iOS源码阅读笔记

// TargetConditionals.h

/*
 *  TARGET_OS_*
 *
 *  These conditionals specify in which Operating System the generated code will
 *  run.  Indention is used to show which conditionals are evolutionary subclasses.
 *
 *  The MAC/WIN32/UNIX conditionals are mutually exclusive.
 *  The IOS/TV/WATCH/VISION conditionals are mutually exclusive.
 *
 *    TARGET_OS_WIN32              - Generated code will run on WIN32 API
 *    TARGET_OS_WINDOWS            - Generated code will run on Windows
 *    TARGET_OS_UNIX               - Generated code will run on some Unix (not macOS)
 *    TARGET_OS_LINUX              - Generated code will run on Linux
 *    TARGET_OS_MAC                - Generated code will run on a variant of macOS
 *      TARGET_OS_OSX                - Generated code will run on macOS
 *      TARGET_OS_IPHONE             - Generated code will run on a variant of iOS (firmware, devices, simulator)
 *        TARGET_OS_IOS                - Generated code will run on iOS
 *          TARGET_OS_MACCATALYST        - Generated code will run on macOS
 *        TARGET_OS_TV                 - Generated code will run on tvOS
 *        TARGET_OS_WATCH              - Generated code will run on watchOS
 *        TARGET_OS_VISION             - Generated code will run on visionOS
 *        TARGET_OS_BRIDGE             - Generated code will run on bridge devices
 *      TARGET_OS_SIMULATOR          - Generated code will run on an iOS, tvOS, watchOS, or visionOS simulator
 *      TARGET_OS_DRIVERKIT          - Generated code will run on macOS, iOS, tvOS, watchOS, or visionOS
 *
 *    TARGET_OS_EMBEDDED           - DEPRECATED: Use TARGET_OS_IPHONE and/or TARGET_OS_SIMULATOR instead
 *    TARGET_IPHONE_SIMULATOR      - DEPRECATED: Same as TARGET_OS_SIMULATOR
 *    TARGET_OS_NANO               - DEPRECATED: Same as TARGET_OS_WATCH
 *
 *    +--------------------------------------------------------------------------------------+
 *    |                                    TARGET_OS_MAC                                     |
 *    | +-----+ +------------------------------------------------------------+ +-----------+ |
 *    | |     | |                  TARGET_OS_IPHONE                          | |           | |
 *    | |     | | +-----------------+ +----+ +-------+ +--------+ +--------+ | |           | |
 *    | |     | | |       IOS       | |    | |       | |        | |        | | |           | |
 *    | | OSX | | | +-------------+ | | TV | | WATCH | | BRIDGE | | VISION | | | DRIVERKIT | |
 *    | |     | | | | MACCATALYST | | |    | |       | |        | |        | | |           | |
 *    | |     | | | +-------------+ | |    | |       | |        | |        | | |           | |
 *    | |     | | +-----------------+ +----+ +-------+ +--------+ +--------+ | |           | |
 *    | +-----+ +------------------------------------------------------------+ +-----------+ |
 *    +--------------------------------------------------------------------------------------+
 */

// objc-runtime-new.h
struct objc_class : objc_object {
    uint32_t getMetaFlags() const {
        ASSERT(!isStubClass());

        uint32_t flags = bits.flags();
        if (flags & RW_META)
            return flags;
        return this->ISA()->bits.flags();
    }

    // 类是否已经初始化完成
    bool isInitialized() const {
        return getMetaFlags() & RW_INITIALIZED;
    }
}

id
objc_loadWeakRetained(id *location)
{
    id obj;
    id result;
    Class cls;

    SideTable *table;
    
 retry:
    obj = *location;
    if (_objc_isTaggedPointerOrNil(obj)) return obj;
    
    table = &SideTables()[obj];
    
    table->lock();
    if (*location != obj) {
        table->unlock();
        goto retry;
    }
    
    result = obj;

    cls = obj->ISA();
    if (! cls->hasCustomRR()) {
        // 执行此逻辑
        if (! obj->rootTryRetain()) {
            result = nil;
        }
    }
    else {
        // 执行不到的逻辑，删掉
    }
        
    table->unlock();
    return result;
}

ALWAYS_INLINE bool 
objc_object::rootTryRetain()
{
    return rootRetain(true, RRVariant::Fast) ? true : false;
}

ALWAYS_INLINE id
objc_object::rootRetain(bool tryRetain, objc_object::RRVariant variant)
{
    if (slowpath(isTaggedPointer())) return (id)this;

    bool sideTableLocked = false;
    bool transcribeToSideTable = false;

    isa_t oldisa;
    isa_t newisa;

    oldisa = LoadExclusive(&isa().bits);

    // ...

    do {
        transcribeToSideTable = false;
        newisa = oldisa;

        // 关键逻辑：
        // 如果正在释放中，并且tryRetain=true，则返回nil
        // don't check newisa.fast_rr; we already called any RR overrides
        if (slowpath(newisa.isDeallocating())) {
            ClearExclusive(&isa().bits);
            if (sideTableLocked) {
                ASSERT(variant == RRVariant::Full);
                sidetable_unlock();
            }
            if (slowpath(tryRetain)) {
                return nil;
            } else {
                return (id)this;
            }
        }
        uintptr_t carry;
        newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry);  // extra_rc++

        if (slowpath(carry)) {
            // newisa.extra_rc++ overflowed
            if (variant != RRVariant::Full) {
                ClearExclusive(&isa().bits);
                return rootRetain_overflow(tryRetain);
            }
            // Leave half of the retain counts inline and 
            // prepare to copy the other half to the side table.
            if (!tryRetain && !sideTableLocked) sidetable_lock();
            sideTableLocked = true;
            transcribeToSideTable = true;
            newisa.extra_rc = RC_HALF;
            newisa.has_sidetable_rc = true;
        }
    } while (slowpath(!StoreExclusive(&isa().bits, &oldisa.bits, newisa.bits)));

    if (variant == RRVariant::Full) {
        if (slowpath(transcribeToSideTable)) {
            // Copy the other half of the retain counts to the side table.
            sidetable_addExtraRC_nolock(RC_HALF);
        }

        if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock();
    } else {
        ASSERT(!transcribeToSideTable);
        ASSERT(!sideTableLocked);
    }

    return (id)this;
}

/***********************************************************************
* objc_destructInstance
* Destroys an instance without freeing memory.
* Calls C++ destructors.
* Calls ARC ivar cleanup.
* Removes associative references.
* Returns `obj`. Does nothing if `obj` is nil.
**********************************************************************/
void *objc_destructInstance(id obj)
{
    if (obj) {
        // Read all of the flags at once for performance.
        bool cxx = obj->hasCxxDtor();
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        if (cxx) object_cxxDestruct(obj);
        if (assoc) _object_remove_associations(obj, /*deallocating*/true);
        obj->clearDeallocating();
    }

    return obj;
}

// class has .cxx_construct/destruct implementations
#define RO_HAS_CXX_STRUCTORS  (1<<2)

static Class realizeClassWithoutSwift(Class cls, Class previously) {
    ...
    
    // Copy some flags from ro to rw
    if (ro->flags & RO_HAS_CXX_STRUCTORS) {
        cls->setHasCxxDtor();
        if (! (ro->flags & RO_HAS_CXX_DTOR_ONLY)) {
            cls->setHasCxxCtor();
        }
    }

    ...
}

// 是否正在释放
bool isDeallocating() {
    return extra_rc == 0 && has_sidetable_rc == 0;
}

ALWAYS_INLINE id 
objc_object::rootRetain()
{
    return rootRetain(false, RRVariant::Fast);
}

// retain最终执行的函数
ALWAYS_INLINE id
objc_object::rootRetain(bool tryRetain, objc_object::RRVariant variant)
{
    // 省略代码
    ... 

    // 在dealloc中这里的执行结果是true, tryRetain是false，
    // 所以会直接返回this
    if (slowpath(newisa.isDeallocating())) {
        ClearExclusive(&isa.bits);
        if (sideTableLocked) {
            ASSERT(variant == RRVariant::Full);
            sidetable_unlock();
        }
        if (slowpath(tryRetain)) {
            return nil;
        } else {
            return (id)this;
        }
    }

    // 省略代码
    ...
}

{
    /// 1. 通知Observers，即将进入RunLoop
    /// 此处有Observer会创建AutoreleasePool: _objc_autoreleasePoolPush();
    __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__(kCFRunLoopEntry);
    do {
 
        /// 2. 通知 Observers: 即将触发 Timer 回调。
        __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__(kCFRunLoopBeforeTimers);
        /// 3. 通知 Observers: 即将触发 Source (非基于port的,Source0) 回调。
        __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__(kCFRunLoopBeforeSources);
        __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__(block);
 
        /// 4. 触发 Source0 (非基于port的) 回调。
        __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__(source0);
        __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__(block);
 
        /// 6. 通知Observers，即将进入休眠
        /// 此处有Observer释放并新建AutoreleasePool: _objc_autoreleasePoolPop(); _objc_autoreleasePoolPush();
        __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__(kCFRunLoopBeforeWaiting);
 
        /// 7. sleep to wait msg.
        mach_msg() -> mach_msg_trap();
        
 
        /// 8. 通知Observers，线程被唤醒
        __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__(kCFRunLoopAfterWaiting);
 
        /// 9. 如果是被Timer唤醒的，回调Timer
        __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__(timer);
 
        /// 9. 如果是被dispatch唤醒的，执行所有调用 dispatch_async 等方法放入main queue 的 block
        __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__(dispatched_block);
 
        /// 9. 如果如果Runloop是被 Source1 (基于port的) 的事件唤醒了，处理这个事件
        __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__(source1);
 
 
    } while (...);
 
    /// 10. 通知Observers，即将退出RunLoop
    /// 此处有Observer释放AutoreleasePool: _objc_autoreleasePoolPop();
    __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__(kCFRunLoopExit);
}

static void *
_dispatch_worker_thread(void *context)
{
    //////////////////////////////////
    // 删减了部分代码
    //////////////////////////////////

	dispatch_queue_global_t dq = context;
	dispatch_pthread_root_queue_context_t pqc = dq->do_ctxt;

    int pending = os_atomic_dec2o(dq, dgq_pending, relaxed);

    // 线程存活5秒钟
	const int64_t timeout = 5ull * NSEC_PER_SEC;
	pthread_priority_t pp = _dispatch_get_priority();
	dispatch_priority_t pri = dq->dq_priority;

    // 从队列中取出任务执行，执行完后等待5秒钟
    // 如果5秒后没有被唤醒则进入超时逻辑，队列释放，线程退出
	do {
        // 任务执行
		_dispatch_root_queue_drain(dq, pri, DISPATCH_INVOKE_REDIRECTING_DRAIN);
		_dispatch_reset_priority_and_voucher(pp, NULL);
	} while (dispatch_semaphore_wait(&pqc->dpq_thread_mediator,
			dispatch_time(0, timeout)) == 0);

#if DISPATCH_USE_INTERNAL_WORKQUEUE
	if (monitored) _dispatch_workq_worker_unregister(dq);
#endif
	(void)os_atomic_inc2o(dq, dgq_thread_pool_size, release);
	_dispatch_root_queue_poke(dq, 1, 0);
	_dispatch_release(dq); // retained in _dispatch_root_queue_poke_slow
	return NULL;
}

// 定义在 libdispatch 仓库中的 init.c 文件中
DISPATCH_VTABLE_INSTANCE(source,
	.do_type        = DISPATCH_SOURCE_KEVENT_TYPE,
	.do_dispose     = _dispatch_source_dispose,
	.do_debug       = _dispatch_source_debug,
	.do_invoke      = _dispatch_source_invoke,

	.dq_activate    = _dispatch_source_activate,
	.dq_wakeup      = _dispatch_source_wakeup,
	.dq_push        = _dispatch_lane_push,
);