Q(uick)BASIC Statements: ON event

The ON event statement only specifies the start of an event-trapping subroutine. Another set of statements determines whether or not the subroutine is called. This set of statements turns event trapping on or off and determines how events are handled when trapping is off. The following list describes these statements in a general way.

For specific information about each of these statements, see the

• ⮜ COM statement ⮞,
• ⮜ KEY statement ⮞,
• ⮜ PEN statement ⮞,
• ⮜ PLAY (event) statement ⮞,
• ⮜ STRIG statement ⮞, or
• ⮜ TIMER statement ⮞.

When an event trap occurs (the subroutine is called), BASIC performs an automatic event STOP that prevents recursive traps. The RETURN from the trapping subroutine automatically performs an event ON statement unless an explicit event OFF is performed inside the subroutine.

The RETURN linenumber or RETURN linelabel forms of RETURN can be used to return to a specific line number from the trapping subroutine. Use this type of return with care, however, because

• any other GOSUB, WHILE, or FOR statements active at the time of the trap remain active. This may produce error messages such as "NEXT without FOR."
• if an event occurs in a procedure, a RETURN linenumber or RETURN linelabel statement cannot get back into the procedure because the line number or label must be in the module-level code.

Differences from BASICA

• If you use BC from the DOS prompt and a program contains ON event statements, you must use /V or /W. These options allow the compiler to function correctly when event-trapping subroutines are included in a program. BASICA does not require additional options.