iOS底层学习-day-10
前言-OC语法篇
我是一名iOS开发者, iOS底层 菜鸟的进阶之路30天。
问题
block的原理是怎样的?本质是什么?
- 封装了函数调用以及调用环境的OC对象
block的引用和移除
- 当block被copy到堆时会调用block内部的copy函数copy函数内部会调用_Block_object_assign函数,_Block_object_assign函数会对__block变量形成强引用(retain)
- 当block从堆中移除时会调用block内部的dispose函数dispose函数内部会调用_Block_object_dispose函数,_Block_object_dispose函数会自动释放引用的__block变量(release)
int no = 20;
__block int age = 10;
NSObject *object = [[NSObject alloc] init];
__weak NSObject *weakObject = object;
MJBlock block = ^{
age = 20;
NSLog(@"%d", no);
NSLog(@"%d", age);
NSLog(@"%p", weakObject);
};
- 调用的本质
- __block int age = 10
- //都会调用__main_block_copy_0 操作,
- __block int age = 10;如果是传入8,就会直接产生一个强引用,就做一次retain(strong,weak的指针操作)
- {_Block_object_assign((void*)&dst->age, (void*)src->age, 8/BLOCK_FIELD_IS_BYREF/);
//简化后
{_Block_object_dispose((void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);
- __weak NSObject *weakObject = object
- __weak NSObject *weakObject = object,传入一个3值,会对对象进行_Block_object_assign的函数调用,如果是__weak就是弱引用,__strong就是强引用,就会对进行,如果直接访问object就是__strong的强引用
- _Block_object_assign((void*)&dst->weakObject, (void*)src->weakObject, 3/BLOCK_FIELD_IS_OBJECT/);}
//简化后
_Block_object_dispose((void*)src->weakObject, 3/*BLOCK_FIELD_IS_OBJECT*/);}
- 总结
- 当block从堆上移除时,都会通过dispose函数来释放它们
- __block变量(假设变量名叫做a)
_Block_object_dispose((void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);
- 对象类型的auto变量(假设变量名叫做p)
_Block_object_dispose((void*)src->p, 3/*BLOCK_FIELD_IS_OBJECT*/);
- ps:当block在栈上时,对它们都不会产生强引用
__forwarding指向block自身的指针
循环引用的解决
ARC
- __weak:不会产生强引用,指向的对象销毁时,会自动让指针置为nil
- __unsafe_unretained:不会产生强引用,不安全,指向的对象销毁时,指针存储的地址值不变
//1.
__weak typeof(person) weakPerson = person;
person.block = ^{
NSLog(@"age is %d", weakPerson.age);
};
//2.
//__block解决循环引用,不好的方案
__block Person *person = [[Person alloc] init];
person.age = 10;
person.block = ^{
NSLog(@"age is %d", person.age);
person = nil;//这两个都可以写
};
person.block();//
MRC
//3.
// MRC不支持__weak的
__block Person *person = [[Person alloc] init];
person.age = 10;
person.block = [^{
NSLog(@"age is %d", person.age);
} copy];
__weak 和 __strong
__weak typeof(self) weakSelf = self;
self.block = ^{
//只是再保证了一次是强指针,还有保证编译器不报错
__strong typeof(weakSelf) strongself = weakSelf;
NSLog(@"age is %d", strongself->_age);
};