public void OnApplicationPause(bool arg0)

bool

When PlayerSettings-runInBackground (Edit \> Project Settings \> Player \> Resolution and Presentation) is disabled, a game running in the Editor's Play mode or in a standalone Player will pause any time the Editor or Player application loses focus. In these cases Unity sends to all MonoBehaviours. The parameter is either (paused) or (running). All MonoBehaviours receive this event while they are initializing, just after , so it will be called (with status ) on first entering Play mode. They receive it again whenever the application pauses or unpauses on losing or regaining focus. Note: Unity does not call in response to toggling the Pause button in the Editor toolbar. The status of the pause button in the Editor is tracked by the PauseState enum. For to trigger in a Player application running separately from the Editor, the running Player must be windowed and smaller than the full screen. If the game is hidden (fully or partly) by another application then it pauses and Unity calls with . When the game regains focus, Unity calls with . can be a co-routine; to do this use the statement in the function. Implemented this way, it is evaluated twice during the initial frame: first as an early notification, and secondly during the normal co-routine update step. On Android, enabling the on-screen keyboard causes an MonoBehaviour.OnApplicationFocus(bool) event with the value . However, if you press "Home" at the moment the keyboard is enabled, the event is not called and is called instead.

using UnityEngine;public class AppPaused : MonoBehaviour{    bool isPaused = false;    void OnGUI()    {        if (isPaused)            GUI.Label(new Rect(100, 100, 50, 30), "Game paused");    }    void OnApplicationFocus(bool hasFocus)    {        isPaused = !hasFocus;    }    void OnApplicationPause(bool pauseStatus)    {        isPaused = pauseStatus;    }}