.h"
#import "XYZShoutingPerson.h"
int main(int argc,char *argv[])
{
@autoreleasepool {
XYZPerson *guy = [XYZPerson init];
[guy sayHello];
//I thought that this change would never be made,but it is everytime I run the code.
guy.firstName = @"Darryl";
[guy sayHello];
XYZShoutingPerson *girl = [XYZShoutingPerson init];
[girl sayHello];
return UIApplicationMain(argc,argv,nil,NSStringFromClass([AppDelegate class]));
}
}
解决方法
考虑这个较短的例子(运行在
CodeRunner btw)：
#import <Foundation/Foundation.h>
@interface Person : NSObject
@property (nonatomic,strong) NSString *name; // strong should be copy
@end
@implementation Person
@end
int main(int argc,char *argv[]) {
@autoreleasepool {
Person *p = [Person new];
NSMutableString *name = [[NSMutableString alloc] initWithString:@"Alice"];
p.name = name;
NSLog(@"%@",p.name); // prints Alice
[name appendString:@"xxx"];
NSLog(@"%@",p.name); // prints Alicexxx
}
}
我将名称指向一个可变字符串,然后附加一些字符.结果,名称在对象内部发生了变化.但是,如果在声明属性时将strong替换为copy,则将仅为Person对象创建新的不可变字符串.
故事的寓意是,当有人传递一个物体然后该物体发生变化时,使用副本可以防止副作用.
消息 – [NSString copy]在传递可变字符串(NSMutableString)时生成副本,或者在不可变(NSString)时保留.因此,在声明NSString属性时始终复制：
@property (nonatomic,copy) NSString *string; // OK @property (nonatomic,strong) NSString *string; // strong should be copy
