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Displaying the Caret in Bidirectional Strings

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In unidirectional text, the caret position has no ambiguity because the leading edge of a character is at the same place as the trailing edge of the previous character. However, in bidirectional text, the caret position between runs of opposing direction is ambiguous. For example, in the left-to-right paragraph "hellosalaam", the last letter of "hello" immediately precedes the first letter of "salaam". The best position in which to display the caret depends on whether it is considered to follow the "o" of "hello" or to precede the "s" of "salaam".

Uniscribe uses the caret positioning conventions shown in the next table.

Situation Visual caret placement
Typing Trailing edge of last character typed.
Pasting Trailing edge of last character pasted.
Caret advancing Trailing edge of last character passed over.
Caret retiring Leading edge of last character passed over.
Home Leading edge of line.
End Trailing edge of line.

 

The caret can be positioned as shown in the following example:

if (fAdvancing) {
    ScriptCPtoX(
        iCharPos - 1, TRUE, 
        cChars, cGlyphs, &wLogClust, &sva, &iAdvance, &sa, &iCaretX
        );
} else {
    ScriptCPtoX(
        iCharPos, FALSE, 
        cChars, cGlyphs, &wLogClust, &sva, &iAdvance, &sa, &iCaretX
        );
}

Positioning of the caret can be simpler, as shown below, given an fAdvancing value restricted to TRUE or FALSE:

ScriptCPtoX(
    iCharPos - fAdvancing, fAdvancing, 
    cChars, cGlyphs, &wLogClust, &sva, &iAdvance, &sa, &iCaretX
    );

ScriptCPtoX handles out-of-range positions logically. It returns the leading edge of the run for iCharPos <0, and the trailing edge of the run for iCharPos >= length. For more information, see Managing Caret Placement and Hit Testing

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