Question

我有一个NSMutableArray，我用下面的代码填充，但是当我在下面的代码中访问数组时，数组是空的？

此代码填充数组

NSMutableArray *videos;
- (void)viewDidLoad
{
    [super viewDidLoad];

    ...

    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *writableDBPath = [documentsDirectory stringByAppendingPathComponent:@"kingdb5.sqlite"];
    db = [FMDatabase databaseWithPath:writableDBPath];
    if (![db open]) {
        return;
    }
    FMResultSet *s = [db executeQuery:@"SELECT * FROM mediaVideos"];
    if ([db hadError]) {
        NSLog(@"DB Error %d: %@", [db lastErrorCode], [db lastErrorMessage]);
    }
    while ([s next]) {
        NSString *url = [s stringForColumnIndex:1];
        NSLog(url);
        [videos addObject:url];
    }


    ...
}

NSLog（url）打印出正确数量的网址（也是正确的等）

然后当用户按下按钮时访问该数组：

- (IBAction)playbutton_onclick:(id)sender {
    for (NSString *s in videos){
        NSLog(s);
    }
    NSString *file = [videos objectAtIndex:2];

    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *dataPath = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"/Videos/%@", file]];
    NSLog(@"Now playing: %@", dataPath);
    NSURL *url = [NSURL fileURLWithPath:dataPath];
    MPMoviePlayerViewController* viewController = [[MPMoviePlayerViewController alloc] initWithContentURL:url];

    [self presentMoviePlayerViewControllerAnimated:viewController];
}

此时数组为空，for循环不打印任何内容，并且在索引点获取对象返回null

Answer 1

首先使videos成为类的实例变量或属性：

@interface MyClass : UIViewController

@property (nonatomic) NSMutableArray *videos;

@end

然后在实现文件中分配并初始化它：

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.videos = [NSMutableArray new];
    ...

}

并将videos的所有引用更改为self.videos。

Answer 2

在viewDidLoad的开头，您必须分配数组：

videos = [[NSMutableArray alloc] init];

为了获得最佳实践，您应该将其作为属性并将其称为self.videos。请参阅trojanfoe添加的答案。

访问时NSMutableArray为空

2 个答案: