Step 5: Managing Access to the Mouse

Microsoft® DirectInput® provides the IDirectInputDevice8::Acquire and IDirectInputDevice8::Unacquire methods to manage device access. Your application must call the Acquire method to gain access to the device before requesting mouse information with the IDirectInputDevice8::GetDeviceState and IDirectInputDevice8::GetDeviceData methods.

Most of the time your application has the device in an acquired state. However, if you have only foreground access, the mouse is automatically unacquired whenever your application moves to the background. You are responsible for reacquiring it when you get the focus back again. This can be done in response to a WM_ACTIVATE message.

Scrawl handles this message by setting a global variable, g_bActive, according to whether the application is gaining or losing the focus. It then calls a helper function, SetAcquire, which acquires the mouse if g_bActive is TRUE and unacquires it otherwise.

case WM_ACTIVATE:
    if (WA_INACTIVE == wParam)
        g_bActive = FALSE;
    else
        g_bActive = TRUE;

    // Set exclusive mode access to the mouse based on active state
    SetAcquire();
    return 0; 

If you have exclusive access, your application might need to unacquire the mouse to enable the user to interact with Microsoft Windows®—for example, to access a menu or a dialog box. In Scrawl this can happen when the user opens the system menu with ALT+SPACEBAR.

The Scrawl window procedure has a handler for WM_ENTERMENULOOP that responds by setting the global variable g_bActive to FALSE and calling the SetAcquire function. This handler enables Windows to have the mouse and display its own cursor.

When the user is done using a menu, Windows sends the application a WM_EXITSIZEMOVE message. In this case, the Scrawl window process sets g_bActive appropriately and calls SetAcquire.

Scrawl also unacquires the mouse in response to a right-button click, which displays a shortcut menu. Although the mouse would get unacquired later, in the WM_ENTERMENULOOP handler, it is unacquired here first so that the position of the Windows cursor can be set before the menu appears.

Finally, Scrawl tries to reacquire the mouse if it receives a DIERR_INPUTLOST error after an attempt to retrieve data. This is in case the device has been unacquired by some mechanism not covered elsewhere; for instance, if the user has pressed CTRL+ALT+DEL.

In summary, your application needs to acquire the mouse before it can get data from it. This needs to be done only once, as long as nothing happens to force your application to surrender access to it. In exclusive mode, you are responsible for giving up control of the mouse when Windows needs it. You are also responsible for reacquiring the mouse whenever your program needs access to it after losing it to Windows or another application.

Once your application can manage access to the mouse, go to Step 6: Retrieving Buffered Data from the Mouse.