探索思路
从哪开始 -> Debug工具 -> 具体流程
程序入口,main
函数
1、在 main
函数中打断点
xcode 窗口说明
调试工具 Debug
1、断点调试: control + ↓(setup into)
逐步执行汇编指令
Control the execution of your running app
2、添加符号断点
Pause execution when events occur
3、源码分析
4、基础调试,跳转方法定义
这里并没有
alloc
的具体实现,所以这里如果要调试,我们需要添加符号断点。
(1)直接添加 alloc
符号断点
调试区点击
Pause/continue execution
继续, 进入如下图所示
(2) 断点调试: control + ↓(setup into)
逐步执行汇编指令
(3)显示汇编信息
探究 alloc 流程
-
1、对照源码,进行探究
-
2、对流程进行具体分析
-
2.1 alloc
发现直接点击 alloc
调用的是 _objc_rootAlloc
// Base class implementation of +alloc. cls is not nil.
// Calls [cls allocWithZone:nil].
id
_objc_rootAlloc(Class cls)
{
return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}
-
2.2 callAlloc
// Call [cls alloc] or [cls allocWithZone:nil], with appropriate
// shortcutting optimizations.
static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__
if (slowpath(checkNil && !cls)) return nil;
if (fastpath(!cls->ISA()->hasCustomAWZ())) {
return _objc_rootAllocWithZone(cls, nil);
}
#endif
// No shortcuts available.
if (allocWithZone) {
return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
}
return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
}
-
2.2.1 编译器优化
slowpath
fastpath
// x 很可能不为 0,希望编译器进行优化
#define fastpath(x) (__builtin_expect(bool(x), 1))
// x 很可能为 0,希望编译器进行优化
#define slowpath(x) (__builtin_expect(bool(x), 0))
-
2.3
_objc_rootAllocWithZone
id
_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)
{
// allocWithZone under __OBJC2__ ignores the zone parameter
return _class_createInstanceFromZone(cls, 0, nil,
OBJECT_CONSTRUCT_CALL_BADALLOC);
}
-
2.4
_class_createInstanceFromZone
这里是 alloc 核心方法
/***********************************************************************
* class_createInstance
* fixme
* Locking: none
*
* Note: this function has been carefully written so that the fastpath
* takes no branch.
**********************************************************************/
static ALWAYS_INLINE id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
int construct_flags = OBJECT_CONSTRUCT_NONE,
bool cxxConstruct = true,
size_t *outAllocatedSize = nil)
{
ASSERT(cls->isRealized());
// Read class's info bits all at once for performance
bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
bool hasCxxDtor = cls->hasCxxDtor();
bool fast = cls->canAllocNonpointer();
size_t size;
size = cls->instanceSize(extraBytes);
if (outAllocatedSize) *outAllocatedSize = size;
id obj;
if (zone) {
obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
} else {
// alloc 开辟内存的地方
obj = (id)calloc(1, size);
}
if (slowpath(!obj)) {
if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
return _objc_callBadAllocHandler(cls);
}
return nil;
}
if (!zone && fast) {
obj->initInstanceIsa(cls, hasCxxDtor);
} else {
// Use raw pointer isa on the assumption that they might be
// doing something weird with the zone or RR.
obj->initIsa(cls);
}
if (fastpath(!hasCxxCtor)) {
return obj;
}
construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;
return object_cxxConstructFromClass(obj, cls, construct_flags);
}
-
2.4.1
size = cls->instanceSize(extraBytes);
先计算出需要的内存空间
-
2.4.2
alloc
向系统申请开启内存,返回地址指针
id obj;
if (zone) {
obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
} else {
// alloc 开辟内存的地方
obj = (id)calloc(1, size);
}
if (slowpath(!obj)) {
if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
return _objc_callBadAllocHandler(cls);
}
return nil;
}
-
2.4.3 关联到相应的类
obj->initInstanceIsa(cls, hasCxxDtor);
if (!zone && fast) {
obj->initInstanceIsa(cls, hasCxxDtor);
} else {
// Use raw pointer isa on the assumption that they might be
// doing something weird with the zone or RR.
obj->initIsa(cls);
}
-
2.4.4 最终总结如下
-
3 init
源码如下
// Replaced by CF (throws an NSException)
+ (id)init {
return (id)self;
}
- (id)init {
return _objc_rootInit(self);
}
id
_objc_rootInit(id obj)
{
// In practice, it will be hard to rely on this function.
// Many classes do not properly chain -init calls.
return obj;
}
这里我们可以看到,这里是直接返回了 obj 没有做其他操作
-
4、new
+ (id)new {
return [callAlloc(self, false/*checkNil*/) init];
}
这里相当于 alloc 方法 加上 init 方法
- Swift入门 swift.org/getting-sta…
- Xcode help.apple.com/xcode/mac/c…
- LLDB lldb.llvm.org/tutorial.ht…