Public Property Get CoffeeMonitor() As CoffeeMonitor

' If the shared CoffeeMonitor object hasn't been created, create it and store a global reference to it.

If gCoffeeMonitor Is Nothing Then

Set gCoffeeMonitor = New CoffeeMonitor

End If

Set CoffeeMonitor = gCoffeeMonitor

End Property

CoffeeMonitor.Cls Option Explicit

' > For an overview of this sample application, search online Help for Coffee. AboutCof.Txt, in the Related Documents folder of CoffWat2.vbp, also contains information about the sample. CoffeeMonitor class monitors an imaginary serial interface to a high-tech coffee pot, using a call-back timer (XTimer object) to determine how often to check the coffee status. When the coffee's ready, the CoffeeMonitor object raises an event to notify clients. (Since the high-tech coffee pot has not yet been invented, this sample application just raises the CoffeeReady event every ten seconds.)

' (See also the CoffeeMonitor2 class, which demonstrates notification by a call-back method instead of an event.)'

' Note that the CoffeeMonitor class's Instancing property is set to PublicNotCreatable. This means that clients cannot create CoffeeMonitors; they can only get a reference to the shared CoffeeMonitor by creating a Connector object and accessing its CoffeeMonitor property.

' By using a code-only timer, this version of CoffeeMonitor fixes the bug described in the topic "Using the Shared CoffeeMonitor," in "Creating an ActiveX Exe Component," in Books Online, whereby multiple CoffeeMonitor objects could sometimes be created.'

' That bug was caused by the 'use count' code the Connector object used to release the global reference to the shared CoffeeMonitor. The use count was required because the step-by-step procedures in Books Online used a hidden form to hold a Timer control; hidden forms keep components from unloading (as described in "Starting and Ending a Component" in Books Online), which keeps global memory from being freed, which keeps gCoffeeMonitor from being set to Nothing.

' Unlike the hidden form with its Timer control, the XTimer object won't keep the Coffee2 component from unloading when the last client releases its last reference. (Note, however, that the global variable gCoffeeMonitor will keep the CoffeeMonitor object alive until all objects provided by Coffee2 are released by their respective clients.)

' =======================================================

' WARNING! Code-only timers are inherently dangerous in the Visual Basic development environment, becaue the system blindly calls back into your code until the timer is turned off with an API call. It's safer to use Timer controls during most of the development process, and only switch to call-back timers at the very end.

' =======================================================

' mXTimer holds a reference to a code-only timer that tells CoffeeMonitor when to check the pot. Because the variable is declared WithEvents, the CoffeeMonitor object receives the XTimer object's Tick events (see Sub mwXTimer_Tick, below). Code for the XTimer object can be found in XTimers.vbp.