Question

我不明白为什么我的NSMutuableArray为空？我错过了什么吗？我在解析中存储了一个字符串。感谢您的帮助。

.h file

@interface IncomeTransactionViewController : UIViewController  <UIPickerViewDataSource, UIPickerViewDelegate>
{
    NSDate *myDate;
    NSMutableArray *accountArray;
    UIPickerView *accountPicker;
}
@property (weak, nonatomic) IBOutlet UITextField *dateField;
@property (weak, nonatomic) IBOutlet UITextField *accountField;
@property (weak, nonatomic) IBOutlet UITextField *categoryField;

- (IBAction)hidekeyboard:(id)sender;

- (IBAction)addTransaction:(id)sender;
@end

.m file

PFQuery *query = [PFQuery queryWithClassName:@"Account"];
    [query whereKey:@"user" equalTo:[PFUser currentUser]];
    [query orderByDescending:@"startingBalance"];
    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        accountArray = [[NSMutableArray alloc]init];
        for (PFObject *object in objects) {
            [accountArray addObject:object[@"name"]];
            NSLog(@"%@", object[@"name"]);

        }

    }
     ];

Answer 1

如果在启动查询后立即检查accountsArray，则它将为nil，因为查询是异步的，并且在完成之前不会调用其回调。

如果这是您的代码：

PFQuery *query = [PFQuery queryWithClassName:@"Account"];
[query whereKey:@"user" equalTo:[PFUser currentUser]];
[query orderByDescending:@"startingBalance"];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    accountArray = [[NSMutableArray alloc]init];
    for (PFObject *object in objects) {
        [accountArray addObject:object[@"name"]];
        NSLog(@"%@", object[@"name"]);

    }

}];
int ct = [accountArray count];

然后ct将为0并且accountArray将为nil，但是当调用回调时，将分配并填充accountArray。之前提到的正确答案，我将在这里用更好的解释进行迭代，是在viewDidLoad中分配数组。或者，您可以在查询调用之前分配它：

accountArray = [[NSMutableArray alloc]init];
PFQuery *query = [PFQuery queryWithClassName:@"Account"];
[query whereKey:@"user" equalTo:[PFUser currentUser]];
[query orderByDescending:@"startingBalance"];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    for (PFObject *object in objects) {
        [accountArray addObject:object[@"name"]];
        NSLog(@"%@", object[@"name"]);

    }
}];
int ct = [accountArray count];

或者您可以执行块内部数组所需的任何操作：

accountArray = [[NSMutableArray alloc]init];
PFQuery *query = [PFQuery queryWithClassName:@"Account"];
[query whereKey:@"user" equalTo:[PFUser currentUser]];
[query orderByDescending:@"startingBalance"];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    for (PFObject *object in objects) {
        [accountArray addObject:object[@"name"]];
        NSLog(@"%@", object[@"name"]);

    }
    int ct = [accountArray count];
    NSLog(@"Array countents: %@", accountArray);
}];

为什么我的nsmutablearray仍为null？

1 个答案: