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消除第三方library警告
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<script src="/assets/js/DPlayer.min.js"> </script><script src="/assets/js/APlayer.min.js"> </script><p>集成第三方库的时候经常会报一些警告，如：<br>1、<code>unable to open object file: No such file or directory</code><br><img src="/images/20170614/warning_1.png" alt=""><br>因为在build静态lib时，CLANG_ENABLE_MODULES没有被设置成NO。<br><img src="/images/20170614/clang_enable_modules.png" alt=""><br>解决办法：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div></pre></td><td class="code"><pre><div class="line">1) Go to Build Settings -> Build Options -> Debug Information Format</div><div class="line">2) Change the Debug setting from "DWARF with dSYM File" to "DWARF"</div><div class="line">3) Leave the Release setting at "DWARF with dSYM File"</div></pre></td></tr></table></figure></p>
<p>但是这不是根本解决办法，根本的还是需要静态库提供者去消除这个警告。</p>
<p>2、<code>ld: pointer not aligned at address 0x13735E9</code><br><img src="/images/20170614/warning_2.png" alt=""><br><img src="/images/20170614/warning_2_1.jpg" alt=""><br>解决办法：<br><img src="/images/20170614/warning_2_solution.png" alt=""></p>
<p>参考：<br><a href="https://pewpewthespells.com/blog/buildsettings.html#clang_enable_modules" target="_blank" rel="external">https://pewpewthespells.com/blog/buildsettings.html#clang_enable_modules</a><br><a href="https://stackoverflow.com/questions/42268117/how-to-remove-the-xcode-warning-apple-mach-o-linker-warning-pointer-not-aligned/43056194#43056194" target="_blank" rel="external">https://stackoverflow.com/questions/42268117/how-to-remove-the-xcode-warning-apple-mach-o-linker-warning-pointer-not-aligned/43056194#43056194</a></p>
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iOS包大小瘦身方案（二）- 基于clang插件iOS瘦身实践
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<script src="/assets/js/DPlayer.min.js"> </script><script src="/assets/js/APlayer.min.js"> </script><p>clang作为LLVM提供的编译器前端，可将用户的源代码(C/C++/Objective-C)编译成语言/目标设备无关的IR(Intermediate Representation)实现。其可提供良好的插件支持，容许用户在编译时，运行额外的自定义动作。<br>本篇是基于clang的插件特性，编写一个clang插件作为一个源代码级别的分析工具(或编译器)生成各种中间文件。编译完成后，还需编写一个工具去分析所有包含源码的方法(包括用户编写，以及引入的第三方库源代码)，检查这些方法中哪些最终可被程序主入口调用，剩余即是疑似无用代码。从而分析出源代码中那些确定无用的代码，便可以有效去除无用的代码从而减少包大小。<br>LLVM工程包含了一组模块化，可复用的编辑器和工具链：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div></pre></td><td class="code"><pre><div class="line">LLVM Core:包含一个现在的源代码/目标设备无关的优化器，一集一个针对很多主流(甚至于一些非主流)的CPU的汇编代码生成支持。</div><div class="line">Clang:一个C/C++/Objective-C编译器，致力于提供令人惊讶的快速编译，极其有用的错误和警告信息，提供一个可用于构建很棒的源代码级别的工具.</div><div class="line">dragonegg: gcc插件，可将GCC的优化和代码生成器替换为LLVM的相应工具。</div><div class="line">LLDB:基于LLVM提供的库和Clang构建的优秀的本地调试器。</div><div class="line">libc++、libc++ ABI: 符合标准的，高性能的C++标准库实现，以及对C++11的完整支持。</div><div class="line">compiler-rt:针对__fixunsdfdi和其他目标机器上没有一个核心IR(intermediate representation)对应的短原生指令序列时，提供高度调优过的底层代码生成支持。</div><div class="line">OpenMP: Clang中对多平台并行编程的runtime支持。</div><div class="line">vmkit:基于LLVM的Java和.NET虚拟机实</div><div class="line">polly: 支持高级别的循环和数据本地化优化支持的LLVM框架。</div><div class="line">libclc: OpenCL标准库的实现</div><div class="line">klee: 基于LLVM编译基础设施的符号化虚拟机</div><div class="line">SAFECode:内存安全的C/C++编译器</div><div class="line">lld: clang/llvm内置的链接器</div></pre></td></tr></table></figure></p>
<p><img src="/images/20170603/llvm_overView.png" alt=""><br><a href="http://llvm.org" target="_blank" rel="external">原文链接</a></p>
