概况
我们都知道+initialize方法会在此类第一次被使用的时候会被调用，那么调用的次数是靠什么来决定的了？
结论一：类第一次被使用的时候，会先调用+initialize方法
NSObject关于initialize源码
源码地址: https://github.com/RetVal/objc-runtime.git
/***********************************************************************
* class_initialize. Send the '+initialize' message on demand to any
* uninitialized class. Force initialization of superclasses first.
**********************************************************************/
void _class_initialize(Class cls)
{
assert(!cls->isMetaClass());
Class supercls;
bool reallyInitialize = NO;
// Make sure super is done initializing BEFORE beginning to initialize cls.
// See note about deadlock above.
supercls = cls->superclass;
if (supercls && !supercls->isInitialized()) {
_class_initialize(supercls);
}
// Try to atomically set CLS_INITIALIZING.
{
monitor_locker_t lock(classInitLock);
if (!cls->isInitialized() && !cls->isInitializing()) {
cls->setInitializing();
reallyInitialize = YES;
}
}
if (reallyInitialize) {
// We successfully set the CLS_INITIALIZING bit. Initialize the class.
// Record that we're initializing this class so we can message it.
_setThisThreadIsInitializingClass(cls);
if (MultithreadedForkChild) {
// LOL JK we don't really call +initialize methods after fork().
performForkChildInitialize(cls, supercls);
return;
}
// Send the +initialize message.
// Note that +initialize is sent to the superclass (again) if
// this class doesn't implement +initialize. 2157218
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: calling +[%s initialize]",
pthread_self(), cls->nameForLogging());
}
// Exceptions: A +initialize call that throws an exception
// is deemed to be a complete and successful +initialize.
//
// Only __OBJC2__ adds these handlers. !__OBJC2__ has a
// bootstrapping problem of this versus CF's call to
// objc_exception_set_functions().
#if __OBJC2__
@try
#endif
{
callInitialize(cls);
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
pthread_self(), cls->nameForLogging());
}
}
#if __OBJC2__
@catch (...) {
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: +[%s initialize] "
"threw an exception",
pthread_self(), cls->nameForLogging());
}
@throw;
}
@finally
#endif
{
// Done initializing.
lockAndFinishInitializing(cls, supercls);
}
return;
}
else if (cls->isInitializing()) {
// We couldn't set INITIALIZING because INITIALIZING was already set.
// If this thread set it earlier, continue normally.
// If some other thread set it, block until initialize is done.
// It's ok if INITIALIZING changes to INITIALIZED while we're here,
// because we safely check for INITIALIZED inside the lock
// before blocking.
if (_thisThreadIsInitializingClass(cls)) {
return;
} else if (!MultithreadedForkChild) {
waitForInitializeToComplete(cls);
return;
} else {
// We're on the child side of fork(), facing a class that
// was initializing by some other thread when fork() was called.
_setThisThreadIsInitializingClass(cls);
performForkChildInitialize(cls, supercls);
}
}
else if (cls->isInitialized()) {
// Set CLS_INITIALIZING failed because someone else already
// initialized the class. Continue normally.
// NOTE this check must come AFTER the ISINITIALIZING case.
// Otherwise: Another thread is initializing this class. ISINITIALIZED
// is false. Skip this clause. Then the other thread finishes
// initialization and sets INITIALIZING=no and INITIALIZED=yes.
// Skip the ISINITIALIZING clause. Die horribly.
return;
}
else {
// We shouldn't be here.
_objc_fatal("thread-safe class init in objc runtime is buggy!");
}
}
先不考虑加锁和多线程问题，_class_initialize主要的就是如下两个步骤：
- 如果父类没有初始化，那么递归初始化父类
- 初始化自己
第二步会调用:
void callInitialize(Class cls)
{
((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
asm("");
}
看到objc_msgSend表明：
结论二：会走OC的消息发送流程
根据以上的推论我们可以得知：
+initialize会被调用0次，一次，多次
- 调用0次：表明这个类没有被使用到
- 调用1次：表明只有这个类被使用到，它的子类要不然是没有被使用到，要不然就是有这个子类自己的
+initialize(子类中+initialize中没有[super initialize]代码)方法 - 调用多次：表明这个类或者它的多个子类被使用了，并且这多个子类没有自己的
+initialize方法，子类会根据OC消息发送流程而调用了它父类的+initialize方法。
或者可以这么理解，任何类在使用之前都会调用它的+initialize，如果这个类没有+initialize，那么就找它的父类的+initialize一直到NSObject类。并且不会自动调用 [super initialize]。
Demo
为了验证上述的结果，做了如下的测试
调用0次
类：RXInitializeParentObject 所有DemoObject的父类
@implementation RXInitializeParentObject
+ (void)initialize {
NSLog(@"Parent initialize");
}
- (void)print {
}
@end
测试类：RXInitializeTestObject
- (void)test_doNoting {
}
当然是什么也没有输出
调用多次
RXInitializeEmptyObject,子类没有自己的initialize
@implementation RXInitializeEmptyObject
@end
测试类：RXInitializeTestObject
- (void)test_empty {
self.rxInitializeEmptyObject = [RXInitializeEmptyObject new];
[self.rxInitializeEmptyObject print];
}
输出的结果：
Parent initialize
Parent initialize
第一行输出：是因为初始化RXInitializeEmptyObject会先初始化其父类RXInitializeParentObject
第二行输出：对于RXInitializeEmptyObject来说，根据OC的消息发送规则，它的initialize方法定位到了父类的initialize方法了
子类自定义的+initialize方法
类RXInitializeCustomObject
@implementation RXInitializeCustomObject
+ (void)initialize {
NSLog(@"Custom initialize");
}
@end
测试类：RXInitializeTestObject
- (void)test_custom {
self.rxInitializeCustomObject = [RXInitializeCustomObject new];
[self.rxInitializeCustomObject print];
}
输出结果
Parent initialize
Custom initialize
第一行输出：是因为初始化RXInitializeCustomObject会先初始化其父类RXInitializeParentObject
第二行输出：对于RXInitializeCustomObject来说，它有自己的initialize方法。
先使用Empty再使用Custom
测试类：RXInitializeTestObject
- (void)test_empty_custom {
self.rxInitializeEmptyObject = [RXInitializeEmptyObject new];
[self.rxInitializeEmptyObject print];
self.rxInitializeCustomObject = [RXInitializeCustomObject new];
[self.rxInitializeCustomObject print];
}
输出结果
Parent initialize
Parent initialize
Custom initialize
第一行输出，是因为使用RXInitializeEmptyObject初始化其父类
第二行输出，是因为使用RXInitializeEmptyObject初始化自己
第三行输出，是因为使用RXInitializeCustomObject
先使用Custom再使用Empty
测试类：RXInitializeTestObject
- (void)test_custom_empty {
self.rxInitializeCustomObject = [RXInitializeCustomObject new];
[self.rxInitializeCustomObject print];
self.rxInitializeEmptyObject = [RXInitializeEmptyObject new];
[self.rxInitializeEmptyObject print];
}
输出结果
Parent initialize
Custom initialize
Parent initialize
大家可以自己尝试分析一下~~~
Custom & Custom2
类RXInitializeCustom2Object，内容跟RXInitializeCustomObject几乎一样
@implementation RXInitializeCustom2Object
+ (void)initialize {
NSLog(@"Custom 2 initialize");
}
@end
测试类：RXInitializeTestObject
- (void)test_custom_custom2
{
self.rxInitializeCustomObject = [RXInitializeCustomObject new];
[self.rxInitializeCustomObject print];
self.rxInitializeCustom2Object = [RXInitializeCustom2Object new];
[self.rxInitializeCustom2Object print];
}
输出结果：
Parent initialize
Custom initialize
Custom2 initialize
结果也是很容易分析的。
super Custom
类RXInitializeSuperCustomObject
@implementation RXInitializeSuperCustomObject
+ (void)initialize
{
[super initialize];
NSLog(@"Super Custom initialize");
}
@end
测试类：RXInitializeTestObject
- (void)test_superCustom {
self.rxInitializeSuperCustomObject = [RXInitializeSuperCustomObject new];
[self.rxInitializeSuperCustomObject print];
}
输出结果
Parent initialize
Parent initialize
Super Custom initialize
其中第二行是因为[super initialize]导致的。
以上就是关于+initialize方法被调用次数探究，包括理论部分和Demo部分。
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