总纲领: OC底层探寻
在上篇文章iOS 程序加载流程分析中分析dyld的过程中, 其中有一幅图来分析_objc_init符号断点图, 如下:
1419656-0cb5ee83eefa4ac6.png
结合这张图我们得知_objc_init调用的流程大致为:
dyld的doModInitFunctions方法调用libSystem.B.dylib的libSystem_initializer方法；接着初始化了libdispatch; libdispatch又调用了_os_object_int,最终来到了_objc_init.
下面我们来看看_objc_init的官方源码(objc4-781)
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(); //读取影响运行时的环境变量。如果需要，还可以打印环境变量帮助. tls_init(); //关于线程key的绑定一比如每线程数据的析构函数. static_init(); //运行C ++静态构造函数。在dyld调用我们的静态构造函数之前, 'libc'会调用_objc_inint(), 因此我们必须自己做. runtime_init(); //runtime运行时环境初始化，里面主要是: unattachedCategories ， allocatedClasses后面会分析 exception_init(); //异常信息的初始化 cache_init(); //缓存条件初始化 _imp_implementationWithBlock_init(); //启动回调机制。通常这不会做什么，因为所有的初始化都是情性的，但是对于某些进程，我们会迫不及待地加载trampolines dylib。 // 什么时候调用? images 镜像文件 // map_images() // load_images() _dyld_objc_notify_register(&map_images, load_images, unmap_image); #if __OBJC2__ didCallDyldNotifyRegister = true; #endif }
1. 环境变量初始化(environ_init)
在不设置环境变量 OBJC_DISABLE_NONPOINTER_ISA 的时候，打印 person 的 isa 信息, 如下:
lldb) x/4gx person 0x1010b5680: 0x001d800100008265 0x0000000000000000 0x1010b5690: 0x0000000000000000 0x0000000000000000 (lldb) p/t 0x001d800100008265 (long) $1 = 0b0000000000011101100000000000000100000000000000001000001001100101
然后设置环境变量OBJC_DISABLE_NONPOINTER_ISA 为YES之后, 再次打印结果如下:
设置环境变量OBJC_DISABLE_NONPOINTER_ISA 为 YES
(lldb) x/4gx person 0x100a09f20: 0x0000000100008260 0x0000000000000000 0x100a09f30: 0x0000000000000000 0x0000000000000000 (lldb) p/t 0x0000000100008260 (long) $1 = 0b0000000000000000000000000000000100000000000000001000001001100000
我们可以看到最后一位发生了变化, 而在isa结构分析这篇文章中, 我们可以得知,最后一位就是 nonpointer 位，表示是否对 isa 指针开启指针优化。 0：纯 isa 指针；1：不止是类对象地址。isa 中包含了类信息、对象的引用计数等。
2. _dyld_objc_notify_register
其实我们是要重点分析的这里, 这里是跨库调用的, 源码在dyld的源码里. 如下是dyld的源码部分:
void registerObjCNotifiers(_dyld_objc_notify_mapped mapped, _dyld_objc_notify_init init, _dyld_objc_notify_unmapped unmapped) { // record functions to call sNotifyObjCMapped = mapped; //map_images sNotifyObjCInit = init; //load_images sNotifyObjCUnmapped = unmapped; //unmap_image // 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()); // 调用了load_image } } }
- 这就是
_objc_init方法里调用了_dyld_objc_notify_register方法在dyld源码中找到了真正调用的地方. - 同样我们在
dyld中也能找到sNotifyObjCMapped的调用.
3. 有关map_images解析
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); }
接着进入map_images_nolock函数看看，这里的核心代码是_read_images方法
if (hCount > 0) { _read_images(hList, hCount, totalClasses, unoptimizedTotalClasses); }
- 条件控制进⾏⼀次的加载
- 修复预编译阶段的 @selector 的混乱问题
- 错误混乱的类处理
- 修复重映射⼀些没有被镜像⽂件加载进来的 类
- 修复⼀些消息!
- 当我们类⾥⾯有协议的时候 : readProtocol
- 修复没有被加载的协议
- 分类处理
- 类的加载处理
- 没有被处理的类 优化那些被侵犯的类
4. 有关readClass解析
Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized) { const char *mangledName = cls->mangledName(); if (missingWeakSuperclass(cls)) { // No superclass (probably weak-linked). // Disavow any knowledge of this subclass. if (PrintConnecting) { _objc_inform("CLASS: IGNORING class '%s' with " "missing weak-linked superclass", cls->nameForLogging()); } addRemappedClass(cls, nil); cls->superclass = nil; return nil; } cls->fixupBackwardDeployingStableSwift(); Class replacing = nil; if (Class newCls = popFutureNamedClass(mangledName)) { // This name was previously allocated as a future class. // Copy objc_class to future class's struct. // Preserve future's rw data block. if (newCls->isAnySwift()) { _objc_fatal("Can't complete future class request for '%s' " "because the real class is too big.", cls->nameForLogging()); } class_rw_t *rw = newCls->data(); const class_ro_t *old_ro = rw->ro(); memcpy(newCls, cls, sizeof(objc_class)); rw->set_ro((class_ro_t *)newCls->data()); newCls->setData(rw); freeIfMutable((char *)old_ro->name); free((void *)old_ro); addRemappedClass(cls, newCls); replacing = cls; cls = newCls; } 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); } // for future reference: shared cache never contains MH_BUNDLEs if (headerIsBundle) { cls->data()->flags |= RO_FROM_BUNDLE; cls->ISA()->data()->flags |= RO_FROM_BUNDLE; } return cls; }
这一步会把class信息从二进制里面读出来, 然后:
- 将
newCls->data()取出来作rw. - 将
newCls->data()再取出来强转为class_ro_t *放到到rw的ro部分. addClassTableEntry这是将类插入到类的集合表中，为了后面调用的快速查找.
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