<p>1、clang源码并编译安装<br>查看<a href="https://luohs.github.io/2017/05/30/20170530/" target="_blank" rel="external">clang源码并编译安装</a><br>2、编写clang插件<br>要实现自定义的Clang插件(以C++为例)，应按照以下步骤:<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div></pre></td><td class="code"><pre><div class="line">1.自定义继承自</div><div class="line"> clang::PluginASTAction(基于consumer的AST前端Action抽象基类)</div><div class="line"> clang::ASTConsumer(用于客户读取AST的抽象基类)，</div><div class="line"> clang::RecursiveASTVisitor(前序或后续地深度优先搜索整个 AST，并访问每一个节点的基类)等基类，</div><div class="line">2.根据自身需要重载</div><div class="line"> PluginASTAction::CreateASTConsumer，</div><div class="line"> PluginASTAction::ParseArgs,</div><div class="line"> ASTConsumer::HandleTranslationUnit</div><div class="line"> RecursiveASTVisitor::VisitDecl</div><div class="line"> RecursiveASTVisitor::VisitStmt</div><div class="line"> 等方法，实现自定义的分析逻辑。</div><div class="line">3.注册插件</div><div class="line"> static FrontendPluginRegistry::Add<MyPlugin> X("my-plugin-name", "my-plugin-description");</div><div class="line">4.编译时载入插件</div><div class="line"> clang++ *** -Xclang -load -Xclang path-of-your-plugin.dylib -Xclang -add-plugin -Xclang your-pluginName -Xclang -plugin-arg-your-pluginName -Xclang your-pluginName-param</div></pre></td></tr></table></figure></p>
<p>编写clang插件：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div><div class="line">50</div></pre></td><td class="code"><pre><div class="line">#include <iostream></div><div class="line">#include "clang/Frontend/FrontendPluginRegistry.h"</div><div class="line">#include "clang/AST/AST.h"</div><div class="line">#include "clang/AST/ASTConsumer.h"</div><div class="line">#include "clang/Frontend/CompilerInstance.h"</div><div class="line">#include "clang/AST/RecursiveASTVisitor.h"</div><div class="line">using namespace clang;</div><div class="line">using namespace std;</div><div class="line">using namespace llvm;</div><div class="line">namespace ClangPlugin</div><div class="line">{</div><div class="line"> class ClangPluginASTVisitor : public RecursiveASTVisitor<ClangPluginASTVisitor></div><div class="line"> {</div><div class="line"> private:</div><div class="line"> ASTContext *context;</div><div class="line"> public:</div><div class="line"> void setContext(ASTContext &context){</div><div class="line"> this->context = &context;</div><div class="line"> }</div><div class="line"> bool VisitDecl(Decl *decl){</div><div class="line"> if(isa<ObjCImplDecl>(decl)){</div><div class="line"> ObjCImplDecl *interDecl = (ObjCImplDecl*)decl;</div><div class="line"> cout<<"[KWLM]Class Implementation Found:"<<interDecl->getNameAsString()<<endl;</div><div class="line"> }</div><div class="line"> return true;</div><div class="line"> }</div><div class="line"> };</div><div class="line"></div><div class="line"> class ClangPluginASTConsumer : public ASTConsumer</div><div class="line"> {</div><div class="line"> private:</div><div class="line"> ClangPluginASTVisitor visitor;</div><div class="line"> void HandleTranslationUnit(ASTContext &context){</div><div class="line"> visitor.setContext(context);</div><div class="line"> visitor.TraverseDecl(context.getTranslationUnitDecl());</div><div class="line"> }</div><div class="line"> };</div><div class="line"></div><div class="line"> class ClangPluginASTAction : public PluginASTAction</div><div class="line"> {</div><div class="line"> public:</div><div class="line"> unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &Compiler,StringRef InFile){</div><div class="line"> return unique_ptr<ClangPluginASTConsumer>(new ClangPluginASTConsumer);</div><div class="line"> }</div><div class="line"> bool ParseArgs(const CompilerInstance &CI, const std::vector<std::string>& args){</div><div class="line"> return true;</div><div class="line"> }</div><div class="line"> };</div><div class="line">}</div><div class="line">static clang::FrontendPluginRegistry::Add<ClangPlugin::ClangPluginASTAction>X("ClangPlugin", "ClangPlugin");</div></pre></td></tr></table></figure></p>
<p>编译生成插件(dylib)：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">clang -std=c++11 -stdlib=libc++ -L/opt/local/lib -L/opt/llvm/llvm_build/lib -I/opt/llvm/llvm_build/tools/clang/include -I/opt/llvm/llvm_build/include -I/opt/llvm/llvm/tools/clang/include -I/opt/llvm/llvm/include -dynamiclib -Wl,-headerpad_max_install_names -lclang -lclangFrontend -lclangAST -lclangAnalysis -lclangBasic -lclangCodeGen -lclangDriver -lclangFrontendTool -lclangLex -lclangParse -lclangSema -lclangEdit -lclangSerialization -lclangStaticAnalyzerCheckers -lclangStaticAnalyzerCore -lclangStaticAnalyzerFrontend -lLLVMX86CodeGen -lLLVMX86AsmParser -lLLVMX86Disassembler -lLLVMExecutionEngine -lLLVMAsmPrinter -lLLVMSelectionDAG -lLLVMX86AsmPrinter -lLLVMX86Info -lLLVMMCParser -lLLVMCodeGen -lLLVMX86Utils -lLLVMScalarOpts -lLLVMInstCombine -lLLVMTransformUtils -lLLVMAnalysis -lLLVMTarget -lLLVMCore -lLLVMMC -lLLVMSupport -lLLVMBitReader -lLLVMOption -lLLVMProfileData -lpthread -lcurses -lz -lstdc++ -fPIC -fno-common -Woverloaded-virtual -Wcast-qual -fno-strict-aliasing -pedantic -Wno-long-long -Wall -Wno-unused-parameter -Wwrite-strings -fno-rtti -fPIC your-clang-plugin-source.cpp -o your-clang-plugin-name.dylib</div></pre></td></tr></table></figure></p>
<p>使用clang编译oc文件(载入生成的your-clang-plugin-name.dylib):<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">/opt/llvm/llvm_build/bin/clang -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator10.0.sdk -I/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1 -mios-version-min=8.0 -Xclang -load -Xclang ~/Desktop/your-clang-plugin-name.dylib -Xclang -add-plugin -Xclang your-clang-plugin-name -c ./ocClsDemo.m</div></pre></td></tr></table></figure></p>
<p>Xcode集成Clang插件:<br>要在Xcode中使用Clang插件，需要Hack Xcode.app。<br><a href="https://raw.githubusercontent.com/kangwang1988/kangwang1988.github.io/master/others/XcodeHacking.zip" target="_blank" rel="external">XcodeHacking.zip</a><br>通过command-line方式或者手动拷贝的方式将XcodeHack拷贝到Xcode目录中，如果Specifications文件不存在则手动创建一个。<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line">sudo mv HackedClang.xcplugin `xcode-select -print- path`/../PlugIns/Xcode3Core.ideplugin/Contents/SharedSupport/Developer/Library/Xcode/Plug-ins</div><div class="line">sudo mv HackedBuildSystem.xcspec `xcode-select -print- path`/../Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Xcode/Specifications</div></pre></td></tr></table></figure></p>
