Be aware that to be able to send an information, the sender needs to know the target. Normally the client knows the server. But the server has no clue about the clients until they try to connect.
Therefore you first have to establish a connection to the server, THEN the server can send data to the client.

In generell, there are a lot of ways to do that.

• Client polls on a regular basis. "Is there anything new?", few seconds later "Is there anything new?"
• Client makes a "long polling" request (request with an extreme high timeout). The server does not immediately answer it but "stores" the request to answer it as soon as something new happens. When the timeout of the client kicks in, he just sends the long polling request again and the server switches to the new one.
• Client sends his address to the server, then the server can connect. Works rarely, because the client quite often does only know the local address (local network), but not the internet address.

For polling, you can use any bidirectional connection to the server (such as TCP or WebSockets (based on TCP)), then the server can push data over this connection.

In your case, the app and the desktop client will very likely get dynamic IP addresses and may even be behind routers which will even more disguise their addresses. Therefore you will very likely need a third central instance (a server) where both can connect to and which will then arrange the communication between mobile app and desktop app

Be aware that to be able to send an information, the sender needs to know the target. Normally the client knows the server. But the server has no clue about the clients until they try to connect.
Therefore you first have to establish a connection to the server, THEN the server can send data to the client.

In generell, there are a lot of ways to do that.

• Client polls on a regular basis. "Is there anything new?", few seconds later "Is there anything new?"
• Client makes a "long polling" request (request with an extreme high timeout). The server does not immediately answer it but "stores" the request to answer it as soon as something new happens. When the timeout of the client kicks in, he just sends the long polling request again and the server switches to the new one.
• Client sends his address to the server, then the server can connect. Works rarely, because the client quite often does only know the local address (local network), but not the internet address.
• For polling, you can use any bidirectional connection to the server (such as TCP or WebSockets (based on TCP)), then the server can push data over this connection.

In your case, the app and the desktop client will very likely get dynamic IP addresses and may even be behind routers which will even more disguise their addresses. Therefore you will very likely need a third central instance (a server) where both can connect to and which will then arrange the communication between mobile app and desktop app

Be aware that to be able to send an information, the sender needs to know the target. Normally the client knows the server. But the server has no clue about the clients until they try to connect.
Therefore you first have to establish a connection to the server, THEN the server can send data to the client.

In generell, there are a lot of ways to do that.

• Client pulls on a regular basis. "Is there anything new?", few seconds later "Is there anything new?"
• Client makes a "long polling" request (request with an extreme high timeout). The server does not immediately answer it but "stores" the request to answer it as soon as something new happens. When the timeout of the client kicks in, he just sends the long polling request again and the server switches to the new one.
• Client sends his address to the server, then the server can connect. Works rarely, because the client quite often does only know the local address (local network), but not the internet address.
• Client opens a bidirectional connection to the server (like WebSockets), then the server can push data over this connection.

In your case, the app and the desktop client will very likely get dynamic IP addresses and may even be behind routers which will even more disguise their addresses. Therefore you will very likely need a third central instance (a server) where both can connect to and which will then arrange the communication between mobile app and desktop app

Be aware that to be able to send an information, the sender needs to know the target. Normally the client knows the server. But the server has no clue about the clients until they try to connect.
Therefore you first have to establish a connection to the server, THEN the server can send data to the client.

In generell, there are a lot of ways to do that.

• Client polls on a regular basis. "Is there anything new?", few seconds later "Is there anything new?"
• Client makes a "long polling" request (request with an extreme high timeout). The server does not immediately answer it but "stores" the request to answer it as soon as something new happens. When the timeout of the client kicks in, he just sends the long polling request again and the server switches to the new one.
• Client sends his address to the server, then the server can connect. Works rarely, because the client quite often does only know the local address (local network), but not the internet address.

For polling, you can use any bidirectional connection to the server (such as TCP or WebSockets (based on TCP)), then the server can push data over this connection.

In your case, the app and the desktop client will very likely get dynamic IP addresses and may even be behind routers which will even more disguise their addresses. Therefore you will very likely need a third central instance (a server) where both can connect to and which will then arrange the communication between mobile app and desktop app