adjust method
Reads an existing value, computes a new one with AdjustFunc and writes the new value to the storage.
Implementation
Future<T?> adjust(AdjustFunc f) async {
if (this.isDisposed) return null;
var oldVal = await this.read();
if (this.isDisposed) return null;
var newVal = f(oldVal);
await this.write(newVal);
if (this.isDisposed) return null;
return newVal;
}