ios - 为什么 becomeFirstResponder 不起作用？

Question

我使用 AGWindowView 添加了一个模态框.在模态视图(使用 IB 构建)中，有一个文本字段。文本字段已连接到 socket 。

这行不通:

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    [self.placesTextField becomeFirstResponder];
}

对 becomeFirstResponder 的调用不起作用，键盘也没有显示。

这个有效:

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    [self.placesTextField performSelector:@selector(becomeFirstResponder) withObject:nil afterDelay:0];
}

但是，如果我使用 performSelector:withObject:afterDelay 手动发送消息，它会起作用。为什么直到运行时才确定此方法？

Answer 1

似乎在 iOS7 中， View /对象还没有附加到 View 层次结构/窗口中。因此通过对象调用方法失败。如果我们进行一些延迟并且它正在工作，则意味着此时对象已附加到窗口。

根据苹果公司的说法，

A responder object only becomes the first responder if the current responder can resign first-responder status (canResignFirstResponder) and the new responder can become first responder.

You may call this method to make a responder object such as a view the first responder. However, you should only call it on that view if it is part of a view hierarchy. If the view’s window property holds a UIWindow object, it has been installed in a view hierarchy; if it returns nil, the view is detached from any hierarchy.

有关详细信息，请参阅 UIResponder Class Reference .