两个问题
1. 使用strong修饰NSArray会有什么问题？
2. 使用copy修饰NSMutableArray会有什么问题?
1. 使用strong修饰NSArray的问题
//
// StrongCopyTest.h
// StudyRuntime
//
// Created by Andy Liu on 2018/5/14.
// Copyright © 2018年 Andy Liu. All rights reserved.
//
#import
@interface StrongCopyTest : NSObject
@property (nonatomic,strong) NSArray *immutableArray;
@property (nonatomic,strong) NSMutableArray *mutableArray;
- (void)doTest;
@end
//
// StrongCopyTest.m
// StudyRuntime
//
// Created by Andy Liu on 2018/5/14.
// Copyright © 2018年 Andy Liu. All rights reserved.
//
#import "StrongCopyTest.h"
@implementation StrongCopyTest
- (void)doTest
{
self.mutableArray = [NSMutableArray arrayWithObject:@"111"];
self.immutableArray = [NSArray array];
NSLog(@"before mutableArray=%@,immutableArray=%@",self.mutableArray,self.immutableArray);
self.immutableArray = self.mutableArray;
[self.mutableArray addObject:@"222"];
NSLog(@"after mutableArray=%@,immutableArray=%@",self.mutableArray,self.immutableArray);
}
@end
输出结果如下
2018-05-14 17:28:15.872188+0800 StudyRuntime[861:40479] before mutableArray=(
111
),immutableArray=(
)
2018-05-14 17:28:15.872308+0800 StudyRuntime[861:40479] after mutableArray=(
111,
222
),immutableArray=(
111,
222
)
Program ended with exit code: 0
可以看出，immutabeArray和mutableArray都被修改了，immutableArray不是预料的结果。
原因分析：
self.immutableArray = self.mutableArray;这行代码将可变数组的指针赋值给了不可变数组指针，由于OC中都是C的指针操作，并且是弱类型，因此这是合法的，编译器不会报错。甚至一个 id x = self.mutableArray也是可以的，因此弱类型是有潜在风险的。此时immutableArray和mutableArray都是指向了同一个内存地址,即mutableArray的内存地址，因此后面修改mutableArray的数据也会导致immutableArray的数据修改，因为是同一块内存地址。
这是一个陷阱。
2. 使用copy修饰NSMutableArray会有什么问题?
//
// StrongCopyTest.h
// StudyRuntime
//
// Created by Andy Liu on 2018/5/14.
// Copyright © 2018年 Andy Liu. All rights reserved.
//
#import
@interface StrongCopyTest : NSObject
//@property (nonatomic,copy) NSArray *immutableArray;
@property (nonatomic,copy) NSMutableArray *mutableArray;
- (void)doTest;
@end
//
// StrongCopyTest.m
// StudyRuntime
//
// Created by Andy Liu on 2018/5/14.
// Copyright © 2018年 Andy Liu. All rights reserved.
//
#import "StrongCopyTest.h"
@implementation StrongCopyTest
- (void)doTest
{
self.mutableArray = [NSMutableArray arrayWithObject:@"111"];
// self.immutableArray = [NSArray array];
//
// NSLog(@"before mutableArray=%@,immutabxrleArray=%@",self.mutableArray,self.immutableArray);
//
// self.immutableArray = self.mutableArray;
[self.mutableArray addObject:@"222"];
//NSLog(@"after mutableArray=%@,immutableArray=%@",self.mutableArray,self.immutableArray);
}
@end
输出结果如下
2018-05-14 17:59:37.794905+0800 StudyRuntime[944:61946] -[__NSSingleObjectArrayI addObject:]: unrecognized selector sent to instance 0x10061c540
2018-05-14 17:59:37.796059+0800 StudyRuntime[944:61946] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSSingleObjectArrayI addObject:]: unrecognized selector sent to instance 0x10061c540'
*** First throw call stack:
(
0 CoreFoundation 0x00007fff2a17032b __exceptionPreprocess + 171
1 libobjc.A.dylib 0x00007fff517eac76 objc_exception_throw + 48
2 CoreFoundation 0x00007fff2a208e04 -[NSObject(NSObject) doesNotRecognizeSelector:] + 132
3 CoreFoundation 0x00007fff2a0e6870 ___forwarding___ + 1456
4 CoreFoundation 0x00007fff2a0e6238 _CF_forwarding_prep_0 + 120
5 StudyRuntime 0x0000000100001b91 -[StrongCopyTest doTest] + 161
6 StudyRuntime 0x0000000100001a85 testStrongCopy + 53
7 StudyRuntime 0x0000000100001ad5 main + 53
8 libdyld.dylib 0x00007fff52404015 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
原因分析:
self.mutableArray = [NSMutableArray arrayWithObject:@"111"];这句代码将得到一个不可变的数组(copy属性导致的）.在此处打一个断点，然后输出其数据类型：
(lldb) po self.mutableArray
<__NSSingleObjectArrayI 0x102a01ed0>(
111
)
后面的异常就显而易见了。
self.mutableArray = [NSMutableArray arrayWithObject:@"111"];
这句话相当于
NSMutableArray *arr = [NSMutableArray arrayWithObject:@"111"];
self.mutableArray = [arr copy];
正确的代码
@property (nonatomic,copy) NSArray *immutableArray;
@property (nonatomic,strong) NSMutableArray *mutableArray;
其他集合
NSArray,NSDictionary,NSSet -> copy
NSMutableArray,NSMutableDictionary,NSMutableSet->strong