Palm OS Programming, 2nd Edition

Types of Form Objects

Now we will discuss each individual type of form object in detail.

Labels

Label objects can be a little bit tricky if you are going to change the label at runtime. They are a snap if the label values don’t switch. Here is the most important label function:

void FrmCopyLabel(FormType *formP, UInt16 labelID, const Char *newLabel)

FrmCopyLabel copies the contents of the new label to the label with the given ID. Unlike almost all the other Form Manager APIs, FrmCopyLabel takes an ID as a parameter, rather than an index.

Note the name of this function includes Copy. This means that the new label will overwrite the contents of the old label. You will have trouble if the length of the new label is longer than the length specified in the resource. Longer strings will definitely cause errors, since FrmCopyLabel will blindly write beyond the space allocated for the label. Thus, this function makes sure you’ve allocated enough space in your resource.

Unfortunately, prior to OS 3.5, FrmCopyLabel would redraw the new label, but would not erase the old one. You can have problems with this in cases where the new text is narrower than the old text; remnants of the old text are left behind like so much screen litter. One way to avoid this problem is to hide the label before doing the copy, and then show it afterward. Here is an example of that:

void SetLabelInForm(FormType *form, UInt16 labelID, const char *newText) { UInt16 labelObjectIndex = FrmGetObjectIndex(form, labelID); ...