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Inside Macintosh: Macintosh Toolbox Essentials /
Chapter 2 - Event Manager / Event Manager Reference
Event Manager Routines / Receiving Events


You can use the EventAvail function to retrieve the next available event from
the Event Manager without removing the returned event from your application's
event stream.

FUNCTION EventAvail (eventMask: Integer;
                     VAR theEvent: EventRecord): Boolean;
A value that indicates which kinds of events are to be returned; this parameter is interpreted as a sum of event mask constants. You can use these constants to specify the event mask:
            mDownMask    = 2;       {mouse-down event    (bit 1)}
            mUpMask      = 4;       {mouse-up event      (bit 2)}
            keyDownMask  = 8;       {key-down event      (bit 3)}
            keyUpMask    = 16;      {key-up event        (bit 4)}
            autoKeyMask  = 32;      {auto-key event      (bit 5)}
            updateMask   = 64;      {update event        (bit 6)}
            diskMask     = 128;     {disk-inserted event (bit 7)}
            activMask    = 256;     {activate event      (bit 8)}
                         = 1024;    {high-level event    (bit 10)}
            osMask       = -32768;  {operating-system    (bit 15)}
To accept all events, you can specify the everyEvent constant as the event mask:
            everyEvent   =  -1;     {every event}
If no event of any of the designated types is available, EventAvail returns a null event.
The next available event of the specified type or types. The EventAvail function does not remove the returned event from the event stream, but does return the information about the event in an event record. The event record includes the type of event received and other information.
EventAvail returns FALSE as its function result if the event being returned is a null event; otherwise, EventAvail returns TRUE.

Like WaitNextEvent, the EventAvail function calls the SystemTask procedure to give time to each open desk accessory or device driver to perform any periodic action defined for it. The EventAvail function also makes the alarm go off if the alarm is set and the current time is the alarm time. The user sets the alarm using the Alarm Clock desk accessory.

If EventAvail returns a low-level event from the Operating System event queue, the event will not be accessible later if, in the meantime, the event queue becomes full and the event is discarded from it; however, this is not a common occurrence.

See "The Event Record," beginning on page 2-79, for a description of the fields in the event record.

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11 JUL 1996

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