修飾符weak引發(fā)的思考

背景

??????在測試中發(fā)現(xiàn)有個(gè)網(wǎng)頁沒有加載出來，查找原因時(shí)，發(fā)現(xiàn)是由于用了weak修飾了一個(gè)NSURL的全局變量哮内，出了方法作用域之后祈噪，這個(gè)NSURL的全局變量的值就變?yōu)?code>nil了泽铛，因此網(wǎng)頁才沒能加載成功。

思考和實(shí)踐

??????對于weak這個(gè)修飾符辑鲤，之前并沒有做的太多的學(xué)習(xí)和研究盔腔，只是知道在修飾UI控件以及代理的時(shí)候使用。
??????做了一個(gè)簡單的測試月褥，分別用weak修飾NSArray弛随、NSMutableArray、NSDictionary宁赤、NSMutableDictionary舀透、NSURL，其中數(shù)組和URL在另一個(gè)方法中調(diào)用是都為nil了决左，但字典仍可以正常的存取值愕够。至于為什么會(huì)出現(xiàn)這種結(jié)果，不得而知佛猛，如果有哪位小伙伴知道原理還請?jiān)谠u(píng)論區(qū)留言惑芭。對于OC中的對象建議還是使用strong或者copy進(jìn)行修飾，以免因?yàn)樽兞繛?code>nil而引起不必要的問題继找。

變量聲明

@property(nonatomic,weak)NSArray * weakArray;
@property(nonatomic,weak)NSMutableArray * weakMutableArray;
@property(nonatomic,weak)NSDictionary * weakDictionary;
@property(nonatomic,weak)NSMutableDictionary * weakMutableDictionary;
@property(nonatomic,weak)NSURL * weakUrl;

運(yùn)行結(jié)果

NSArray * weakArray = [NSArray arrayWithObject:@"weakArray"];
self.weakArray = weakArray;

NSLog(@"viewDidLoad方法中 weakArray = %@ %p",self.weakArray,self.weakArray);// viewDidLoad方法中 weakArray = (weakArray) 0x600003534a10

NSMutableArray * weakMutableArray = [NSMutableArray arrayWithObject:@"weakMutableArray"];
self.weakMutableArray = weakMutableArray;

NSLog(@"viewDidLoad方法中 weakMutableArray = %@ %p",self.weakMutableArray,self.weakMutableArray);//viewDidLoad方法中 weakMutableArray = (weakMutableArray) 0x60000396e130

NSLog(@"字典");

NSDictionary * weakDictionary = [NSDictionary dictionaryWithObjectsAndKeys:@"weakDictionary",@"weak",nil];
self.weakDictionary = weakDictionary;

NSLog(@"viewDidLoad方法中 weakDictionary = %@ %p",self.weakDictionary,self.weakDictionary);//viewDidLoad方法中 weakDictionary = {weak = weakDictionary;} 0x6000022611c0

NSMutableDictionary * weakMutableDictionary = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"weakMutableDictionary",@"weak",nil];
self.weakMutableDictionary = weakMutableDictionary;

NSLog(@"viewDidLoad方法中 weakMutableDictionary = %@ %p",self.weakMutableDictionary,self.weakMutableDictionary);// viewDidLoad方法中 weakMutableDictionary = {weak = weakMutableDictionary;} 0x6000037701a0

NSURL * weakUrl = [NSURL URLWithString:@"weakUrl"];
self.weakUrl = weakUrl;

NSLog(@"viewDidLoad方法中 weakUrl = %@ %p",self.weakUrl,self.weakUrl);// viewDidLoad方法中 weakUrl = weakUrl 0x600001419a40