<p><img src="/images/20170603/XcodeHack_plugin.png" alt=""><br><img src="/images/20170603/XcodeHack_spec.png" alt=""><br>之后在Xcode->Target-Build Settings->Build Options->Compiler for C/C++/Objective-C选择Clang LLVM Trunk即可使得Xcode使用1生成的的Clang来编译。至于其他的命令行参数，均可以通过Xcode设置完成。<br><img src="/images/20170603/XcodeHack_buildsetting.png" alt=""><br>3、clang插件在Xcode工程的配置：<br><a href="https://github.com/kangwang1988/XcodeZombieCode.git" target="_blank" rel="external">clang插件示例工程</a><br>a、首先通过Xcode将自定义clang插件编译成动态库（dylib）。<br><img src="/images/20170603/clang_plugin_out.png" alt=""><br><img src="/images/20170603/clang_plugin_out_2.png" alt=""><br>b、其次编译生成分析工具XcodeCodeAnalyser<br><img src="/images/20170603/clang_analyzer_out.png" alt=""><br><img src="/images/20170603/clang_analyzer_out_2.png" alt=""><br>c、设置工程<br>使用用户编译的Clang载入ClangPlugin去编译并生成各种中间文件。编译完成之后使用Xcode提供的post_build_action_shell机制调用分析工具，生成最终结果。<br><img src="/images/20170603/post_build_action_shell_1.png" alt=""><br><img src="/images/20170603/post_build_action_shell_2.png" alt=""><br><img src="/images/20170603/post_build_action_shell_script.png" alt=""></p>
<p>参考：<br><a href="http://railsware.com/blog/2014/02/28/creation-and-using-clang-plugin-with-xcode/" target="_blank" rel="external">http://railsware.com/blog/2014/02/28/creation-and-using-clang-plugin-with-xcode/</a><br><a href="http://blog.mrriddler.com/2017/02/24/Clang插件-Sherlock/" target="_blank" rel="external">http://blog.mrriddler.com/2017/02/24/Clang插件-Sherlock/</a><br><a href="https://lmsgsendnilself.github.io/blog/2017/02/28/clangtojszi-yuan/" target="_blank" rel="external">https://lmsgsendnilself.github.io/blog/2017/02/28/clangtojszi-yuan/</a><br><a href="http://kangwang1988.github.io/tech/2016/11/01/validate-ios-api-using-clang-plugin.html" target="_blank" rel="external">http://kangwang1988.github.io/tech/2016/11/01/validate-ios-api-using-clang-plugin.html</a><br><a href="https://github.com/kangwang1988/XcodeValidAPI" target="_blank" rel="external">https://github.com/kangwang1988/XcodeValidAPI</a><br><a href="http://www.cocoachina.com/ios/20151211/14562.html" target="_blank" rel="external">http://www.cocoachina.com/ios/20151211/14562.html</a><br><a href="http://mp.weixin.qq.com/s?__biz=MzA3ODg4MDk0Ng==&mid=2651112096&idx=1&sn=ce8fccce7d5f70e30c078e63e8ea0d15&scene=21#wechat_redirect" target="_blank" rel="external">http://mp.weixin.qq.com/s?__biz=MzA3ODg4MDk0Ng==&mid=2651112096&idx=1&sn=ce8fccce7d5f70e30c078e63e8ea0d15&scene=21#wechat_redirect</a></p>
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<script src="/assets/js/DPlayer.min.js"> </script><script src="/assets/js/APlayer.min.js"> </script><p>要想将iOS安装包瘦身，首先得熟悉iOS安装包的构成，然后根据各个部分逐个优化。<br>1、iOS安装包构成：<br>iOS安装包是以后缀名为“ipa”一个压缩包，该压缩包内容大致包含三块内容：可执行文件、资源文件、签名文件<br>构成如下图：<br><img src="/images/20170601/ipa_archive.png" alt=""><br>2、可执行文件构成：<br>可执行文件是通过编译器、连接器将我们编写的代码、静态库、动态库编译成的文件，大致可分为代码段和数据段<br>构成如下：<br><img src="/images/20170601/ipa_mach.png" alt=""><br>3、可执行文件支持的cpu架构以及安装包大小关系<br>以“滴滴打车”为例，对比去年3.9.7版本以及最新4.3.10版本<br>3.9.7版本：<br><img src="/images/20170601/didi_3_9_7.png" alt=""><br><img src="/images/20170601/didi_3_9_7_2.png" alt=""><br><img src="/images/20170601/didi_3_9_7_3.png" alt=""><br>4.3.10版本：<br><img src="/images/20170601/didi_4_3_10_1.png" alt=""><br><img src="/images/20170601/didi_4_3_10_2.png" alt=""><br><img src="/images/20170601/didi_4_3_10_3.png" alt=""><br>由此可见支持的CPU体系架构越多，对安装包大小成正比例影响。