iOS8: What's going on with moving views during

问题:

After switching to iOS8, I'm getting weird behavior when I move views during a keyboard transition. Can anyone explain what's going on?

Here's a minimal example to demonstrate the problem. I have a simple view with a UITextField and a UIButton. The function nudgeUp moves the text field and the button up by 10 points. It is triggered either by the buttonPressed callback, or the keyboardWillShow callback.

When I tap the button, the code works as expected: buttonPressed calls nudgeUp and the button and text field jump up by 10 points.

When I tap the text field, keyboardWillShow calls nudgeUp, but the behaviour is very different. The button and text field immediately jump down by 10 points, and then slide back up to their original position as the keyboard shows itself.

Why is this happening? How can I regain control of animations during keyboard presentation in iOS8?

#import "ViewController.h"

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillShow:)
                                                 name:UIKeyboardWillShowNotification
                                               object:nil];

}

- (void)keyboardWillShow:(NSNotification *)notification
{
    // Called when the keyboard appears.
    [self nudgeUp];
}

- (IBAction)buttonPressed:(id)sender {
    [self nudgeUp];
}

- (void)nudgeUp
{
    CGRect newTextFieldFrame = self.textField.frame;
    newTextFieldFrame.origin.y -= 10;
    self.textField.frame = newTextFieldFrame;

    CGRect newButtonFrame = self.button.frame;
    newButtonFrame.origin.y -= 10;
    self.button.frame = newButtonFrame;
}
@end

回答1:

It's AutoLayout. Something changed in iOS8 and you can't just change frame or center points anymore if you have AutoLayout enabled. You have to create an outlet(s) of your constraint (vertical space) and update it accordingly instead of changing frame position. Constraints are like any other ui control and can have an outlet. Constraint change can be animated.

Example:

[UIView animateWithDuration:[notification.userInfo[UIKeyboardAnimationDurationUserInfoKey] doubleValue] delay:0 options:[[[notification userInfo] objectForKey:UIKeyboardAnimationCurveUserInfoKey] integerValue] animations:^{        
    self.bottomSpaceConstraint.constant = adjustmentedValue;
    [self.view layoutIfNeeded];        
} completion:^(BOOL finished) {
}];

回答2:

You should use UIKeyboardDidShowNotification (you're using will version) and everything will work as you expect:

- (void)viewDidLoad {
    [super viewDidLoad];
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardDidShow:)
                                                 name:UIKeyboardDidShowNotification
                                               object:nil];

}

- (void)keyboardDidShow:(NSNotification *)notification
{
    // Called when the keyboard finished showing up
    [self nudgeUp];
}

The explanation is that with UIKeyboardWillShowNotification you are changing the frames too early. After your changes the system will relayout everything to accomodate the keyboard and your changes won't have any effect.

Also, I recommend you to switch to autolayout and forget about frames.

回答3:

Try using the UIKeyboardWillShowNotification userInfo to give you the frame of the keyboard. Then move the onscreen elements based on that.