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Palm OS SDK Reference

Palm Logo 17 Scroll Bars

This chapter provides reference material for the scroll bar API.

The header file ScrollBar.h declares the API that this chapter describes. For more information on scroll bars, see the section "Scroll Bars" in the Palm OS Programmer's Companion.

Scroll Bar Data Structures

ScrollBarAttrType

The ScrollBarAttrType bit field defines a scroll bar's visible characteristics.

typedef struct {
    UInt16 usable : 1;
    UInt16 visible : 1;
    UInt16 hilighted : 1;
    UInt16 shown : 1;
    UInt16 activeRegion: 4;
} ScrollBarAttrType;

Field Descriptions

usable If not set, the scroll bar object is not considered part of the current interface of the application, and it doesn't appear on screen.
visible If set, the scroll bar is allowed to be displayed on the screen. If both visible and shown are true, then the scroll bar is actually displayed on the screen.
hilighted true if either the up arrow or the down arrow is highlighted.
shown Set if the scroll bar is visible and if maxValue > minValue. (See ScrollBarType.)
activeRegion The region of the scroll bar that is receiving the pen down events. Possible values are:
  sclUpArrow
The up arrow.
  sclDownArrow
The down arrow.
  sclUpPage
The region between the scroll car and the up arrow.
  sclDownPage
The region between the scroll car and the down arrow.
  sclCar
The scroll car.

ScrollBarPtr

The ScrollBarPtr type defines a pointer to a ScrollBarType structure.

typedef ScrollBarType *ScrollBarPtr;

You pass the ScrollBarPtr as an argument to all scroll bar functions. You can obtain the ScrollBarPtr using the function FrmGetObjectPtr in this way:

scrollBarPtr = FrmGetObjectPtr(frm,
FrmGetObjectIndex(frm, scrollBarID));

where scrollBarID is the resource ID assigned when you created the scroll bar.

ScrollBarType

The ScrollBarType represents a scroll bar.

typedef struct {
    RectangleType bounds;
    UInt16 id;
    ScrollBarAttrType attr;
    Int16 value;
    Int16 minValue;
    Int16 maxValue;
    Int16 pageSize;
    Int16 penPosInCar;
    Int16 savePos;
} ScrollBarType;

Your code should treat the ScrollBarType structure as opaque. Use the functions described in this chapter to retrieve and set each value. Do not attempt to change structure member values directly.

Field Descriptions

bounds Position (using absolute coordinates) and size (in pixels) of the scroll bar on the screen.
id ID value you specified when you created the scroll bar object.
attr Scroll bar's attributes. See ScrollBarAttrType.
value Current value of the scroll bar. This value is used to determine where to position the scroll car (the dark region in the scroll bar that indicates the position in the document).
The number given is typically a number relative to minValue and maxValue. These values have nothing to do with any physical characteristics of the object that the scroll bar is attached to, such as the number of lines in the object.
This value is typically set to 0 initially and then adjusted programmatically with SclSetScrollBar.
minValue Minimum value. When value equals minValue, the scroll car is positioned at the very top of the scrolling region. This value is typically 0.
maxValue Maximum value. When value equals maxValue, the scroll car is positioned at the very bottom of the scrolling region. This value is typically set to 0 initially and then adjusted programmatically with SclSetScrollBar.
pageSize Number of lines to scroll when user scrolls one page.
penPosInChar Used internally.
savePos Used internally.

Scroll Bar Resources

The Scroll Bar Resource (tSCL) represents a scroll bar.

Scroll Bar Functions




SclDrawScrollBar

Purpose

Draw a scroll bar.

Prototype

void SclDrawScrollBar (const ScrollBarPtr bar)

Parameters

  -> bar
 Pointer to a scroll bar structure (see ScrollBarType).

Result

Returns nothing.

Comments

This function is called internally by SclSetScrollBar and FrmDrawForm. You rarely need to call it yourself.

Compatibility

Implemented only if 2.0 New Feature Set is present.



SclGetScrollBar

Purpose

Retrieve a scroll bar's current position, its range, and the size of a page.

Prototype

void SclGetScrollBar (const ScrollBarPtr bar, Int16 *valueP, Int16 *minP, Int16 *maxP, Int16 *pageSizeP)

Parameters

  -> bar
 Pointer to a scroll bar structure (see ScrollBarType).
  <- valueP
 A value representing the scroll car's current position. (The scroll car is the dark region that indicates the position in the document.)
  <-minP
 A value representing the top of the user interface object.
  <-maxP
 A value representing the bottom of the user interface object.
  <-pageSizeP
 Pointer to size of a page (used when page scrolling).

Result

Returns the scroll bar's current values in valueP, minP, maxP, and pageSizeP.

Comments

You might use this function immediately before calling SclSetScrollBar to update the scroll bar. SclGetScrollBar returns the scroll bar's current values, which you can then adjust as necessary and pass to SclSetScrollBar.

Compatibility

Implemented only if 2.0 New Feature Set is present.

See Also

SclSetScrollBar



SclHandleEvent

Purpose

Handles events that affect a scroll bar.

Prototype

Boolean SclHandleEvent (const ScrollBarPtr bar, const EventType *event)

Parameters

  -> bar
 Pointer to a scroll bar structure (see ScrollBarType).
  -> event
 Pointer to an event (EventType).

Result

Returns true if the event was handled.

Comment

When a penDownEvent occurs, the scroll bar sends an sclEnterEvent to the event queue.

When an sclEnterEvent occurs, the scroll bar determines what its new value should be based on which region of the scroll bar is receiving the pen down events. It then sends either an sclRepeatEvent or an sclExitEvent to the event queue.

When the user holds and drags the scroll bar with the pen, the scroll bar sends a sclRepeatEvent. Applications that implement dynamic scrolling should catch this event and move the text each time one arrives.

When the user releases the pen from the scroll bar, the scroll bar sends a sclExitEvent. Applications that implement non-dynamic scrolling should catch this event and move the text when sclExitEvent arrives. Applications that implement dynamic scrolling can ignore this event.

Compatibility

Implemented only if 2.0 New Feature Set is present.



SclSetScrollBar

Purpose

Set the scroll bar's current position, its range, and the size of a page. If the scroll bar is visible and its minimum and maximum values are not equal, it's redrawn.

Prototype

void SclSetScrollBar (const ScrollBarPtr bar, Int16 value, const Int16 min, const Int16 max, const Int16 pageSize)

Parameters

  -> bar
 Pointer to a scroll bar structure (see ScrollBarType).
  -> value
 The position the scroll car should move to. (The scroll car is the dark region that indicates the position in the document.)
  -> min
 Minimum value.
  -> max
 Maximum value.
  -> pageSize
 Number of lines of text that can be displayed on a the screen at one time (used when page scrolling).

Result

Returns nothing. May display a fatal error message if the min parameter is greater than the max parameter.

Comments

Call this function when the user adds or deletes text in a field or when a table row is added or deleted.

For scrolling fields, your application should catch the fldChangedEvent and update the scroll bar at that time.

The max parameter is computed as:

number of lines of text - page size + overlap

where number of lines of text is the total number of lines or rows needed to display the entire object, page size is the number of lines or rows that can be displayed on the screen at one time, and overlap is the number of lines or rows from the bottom of one page to be visible at the top of the next page.

For example, if you have 100 lines of text and 10 lines show on a page, the max value would be 90 or 91, depending on the overlap. So if value is greater than or equal to 90 or 91, the scroll car is at the very bottom of the scrolling region.

You can use the FldGetScrollValues function to compute the values you pass for value, min, and max. For example:

FldGetScrollValues (fld, &scrollPos,
    &textHeight, &fieldHeight);

if (textHeight > fieldHeight)
    maxValue = textHeight - fieldHeight;
else if (scrollPos)
    maxValue = scrollPos;
else
    maxValue = 0;

SclSetScrollBar (bar, scrollPos, 0, maxValue,
    fieldHeight-1);

In this case, textHeight is the number of lines of text and fieldHeight is the page size. No lines overlap when you scroll one page. Notice that if the page size is greater than the lines of text, then max equals min, which means that the scroll bar is not displayed.

For scrolling tables, there is no equivalent to FldGetScrollValues. Your application must scroll the table itself and keep track of the scroll values. See the ListViewUpdateScrollers function in the Memo example application (MemoMain.c) for an example of setting scroll bar values for a table.

Compatibility

Implemented only if 2.0 New Feature Set is present.

See Also

SclGetScrollBar


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