百度APP iOS端包体积50M优化实践(三) 资源优化
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百度Geek说约 11 分钟
百度Geek说约 11 分钟GEEK TALK
01
前言
GEEK TALK
02
Mach-O文件详解
2.1 简介
2.2 分析Mach-O文件的工具
2.2.1 MachOView分析
MachOView下载地址:http://sourceforge.net/projects/machoview/ MachOView源码地址：https://github.com/gdbinit/MachOView
Contents of (__DATA,__objc_classlist) section0000000100008238 0x100009980isa 0x1000099a8superclass 0x0 _OBJC_CLASS_$_UIViewControllercache 0x0 __objc_empty_cachevtable 0x0data 0x1000083e8flags 0x90instanceStart 8instanceSize 8reserved 0x0ivarLayout 0x0name 0x100007349 ViewControllerbaseMethods 0x1000082d8entsize 24count 11name 0x100006424 test4types 0x1000073e4 v16@0:8imp 0x100004c58name 0x1000063b4 viewDidLoad*****
下面列举otool常见命令：
2.3 查看文件格式
~ % file /Users/ycx/Desktop/demo.app/demo/Users/ycx/Desktop/demo.app/demo: Mach-O 64-bit executable arm64~ % lipo -info /Users/ycx/Desktop/demo.app/demoNon-fat file: /Users/ycx/Desktop/demo.app/demo is architecture: arm64
2.4 文件结构
2.4.1 总体结构
2.4.2 Header(头部)
2.4.2.1 数据结构
Header(头部): 用于描述当前Mach-O文件的基本信息（CPU类型、文件类型等)，XNU代码路径：EXTERNAL_HEADERS/mach-o/loader.h，数据结构如下所示：
struct mach_header_64 {uint32_t magic; /* mach magic number identifier */cpu_type_t cputype; /* cpu specifier */cpu_subtype_t cpusubtype; /* machine specifier */uint32_t filetype; /* type of file */uint32_t ncmds; /* number of load commands */uint32_t sizeofcmds; /* the size of all the load commands */uint32_t flags; /* flags */uint32_t reserved; /* reserved */};
2.4.2.2 查看字段值
% otool -hv demodemo:Mach headermagic cputype cpusubtype caps filetype ncmds sizeofcmds flagsMH_MAGIC_64 ARM64 ALL 0x00 EXECUTE 22 3040 NOUNDEFS DYLDLINK TWOLEVEL PIE
2.4.2.3 字段具体含义
各个字段具体含义如下所示：
2.4.3 LoadCommands(加载命令)
2.4.3.1 数据结构
struct load_command {uint32_t cmd; /* type of load command */uint32_t cmdsize; /* total size of command in bytes */};/* Constants for the cmd field of all load commands, the type */#define LC_SEGMENT 0x1 /* segment of this file to be mapped */#define LC_SYMTAB 0x2 /* link-edit stab symbol table info */#define LC_SYMSEG 0x3 /* link-edit gdb symbol table info (obsolete) */#define LC_THREAD 0x4 /* thread */#define LC_UNIXTHREAD 0x5 /* unix thread (includes a stack) */#define LC_LOADFVMLIB 0x6 /* load a specified fixed VM shared library */#define LC_IDFVMLIB 0x7 /* fixed VM shared library identification */#define LC_IDENT 0x8 /* object identification info (obsolete) */#define LC_FVMFILE 0x9 /* fixed VM file inclusion (internal use) */#define LC_PREPAGE 0xa /* prepage command (internal use) */#define LC_DYSYMTAB 0xb /* dynamic link-edit symbol table info */#define LC_LOAD_DYLIB 0xc /* load a dynamically linked shared library */#define LC_ID_DYLIB 0xd /* dynamically linked shared lib ident */#define LC_LOAD_DYLINKER 0xe /* load a dynamic linker */#define LC_ID_DYLINKER 0xf /* dynamic linker identification */#define LC_PREBOUND_DYLIB 0x10 /* modules prebound for a dynamically */*****
