关于Objective-c 对象 getter 初始化

昨天写了一个计算器 出现一个bug，导致对象为null并且调试也无法找出错误
最后终于发现原来是getter的函数名写错了
所以导致初始化失败。

//
//  CalculatorViewController.m
//  Calculator
//
//  Created by linqianqiu on 12-7-19.
//  Copyright (c) 2012年 __MyCompanyName__. All rights reserved.
//

#import "CalculatorViewController.h"
#import "CalculatorBrain.h"

@interface CalculatorViewController()
@property (nonatomic) BOOL userIsInTheMiddleOfEnteringANumber;
@property (nonatomic, strong) CalculatorBrain *brain;
@end

@implementation CalculatorViewController

@synthesize display = _display;
@synthesize userIsInTheMiddleOfEnteringANumber = _userIsInTheMiddleOfEnteringANumber;
@synthesize brain = _brain;

- (CalculatorBrain *)barin {
    if (!_brain) {
        _brain = [[CalculatorBrain alloc] init];   
    }
    return _brain;
}

- (IBAction)digitPressed:(UIButton *)sender {
    NSString* digit = sender.currentTitle;
    if (self.userIsInTheMiddleOfEnteringANumber) {
        self.display.text = [self.display.text stringByAppendingFormat:digit];
    } else {
        self.display.text = digit;
        self.userIsInTheMiddleOfEnteringANumber = YES;
    }
}

- (IBAction)operationPressed:(UIButton *)sender {
    if (self.userIsInTheMiddleOfEnteringANumber) [self enterPressed];
    double result = [self.brain performOperation:sender.currentTitle];
    NSString* resultString = [NSString stringWithFormat:@"%g", result];
    self.display.text = resultString;
}
- (IBAction)enterPressed {
    [self.brain pushOperand:[self.display.text doubleValue]];
    self.userIsInTheMiddleOfEnteringANumber = NO;
}

- (IBAction)clearPressed {
    self.display.text = @"0";
}

@end