指定根视图

    self.window.rootViewController = [RootViewController new];

方法实现:

#import "RootViewController.h"
#define kScreenHeight [UIScreen mainScreen].bounds.size.height
#define kScreenWidth [UIScreen mainScreen].bounds.size.width
@interface RootViewController ()
@property (nonatomic, strong) UITextField *textField;
@end
@implementation RootViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.view.backgroundColor = [UIColor greenColor];

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(changeFrame:) name:UIKeyboardWillShowNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(hiddenFrame2:) name:UIKeyboardWillHideNotification object:nil];

    self.textField = [[UITextField alloc] initWithFrame:CGRectMake(50, kScreenHeight - 100, kScreenWidth - 100, 35)];
    self.textField.borderStyle = UITextBorderStyleRoundedRect;
    [self.view addSubview:self.textField];
}

- (void)changeFrame:(NSNotification *)sender
{
    CGRect frame = self.textField.frame;
    frame.origin.y = 100;
    [UIView animateWithDuration:2 animations:^{
        self.textField.frame = frame;
    }];
}

- (void)hiddenFrame2:(NSNotification *)sender
{
    [UIView animateWithDuration:2 animations:^{
        CGRect frame = self.textField.frame;
        frame.origin.y = kScreenHeight - 100;

        self.textField.frame = frame;
    }];
}

释放:

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillHideNotification object:nil];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillShowNotification object:nil];
}

最终效果:

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