Bitcoin – Find Pending Transaction

Solution for Bitcoin – Find Pending Transaction
is Given Below:

I want to implement a bitcoin payment gateway. So the idea is that I give the public address to the users and check the balance of the address using any API.

But i have to know when the user start the transaction, becauce i want give him a response that the coins are on the way. For this reason I thought that with the help of my public address on the blockchain I could search the pending transactions.
Any ideas how to do that? Because i didnt find any api for this particular issue

Just for info, i programm it in dart(flutter)