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iOS Cateogry的深入理解&&initialize方法调用理解

  • 上一篇文章我们讲到了load方法,今天我们来看看initialize

新建项目,新建类   类结构图如下

c996112f54482f925b7f7413ef7ceda0.png

将原来的load方法换成initialize

f61b27bbbefa89e9c5f1c753ae33d05e.png1d6a13778eb2413fbe8f9ec8ae780ff7.pnge9e08befd5459f9851c574fce4974c11.png

先告诉大家initialize方法调用的时间,以便大家带着答案去理解initialize:在类第一次接收到消息的时候调用,它区别于load(运行时加载类的时候调用),下面我们来深入理解initialize

相信大家在想什么叫第一次接收消息了,我们回到main()

3a061f5098a014a1549375a6d58e0feb.png

运行程序,输出结果:
0a0802f3c00c41aab626683640a5a308.png


说明:NSLog(@"---")是由于我建的是命令行工程,不写这个,貌似不能显示控制台,Xcode版本是12,当然你们的要显示控制台,直接去掉这行代码

从输出结果可以看到没有任何关于initialize的打印,程序直接退出

  • 2.initialize的打印
int main(int argc, const char * argv[]) {
@autoreleasepool {

[TCPerson alloc];
}
return 0;
}


运行结果:
2020-12-04 14:59:17.417072+0800 TCCateogry[1616:79391] TCPerson (TCtest2) +initialize
Program ended with exit code: 0

从上面的输出结果我们可以看到,TCPerson (TCtest2) +initialize打印

load是直接函数指针直接调用,类,分类,继承等等

[TCPerson alloc]就是相当于该类发送消息,但是它只会调用类,分类的其中一个(取决于编译顺序,从输出结果可以看出,initialize走的是objc_msgSend,而load直接通过函数指针直接调用,所以initialize通过isa方法查找调用

多次向TCPerson发送消息的输出结果
int main(int argc, const char * argv[]) {
@autoreleasepool {

[TCPerson alloc];
[TCPerson alloc];
[TCPerson alloc];
[TCPerson alloc];
}
return 0;
}

输出结果:
2020-12-04 15:11:12.246442+0800 TCCateogry[1659:85317] TCPerson (TCtest2) +initialize
Program ended with exit code: 0

initialize只会调用一次

我们再来看看继承关系中,initialize的调用

int main(int argc, const char * argv[]) {
@autoreleasepool {
[TCStudent alloc];

}
return 0;
}

输出结果:

2020-12-04 15:14:58.705423+0800 TCCateogry[1705:87507] TCPerson (TCtest2) +initialize
2020-12-04 15:14:58.705750+0800 TCCateogry[1705:87507] TCStudent (TCStudentTest2) +initialize
Program ended with exit code: 0
从输出结果来看,子类调用initialize之前,会先调用父类的initialize,再调用自己的initialize,当然无论父类调用initialize,还是子类调用initialize,如果有多个分类(这里指的是父类调用父类的分类,子类调用子类的分类),调用initialize取决于分类的编译顺序(调用后编译分类中的initialize,类似于压栈,先进后出),值得注意的是,无论父类子类的initialize,都只调用一次

int main(int argc, const char * argv[]) {
@autoreleasepool {
[TCPerson alloc];
[TCPerson alloc];
[TCStudent alloc];
[TCStudent alloc];
}
return 0;
}

输出结果:

020-12-04 15:23:27.168243+0800 TCCateogry[1731:91248] TCPerson (TCtest2) +initialize
2020-12-04 15:23:27.168601+0800 TCCateogry[1731:91248] TCStudent (TCStudentTest2) +initialize
Program ended with exit code: 0

如果子类(子类的分类也不实现)不实现initialize,则父类的initialize就调用多次

#import "TCStudent.h"

@implementation TCStudent
//+ (void)initialize{
// NSLog(@"TCStudent +initialize");
//}
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
[TCPerson alloc];
[TCStudent alloc];
}
return 0;
}


输出结果:

2020-12-04 15:37:09.055459+0800 TCCateogry[1822:98237] TCPerson (TCtest2) +initialize
2020-12-04 15:37:09.055775+0800 TCCateogry[1822:98237] TCPerson (TCtest2) +initialize
Program ended with exit code: 0


如果子类(子类的分类实现initialize)不实现initialize,则子类的initialize不会调用,调用子类分类的initialize(当然多个分类的话,调用哪个的initialize取决于编译顺序)

#import "TCStudent.h"

@implementation TCStudent
+ (void)initialize{
NSLog(@"TCStudent +initialize");
}
@end
#import "TCStudent+TCStudentTest1.h"

@implementation TCStudent (TCStudentTest1)
+ (void)initialize{
NSLog(@"TCStudent (TCStudentTest1) +initialize");
}
@end#import "TCStudent+TCStudentTest2.h"

@implementation TCStudent (TCStudentTest2)
+ (void)initialize{
NSLog(@"TCStudent (TCStudentTest2) +initialize");
}
@end
输出结果:

2020-12-04 15:41:21.863260+0800 TCCateogry[1868:100750] TCPerson (TCtest2) +initialize
2020-12-04 15:41:21.863568+0800 TCCateogry[1868:100750] TCStudent (TCStudentTest2) +initialize
Program ended with exit code: 0

作者:枫紫_6174
链接:https://www.jianshu.com/p/f0150edc0f42




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