Coding for Lefty Pilots

Objective: make your Pilot application just as ergonomically well-designed for users who hold the stylus in their left hand as it is for users who use their right.

Strategy: move control objects which must be dragged to the left of the objects they control -- for example, move scrollbars to the left of the scrolling content area. Leave all other interface elements (buttons, menus, etc.) in their standard positions. See Lefty Pilots for more discussion.

Methods: either use the FrmSetObjectPosition system call directly, or include "Lefty.h" (a header file provided below) and use its higher-level functions to rearrange your form. This should be done after the form is initialized (FrmInitForm) but before it is displayed (FrmDrawForm).

Using FrmSetObjectPosition

	FrmSetObjectPosition(FormPtr frm, Word objIndex, SWord x, SWord y);

Using Lefty.h and Lefty.c

• Boolean UserIsLefty()
  This function returns TRUE if the user has a 'Lfty' preference and, this preference setting designates the user as a lefty; FALSE otherwise.

• void LeftyFlip(Word objIndex, SWord YAxis)
  LeftyFlip shifts the specified form object such that its maximum distance from the specified vertical line (at y=YAxis) remains the same, but is now on the other side of the line. For example: if YAxis is 80 and the specified object's left edge is at position 50 (i.e. 80-30), then it will be repositioned to put its right edge at position 110 (i.e. 80+30). See diagram. Note that the parameters to this function are object indices; more often, you'd use the next function instead:

• void LeftyFlipByID(Word objID, SWord YAxis)
  This does the same as the above, but the object is specified by the unique object ID as defined in your resource header.

• SWord LeftyFindAxis(Word objIndex1, Word objIndex2)
  LeftyFindAxis finds the middle of the two specified form objects; that is, the point halfway between the leftmost left edge and the rightmost right edge.

• SWord LeftyFindAxisByID(Word objID1, Word objID2)
  As above, but using object IDs instead of index numbers.

• void LeftySwap(Word objIndex1, Word objIndex2)
  This function swaps the position of two objects on the form, such that the left edge of object 2 is moved to where the left edge of object 1 used to be, and the right edge of object 1 is moved to where the right edge of object 2 used to be (see diagram). This is functionally equivalent to calling LeftyFindAxis on the two objects, followed by LeftyFlip on each one.

• Void LeftySwapByID(Word objID1, Word objID2)
  This function does the same as LeftySwap, but each parameter is the unique object ID as defined in your resource header.

Most developers will never need to do more than call UserIsLefty to determine if the user is a lefty, and if so, then call LeftySwapByID to swap the position of a scrollbar and its content. If you have additional form objects to shift around, then you may need to use also the other Lefty functions above.

Example. The image at the left shows a modified version of the Memo example in the Examples folder of the SDK. A couple of lines were added to MemoMain.c, in the ApplicationHandleEvent function, in the case where the form just initialized is "EditView". The added lines are:

    if (UserIsLefty()) {
        LeftySwapByID( frm, EditMemoField, EditMemoScrollBar );
    }

A common mistake (for me, at least) is to pass an object index where I should be passing an ID, or vice versa. If you have any trouble while testing your lefty interface, I suggest you first check whether you're using the "ByID" form of the Lefty function calls, and if so, make sure you're passing IDs rather than index numbers!

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