项目中遇到了一个需求：
要求根据某一个UITextField.text的值的改变随时的计算另一个UITextField.text的值。

我们都知道监听UITextField的值改变的几种方法：
1.通过UITextFieldDelegate监听textField的值的改变；
2.通过NSNotificationCenter监听；
3.通过KVO监听；
4.通过addarget:直接监听；
5.通过三方库ReactiveCocoa添加监听；

目前功能实现使用的监听方式是第四种直接监听，以下是代码实现：

#import "HHTableViewController.h"

@interface HHTableViewController ()
@property (weak, nonatomic) IBOutlet UILabel *totalLab;
@property (weak, nonatomic) IBOutlet UITextField *firstTF;
@property (weak, nonatomic) IBOutlet UITextField *secTF;
@property (weak, nonatomic) IBOutlet UILabel *msgLab;

@end

@implementation HHTableViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.firstTF addTarget:self action:@selector(textFieldWillChange:) forControlEvents:UIControlEventEditingChanged];
    [self.secTF addTarget:self action:@selector(textFieldWillChange:) forControlEvents:UIControlEventEditingChanged];
}

- (void)textFieldWillChange:(UITextField *)textField {
    if ([textField.text hasPrefix:@"."]) {
        textField.text = [@"0" stringByAppendingString:textField.text];
    }
    
    if ([self validateMoney:textField.text]) {
        
        if (textField == self.firstTF) {
            if ([textField.text doubleValue] > [self.totalLab.text doubleValue]) {
                self.msgLab.text = @"error,首付金额大于总价格!";
                return;
            }
            self.secTF.text = [self decimalCalculateWithTotalValue:self.totalLab.text type:0 textValue:textField.text];
        }
        else {
            if ([textField.text doubleValue] > 1.00) {
                self.msgLab.text = @"首付所占利率比例最大为1";
                return;
            }
            self.firstTF.text = [self decimalCalculateWithTotalValue:self.totalLab.text type:1 textValue:textField.text];
            
        }
        self.msgLab.text = @"成功!";
    }
    else {
        
        if (textField == self.firstTF) {
            self.secTF.text = @"";
            self.msgLab.text = @"首付金额输入值有误！";
        }
        else {
            self.firstTF.text = @"";
            self.msgLab.text = @"换算比率输入值有误！";
        }
    }
}

-(BOOL)validateMoney:(NSString *)money
{
    NSString *phoneRegex = @"^[0-9]+(\\.[0-9]{1,2})?$";
    NSPredicate *phoneTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@",phoneRegex];
    return [phoneTest evaluateWithObject:money];
}

/**
 * 根据总价计算首付及首付比率
 * textValue: 目前处于编辑状态的textField的text值
 * type: = 0 除
 *       = 1 乘
 * totalValue: 总的价格
 * return: 返回计算结果(String类型)
 */
- (NSString *)decimalCalculateWithTotalValue:(NSString *)totalValue type:(short)type textValue:(NSString *)textValue {
    NSDecimalNumber *decimal = [NSDecimalNumber decimalNumberWithString:textValue];
    
    NSDecimalNumberHandler*roundUp = [NSDecimalNumberHandler  decimalNumberHandlerWithRoundingMode:NSRoundBankers scale:9 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:YES];
    
    switch (type) {
        case 0: // 除
        {
            double divideValue = [[decimal decimalNumberByDividingBy:[NSDecimalNumber decimalNumberWithString:totalValue] withBehavior:roundUp] doubleValue];
            return [NSString stringWithFormat:@"%.2f", divideValue];
        }
            break;
        case 1: // 乘
        {
            double multiplyValue = [[decimal decimalNumberByMultiplyingBy:[NSDecimalNumber decimalNumberWithString:totalValue] withBehavior:roundUp] doubleValue];
            return [NSString stringWithFormat:@"%.2f", multiplyValue];
        }
            break;
        default:
            break;
    }
    return nil;
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

运行结果示意图：

运行结果图