常见问题解惑
1.问题一
1.1以下代码打印结果
- (void)viewDidLoad {
[super viewDidLoad];
dispatch_queue_t queue = dispatch_queue_create("myqueue", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
NSLog(@"task 1");
[self performSelector:@selector(task2) withObject:nil afterDelay:.0];
NSLog(@"task 3");
});
/*
执行结果：
线程[51489:6875868] task 1
线程[51489:6875868] task 3
原因：
- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay;
1.上述方法在子线程中 运行本质本质是往Runloop中添加NSTimer
2.子线程默认没有启动Runloop
*/
}
- (void)task2 {
NSLog(@"task 2");
}
1.2.如何修复
1.
- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay;
此方法本质是：往Runloop里面添加NStimer
启动Runloop
2. 顺便记一下
- (id)performSelector:(SEL)aSelector withObject:(id)object;
此方法本质是：objc_msgsend(self,@SEL);
dispatch_queue_t queue = dispatch_queue_create("myqueue", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
NSLog(@"task 1");
[self performSelector:@selector(task2) withObject:nil afterDelay:.0];
NSLog(@"task 3");
//[[NSRunLoop currentRunLoop] addPort:[[NSPort alloc]init] forMode:NSDefaultRunLoopMode]; 这行代码可删除上行代码已经往Runloop中添加了定时器
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
});
/*
执行结果：
线程[51489:6875868] task 1
线程[51489:6875868] task 3
线程[51489:6875868] task 2
*/
问题二
2.1一下代码运行结果
- (void)viewDidLoad {
[super viewDidLoad];
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
NSThread *thread = [[NSThread alloc] initWithBlock:^{
NSLog(@"task1");
}];
[thread start];
[self performSelector:@selector(task2) onThread:thread withObject:nil waitUntilDone:YES];
}
- (void)task2 {
NSLog(@"task2");
}
/*
1.崩溃信息如下：
*** Terminating app due to uncaught exception 'NSDestinationInvalidException', reason: '*** -[ViewController performSelector:onThread:withObject:waitUntilDone:modes:]: target thread exited while waiting for the perform'
2. 原因：
thread调用start 就会再子线程中执行block中的代码 执行完毕后线程就退出
没有办法在当前thread线程中执行task2方法
*/
2.2如何修复
- (void)viewDidLoad {
[super viewDidLoad];
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
NSThread *thread = [[NSThread alloc] initWithBlock:^{
NSLog(@"task1");
[[NSRunLoop currentRunLoop]addPort:[[NSPort alloc]init] forMode:NSDefaultRunLoopMode];
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
}];
[thread start];
[self performSelector:@selector(task2) onThread:thread withObject:nil waitUntilDone:YES];
}
- (void)task2 {
NSLog(@"task2");
}
/*
打印结果
线程[58654:7033043] task1
线程[58654:7033043] task2
*/
@end