<br>4、可执行文件各段所占大小<br>64位架构：<br><img src="/images/20170601/ipa_archive_linkmap_arm64_1.png" alt=""><br><img src="/images/20170601/ipa_archive_linkmap_arm64_2.png" alt=""><br><img src="/images/20170601/ipa_archive_linkmap_arm64_3.png" alt=""><br>32位架构：<br><img src="/images/20170601/ipa_archive_linkmap_arm7_1.png" alt=""><br><img src="/images/20170601/ipa_archive_linkmap_arm7_2.png" alt=""><br><img src="/images/20170601/ipa_archive_linkmap_arm7_3.png" alt=""><br>5、第三方SDK大小占比<br><img src="/images/20170601/ipa_archive_sdk_linkmap_arm64.png" alt=""><br><img src="/images/20170601/ipa_archive_sdk_linkmap_arm7.png" alt=""><br><img src="/images/20170601/ipa_archive_sdk_size_1.png" alt=""><br><img src="/images/20170601/ipa_archive_sdk_size_2.png" alt=""><br><img src="/images/20170601/ipa_archive_size_1.png" alt=""><br><img src="/images/20170601/ipa_archive_size_2.png" alt=""><br>通过比较不难看出第三方SDK编译出来的可执行文件占比达到62.5%<br>6、资源文件对安装包的影响<br>未对图片资源进行无损压缩之后包大小<br><img src="/images/20170601/ipa_archive_size_4.png" alt=""><br>对图片资源进行无损压缩之后包大小<br><img src="/images/20170601/ipa_archive_size_3.png" alt=""><br>下图为对我们app中图片资源进行无损压缩的效果：<br><img src="/images/20170601/imageOptim.png" alt=""></p>
<p>Tips：<br>1、查看可执行文件大小：<br><img src="/images/20170601/linkmap_setting_1.png" alt=""><br><img src="/images/20170601/linkmap_setting_2.png" alt=""><br><img src="/images/20170601/linkmap_used.png" alt=""><br><a href="https://github.com/luohs/luohs.github.io/blob/master/images/20170601/linkmap.js" target="_blank" rel="external">linkmap.js</a><br>2、MachOView使用：<br><a href="http://sourceforge.net/projects/machoview/" target="_blank" rel="external">MachOView下载地址</a><br><a href="https://github.com/gdbinit/MachOView" target="_blank" rel="external">MachOView源码地址</a><br>3、ImageOptim使用：<br><a href="https://imageoptim.com/mac" target="_blank" rel="external">ImageOptim下载地址</a><br>4、去除工程中多余的图片资源：<br>a、脚本<br>详见<a href="https://stackoverflow.com/questions/6113243/how-to-find-unused-images-in-an-xcode-project/6113449#6113449" target="_blank" rel="external">链接</a><br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line">#!/bin/sh</div><div class="line">PROJ=`find . -name '*.xib' -o -name '*.[mh]'`</div><div class="line">for png in `find . -name '*.png'`</div><div class="line">do</div><div class="line">name=`basename $png`</div><div class="line">if ! grep -qhs "$name" "$PROJ"; then</div><div class="line"> echo "$png is not referenced"</div><div class="line">fi</div><div class="line">done</div></pre></td></tr></table></figure></p>
<p>b、Unused<br><a href="http://jeffhodnett.github.io/Unused/" target="_blank" rel="external">Unused</a><br>c、LSUnusedResources<br><a href="https://github.com/tinymind/LSUnusedResources/" target="_blank" rel="external">LSUnusedResources</a></p>
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深入理解"weak-strong dance"
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<script src="/assets/js/DPlayer.min.js"> </script><script src="/assets/js/APlayer.min.js"> </script><p>使用block时利用”weak-strong dance”解决循环引用大家都知道，但这里有三点值得注意：<br>1、当self指向的对象已经被废弃的情况下，持有block成员变量也不存在了，这个时候block对象应该已经没有被变量所持有了，它的引用计数应该已经为0了，它应该被废弃了啊，为什么它还能继续存在并执行。<br>如以下代码，在 block 执行前退出这个页面的话，该 Controller 实例会被废弃，但 Block 还是会执行，会打印“self is (null)”。