前言

在dyld加载过程中，我们知道会调用_objc_init方法，那么在_objc_init方法中究竟做了什么呢？我们来探究下。

_objc_init方法

_objc_init方法实现

void _objc_init(void)
{
static bool initialized = false;
if (initialized) return;
initialized = true;

// fixme defer initialization until an objc-using image is found?<br />    environ_init();<br />    tls_init();<br />    static_init();<br />    runtime_init();<br />    exception_init();<br />    cache_init();<br />    _imp_implementationWithBlock_init();
_dyld_objc_notify_register(&map_images, load_images, unmap_image);

if OBJC2
didCallDyldNotifyRegister = true;
#endif
}

从_objc_init实现中我们分析下该方法主要做了什么

environ_init()

该方法主要是读取运行时的环境变量，我们可以通过设置DYLD_PRINT_STATISTICS = YES来打印APP启动到main()函数之前的时长，进而可以进行APP启动优化。具体的environ_init()简介可参考博客iOS-底层原理 16：dyld与objc的关联中有关nviron_init()部分的介绍

tls_init()

主要用于关于线程key的绑定，比如每线程数据的析构函数。

void tls_init(void)
{
#if SUPPORT_DIRECT_THREAD_KEYS
pthread_key_init_np(TLS_DIRECT_KEY, &_objc_pthread_destroyspecific);
#else
_objc_pthread_key = tls_create(&_objc_pthread_destroyspecific);
#endif
}

static_init()

主要是C++静态构造函数

static void static_init()
{
size_t count;
auto inits = getLibobjcInitializers(&_mh_dylib_header, &count);
for (size_t i = 0; i < count; i++) {
initsi;
}
}

runtime_init()

主要是运行时的初始化，主要分为两部分：分类初始化和类的表初始化

void runtime_init(void)
{
objc::unattachedCategories.init(32);
objc::allocatedClasses.init();
}
复制代码

exception_init()

初始化libobjc异常处理

/*
exception_init
Initialize libobjc’s exception handling system.
Called by map_images().
*/
void exception_init(void)
{
old_terminate = std::set_terminate(&_objc_terminate);
}

cache_init()

主要是缓存初始化

void cache_init()
{
#if HAVE_TASK_RESTARTABLE_RANGES
mach_msg_type_number_t count = 0;
kern_return_t kr;

while (objc_restartableRanges[count].location) {<br />        count++;<br />    }
kr = task_restartable_ranges_register(mach_task_self(),<br />                                          objc_restartableRanges, count);<br />    if (kr == KERN_SUCCESS) return;<br />    _objc_fatal("task_restartable_ranges_register failed (result 0x%x: %s)",<br />                kr, mach_error_string(kr));<br />#endif // HAVE_TASK_RESTARTABLE_RANGES<br />}

_imp_implementationWithBlock_init()

主要用来启动机制回调

/// everything is initialized lazily, but for certain processes we eagerly load
/// the trampolines dylib.
void
_imp_implementationWithBlock_init(void)
{
#if TARGET_OS_OSX
// Eagerly load libobjc-trampolines.dylib in certain processes. Some
// programs (most notably QtWebEngineProcess used by older versions of
// embedded Chromium) enable a highly restrictive sandbox profile which
// blocks access to that dylib. If anything calls
// imp_implementationWithBlock (as AppKit has started doing) then we’ll
// crash trying to load it. Loading it here sets it up before the sandbox
// profile is enabled and blocks it.
//
// This fixes EA Origin (rdar://problem/50813789)
// and Steam (rdar://problem/55286131)
if (progname &&
(strcmp(progname, “QtWebEngineProcess”) == 0 ||
strcmp(__progname, “Steam Helper”) == 0)) {
Trampolines.Initialize();
}
#endif
}

dyld与objc关联

_dyld_objc_notify_register(&map_images, load_images, unmap_image)

主要是dyld注册 实际代码实现

void _dyld_objc_notify_register(_dyld_objc_notify_mapped mapped,
_dyld_objc_notify_init init,
_dyld_objc_notify_unmapped unmapped)
{
dyld::registerObjCNotifiers(mapped, init, unmapped);
}

从上文正中我们可以看出

• mapped即map_images
  
• init即load_images
  
• unmapped即unmap_image
  

map_images()函数分析

/*
map_images
Process the given images which are being mapped in by dyld.
Calls ABI-agnostic code after taking ABI-specific locks.

