Question

如何在以下或类似情况下避免回调地狱，

[self saveSomethingToTheServerWithCompletion:^(BOOL saveSucceeded) {
    dispatch_async(dispatch_get_main_queue(), ^{
        [self dismissViewControllerAnimated:YES completion:^{
            dispatch_async(dispatch_get_main_queue(), ^{
                if (saveSucceeded) {
                    [self showAlertControllerWithTitle:@"Message" message:@"Save successful."];
                } else {
                    [self showAlertControllerWithTitle:@"Message" message:@"Save failed."];
                }
            });
        }];
    });
}];

我的理解是dismissViewControllerAnimated:completion:和showAlertControllerWithTitle:message:必须在主线程上执行。

Answer 1

这个问题相当含糊，所以我正在猜测你的问题。我唯一能想到的是你对嵌套调用不满意。您可以在一定程度上整理方法。例如你知道总是需要在主线程上显示警告，所以将主线程代码移动到你的警报方法：

[self saveSomethingToTheServerWithCompletion:^(BOOL saveSucceeded) {
    dispatch_async(dispatch_get_main_queue(), ^{
        [self dismissViewControllerAnimated:YES completion:^{
            NSString *message = saveSucceeded ? @"Save successful." : @"Save failed.";
            [self showAlertControllerWithTitle:@"Message" message:message];
        }];
    });
}];

- (void)showAlertControllerWithTitle:(NSString *)title message:(NSString *)message {
    dispatch_async(dispatch_get_main_queue(), ^{
         //Show alert;
    }
}

此外，您可以考虑从dismissViewControllerAnimated调用中删除完成块。即你的警报显示真的取决于视图控制器是否已被解雇？

[self saveSomethingToTheServerWithCompletion:^(BOOL saveSucceeded) {
    dispatch_async(dispatch_get_main_queue(), ^{
        NSString *message = saveSucceeded ? @"Save successful." : @"Save failed.";
        [self showAlertControllerWithTitle:@"Message" message:message];
        [self dismissViewControllerAnimated:YES completion:nil];
    });
}];

最终，有时你只需要嵌套回调。这没有什么不妥。

编辑：我还用三元代替了你的if语句而没有真正考虑它。不确定你是否算作“回调地狱”。

Answer 2

您可以尝试这样的事情

[self saveSomethingToTheServerWithCompletion:^(BOOL saveSucceeded) {

    dispatch_async(dispatch_get_main_queue(), ^{

        NSString *strMsg = saveSucceeded ? @"Save successful." : @"Save failed.";

        UIAlertAction *okAction = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {

            dispatch_async(dispatch_get_main_queue(), ^{

                [self dismissViewControllerAnimated:YES completion:^{

                }];

            });
        }];

        [self showAlertWithTitle:@"Message" message:strMsg actions:@[okAction]];
    });

}];

- (void)showAlertWithTitle:(NSString * _Nonnull)title message:(NSString * _Nonnull)msg actions:(NSArray * _Nonnull)actions {

    UIAlertController *alertVC = [UIAlertController alertControllerWithTitle:title message:msg preferredStyle:UIAlertControllerStyleAlert];

    for (UIAlertAction *action in actions) {
        [alertVC addAction:action];
    }

    [self presentViewController:alertVC animated:true completion:nil];
}

因此，在回调时，它将显示带有消息的警报，并且在ok按钮上，顶部警报将解除并且还将关闭所呈现的视图控制器。所以它将在主线上。

Answer 3

你不必在主线程中使用回调包装dismissViewControllerAnimated：因为它已经在主线程中，也试图避免重复的代码：

View inflate = View.inflate(getContext(), R.layout.dialog_add_url, null);
dialogViewHolder = new DialogViewHolder(inflate);

如何在iOS中避免回调地狱？

3 个答案: