关于 performSelector 的一些小探讨

考虑以下代码,最终会输出什么?

例子①:

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- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 - %@", [NSThread currentThread]);
[self performSelector:@selector(test)
withObject:nil];
NSLog(@"4 - %@", [NSThread currentThread]);
});
}
- (void)test
{
NSLog(@"3 - %@", [NSThread currentThread]);
}
  • 输出结果:1,2,3,4
  • 原因:因为 performSelector:withObject: 会在当前线程立即执行指定的 selector 方法。

例子②:

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- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 - %@", [NSThread currentThread]);
[self performSelector:@selector(test)
withObject:nil
afterDelay:0];
NSLog(@"4 - %@", [NSThread currentThread]);
});
}
- (void)test
{
NSLog(@"3 - %@", [NSThread currentThread]);
}
  • 输出结果:1,2,4
  • 原因:因为 performSelector:withObject:afterDelay: 实际是往 RunLoop 里面注册一个定时器,而在子线程中,RunLoop 是没有开启(默认)的,所有不会输出 3。官网 API 作如下解释:
    PerformSelectorAfterDelay.png

例子③:

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- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 - %@", [NSThread currentThread]);
[self performSelector:@selector(test)
withObject:nil
afterDelay:0];
[[NSRunLoop currentRunLoop] run];
NSLog(@"4 - %@", [NSThread currentThread]);
});
}
- (void)test
{
NSLog(@"3 - %@", [NSThread currentThread]);
}
  • 输出结果:1,2,3,4
  • 原因:由于 [[NSRunLoop currentRunLoop] run]; 会创建的当前子线程对应的 RunLoop 对象并启动了,因此可以执行 test 方法;并且 test 执行完后,RunLoop 中注册的定时器已经无效,所以还可以输出 4 (对比 例子⑥例子)。

例子④:

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- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 - %@", [NSThread currentThread]);
[self performSelector:@selector(test)
onThread:[NSThread currentThread]
withObject:nil
waitUntilDone:YES];
NSLog(@"4 - %@", [NSThread currentThread]);
});
}
- (void)test
{
NSLog(@"3 - %@", [NSThread currentThread]);
}
  • 输出结果:1,2,3,4
  • 原因:因为 performSelector:onThread:withObject:waitUntilDone: 会在指定的线程执行,而执行的策略根据参数 wait 处理,这里传 YES 表明将会立即阻断 指定的线程 并执行指定的 selector。官网 API 解释如下:

    PerformSelectorOnThreadWaitYes.png

例子⑤:

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- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 - %@", [NSThread currentThread]);
[self performSelector:@selector(test)
onThread:[NSThread currentThread]
withObject:nil
waitUntilDone:NO];
NSLog(@"4 - %@", [NSThread currentThread]);
});
}
- (void)test
{
NSLog(@"3 - %@", [NSThread currentThread]);
}
  • 输出结果:1,2,4
  • 原因:因为 performSelector:onThread:withObject:waitUntilDone: 会在指定的线程执行,而执行的策略根据参数 wait 处理,这里传 NO 表明不会立即阻断 指定的线程 而是将 selector 添加到指定线程的 RunLoop 中等待时机执行。(该例子中,子线程 RunLoop 没有启动,所有没有输出 3)官网 API 解释如下:

    PerformSelectorOnThreadWaitNo.png

例子⑥:

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- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 - %@", [NSThread currentThread]);
[self performSelector:@selector(test)
onThread:[NSThread currentThread]
withObject:nil
waitUntilDone:NO];
[[NSRunLoop currentRunLoop] run];
NSLog(@"4 - %@", [NSThread currentThread]);
});
}
- (void)test
{
NSLog(@"3 - %@", [NSThread currentThread]);
}
  • 输出结果:1,2,3
  • 原因:由于 [[NSRunLoop currentRunLoop] run]; 已经创建的当前子线程对应的 RunLoop 对象并启动了,因此可以执行 test 方法;但是 test 方法执行完后,RunLoop 并没有结束(使用这种启动方式,RunLoop 会一直运行下去,在此期间会处理来自输入源的数据,并且会在 NSDefaultRunLoopMode 模式下重复调用 runMode:beforeDate: 方法)所以无法继续输出 4

例子⑦

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- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 - %@", [NSThread currentThread]);
[self performSelector:@selector(test)
onThread:[NSThread currentThread]
withObject:nil
waitUntilDone:NO];
[[NSRunLoop currentRunLoop] runUntilDate:[NSDate distantFuture]];
NSLog(@"4 - %@", [NSThread currentThread]);
});
}
- (void)test
{
NSLog(@"3 - %@", [NSThread currentThread]);
}
  • 输出结果:1,2,3
  • 原因:由于 [[NSRunLoop currentRunLoop] runUntilDate:[NSDate distantFuture]]; 已经创建的当前子线程对应的 RunLoop 对象并启动了,因此可以执行 test 方法;但是 test 方法执行完后,RunLoop 并没有结束(使用这种启动方式,可以设置超时时间,在超时时间到达之前,runloop会一直运行,在此期间runloop会处理来自输入源的数据,并且会在 NSDefaultRunLoopMode 模式下重复调用 runMode:beforeDate: 方法)所以无法继续输出 4

例子⑧:

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- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2 - %@", [NSThread currentThread]);
[self performSelector:@selector(test)
onThread:[NSThread currentThread]
withObject:nil
waitUntilDone:NO];
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode
beforeDate:[NSDate distantFuture]];
NSLog(@"4 - %@", [NSThread currentThread]);
});
}
- (void)test
{
NSLog(@"3 - %@", [NSThread currentThread]);
}
  • 输出结果:1,2,3,4
  • 原因:由于 [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]; 已经创建的当前子线程对应的 RunLoop 对象并启动了,因此可以执行 test 方法;而且 test 方法执行完后,RunLoop 立刻结束(使用这种启动方式 ,RunLoop 会运行一次,超时时间到达或者第一个 input source 被处理,则 RunLoop 就会退出)所以可以继续输出 4

小结:

  1. 常用 performSelector 方法

    • 常用的 perform,是 NSObject.h 头文件下的方法:

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      - (id)performSelector:(SEL)aSelector;
      - (id)performSelector:(SEL)aSelector withObject:(id)object;
      - (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;
    • 可以 delayperform,是 NSRunLoop.h 头文件下的方法:

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      - (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray<NSRunLoopMode> *)modes;
      - (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay;
    • 可以 指定线程perform,是 NSThread 头文件下的方法:

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      - (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray<NSString *> *)array;
      - (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait;
      - (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray<NSString *> *)array;
      - (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait;
      - (void)performSelectorInBackground:(SEL)aSelector withObject:(nullable id)arg;
  2. RunLoop 退出方式:

    • 使用 - (void)run; 启动,RunLoop 会一直运行下去,在此期间会处理来自输入源的数据,并且会在 NSDefaultRunLoopMode 模式下重复调用 runMode:beforeDate: 方法;
    • 使用 - (void)runUntilDate:(NSDate *)limitDate;启动,可以设置超时时间,在超时时间到达之前,RunLoop 会一直运行,在此期间 RunLoop 会处理来自输入源的数据,并且也会在 NSDefaultRunLoopMode 模式下重复调用 runMode:beforeDate: 方法;
    • 使用 - (void)runMode:(NSString *)mode beforeDate:(NSDate *)limitDate;启动,RunLoop 会运行一次,超时时间到达或者第一个 input source 被处理,则 RunLoop 就会退出。

更多关于 NSRunLoop的退出方式 可以看这篇博文

参考

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