iphone - (id)sender = nil?

Question

我有以下代码用当前时间更新文本字段的值。我的问题是:为什么在代码的底部发送 nil 作为发件人？ [self showCurrentTime:nil];
CurrentTimeViewController.m

- (IBAction)showCurrentTime:(id)sender
{
NSDate *now = [NSDate date];

static NSDateFormatter *formatter = nil;

if(!formatter) {
    formatter = [[NSDateFormatter alloc] init];
    [formatter setTimeStyle:NSDateFormatterShortStyle];
}

[timeLabel setText:[formatter stringFromDate:now]];

}

...

- (void)viewWillAppear:(BOOL)animated
{
    NSLog(@"CurrentTimeViewController will appear");
    [super viewWillAppear:animated];
    [self showCurrentTime:nil];
}

Answer 1

因为通常在调用 Action 处理程序时，它们会将发起调用的对象传递给方法，例如 UIButton , 或 UISegmentedControl .但是，当您想从代码中调用方法时，而不是作为操作的结果，您不能将人类作为 sender 传递。 , 所以你通过 nil .

此外，- (IBAction)表示此方法可以通过 Interface Builder 连接到事件通过拖动 Touch Up Inside (或触碰外部/等)事件从按钮(或任何其他具有某种事件的控件)到 File's Owner并选择 thumbTapped: .

例如:

UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
button.tag = 1001;
[button addTarget:self action:@selector(thumbTapped:) forControlEvents:UIControlEventTouchUpInside];

当触摸被释放(并且触摸仍在按钮内部)时，将调用 thumbTapped: ，通过 button对象为 sender

- (IBAction)thumbTapped:(id)sender {
    if ([sender isKindOfClass:[UIButton class]] && ((UIButton *)sender).tag == 1001) {
        iPhoneImagePreviewViewController *previewVC = [[iPhoneImagePreviewViewController alloc] initWithNibName:@"iPhoneImagePreviewViewController" bundle:nil];
        [self.navigationController pushViewController:previewVC animated:YES];
        [previewVC release];
    } else {
        [[[[UIAlertView alloc] initWithTitle:[[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleDisplayName"] 
                                     message:@"This method was called from somewhere in user code, not as the result of an action!" 
                                    delegate:self 
                           cancelButtonTitle:@"OK" 
                           otherButtonTitles:nil] autorelease] show];
    }
}