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Accepted
Lucas Tomic
Asked: 2022-08-17 03:02:13 +0800 CST 2022-08-17 03:02:13 +0800 CST

How to wait for a DB value to change before continuing?

  • 772

I have certain products and I need them not to be purchased by several users at the same time as they could buy more than the available amount. For this, every time a user starts the process of buying a product, I change a property of my database ( isBeingBought/idDelProducto) to true. So, if a user initiates a purchase, I want the app to confirm that no one else is buying the product ( isBeingBought/idDelProducto == false) and if someone else is ( isBeingBought/idDelProducto == true), I want the app to wait until the property returns to false.

For this I made the function awaitNobodyIsBuyingthat is a Futurethat waits until the property istrue

 static Future<void> awaitNobodyIsBuying(String id, String weight) async {
        DatabaseReference ref = await FirebaseDatabase.instance.ref()
.child("isBeingBought/$id");
        final body = await ref.get();
        if(body.value!=null && body.value as bool){
          await Future.delayed(Duration(seconds: 1)).then((event) {
              awaitNobodyIsBuying(id, weight);
          });
        }
    }

What this function does is ask if the property is trueand if it is, wait a second and ask again.

I call it when the buy button is pressed:

onPressed:() async {
await ProductDatabaseService.awaitNobodyIsBuying(buyProducProvider.productSelected!.id, buyProducProvider.caliberSelected!.weight);

//Realizar compra....
}

the problem is that the method works correctly, when checking it with the Debugger it keeps looping until I manually change the property to false. However, the application does not wait and while the method awaitNobodyIsBuyingis still executing, the other methods are still executing.

I use firebase realtime database.

How can I solve that? Is there a more efficient method with a listener or something?

firebase

1 Answers

  1. Best Answer

    When you use it, it awaitstops the flow until you get a response, but when you use then, what you do is receive the response asynchronously while the flow continues, you would have to wait and return, like this:

    
 static Future<void> awaitNobodyIsBuying(String id, String weight) async {
        DatabaseReference ref = await FirebaseDatabase.instance.ref()
.child("isBeingBought/$id");
        final body = await ref.get();
        if(body.value!=null && body.value as bool){
          await Future.delayed(Duration(seconds: 1));
          return awaitNobodyIsBuying(id, weight);
        }
    }
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