布局圖片:

// ViewController.h
#import <UIKit/UIKit.h>

@interface ViewController : UIViewController


@end

// ViewController.m

#import "ViewController.h"

@interface ViewController ()
/** 文本框底部約束*/
@property (weak, nonatomic) IBOutlet NSLayoutConstraint *bottomSpace;
/** 文本框*/
@property (weak, nonatomic) IBOutlet UITextField *textField;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillChange:) name:UIKeyboardWillChangeFrameNotification object:nil];
}

/**
 *  監(jiān)聽鍵盤的frame即將發(fā)生改變的時候調(diào)用
 */
- (void)keyboardWillChange:(NSNotification *)note
{
    // 獲得鍵盤的frame
    CGRect frame = [note.userInfo[UIKeyboardFrameEndUserInfoKey] CGRectValue];

    // 修改底部約束
    self.bottomSpace.constant = self.view.frame.size.height - frame.origin.y;

    //  執(zhí)行動畫
    CGFloat duration = [note.userInfo[UIKeyboardAnimationDurationUserInfoKey] doubleValue];
    // 約束動畫，利用duration時間去更新約束
    [UIView animateWithDuration:duration animations:^{
        // 強(qiáng)制布局
        [self.view layoutIfNeeded];
    }];
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self.view endEditing:YES];
}

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}
@end

程序一啟動效果圖片: