iOS 淺談：深.淺拷貝與copy.strong

淺談：深.淺拷貝與copy.strong

深.淺拷貝

• copy mutableCopy NSString

NSString *string = @"漢斯哈哈哈";
// 沒有產生新對象
NSString *copyString = [string copy];
// 產生新對象
NSMutableString *mutableCopyString = [string mutableCopy];

NSLog(@"string = %p copyString = %p mutableCopyString = %p", string, copyString, mutableCopyString);

• copy mutableCopy NSMutableString

NSMutableString *string = [NSMutableString stringWithString:@"漢斯哈哈哈"];

// 產生新對象
NSString *copyString = [string copy];
// 產生新對象
NSMutableString *mutableCopyString = [string mutableCopy];

NSLog(@"string = %p copyString = %p mutableCopyString = %p", string, copyString, mutableCopyString);

結論：

注意：其他對象NSArray、NSMutableArray 、NSDictionary、NSMutableDictionary一樣適用

• copy NSObject

HSPerson *p = [[HSPerson alloc] init];
p.age = 20;
p.height = 170.0;

HSPerson *copyP = [p copy]; // 這裏崩潰

崩潰：

看崩潰信息HSPerson應該先實現：

- (id)copyWithZone:(NSZone *)zone;

測試：

#import "HSPerson.h"

@interface HSPerson()<NSCopying>

@end

@implementation HSPerson

- (id)copyWithZone:(NSZone *)zone
{

    return @"漢斯哈哈哈";
}

@end

HSPerson *p = [[HSPerson alloc] init];
p.age = 20;
p.height = 170.0;

HSPerson *copyP = [p copy];
NSLog(@"copyP: %@", copyP);

可以看出copyWithZone重新分配新的內存空間，則：

- (id)copyWithZone:(NSZone *)zone
{
    HSPerson *person = [[HSPerson allocWithZone:zone] init];
    return person;

// 有些人可能下面alloc,重新初始化空間，但這方法已給你分配了zone，自己就無需再次alloc內存空間了
//    HSPerson *person = [[HSPerson alloc] init];
}

HSPerson *p = [[HSPerson alloc] init];
p.age = 20;
p.height = 170.0;

HSPerson *copyP = [p copy];
NSLog(@"p = %p copyP = %p", p, copyP);

NSLog(@"age = %d height = %f", copyP.age, copyP.height);

雖然copy了份新的對象，然而age,height值並未copy，那麼：

- (id)copyWithZone:(NSZone *)zone
{
    HSPerson *person = [[HSPerson allocWithZone:zone] init];
    person.age = self.age;
    person.height = self.height;
    // 這裏self其實就要被copy的那個對象，很顯然要自己賦值給新對象，所以這裏可以控制copy的屬性
    return person;
}

這時你會想，有NSMutableCopying？沒錯，是有這貨：

- (id)mutableCopyWithZone:(NSZone *)zone
{
    HSPerson *person = [[HSPerson allocWithZone:zone] init];
    person.age = self.age;
    person.height = self.height;

    return person;
}

NSCopying、NSMutableCopying有啥區別？
其實感覺沒必要有NSMutableCopying，因爲壓根就沒可變的HSPerson，但如果該對象有其他行爲，可以借用NSMutableCopying實現，哈哈哈

copy.strong

說完深淺拷貝，理解copy.strong就輕鬆多了！

• copy

#import <Foundation/Foundation.h>

@interface HSPerson : NSObject

@property (nonatomic, copy) NSString *name;

@end

NSMutableString *string = [NSMutableString stringWithFormat:@"漢斯哈哈哈"];

HSPerson *person = [[HSPerson alloc] init];
person.name = string;

// 不能改變person.name的值，因爲其內部copy新的對象
[string appendString:@" hans"];

 NSLog(@"name = %@", person.name);

property copy 實際上就對name幹了這個：

- (void)setName:(NSString *)name
{
    _name = [name copy];
}

假設name爲NSMutableString，會發生什麼事？

@property (nonatomic, copy) NSMutableString *name;

這樣會捱罵哦，實際上內部還是：

- (void)setName:(NSMutableString *)name
{
    _name = [name copy];
}

copy出來的仍然是不可變字符！如果有人用NSMutableString的方法，就會崩潰:

• strong

@property (nonatomic, strong) NSString *name;

NSMutableString *string = [NSMutableString stringWithFormat:@"漢斯哈哈哈"];

HSPerson *person = [[HSPerson alloc] init];
person.name = string;

// 可以改變person.name的值，因爲其內部沒有生成新的對象
[string appendString:@" hans"];

NSLog(@"name = %@", person.name);

總結：用copy與strong取決於需求，如果不希望被外界更改用copy，反之用strong