Deep/Shallow Copy

原文链接拷贝是我们在开发中经常使用的技巧，这里指的不是到Github上去复制粘贴代码，而是对内存中对象的操作 (逃深拷贝与浅拷贝的区别？深拷贝是指我们拷贝出来的对象拥有自己单独的内存地址，修改新对象不影响源对象，浅拷贝指的是在copy指针的引用，修改新对象会影响到源对象在ObjC里面主要有两个方法对对象进行拷贝- (id)copy;- (id)mutableCopy;要对象能够使用这两个方法需要遵守协议 NSCopying, NSMutableCopying那么，该何时使用这两种方法呢，先说结论，只有不可变对象调用copy方法的时候才是浅拷贝，其他情况均为深拷贝新建一个工程验证一下吧Xcode -> New -> MacOS -> CommandLine -> main.m由于NSString 同时实现了 NSCopying, NSMutableCopying 两个协议，我们就用他来做实验NSString *str1 = @“str1”;NSString *str2 = str1.copy;NSString *str3 = str1.mutableCopy;NSLog(@"%p %p %p",str1, str2, str3);运行之后可以看到如下输出0x1000020b8 0x1000020b8 0x100508e00由此可以得出结论，不可变对象使用 mutableCopy 为深拷贝，copy 为浅拷贝下面验证一下可变对象 NSMutableStringNSMutableString *str1 = [NSMutableString stringWithFormat:@“str1”];NSString *str2 = str1.copy;NSString *str3 = str1.mutableCopy;NSLog(@"%p %p %p",str1, str2, str3);0x100505260 0x50602036b4ec25f 0x100505290得出结论，可变对象无论使用 copy 还是 mutableCopy ，均为深拷贝这些是非集合类型的拷贝结论，那么对于集合类型来说呢新建Person类Person.h#import <Foundation/Foundation.h>NS_ASSUME_NONNULL_BEGIN@interface Person : NSObject@property (nonatomic ,copy) NSString *name;@property (nonatomic ,assign) NSUInteger age;@property (nonatomic ,copy) NSArray<Person *> *friends;@endNS_ASSUME_NONNULL_ENDPerson.m#import “Person.h”@implementation Person- (instancetype)init { self = [super init]; if (self) { _name = @""; _age = 0; _friends = @[]; } return self;}// 重写以便打印对象的属性- (NSString *)description { return [NSString stringWithFormat:@"- name: %@, age: %ld, friends: %@",self.name, self.age, self.friends];}@end如果正常按照上面的来想，集合类型应该也是一样只有不可变对象调用copy方法的时候才是浅拷贝，其他情况均为深拷贝下面创建Array来验证想法Person *Alice = Person.new;Alice.name = @“Alice”;Alice.age = 18;NSArray *arr1 = @[Alice];NSArray *arr2 = arr1.copy;NSArray *arr3 = arr1.mutableCopy;NSLog(@"%p %p %p",arr1, arr2, arr3);0x1030064c0 0x1030064c0 0x103004a90NSArray吻合我们上面的结论，下面看下NSMutableArrayPerson *Alice = Person.new;Alice.name = @“Alice”;Alice.age = 18;NSMutableArray *arr1 = [NSMutableArray arrayWithArray:@[Alice]];NSArray *arr2 = arr1.copy;NSArray *arr3 = arr1.mutableCopy;NSLog(@"%p %p %p",arr1, arr2, arr3);0x1030051f0 0x103006a00 0x1030054a0也温和上面的结论，但是稍等NSMutableArray *arr1 = [NSMutableArray arrayWithArray:@[Alice]];NSArray arr2 = arr1.copy;NSLog(@"%p %p",arr1.firstObject, arr2.firstObject);我们可以打印arr1 和 arr2 的第一个元素的地址，发现他们还是指向同一个地址！查看Apple关于DeepCopy的文档我们发现，原来要对一个集合类型实现真正的深拷贝需要用这种方法, 而上面这种官方名称为【单层深拷贝】NSArray trueDeepCopyArray = [NSKeyedUnarchiver unarchiveObjectWithData:[NSKeyedArchiver archivedDataWithRootObject:oldArray]];好，那么我们来改造代码Person *Alice = Person.new;Alice.name = @“Alice”;Alice.age = 18;NSError *err;NSMutableArray *arr1 = [NSMutableArray arrayWithArray:@[Alice]];NSArray *arr2 = [NSKeyedUnarchiver unarchivedObjectOfClass:[NSArray class] fromData:[NSKeyedArchiver archivedDataWithRootObject:arr1 requiringSecureCoding:false error:&err] error:&err];if (err) { NSLog(@"%@",err);}NSLog(@"%p %p",arr1.firstObject, arr2.firstObject);奇怪的是输出arr2为nil，看一下控制台输出-[Person encodeWithCoder:]: unrecognized selector sent to instance 0x100706ac0原来是我们的Person没有遵守协议 NSCoding , 除此之外我们还需要遵守 NSSecureCoding 协议改造后的Person.m#import “Person.h”@interface Person () <NSCoding, NSSecureCoding>@end@implementation Person+ (BOOL)supportsSecureCoding { return true;}- (void)encodeWithCoder:(NSCoder *)aCoder { [aCoder encodeObject:_name forKey:@“name”]; [aCoder encodeInteger:_age forKey:@“age”];}- (instancetype)initWithCoder:(NSCoder *)aDecoder { self = [super init]; if (self) { self.name = [aDecoder decodeObjectForKey:@“name”]; self.age = [aDecoder decodeIntegerForKey:@“age”]; } return self;}- (instancetype)init { self = [super init]; if (self) { _name = @""; _age = 0; _friends = @[]; } return self;}现在我们打开main.m试一下Person *Alice = Person.new;Alice.name = @“Alice”;Alice.age = 18;NSError *err;NSMutableArray *arr1 = [NSMutableArray arrayWithArray:@[Alice]];NSArray *arr2 = [NSKeyedUnarchiver unarchivedObjectOfClasses:[NSSet setWithObjects:[NSArray class],[Person class], nil] fromData:[NSKeyedArchiver archivedDataWithRootObject:arr1 requiringSecureCoding:true error:&err] error:&err];if (err) { NSLog(@"%@",err);}NSLog(@"%p %p",arr1.firstObject, arr2.firstObject);0x100709880 0x100705c60我们发现数组里面的对象地址也全都不一样了，这才是集合类型的真正深拷贝thanks.