The PowerBuilder garbage collection mechanism checks memory automatically for unreferenced and orphaned objects and removes any it finds, thus taking care of most memory leaks. You can use garbage collection to destroy objects instead of explicitly destroying them using the DESTROY statement. This lets you avoid runtime errors that occur when you destroy an object that was being used by another process or had been passed by reference to a posted event or function.
A reference to an object is any variable whose value is the object. When the variable goes out of scope, or when it is assigned a different value, PowerBuilder removes a reference to the object and counts the remaining references, and the garbage collection process destroys the object if no references remain.
Garbage collection occurs:
When the garbage collection interval has been exceeded and the PowerBuilder application becomes idle and
When you explicitly call the GarbageCollect function.
When PowerBuilder completes the execution of a system-triggered event, it makes a garbage collection pass if the set interval between garbage collection passes has been exceeded. The default interval is 0.5 seconds. Note that this system-triggered garbage collection pass only occurs when the PowerBuilder application is idle, therefore if a long computation or process is in progress when the interval is exceeded, garbage collection does not occur immediately.
You can force immediate garbage collection by invoking the GarbageCollect function. When you use dot notation and OLEObjects, temporary variables are created. These temporary variables are released only during the garbage collection process. You might want to invoke GarbageCollect inside a loop that appears to be causing memory leaks.
The garbage collection pass removes any objects and classes that cannot be referenced, including those containing circular references (otherwise unreferenced objects that reference each other).
Posting events and functions
When you post an event or function and pass an object reference,
PowerBuilder adds an internal reference to the object to prevent
its memory from being reclaimed by the garbage collector between
the time of the post and the actual execution of the event or function.
This reference is removed when the event or function is executed.
There are a few objects that are prevented from being collected:
Visual objects Any object that is visible on your screen is not collected because when the object is created and displayed on your screen, an internal reference is added to the object. When any visual object is closed, it is explicitly destroyed.
Timing objects Any Timing object that is currently running is not collected because the Start function for a Timing object adds an internal reference. The Stop function removes the reference.
Shared objects Registered shared objects are not collected because the SharedObjectRegister function adds an internal reference. SharedObjectUnregister removes the internal reference.
Garbage collection occurs automatically in PowerBuilder, but you can use functions to force immediate garbage collection or to change the interval between reference count checks. Three functions let you control when garbage collection occurs: GarbageCollect, GarbageCollectGetTimeLimit, and GarbageCollectSetTimeLimit.
For information about these functions, see the PowerScript Reference. For an example illustrating their use, see the Code Examples sample application, described in Chapter 1, “Using Sample Applications.”
You can use tracing and profiling to examine the effect of changing the garbage collection interval on performance.
For information about tracing and profiling, see the PowerBuilder Users Guide.
