《iOS底层原理文章汇总》

上一篇文章《iOS-底层原理12-应用程序加载》,之后了load_images中调用类，分类中的load方法，本文介绍类中属性，方法，协议，Categories,是如何加载到ro,rw,rwe中的。

由上一篇文章中知道dyld-->libSystem_initializer->libdispatch_init->_os_object_init->_objc_init->回到dyld中的notifySingle->(*sNotifyObjCInit)(image->getRealPath(), image->machHeader());从而执行load_images,_objc_init中还有其他函数的作用呢？

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
}

void registerObjCNotifiers(_dyld_objc_notify_mapped mapped, _dyld_objc_notify_init init, _dyld_objc_notify_unmapped unmapped)
{
    // record functions to call
    sNotifyObjCMapped   = mapped;
    sNotifyObjCInit     = init;
    sNotifyObjCUnmapped = unmapped;

    // call 'mapped' function with all images mapped so far
    try {
        notifyBatchPartial(dyld_image_state_bound, true, NULL, false, true);
    }
    catch (const char* msg) {
        // ignore request to abort during registration
    }

    // <rdar://problem/32209809> call 'init' function on all images already init'ed (below libSystem)
    for (std::vector<ImageLoader*>::iterator it=sAllImages.begin(); it != sAllImages.end(); it++) {
        ImageLoader* image = *it;
        if ( (image->getState() == dyld_image_state_initialized) && image->notifyObjC() ) {
            dyld3::ScopedTimer timer(DBG_DYLD_TIMING_OBJC_INIT, (uint64_t)image->machHeader(), 0, 0);
            (*sNotifyObjCInit)(image->getRealPath(), image->machHeader());
        }
    }
}

1.environ_init():环境变量的初始化,打印下都有哪些环境变量

1.在Xcode可进行环境变量的设置OBJC_DISABLE_NONPOINTER_ISA，设置为YES则去掉为nonpointer的isa指针，另一个环境变量OBJC_PRINT_LOAD_METHODS，设值后，所有load方法都会被打印

可配置的环境变量@2x.png

objc[7503]: OBJC_PRINT_IMAGES: log image and library names as they are loaded
objc[7503]: OBJC_PRINT_IMAGES is set
objc[7503]: OBJC_PRINT_IMAGE_TIMES: measure duration of image loading steps
objc[7503]: OBJC_PRINT_IMAGE_TIMES is set
objc[7503]: OBJC_PRINT_LOAD_METHODS: log calls to class and category +load methods
objc[7503]: OBJC_PRINT_LOAD_METHODS is set
objc[7503]: OBJC_PRINT_INITIALIZE_METHODS: log calls to class +initialize methods
objc[7503]: OBJC_PRINT_INITIALIZE_METHODS is set
objc[7503]: OBJC_PRINT_RESOLVED_METHODS: log methods created by +resolveClassMethod: and +resolveInstanceMethod:
objc[7503]: OBJC_PRINT_RESOLVED_METHODS is set
objc[7503]: OBJC_PRINT_CLASS_SETUP: log progress of class and category setup
objc[7503]: OBJC_PRINT_CLASS_SETUP is set
objc[7503]: OBJC_PRINT_PROTOCOL_SETUP: log progress of protocol setup
objc[7503]: OBJC_PRINT_PROTOCOL_SETUP is set
objc[7503]: OBJC_PRINT_IVAR_SETUP: log processing of non-fragile ivars
objc[7503]: OBJC_PRINT_IVAR_SETUP is set
objc[7503]: OBJC_PRINT_VTABLE_SETUP: log processing of class vtables
objc[7503]: OBJC_PRINT_VTABLE_SETUP is set
objc[7503]: OBJC_PRINT_VTABLE_IMAGES: print vtable images showing overridden methods
objc[7503]: OBJC_PRINT_VTABLE_IMAGES is set
objc[7503]: OBJC_PRINT_CACHE_SETUP: log processing of method caches
objc[7503]: OBJC_PRINT_CACHE_SETUP is set
objc[7503]: OBJC_PRINT_FUTURE_CLASSES: log use of future classes for toll-free bridging
objc[7503]: OBJC_PRINT_FUTURE_CLASSES is set
objc[7503]: OBJC_PRINT_PREOPTIMIZATION: log preoptimization courtesy of dyld shared cache
objc[7503]: OBJC_PRINT_PREOPTIMIZATION is set
objc[7503]: OBJC_PRINT_CXX_CTORS: log calls to C++ ctors and dtors for instance variables
objc[7503]: OBJC_PRINT_CXX_CTORS is set
objc[7503]: OBJC_PRINT_EXCEPTIONS: log exception handling
objc[7503]: OBJC_PRINT_EXCEPTIONS is set
objc[7503]: OBJC_PRINT_EXCEPTION_THROW: log backtrace of every objc_exception_throw()
objc[7503]: OBJC_PRINT_EXCEPTION_THROW is set
objc[7503]: OBJC_PRINT_ALT_HANDLERS: log processing of exception alt handlers
objc[7503]: OBJC_PRINT_ALT_HANDLERS is set
objc[7503]: OBJC_PRINT_REPLACED_METHODS: log methods replaced by category implementations
objc[7503]: OBJC_PRINT_REPLACED_METHODS is set
objc[7503]: OBJC_PRINT_DEPRECATION_WARNINGS: warn about calls to deprecated runtime functions
objc[7503]: OBJC_PRINT_DEPRECATION_WARNINGS is set
objc[7503]: OBJC_PRINT_POOL_HIGHWATER: log high-water marks for autorelease pools
objc[7503]: OBJC_PRINT_POOL_HIGHWATER is set
objc[7503]: OBJC_PRINT_CUSTOM_CORE: log classes with custom core methods
objc[7503]: OBJC_PRINT_CUSTOM_CORE is set
objc[7503]: OBJC_PRINT_CUSTOM_RR: log classes with custom retain/release methods
objc[7503]: OBJC_PRINT_CUSTOM_RR is set
objc[7503]: OBJC_PRINT_CUSTOM_AWZ: log classes with custom allocWithZone methods
objc[7503]: OBJC_PRINT_CUSTOM_AWZ is set
objc[7503]: OBJC_PRINT_RAW_ISA: log classes that require raw pointer isa fields
objc[7503]: OBJC_PRINT_RAW_ISA is set
objc[7503]: OBJC_DEBUG_UNLOAD: warn about poorly-behaving bundles when unloaded
objc[7503]: OBJC_DEBUG_UNLOAD is set
objc[7503]: OBJC_DEBUG_FRAGILE_SUPERCLASSES: warn about subclasses that may have been broken by subsequent changes to superclasses
objc[7503]: OBJC_DEBUG_FRAGILE_SUPERCLASSES is set
objc[7503]: OBJC_DEBUG_NIL_SYNC: warn about @synchronized(nil), which does no synchronization
objc[7503]: OBJC_DEBUG_NIL_SYNC is set
objc[7503]: OBJC_DEBUG_NONFRAGILE_IVARS: capriciously rearrange non-fragile ivars
objc[7503]: OBJC_DEBUG_NONFRAGILE_IVARS is set
objc[7503]: OBJC_DEBUG_ALT_HANDLERS: record more info about bad alt handler use
objc[7503]: OBJC_DEBUG_ALT_HANDLERS is set
objc[7503]: OBJC_DEBUG_MISSING_POOLS: warn about autorelease with no pool in place, which may be a leak
objc[7503]: OBJC_DEBUG_MISSING_POOLS is set
objc[7503]: OBJC_DEBUG_POOL_ALLOCATION: halt when autorelease pools are popped out of order, and allow heap debuggers to track autorelease pools
objc[7503]: OBJC_DEBUG_POOL_ALLOCATION is set
objc[7503]: OBJC_DEBUG_DUPLICATE_CLASSES: halt when multiple classes with the same name are present
objc[7503]: OBJC_DEBUG_DUPLICATE_CLASSES is set
objc[7503]: OBJC_DEBUG_DONT_CRASH: halt the process by exiting instead of crashing
objc[7503]: OBJC_DEBUG_DONT_CRASH is set
objc[7503]: OBJC_DISABLE_VTABLES: disable vtable dispatch
objc[7503]: OBJC_DISABLE_VTABLES is set
objc[7503]: OBJC_DISABLE_PREOPTIMIZATION: disable preoptimization courtesy of dyld shared cache
objc[7503]: OBJC_DISABLE_PREOPTIMIZATION is set
objc[7503]: OBJC_DISABLE_TAGGED_POINTERS: disable tagged pointer optimization of NSNumber et al.
objc[7503]: OBJC_DISABLE_TAGGED_POINTERS is set
objc[7503]: OBJC_DISABLE_TAG_OBFUSCATION: disable obfuscation of tagged pointers
objc[7503]: OBJC_DISABLE_TAG_OBFUSCATION is set
objc[7503]: OBJC_DISABLE_NONPOINTER_ISA: disable non-pointer isa fields
objc[7503]: OBJC_DISABLE_NONPOINTER_ISA is set
objc[7503]: OBJC_DISABLE_INITIALIZE_FORK_SAFETY: disable safety checks for +initialize after fork
objc[7503]: OBJC_DISABLE_INITIALIZE_FORK_SAFETY is set

我们先不设置环境变量，查看LGPerson中的isa的内存地址的值,打印出来确实为nonpointisa，最后一个数为1表示nonpointerisa,p/t表示二进制打印

nonpointerisa.png

进行环境变量设值后，将nonpointerisa舍弃掉，查看person中的isa的内存地址,说明环境变量的设值生效了

OBJC_DISABLE_NONPOINTER_ISA@2x.png
非nonpointerisa.png
设置环境变量打印load方法，将load方法都打印出来，新增LGPerson的load方法，也会打印。
OBJC_PRINT_LOAD_METHODS.png LGPersonload方法.png

2.通过终端输出环境变量export OBJC_HELP = 1

export-OBJC_HELP@2x.png

2.tls_init()本地线程池,runloop,autoreleasepool都会依赖于线程，一个初始setter方法，一个析构getter方法

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
}

3.static_init,当前objc-os中的静态构造函数初始化，比dyld的调用还早，在当前dyld调用之前必须有这些静态方法，主要是当前的环境的处理

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

4.runtime_init，objc::unattachedCategories.init(32)为了进行分类而进行的初始化，当前初始化class的一张表，用来储存我们加载完毕的类

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

5.exception_init,crash并不是真正的崩溃，系统发的不允许的指令，是违反系统规定，系统给信号Crash，传入发生异常的函数

void exception_init(void)
{
    old_terminate = std::set_terminate(&_objc_terminate);
}
static void _objc_terminate(void)
{
    if (PrintExceptions) {
        _objc_inform("EXCEPTIONS: terminating");
    }

    if (! __cxa_current_exception_type()) {
        // No current exception.
        (*old_terminate)();
    }
    else {
        // There is a current exception. Check if it's an objc exception.
        @try {
            __cxa_rethrow();
        } @catch (id e) {
            // It's an objc object. Call Foundation's handler, if any.
            (*uncaught_handler)((id)e);
            (*old_terminate)();
        } @catch (...) {
            // It's not an objc object. Continue to C++ terminate.
            (*old_terminate)();
        }
    }
}
static objc_uncaught_exception_handler uncaught_handler = _objc_default_uncaught_exception_handler;

