iOS代码面试题

id __weak obj = nil;
{
    id __strong obj1 = [[NSObject alloc] init];
    obj = obj1;
 }
NSLog(@"%@",obj);

• 输出 nil， __weak 修饰符的变量不持有对象，在超出变量作用域时，对象就会被释放并会被置为 nil。

//不使用第三个变量，交换两个变量的值
int a = 6;
int b = 8;

a = a + b;   
b = a - b;   
a = a - b;  

#import <Foundation/Foundation.h>

@interface Person : NSObject

@end

@interface Student : Person

@end

#import "Person.h"

@implementation Person

@end

@implementation Student

- (instancetype)init {
    self = [super init];
    if (self) {
        NSLog(@"self.class = %@", self.class);
        NSLog(@"super.class = %@", super.class);
        NSLog(@"self.superclass = %@", self.superclass);
        NSLog(@"super.superclass = %@", super.superclass);
    }
    return self;
}

@end

self.class = Student
super.class = Student
self.superclass = Person
super.superclass = Person

• objc_msgSend 函数
• super 本质是调用 objc_msgSendSuper2 函数

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"1");
        [self performSelector:@selector(text) withObject:nil afterDelay:0];
        NSLog(@"3");
    });
}

- (void)test {
    NSLog(@"2");
}

1  
3

• - (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay; 本质是向 runloop 中添加定时器
• 子线程默认未开启 runloop

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    NSLog(@"1");
    [self performSelectorOnMainThread:@selector(test) withObject:nil waitUntilDone:YES];
    NSLog(@"3");
}

- (void)test {
    NSLog(@"2");
}

//waitUntilDone:YES
1 
2
3

//waitUntilDone:NO
1 
3 
2

• waitUntilDone 表示是否等待当前 selector 任务完成后再执行后续任务

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    NSThread *thread = [[NSThread alloc] initWithBlock:^{
        NSLog(@"1");
    }];
    [thread start];
    [self performSelector:@selector(text) onThread:thread withObject:nil waitUntilDone:YES];
}

- (void)test {
    NSLog(@"2");
}

1
*** Terminating app due to uncaught exception 'NSDestinationInvalidException', reason: '*** -[ViewController performSelector:onThread:withObject:waitUntilDone:modes:]: target thread exited while waiting for the perform'

• [thread start]; 执行 NSLog(@"1"); 后 thread 就销毁了
• waitUntilDone 为 YES 时候需注意所在线程生命周期是否正常

for (NSInteger i = 0; i < 100; i++) {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        self.name = [NSString stringWithFormat:@"%@", @"qwertyuiopasdfghjkl"];
    });
}

//多个线程同时调用[_name release];
//解决办法加锁。
EXC_BAD_ACCESS

• self.name 本质

- (void)setName:(NSString *)name {
    if (_name != name) {
        [_name release];
        _name = [name copy];
    }
}

for (NSInteger i = 0; i < 100; i++) {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        self.name = [NSString stringWithFormat:@"%@", @"qwe"];
    });
}

• Tagged Pointer 数据存储在指针中

NSInteger a = 10;
static NSInteger b = 100;
void (^block)(void) = ^ {
    NSLog(@"a = %ld", a);
    NSLog(@"b = %ld", b);
};
a = 20;
b = 200;
block();

a = 10
b = 200

• 局部 auto 变量，block 值捕获
• 局部 static 变量，block 指针捕获
• 全局变量，block 不捕获，直接使用