<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line">- (void)viewDidLoad {</div><div class="line"> [super viewDidLoad];</div><div class="line"></div><div class="line"> __weak typeof(self) weakSelf = self;</div><div class="line"> self.handler = ^{</div><div class="line"> __strong typeof(weakSelf) strongSelf = weakSelf;</div><div class="line"> NSLog(@"self is %@", strongSelf);</div><div class="line"> };</div><div class="line"></div><div class="line"> NSTimeInterval interval = 6.0;</div><div class="line"> dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(interval * NSEC_PER_SEC)), dispatch_get_main_queue(), weakSelf.handler);</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>理解这个问题需要了解block原理，简单一点的说，block有三种类型：<br>NSConcreteStackBlock（栈）<br>NSConcreteGlobalBlock（全局）<br>NSConcreteMallocBlock（堆）</p>
<p>如何实现这三种类型的block：<br><img src="/images/20170531/block_objc.png" alt=""><br>使用<code>clang -rewrite-objc $filePath</code>将OC代码转化为C++代码实现。<a href="https://luohs.github.io/2017/05/30/20170530/" target="_blank" rel="external">如何安装编译clang</a><br><img src="/images/20170531/clang_rewrite.png" alt=""><br><img src="/images/20170531/block_clang.png" alt=""><br>如果 block 在记述全局变量的地方被设置或者 block 没有捕获外部变量，那就生成一个 NSConcreteGlobalBlock 实例。其它情况都会生成一个 NSConcreteStackBlock 实例，也就是说，它是在栈上的，所以一旦它所属的变量超出了变量作用域，该 block 就被废弃了。而当发生以下任一情况时：<br>a、手动调用 block 的实例方法copy。<br>b、block 作为函数返回值返回。<br>c、将 block 赋值给附有__strong修饰符的成员变量。<br>d、在方法名中含有usingBlock的 Cocoa 框架方法或 GCD 的 API 中传递 block。<br>所以NSConcreteMallocBlock 类型的 block 通常不会在源码中直接出现。<br><a href="http://www.galloway.me.uk/2013/05/a-look-inside-blocks-episode-3-block-copy/" target="_blank" rel="external">看一下Block_copy()实现</a>：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div></pre></td><td class="code"><pre><div class="line">static void *_Block_copy_internal(const void *arg, const int flags) {</div><div class="line"> struct Block_layout *aBlock;</div><div class="line"> const bool wantsOne = (WANTS_ONE & flags) == WANTS_ONE;</div><div class="line"></div><div class="line"> // 1</div><div class="line"> if (!arg) return NULL;</div><div class="line"></div><div class="line"> // 2</div><div class="line"> aBlock = (struct Block_layout *)arg;</div><div class="line"></div><div class="line"> // 3</div><div class="line"> if (aBlock->flags & BLOCK_NEEDS_FREE) {</div><div class="line"> // latches on high</div><div class="line"> latching_incr_int(&aBlock->flags);</div><div class="line"> return aBlock;</div><div class="line"> }</div><div class="line"></div><div class="line"> // 4</div><div class="line"> else if (aBlock->flags & BLOCK_IS_GLOBAL) {</div><div class="line"> return aBlock;</div><div class="line"> }</div><div class="line"></div><div class="line"> // 5</div><div class="line"> struct Block_layout *result = malloc(aBlock->descriptor->size);</div><div class="line"> if (!result) return (void *)0;</div><div class="line"></div><div class="line"> // 6</div><div class="line"> memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first</div><div class="line"></div><div class="line"> // 7</div><div class="line"> result->flags &= ~(BLOCK_REFCOUNT_MASK); // XXX not needed</div><div class="line"> result->flags |= BLOCK_NEEDS_FREE | 1;</div><div class="line"></div><div class="line"> // 8</div><div class="line"> result->isa = _NSConcreteMallocBlock;</div><div class="line"></div><div class="line"> // 9</div><div class="line"> if (result->flags & BLOCK_HAS_COPY_DISPOSE) {</div><div class="line"> (*aBlock->descriptor->copy)(result, aBlock); // do fixup</div><div class="line"> }</div><div class="line"></div><div class="line"> return result;</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>综上所述，已经可以解答问题1。