2.4.3.2 查看字段值
2.4.3.3 cmd类型及其具体作用 常见的cmd类型及其具体作用如下面表格所示:
2.4.3.4 LC_SEGMENT_64
2.4.3.4.1 数据结构
在众多cmd命令中，我们需要重点关注的是LC_SEGMENT/LC_SEGMENT_64，LC_SEGMENT是32位，LC_SEGMENT_64是64位，目前主流机型是LC_SEGMENT_64。LC_SEGMENT_64作用是如何将Data中的各个Segment加载入内存中，而和我们APP相关的代码及数据，大部分位于各个Segment中。其数据结构名称是segment_command_64，XNU代码路径：EXTERNAL_HEADERS/mach-o/loader.h，源码如下所示：
struct segment_command_64 { /* for 64-bit architectures */uint32_t cmd; /* LC_SEGMENT_64 */uint32_t cmdsize; /* includes sizeof section_64 structs */char segname[16]; /* segment name */uint64_t vmaddr; /* memory address of this segment */uint64_t vmsize; /* memory size of this segment */uint64_t fileoff; /* file offset of this segment */uint64_t filesize; /* amount to map from the file */vm_prot_t maxprot; /* maximum VM protection */vm_prot_t initprot; /* initial VM protection */uint32_t nsects; /* number of sections in segment */uint32_t flags; /* flags */};
2.4.3.4.4 _DATA
2.4.4 Data(数据段)
GEEK TALK
03
资源优化
3.1 简介
3.2 大资源优化
3.2.1 获取大资源
def findBigResources(path,threshold):pathDir = os.listdir(path)for allDir in pathDir:child = os.path.join('%s%s' % (path, allDir))if os.path.isfile(child):# 获取读到的文件的后缀end = os.path.splitext(child)[-1]# 过滤掉dylib系统库和asset.carif end != ".dylib" and end != ".car":temp = os.path.getsize(child)# 转换单位：B -> KBfileLen = temp / 1024if fileLen > threshold:#print(end)print(child + " length is " + str(fileLen));else:# 递归遍历子目录child = child + "/"findBigResources(child,threshold)
3.2.2 优化方法
异步下载：只要APP首次启动时不需要加载该资源，或者即使首次启动需要加载但是使用频率不高，那么该资源就可以走异步下载; 资源压缩：当APP首次启动需要加载且频率较高的情况下，可以对大块资源先进行压缩内置APP，启动阶段异步线程解压再使用；
3.2 无用的配置文件
3.3.1 获取配置文件
def findProfileResources(path):pathDir = os.listdir(path)for allDir in pathDir:child = os.path.join('%s%s' % (path, allDir))if os.path.isfile(child):# 获取读到的文件的后缀end = os.path.splitext(child)[-1]if end != ".dylib" and end != ".car" and end != ".png" and end != ".webp" and end != ".gif" and end != ".js" and end != ".css":print(child + " 后缀 " + end)else:# 递归遍历子目录child = child + "/"findProfileResources(child)
lines = os.popen('/usr/bin/otool -v -s __TEXT __cstring %s' % path).readlines()3.3.3 获取无用配置文件
前面获取的集合做diff，获取无用配置文件，确认无误后删除以减少包体积。如果你的资源名是拼接使用的，就无法命中，所以删除资源一定要逐个确认。
3.3.4 JS&CSS无用文件排查
JS&CSS文件具有特殊性，OC代码可以引用，HTML文件也可以加载引用，图片也是这种情况，但是上面提到的mach-o文件中TEXT字段只能覆盖OC文件的引用方式，而HTML加载才是主流场景，为此针对这种case百度APP采用跟无用图片检测类似的解决方案。
3.4 重复资源优化
def get_file_library(path, file_dict):pathDir = os.listdir(path)for allDir in pathDir:child = os.path.join('%s/%s' % (path, allDir))if os.path.isfile(child):md5 = img_to_md5(child)# 将md5存入字典key = md5file_dict.setdefault(key, []).append(allDir)continueget_file_library(child, file_dict)def img_to_md5(path):fd = open(path, 'rb')fmd5 = hashlib.md5(fd.read()).hexdigest()fd.close()return fmd5
GEEK TALK
04
总结
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