becomeFirstResponder works in viewDidAppear but do

问题:

My Application has a modal view controller, including a search bar. When the view comes up, I want the search bar to be focused. I tried [self.searchBar becomeFirstResponder] in viewDidLoad, but it didn't work. Later on I put it in viewDidAppear, it worked. But with this workaround, there is a visible delay. (after the view fully appeared, the keyboard began to appear)

I can ensure both viewDidAppear and viewDidLoad have been invoked.

What should I do if I want the search bar to be focused instantly with the view appear?

(I'm using StoryBoard)

Followed the answers, I tried to put the code in viewWillLoad, but still didn't work. (in viewWillLoad, self.searchBar.window is nil)

回答1:

You should call in viewDidLayoutSubviews(), the code below set textField becomeFirstResponder only at the first time view layout subviews, and it should be.

var isFirstLayout: Bool = true
override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    if isFirstLayout {
        defer { isFirstLayout = false }
        textField.becomeFirstResponder()
    }
}

回答2:

Possibly it does not work in viewDidLoad, as view does not added into view hierarchy yet. But according to apple documentation becomeFirstResponder should be called only on objects attached to UIWindow:

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.

So, i assume, the best place to achieve necessary behavior is to place call into viewWillAppear method.

Update.

So, in viewWillAppear controller's view not yet attached to UIWindow... it only notify, that view will be added to view hierarchy

It may be some tricky, but you can make some small delay in viewWillAppear:

 - (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    double delayInSeconds = 0.05;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^{ 
        make first responder here
    });
 }

But I believe there is should be a better solution

回答3:

all the IBOutlet objects are loaded in viewDidLoad,if you are calling the method in viewDidLoad then that action not performed because before the objects are loaded we can't do anything that's whybetter to write that code in

-(void)viewWillAppear:(BOOL)animated{
//write here
} 

then it works fine.

回答4:

This will help:

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    dispatch_async(dispatch_get_main_queue(), ^{
        [self.quantifyTextField becomeFirstResponder];
    });
}

回答5:

Making text field/view first responder should be done after all UIViewController animations, which take place when view is loaded and presented. So the best place is viewDidAppear.

回答6:

Write viewWillAppear instad of viewDidAppear/viewDidLoad.

BecauseviewWillAppear method is call at the time of View will appear (in process), for more information about viewWillAppear read this official Document.

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

回答7:

I know it is a bit old thread, but I think it can help someone that is facing some keyboard issue when adding this code.

Remember to set the textfield delegate to nil in viewWillDisappear, otherwise the keyboard will not be shown again if you pop/dismiss the view controller without closing the keyboard.