iOS底层原理之—dyld与objc的关联

原文作者：我是小菜
原文链接：https://juejin.im/post/6884147035172405256

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?
    environ_init();
    tls_init();
    static_init();
    runtime_init();
    exception_init();
    cache_init();
    _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++) {
        inits[i]();
    }
}

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) {
        count++;
    }

    kr = task_restartable_ranges_register(mach_task_self(),
                                          objc_restartableRanges, count);
    if (kr == KERN_SUCCESS) return;
    _objc_fatal("task_restartable_ranges_register failed (result 0x%x: %s)",
                kr, mach_error_string(kr));
#endif // HAVE_TASK_RESTARTABLE_RANGES
}

_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;
    auto processCatlist = [&](category_t * const *catlist) {
        for (unsigned i = 0; i < count; i++) {
            category_t *cat = catlist[i];
            Class cls = remapClass(cat->cls);
            locstamped_category_t lc{cat, hi};

            if (!cls) {
                // Category's target class is missing (probably weak-linked).
                // Ignore the category.
                if (PrintConnecting) {
                    _objc_inform("CLASS: IGNORING category \?\?\?(%s) %p with "
                                 "missing weak-linked target class",
                                 cat->name, cat);
                }
                continue;
            }

            // Process this category.
            if (cls->isStubClass()) {
                // Stub classes are never realized. Stub classes
                // don't know their metaclass until they're
                // initialized, so we have to add categories with
                // class methods or properties to the stub itself.
                // methodizeClass() will find them and add them to
                // the metaclass as appropriate.
                if (cat->instanceMethods ||
                    cat->protocols ||
                    cat->instanceProperties ||
                    cat->classMethods ||
                    cat->protocols ||
                    (hasClassProperties && cat->_classProperties))
                {
                    objc::unattachedCategories.addForClass(lc, cls);
                }
            } else {
                // First, register the category with its target class.
                // Then, rebuild the class's method lists (etc) if
                // the class is realized.
                if (cat->instanceMethods ||  cat->protocols
                    ||  cat->instanceProperties)
                {
                    if (cls->isRealized()) {
                        attachCategories(cls, &lc, 1, ATTACH_EXISTING);
                    } else {
                        objc::unattachedCategories.addForClass(lc, cls);
                    }
                }

                if (cat->classMethods  ||  cat->protocols
                    ||  (hasClassProperties && cat->_classProperties))
                {
                    if (cls->ISA()->isRealized()) {
                        attachCategories(cls->ISA(), &lc, 1, ATTACH_EXISTING | ATTACH_METACLASS);
                    } else {
                        objc::unattachedCategories.addForClass(lc, cls->ISA());
                    }
                }
            }
        }
    };

    processCatlist(_getObjc2CategoryList(hi, &count));
    processCatlist(_getObjc2CategoryList2(hi, &count));
}

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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