這里的單例無(wú)效，指的是雖然用了singleton模式，本該在APP生命周期內(nèi)，只有一個(gè)單例對(duì)象，一個(gè)內(nèi)存地址，卻出現(xiàn)多個(gè)對(duì)象的問(wèn)題。如下是singleton模式代碼：

@implementation AObject

+ (instancetype)shareInstance {
    static dispatch_once_t onceToken;
    static AObject *instance;
    dispatch_once(&onceToken, ^{
        instance = [AObject new];
        NSLog(@"AObject: %p", instance);
        instance.name = @"A";
    });
    return instance;
}
@end

基本上通過(guò)[AObject shareInstance]，在工程任何地方取得的對(duì)象，都是同一個(gè)內(nèi)存地址的單例對(duì)象。但發(fā)現(xiàn)某種存在動(dòng)態(tài)庫(kù)情況下，出現(xiàn)[AObject shareInstance]取到多個(gè)不同對(duì)象的情況。

單例無(wú)效demo測(cè)試

有個(gè)簡(jiǎn)單的測(cè)試demo

目錄結(jié)構(gòu)：
.
├── LA0
├── LB0
├── LC0
├── LC1
├── MainProj
└── Test.xcworkspace
工程結(jié)構(gòu)：
MainProj
------------------
LC0    |    LC1
------------------
LB0
------------------
LA0
------------------

Test workspace中，MainProj是主工程，LC0、LC1、LB0、LA0都是依賴(lài)的framework。如圖示，MainProj依賴(lài)LC0、LC1；LC0、LC1依賴(lài)LB0；LB0依賴(lài)LA0。但在build phases中，上層的link binary with libraries不僅將直屬下層加入，同時(shí)也將下層的下層加入。例如LC0的配置：

WX20180306-093802@2x.png

單例類(lèi)AObject在LA0中，上層庫(kù)、主工程都會(huì)調(diào)用單例對(duì)象。

1. 所有framework都是static library

設(shè)置LC0、LC1、LB0、LA0的Build Settings->Mach-O Type->static library。
運(yùn)行工程（注：運(yùn)行前需要清空products文件夾下之前編譯好的framework、.app，保證環(huán)境clear），console：

2018-03-01 16:06:31.850360+0800 MainProj[24755:2092911] AObject: 0x6000000123d0

單例對(duì)象[AObject shareInstance]只有一個(gè)內(nèi)存地址。單例模式?jīng)]有問(wèn)題。

2. 設(shè)置LA0為dynamic library，其他framework都是static library

2018-03-01 16:08:29.750429+0800 MainProj[24800:2095054] AObject: 0x600000012a80

單例模式?jīng)]有問(wèn)題

3. 設(shè)置LB0為dynamic library，其他framework都是static library

objc[50214]: Class AObject is implemented in both /Users/hotacool/Library/Developer/Xcode/DerivedData/Test-fsrnousjgrbqxngqgmulsskbrcln/Build/Products/Debug-iphonesimulator/LB0.framework/LB0 (0x10daf6418) and /Users/hotacool/Library/Developer/CoreSimulator/Devices/E925C3DF-D46D-4DCF-9936-9239176C4189/data/Containers/Bundle/Application/BA8871EE-BFBA-4F94-9A05-EE56FF8B5758/MainProj.app/MainProj (0x10d8112b0). One of the two will be used. Which one is undefined.
(void *) $0 = 0x00006040001ad580
2018-03-06 09:39:58.536791+0800 MainProj[50214:3929509] AObject: 0x6040002033f0
2018-03-06 09:39:58.537293+0800 MainProj[50214:3929509] AObject: 0x6000000197e0

單例對(duì)象[AObject shareInstance]出現(xiàn)兩個(gè)不同的內(nèi)存地址。根據(jù)console提示信息，AObject類(lèi)在LB0.framework和.app中都有實(shí)現(xiàn)。

4. 設(shè)置LC0為dynamic library，其他framework都是static library

objc[50340]: Class AObject is implemented in both /Users/hotacool/Library/Developer/Xcode/DerivedData/Test-fsrnousjgrbqxngqgmulsskbrcln/Build/Products/Debug-iphonesimulator/LC0.framework/LC0 (0x106ed5590) and /Users/hotacool/Library/Developer/CoreSimulator/Devices/E925C3DF-D46D-4DCF-9936-9239176C4189/data/Containers/Bundle/Application/8C4343C0-DF3F-442B-9CF2-3898EC678F39/MainProj.app/MainProj (0x106bef2b0). One of the two will be used. Which one is undefined.
objc[50340]: Class BObject is implemented in both /Users/hotacool/Library/Developer/Xcode/DerivedData/Test-fsrnousjgrbqxngqgmulsskbrcln/Build/Products/Debug-iphonesimulator/LC0.framework/LC0 (0x106ed55e0) and /Users/hotacool/Library/Developer/CoreSimulator/Devices/E925C3DF-D46D-4DCF-9936-9239176C4189/data/Containers/Bundle/Application/8C4343C0-DF3F-442B-9CF2-3898EC678F39/MainProj.app/MainProj (0x106bef300). One of the two will be used. Which one is undefined.
(void *) $0 = 0x00006000001bdf80
2018-03-06 09:47:11.090783+0800 MainProj[50340:3937289] AObject: 0x604000011dd0
2018-03-06 09:47:11.091422+0800 MainProj[50340:3937289] AObject: 0x604000011d70

同樣出現(xiàn)單例對(duì)象多個(gè)地址問(wèn)題。并且根據(jù)提示信息，AObject 、BObject都出現(xiàn)重復(fù)實(shí)現(xiàn)。

5. 所有framework都是dynamic library

2018-03-06 09:55:09.844002+0800 MainProj[50456:3944829] AObject: 0x604000015eb0

單例模式?jīng)]有問(wèn)題

推測(cè)結(jié)論

通過(guò)上述測(cè)試，只有當(dāng)dynamic library依賴(lài)static library，上層同時(shí)添加dynamic library和static library到link binary library中時(shí)，會(huì)發(fā)生單例對(duì)象多個(gè)地址，如上面demo中static library中對(duì)象在.app和dynamic library多個(gè)實(shí)現(xiàn)的問(wèn)題。

原因推測(cè)是：編譯dynamic library時(shí)，會(huì)將依賴(lài)static library編譯為二進(jìn)制集成到dynamic library中，同時(shí).app也會(huì)將static library編譯到二進(jìn)制中，導(dǎo)致有多個(gè)二進(jìn)制實(shí)現(xiàn)，單例對(duì)象的調(diào)用地址會(huì)在編譯時(shí)在二進(jìn)制中寫(xiě)好，導(dǎo)致調(diào)用時(shí)有多個(gè)對(duì)象地址。

避免上述問(wèn)題，一方面是梳理清楚依賴(lài)關(guān)系，如demo中，有很多多余的依賴(lài)，例如LC0的link binary libraries中只需加入LB0即可，無(wú)需LA0，通過(guò)刪除多余的依賴(lài)，也可以保證不出現(xiàn)單例無(wú)效的問(wèn)題。另一方面，在項(xiàng)目架構(gòu)設(shè)計(jì)中，盡量避免dynamic和static相互依賴(lài)的情況，在物理層面上完全杜絕。