-(void)viewWillAppear:(BOOL)animated{
    [super viewWillAppear:animated];
    NSLog(@"viewWillAppear方法中 weakArray = %@ %p",self.weakArray,self.weakArray);// viewWillAppear方法中 weakArray = (null) 0x0
    NSLog(@"viewWillAppear方法中 weakMutableArray = %@ %p",self.weakMutableArray,self.weakMutableArray);// viewWillAppear方法中 weakMutableArray = (null) 0x0
    NSLog(@"viewWillAppear方法中 weakDictionary = %@ %p",self.weakDictionary,self.weakDictionary);//viewWillAppear方法中 weakDictionary = {weak = weakDictionary;} 0x6000022611c0
    NSLog(@"viewWillAppear方法中 weakMutableDictionary = %@ %p",self.weakMutableDictionary,self.weakMutableDictionary);// viewWillAppear方法中 weakMutableDictionary = {weak = weakMutableDictionary;} 0x6000037701a0
    NSLog(@"viewWillAppear方法中 weakUrl = %@ %p",self.weakUrl,self.weakUrl);// viewWillAppear方法中 weakUrl = (null) 0x0
}

viewdidload運(yùn)行結(jié)果

viewwillappear運(yùn)行結(jié)果

copy和strong修飾數(shù)組

當(dāng)源對象為不可變數(shù)組時(shí)强衡，不管是可變還是不可變數(shù)組，使用copy或strong進(jìn)行修飾時(shí)，最終數(shù)組都會(huì)變?yōu)椴豢勺償?shù)組漩勤，內(nèi)存地址不變感挥。
當(dāng)源對象為可變數(shù)組時(shí)

使用copy修飾可變數(shù)組或者不可變數(shù)組時(shí)，即使聲明的是可變數(shù)組越败，最終變?yōu)橐粋€(gè)新的不可變數(shù)組触幼，如果這時(shí)調(diào)用可變數(shù)組的方法，像addObject究飞、removeObject等方法時(shí)置谦，會(huì)導(dǎo)致程序崩潰，因?yàn)椴豢勺償?shù)組沒有這些方法亿傅。
使用strong修飾不可變數(shù)組時(shí)媒峡，最終數(shù)組都會(huì)變?yōu)椴豢勺償?shù)組，內(nèi)存地址不變葵擎。這時(shí)候仍然可以調(diào)用可變數(shù)組的方法谅阿，不過不能直接調(diào)用，需要強(qiáng)轉(zhuǎn)或者使用performSelector:withObject:這種方式酬滤。
使用strong修飾可變數(shù)組時(shí)签餐，數(shù)組依然是可變數(shù)組，內(nèi)存地址不變盯串。

@property(nonatomic,copy)NSArray * coArray;
@property(nonatomic,copy)NSMutableArray * coMutableArray;
@property(nonatomic,strong)NSArray * strongArray;
@property(nonatomic,strong)NSMutableArray * strongMutableArray;

NSArray * array = [NSArray arrayWithObject:@"array"];
NSLog(@"%@  %p",[array class],array); // __NSSingleObjectArrayI 0x6000015937a0

self.coArray = array;
self.coMutableArray = array;
self.strongArray = array;
self.strongMutableArray = array;

NSLog(@"%@  %p",[self.coArray class],self.coArray); // __NSSingleObjectArrayI 0x6000015937a0
NSLog(@"%@  %p",[self.coMutableArray class],self.coMutableArray);// __NSSingleObjectArrayI 0x6000015937a0
NSLog(@"%@  %p",[self.strongArray class],self.strongArray);// __NSSingleObjectArrayI 0x6000015937a0
NSLog(@"%@  %p",[self.strongMutableArray class],self.strongMutableArray);// __NSSingleObjectArrayI 0x6000015937a0

NSMutableArray * mutableArray = [NSMutableArray arrayWithObject:@"mutableArray"];
NSLog(@"%@  %p",[mutableArray class],mutableArray);// __NSArrayM 0x6000019dd8c0

self.coArray = mutableArray;
self.coMutableArray = mutableArray;
self.strongArray = mutableArray;
self.strongMutableArray = mutableArray;

NSLog(@"%@  %p",[self.coArray class],self.coArray);// __NSSingleObjectArrayI 0x6000015935c0
NSLog(@"%@  %p",[self.coMutableArray class],self.coMutableArray);// __NSSingleObjectArrayI 0x600001593590

[self.coMutableArray addObject:@"coMutableArray"];// -[__NSSingleObjectArrayI addObject:]: unrecognized selector sent to instance

NSLog(@"%@  %p",[self.strongArray class],self.strongArray);// __NSArrayM 0x6000019dd8c0
NSLog(@"%@  %p",[self.strongMutableArray class],self.strongMutableArray);// __NSArrayM 0x6000019dd8c0