Locking: write-locks runtimeLock
**/
void
map_images(unsigned count, const char const paths[],
const struct mach_header * const mhdrs[])
{
mutex_locker_t lock(runtimeLock);
return map_images_nolock(count, paths, mhdrs);
}

从map_images函数中我们发现map_images_nolock函数是重点，我们进入map_images_nolock函数

map_images_nolock

我们查看代码实现

从截图中我们可以看出_read_images是我们要重点研究的方法

_read_images函数分析

是否是第一次加载

修复预编译时**@selector**的错乱问题

错误类处理，通过**readClass**读取出来类的信息

重新设置映射镜像

消息处理

类中如果有协议，读取协议

映射协议

加载分类

注意在分类处理中主要是通过load_categories_nolock处理，我们进入load_categories_nolock函数中

**load_categories_nolock**函数

static void load_categories_nolock(header_info *hi) {
bool hasClassProperties = hi->info()->hasCategoryClassProperties();

size_t count;<br />    auto processCatlist = [&](category_t * const *catlist) {<br />        for (unsigned i = 0; i < count; i++) {<br />            category_t *cat = catlist[i];<br />            Class cls = remapClass(cat->cls);<br />            locstamped_category_t lc{cat, hi};
        if (!cls) {<br />                // Category's target class is missing (probably weak-linked).<br />                // Ignore the category.<br />                if (PrintConnecting) {<br />                    _objc_inform("CLASS: IGNORING category \?\?\?(%s) %p with "<br />                                 "missing weak-linked target class",<br />                                 cat->name, cat);<br />                }<br />                continue;<br />            }
        // Process this category.<br />            if (cls->isStubClass()) {<br />                // Stub classes are never realized. Stub classes<br />                // don't know their metaclass until they're<br />                // initialized, so we have to add categories with<br />                // class methods or properties to the stub itself.<br />                // methodizeClass() will find them and add them to<br />                // the metaclass as appropriate.<br />                if (cat->instanceMethods ||<br />                    cat->protocols ||<br />                    cat->instanceProperties ||<br />                    cat->classMethods ||<br />                    cat->protocols ||<br />                    (hasClassProperties && cat->_classProperties))<br />                {<br />                    objc::unattachedCategories.addForClass(lc, cls);<br />                }<br />            } else {<br />                // First, register the category with its target class.<br />                // Then, rebuild the class's method lists (etc) if<br />                // the class is realized.<br />                if (cat->instanceMethods ||  cat->protocols<br />                    ||  cat->instanceProperties)<br />                {<br />                    if (cls->isRealized()) {<br />                        attachCategories(cls, &lc, 1, ATTACH_EXISTING);<br />                    } else {<br />                        objc::unattachedCategories.addForClass(lc, cls);<br />                    }<br />                }
            if (cat->classMethods  ||  cat->protocols<br />                    ||  (hasClassProperties && cat->_classProperties))<br />                {<br />                    if (cls->ISA()->isRealized()) {<br />                        attachCategories(cls->ISA(), &lc, 1, ATTACH_EXISTING | ATTACH_METACLASS);<br />                    } else {<br />                        objc::unattachedCategories.addForClass(lc, cls->ISA());<br />                    }<br />                }<br />            }<br />        }<br />    };
processCatlist(_getObjc2CategoryList(hi, &count));<br />    processCatlist(_getObjc2CategoryList2(hi, &count));<br />}

从load_categories_nolock函数实现中，我们可以看到该函数将类、实例方法、协议、属性、类方法等再次链接了一次。

非懒加载类处理

处理没有使用的类

dyld与objc关联总结

• dyld_start调用_objc_init来初始化，_objc_init中通过dyld调用_dyld_objc_notify_register函数，传入map_images跟load_images这两个参数来处理
  
• map_images通过map_images_nolock函数调用_read_images函数
  
• 在_read_images函数中处理类信息、属性、协议、分类等
  
• 当一切准备妥当，则再次返回dyld_start中，此时dyld跟objc关联了起来
  

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