The application calls this function to get a set of hit-test metrics corresponding to a range of text positions. One of the main usages is to implement highlight selection of the text string. The function returns E_NOT_SUFFICIENT_BUFFER, which is equivalent to HRESULT_FROM_WIN32(ERROR_INSUFFICIENT_BUFFER), when the buffer size of hitTestMetrics is too small to hold all the regions calculated by the function. In this situation, the function sets the output value *actualHitTestMetricsCount to the number of geometries calculated. The application is responsible for allocating a new buffer of greater size and calling the function again. A good value to use as an initial value for maxHitTestMetricsCount may be calculated from the following equation: maxHitTestMetricsCount = lineCount * maxBidiReorderingDepth where lineCount is obtained from the value of the output argument *actualLineCount (from the function IDWriteTextLayout::GetLineLengths), and the maxBidiReorderingDepth value from the DWRITE_TEXT_METRICS structure of the output argument *textMetrics (from the function IDWriteFactory::CreateTextLayout).

HRESULT IDWriteTextLayout::HitTestTextRange([In] unsigned int textPosition,[In] unsigned int textLength,[In] float originX,[In] float originY,[Out, Buffer, Optional] DWRITE_HIT_TEST_METRICS* hitTestMetrics,[In] unsigned int maxHitTestMetricsCount,[Out] unsigned int* actualHitTestMetricsCount)