Script Type Function Block: Resetting the Present Flag

The Present Flag

One of the most common actions in a function block script is to wait for receipt of a message before executing a block of code. To aid in this, each message has associated with it a property called the Present flag. This variable is set to False (0) when Vehicle Spy starts, and then changed to True (1) when a message is received, indicating that the message is now present.

Many actions need to be triggered only once for each copy of a particular message that is received. However, once the Present flag is set, it will remain set as long as Vehicle Spy stays online (because a received message remains "present"; it doesn't disappear on its own.) To ensure that a script section runs only once for each message copy, it is necessary to reset (or clear) the Present flag after each time it is seen as True, so that only a new message will set it again.

There are two common ways of resetting the Present flag.

Resetting the Present Flag using the Set Value command

The most precise method is to directly target the Present flag itself, simply setting its value to 0 using a Set Value command. This allows you to clear the Present flag without affecting other message properties. An example is shown in Figure 1.

Figure 1: Using the Set Value command to reset the present flag.

You can also use Set Value If in a similar fashion, when that would be more useful.

Resetting the Present Flag using the Clear Stats command

A secondary method is to use the Clear Stats command targeting a message. An example of this can be seen in Figure 1.

Important Note: Using Clear Stats in this manner resets not only the Present flag, but also all other information related to the message, including its data. If you will need to work with other properties or data in the message, it is recommended that you instead use Set Value as described above.

Figure 1: Using the Clear Stats command to reset the Present flag of a receive message.