把 block 赋值给self.handler的时候，在栈上生成的 block 被复制了一份放到堆上。而之后如果你把这个 block 当作 GCD 参数使用，GCD 函数内部会把该 block 再 copy 一遍，而此时 block 已经在堆上，则该 block 的引用计数加1。所以此时 block 的引用计数是大于1的，即使self对象被废弃 block 会被 release 一次，但它的引用计数仍然大于0，故而不会被废弃。<br>Tips:<br>其实在 ARC 开启的情况下，将只会有 NSConcreteGlobalBlock 和 NSConcreteMallocBlock 类型的 block。<br><img src="/images/20170531/block_work_in_ARC.png" alt=""><br>更多请查看<a href="https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html" target="_blank" rel="external">苹果官方文档</a></p>
<p>2、在 block 内部使用weakSelf就是为了让 block 对象不持有self指向的对象，那在 block 内部又把weakSelf赋给strongSelf不就又持有self对象了么？又循环引用了？<br>理解这个问题就需要了解block捕获对象变量。<br>使用<strong>weak修饰的变量时</strong>weak是不会持有对象，它用一张 weak 表来管理对象和变量。赋值的时候它会以赋值对象的地址作为 key，变量的地址为 value，注册到 weak 表中。一旦该对象被废弃，就通过对象地址在 weak 表中找到变量的地址，赋值为 nil，然后将该条记录从 weak 表中删除。<br>那使用 “weak-strong dance” 的时候是怎么个情况呢？<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div></pre></td><td class="code"><pre><div class="line">struct __block_impl {</div><div class="line"> void *isa;</div><div class="line"> int Flags;</div><div class="line"> int Reserved;</div><div class="line"> void *FuncPtr;</div><div class="line">};</div><div class="line"></div><div class="line">struct __xx_block_impl_y {</div><div class="line"> struct __block_impl impl;</div><div class="line"> __weak OCClass *occlass;</div><div class="line"> // ...</div><div class="line">};</div><div class="line"></div><div class="line">static void __xx_block_func_y(struct __xx_block_impl_y *__cself) {</div><div class="line"> OCClass *occlass = __cself -> occlass;</div><div class="line"> // ...</div><div class="line">}</div></pre></td></tr></table></figure></p>
<p>每次使用<strong>weak变量的时候，都会取出该变量指向的对象并 retain，然后将该对象注册到 autoreleasepool 中。通过上述代码我们可以发现，在</strong>xx_block_func_y中，局部变量occlass会持有捕获的对象，然后对象会被注册到 autoreleasepool。这是延长对象生命周期的关键（保证在执行 Block 期间对象不会被废弃），但这不会造成循环引用，当函数执行结束，变量occlass超出作用域，过一会儿（一般一次 RunLoop 之后），对象就被释放了。所以 weak-strong dance 的行为非常符合预期：延长捕获对象的生命周期，一旦 Block 执行完，对象被释放，而 Block 也会被释放（如果被 GCD 之类的 API copy 过一次增加了引用计数，那最终也会被 GCD 释放）。</p>
<p>3、使用”weak-strong dance”一定安全吗？<br>其实通过问题2就可以理解，”weak-strong dance”并不是安全的，”weak-strong dance”只实用与block已经捕获到对象的情况，”weak-strong dance”并不能保证 block 所引用对象的释放时机在执行之后， 更安全的做法应该是在 block 内部使用 strongSelf 时进行 nil检测，这样可以避免上述情况。</p>
<p>参考：<br><a href="https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html" target="_blank" rel="external">https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html</a><br><a href="http://www.galloway.me.uk/2013/05/a-look-inside-blocks-episode-3-block-copy/" target="_blank" rel="external">http://www.galloway.me.uk/2013/05/a-look-inside-blocks-episode-3-block-copy/</a><br><a href="http://blog.devtang.com/2013/07/28/a-look-inside-blocks/" target="_blank" rel="external">http://blog.devtang.com/2013/07/28/a-look-inside-blocks/</a><br><a href="http://albertodebortoli.com/blog/2013/08/03/objective-c-blocks-caveat/" target="_blank" rel="external">http://albertodebortoli.com/blog/2013/08/03/objective-c-blocks-caveat/</a><br><a href="http://www.jianshu.com/p/737999a30544" target="_blank" rel="external">http://www.jianshu.com/p/737999a30544</a></p>
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clang源码编译安装
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