CS193P作业1（Matchismo）始终有一个主题问题

Question

我是iOS开发的新手，并开始使用CS193P（2013年秋季）学习它。当我试图完成作业1时，我遇到了一个奇怪的Thread问题。

发生在PlayingCard.m上，当我在刺激器中点击卡背面时出现了这个问题。

有人可以告诉我出了什么问题，或者我该如何解决这个Thread问题。 the Thread issue

it appear on setRank: method(which exactly the same as the slide)

#import "PlayingCard.h"

@implementation PlayingCard
- (NSString *)contents
{
    NSArray *rankStrings = [PlayingCard rankStrings];
    return [rankStrings[self.rank] stringByAppendingString:self.suit];
}

@synthesize suit= _suit;

+ (NSArray *)validSuits
{
    return @[@"♥",@"♦",@"♠",@"♣"];
}

- (void)setSuit:(NSString *)suit
{
    if ([[PlayingCard validSuits] containsObject:suit]) {
        _suit = suit;
    }
}

- (NSString *)suit
{
    return _suit ? _suit : @"?";
}

+ (NSArray *)rankStrings
{
    return @[@"?",@"A",@"2",@"3",@"4",@"5",@"6",@"7",@"8",@"9",@"10",@"J",@"Q",@"K"];
}

+ (NSUInteger)maxRank { return [[self rankStrings] count]-1; }

- (void)setRank:(NSUInteger)rank
{
    if (rank <= [PlayingCard maxRank]) {
        _rank = rank;
    }
}

@end`

我为作业编写的viewController.m。

`#import "ViewController.h"
#import "PlayingCardDeck.h"
#import "PlayingCard.h"


@interface ViewController ()
@property (weak, nonatomic) IBOutlet UILabel *flipsLabel;
@property(nonatomic) int flipCount;
@property (strong, nonatomic) Deck *deck;


@end

@implementation ViewController

-(Deck *)deck{
    if(!_deck) _deck = [[PlayingCardDeck alloc] init];
    return _deck;
}
- (void)setFlipCount:(int)flipCount
{
    _flipCount = flipCount;
    self.flipsLabel.text = [NSString stringWithFormat:@"Flips: %d", self.flipCount];
    NSLog(@"flipCount changed to %d", self.flipCount);
}
- (IBAction)touchCardButton:(UIButton *)sender {

    if ([sender.currentTitle length]) {

        [sender setBackgroundImage:[UIImage imageNamed:@"cardback"]
                          forState:UIControlStateNormal];
        [sender setTitle:@"" forState:UIControlStateNormal];

    }else{
        NSString *cardContent = [[self.deck drawRandomCard] contents];
        if (cardContent) {
            [sender setBackgroundImage:[UIImage imageNamed:@"cardfront"]
                              forState:UIControlStateNormal];

            [sender setTitle:@"" forState:UIControlStateNormal];

        }

    }
    self.flipCount++;
}



@end`