重写传入捕获异常的方法fn------《iOS-底层原理38-Crash分析》

/***********************************************************************
* objc_setUncaughtExceptionHandler
* Set a handler for uncaught Objective-C exceptions. 
* Returns the previous handler. 
**********************************************************************/
objc_uncaught_exception_handler 
objc_setUncaughtExceptionHandler(objc_uncaught_exception_handler fn)
{
    objc_uncaught_exception_handler result = uncaught_handler;
    uncaught_handler = fn;
    return result;
}

6.cache_init()缓存的处理

7._imp_implementationWithBlock_init()实现的处理

回到dyld的执行，_dyld_objc_notify_register(&map_images, load_images, unmap_image)将map_images、load_images、unmap_image传入到dyld中，dyld分别用回调函数来接收

map_images和load_images.png

void registerObjCNotifiers(_dyld_objc_notify_mapped mapped, _dyld_objc_notify_init init, _dyld_objc_notify_unmapped unmapped)
{
    // record functions to call
    sNotifyObjCMapped   = mapped;
    sNotifyObjCInit     = init;
    sNotifyObjCUnmapped = unmapped;
}

dyld的流程接着往下走，dyld中的notifySingle方法调用了*sNotifyObjCInit，从而执行了load_images方法，map_images方法什么时候执行呢？

notifySingle@2x.png

我们在dyld的源码里面搜索sNotifyObjCMapped，找到调用是在notifyBatchPartial方法，继续搜索notifyBatchPartial方法

notifyBatchPartial@2x.png
在notifyBatch方法中调用notifyBatchPartial方法

static void notifyBatch(dyld_image_states state, bool preflightOnly)
{
    notifyBatchPartial(state, false, NULL, preflightOnly, false);
}

搜索notifyBatch，在runInitializers调用context.notifyBatch(dyld_image_state_initialized, false)，context.notifyBatch调用notifyBatchPartial中调用(*sNotifyObjCMapped)(objcImageCount, paths, mhs);流程如下

void ImageLoader::runInitializers(const LinkContext& context, InitializerTimingList& timingInfo)
{
    uint64_t t1 = mach_absolute_time();
    mach_port_t thisThread = mach_thread_self();
    ImageLoader::UninitedUpwards up;
    up.count = 1;
    up.imagesAndPaths[0] = { this, this->getPath() };
    processInitializers(context, thisThread, timingInfo, up);
    context.notifyBatch(dyld_image_state_initialized, false);
    mach_port_deallocate(mach_task_self(), thisThread);
    uint64_t t2 = mach_absolute_time();
    fgTotalInitTime += (t2 - t1);
}
static void notifyBatch(dyld_image_states state, bool preflightOnly)
{
    notifyBatchPartial(state, false, NULL, preflightOnly, false);
}

程序结束，从dyld链接卸载才会调用unmap_image(*sNotifyObjCUnmapped)(image->getRealPath(), image->machHeader());

unmap_image@2x.png main()函数之前类的加载.png

2.map_images和load_images分别做了什么呢？

read_images流程如下:
1: 条件控制进行一次的加载
2: 修复预编译阶段的 @selector 的混乱问题
3: 错误混乱的类处理
4:修复重映射一些没有被镜像文件加载进来的 类
5: 修复一些消息!
6: 当我们类里面有协议的时候 : readProtocol
7: 修复没有被加载的协议
8: 分类处理
9: 类的加载处理
10 : 没有被处理的类 优化那些被侵犯的类

1.小对象指针经过了一层小小的处理，并不是简单的指针，比如int类型，NSNumber类型

static void
initializeTaggedPointerObfuscator(void)
{
    if (sdkIsOlderThan(10_14, 12_0, 12_0, 5_0, 3_0) ||
        // Set the obfuscator to zero for apps linked against older SDKs,
        // in case they're relying on the tagged pointer representation.
        DisableTaggedPointerObfuscation) {
        objc_debug_taggedpointer_obfuscator = 0;
    } else {
        // Pull random data into the variable, then shift away all non-payload bits.
        arc4random_buf(&objc_debug_taggedpointer_obfuscator,
                       sizeof(objc_debug_taggedpointer_obfuscator));
        objc_debug_taggedpointer_obfuscator &= ~_OBJC_TAG_MASK;
    }
}

2.创建一些表，类存表,方便查找

        // namedClasses
        // Preoptimized classes don't go in this table.
        // 4/3 is NXMapTable's load factor
        int namedClassesSize = 
            (isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
        gdb_objc_realized_classes =
            NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);

3.地址统一，在不同的动态库中存在的类的地址不一样，比如[LGPerson load]，[LGPerson class]方法在libdyld，libSystem,libDispatch的库，在这些库中所在位置的坐标是不一样的，最后都加载到内存中，要确保类是同一个，地址统一，进行统一调度，sel中不仅存的地址值，也存的类的字符串，发现不一致的进行统一，SEL不是一个简单的字符串，是带地址的字符串

static void
initializeTaggedPointerObfuscator(void)
{
    if (sdkIsOlderThan(10_14, 12_0, 12_0, 5_0, 3_0) ||
        // Set the obfuscator to zero for apps linked against older SDKs,
        // in case they're relying on the tagged pointer representation.
        DisableTaggedPointerObfuscation) {
        objc_debug_taggedpointer_obfuscator = 0;
    } else {
        // Pull random data into the variable, then shift away all non-payload bits.
        arc4random_buf(&objc_debug_taggedpointer_obfuscator,
                       sizeof(objc_debug_taggedpointer_obfuscator));
        objc_debug_taggedpointer_obfuscator &= ~_OBJC_TAG_MASK;
    }
}

unfixedSelectors@2x.png

3.readClass,地址赋上名字，MachO文件在MachView中显示会出现很多内存地址，内存地址怎么变成类呢？cls的地址赋上名字,走到cls,readClass精简代码如下:

cls是内存地址，通过调用addNamedClass(cls, mangledName, replacing)将类的名字和内存地址进行绑定，存放在刚开始进入read_images方法中时，创建的表gdb_objc_realized_classes中,通过调用addClassTableEntry(cls)将类和类的元类加到要开辟的类的表中，表被初始化在_objc_init() -> runtime_init() -> objc::allocatedClasses.init();中，此时这个类在共享缓存中变为已知类。

Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
    const char *mangledName = cls->mangledName();
    const char *LGPersonName = "LGPerson";
    if (strcmp(LGPersonName, mangledName) == 0) {
        printf("%s -哎唷不错！- %s",__func__,mangledName);
        }
            if (headerIsPreoptimized  &&  !replacing) {
        // class list built in shared cache
        // fixme strict assert doesn't work because of duplicates
        // ASSERT(cls == getClass(name));
        ASSERT(getClassExceptSomeSwift(mangledName));
    } else {
        addNamedClass(cls, mangledName, replacing);
        addClassTableEntry(cls);
    }
}
static void addNamedClass(Class cls, const char *name, Class replacing = nil)
{
    runtimeLock.assertLocked();
    Class old;
    if ((old = getClassExceptSomeSwift(name))  &&  old != replacing) {
        inform_duplicate(name, old, cls);

        // getMaybeUnrealizedNonMetaClass uses name lookups.
        // Classes not found by name lookup must be in the
        // secondary meta->nonmeta table.
        addNonMetaClass(cls);
    } else {
        NXMapInsert(gdb_objc_realized_classes, name, cls);
    }
    ASSERT(!(cls->data()->flags & RO_META));

    // wrong: constructed classes are already realized when they get here
    // ASSERT(!cls->isRealized());
}

这就是为什么前面读到的是地址，后面读到的是类名

地址关联类@2x.png

拦截特定的类

LGPerson@2x.png
LGPerson方法实现@2x.png
readClass@2x.png

类从MachO文件中读取到内存中并和类名进行绑定，并添加到已知类的表中

类的加载上.png