數(shù)組執(zhí)行結(jié)果

對象的copy與mutableCopy

當(dāng)源對象為不可變時(shí),使用copy時(shí)不會(huì)生成新的對象氯檐，即指針拷貝（淺拷貝）；使用mutableCopy會(huì)生成一個(gè)新的可變對象体捏，即對象拷貝（深拷貝）冠摄。
當(dāng)源對象為可變時(shí)，使用copy和mutableCopy時(shí)都會(huì)生成一個(gè)新的對象几缭，即對象拷貝（深拷貝）河泳，區(qū)別在于：使用copy生成的對象為不可變的，而mutableCopy生成的對象是可變的奏司。

測試代碼如下：

NSString * string = @"string";

NSLog(@"string = %@  %p",[string class],string);
// __NSCFConstantString  0x10eff20a0

NSLog(@"copyString = %@  %p",[[string copy] class],[string copy]);
// __NSCFConstantString  0x10eff20a0

NSLog(@"mutableCopyString = %@  %p",[[string mutableCopy] class],[string mutableCopy]);
// __NSCFString  0x600003f8e250

NSLog(@"NSMutableString\n");

NSMutableString * mutableString = [NSMutableString stringWithString:@"mutableString"];

NSLog(@"mutableString = %@  %p",[mutableString class],mutableString);
// __NSCFString  0x600003dfa670


NSLog(@"copy_mutableString = %@  %p",[[mutableString copy] class],[mutableString copy]);
// __NSCFString  0x6000033fc340
/**
    這個(gè)地方不知道為什么打印的結(jié)果是可變字符串 乔询，做了一個(gè)測試樟插，調(diào)用可變字符串的appendString方法韵洋，崩潰了
    NSMutableString * copy_mutableString = [mutableString copy];
    [copy_mutableString appendString:@"append"];
    NSLog(@"%@",copy_mutableString); // Attempt to mutate immutable object with appendString:
 */

NSLog(@"mutableCopy_mutableString = %@  %p",[[mutableString mutableCopy] class],[mutableString mutableCopy]);
// __NSCFString  0x600003dfa940

NSLog(@"NSArray\n");

NSArray * array = [NSArray arrayWithObject:@"array"];

NSLog(@"array = %@  %p",[array class],array);
// __NSSingleObjectArrayI  0x6000031a9960

NSLog(@"copy_array = %@  %p",[[array copy] class],[array copy]);
// __NSSingleObjectArrayI  0x6000031a9960

NSLog(@"mutableCopy_array = %@  %p",[[array mutableCopy] class],[array mutableCopy]);
// __NSArrayM  0x600003dfa310

NSLog(@"NSMutableArray\n");

NSMutableArray * mutablearray = [NSMutableArray arrayWithObject:@"mutablearray"];

NSLog(@"mutablearray = %@  %p",[mutablearray class],mutablearray);
// __NSArrayM  0x600003dfdb00

NSLog(@"copy_mutablearray = %@  %p",[[mutablearray copy] class],[mutablearray copy]);
// __NSSingleObjectArrayI  0x6000031bd0a0

NSLog(@"mutableCopy_mutablearray = %@  %p",[[mutablearray mutableCopy] class],[mutablearray mutableCopy]);
// __NSArrayM  0x600003dfd710

字符串

數(shù)組

注：本文中的測試SDK版本為12.2，Xcode 10.2 黄锤，如有錯(cuò)誤歡迎留言指正搪缨。

參考文章

iOS內(nèi)存管理（6）--NSArray與NSMutableArray用copy修飾還是strong
assign weak retain strong copy關(guān)鍵字的區(qū)別
 整理一下OC中的那些屬性修飾符

iOS Copy與MutableCopy 和 Copy與Strong 深度解析
 iOS中copy和mutableCopy詳解
 聊聊NSString為什么用copy

iOS學(xué)習(xí)——屬性引用self.xx與_xx的區(qū)別
 iOS 同時(shí)重寫setter和getter時(shí)候報(bào)錯(cuò)：Use of undeclared identifier '_name';did you mean 'name'
ios中set和get方法

iOS 底層解析weak的實(shí)現(xiàn)原理（包含weak對象的初始化，引用鸵熟，釋放的分析)