Direct2D CreateTextLayout() - How to get caret coordinates - c++

Question

I am rendering Text using Direct2D starting with a text Layout
HRESULT hr = m_spWriteFactory->CreateTextLayout(
m_wsText.c_str( ),
m_wsText.length( ),
m_spWriteTextFormat.Get( ),
m_rect.right - m_rect.left - m_spacing.right - m_spacing.left,
m_rect.bottom - m_rect.top - m_spacing.top - m_spacing.bottom,
&m_spTextLayout
);
and then rendering it to a bitmap which I later use with Direct3D
m_sp2DDeviceContext->DrawTextLayout(
D2D1::Point2F( m_spacing.left, m_spacing.top ),
m_spTextLayout.Get( ),
m_spTextBrush.Get( )
);
I would like to draw a simple thin flashing line as a caret. I know how to draw a line and how to make it appear / disappear.
Question: How do I get the starting point and the end point coordinates for my caret line?
Simplification: If it is much easier to assume that the text consists of one line only, then that's ok. But of course a more general solution is appreciated.

Answer 1

Use IDWriteTextLayout's hit-testing functions to determine these:
HitTestTextPosition for mapping a text position index (relative to the first character) to a rectangle.
HitTestTextRange for getting a whole range of rectangles such as for selection.
HitTestPoint for mapping a mouse coordinate to a text position index.
For carets, this below works for all horizontal reading directions and proportional/monospace fonts:
...
DWRITE_HIT_TEST_METRICS hitTestMetrics;
float caretX, caretY;
bool isTrailingHit = false; // Use the leading character edge for simplicity here.
// Map text position index to caret coordinate and hit-test rectangle.
textLayout->HitTestTextPosition(
textPosition,
isTrailingHit,
OUT &caretX,
OUT &caretY,
OUT &hitTestMetrics
);
// Respect user settings.
DWORD caretWidth = 1;
SystemParametersInfo(SPI_GETCARETWIDTH, 0, OUT &caretWidth, 0);
DWORD halfCaretWidth = caretWidth / 2u;
// Draw a thin rectangle.
D2D1::RectF caretRect = {
layoutOriginX + caretX - halfCaretWidth,
layoutOriginY + hitTestMetrics.top,
layoutOriginX + caretX + (caretWidth - halfCaretWidth),
layoutOriginY + hitTestMetrics.top + hitTestMetrics.height
};
solidColorBrush->SetColor(D2D1::ColorF::AliceBlue);
d2dRenderTarget->FillRectangle(&caretRect, solidColorBrush);
Notes:
The above code as-is doesn't account for vertical reading directions such as in Japanese newspapers. You would need to draw a wide flat caret instead of the tall thin one here when the DWRITE_READING_DIRECTION was either top-to-bottom or bottom-to-top.
IDWriteTextLayout::GetMetrics only gives the overall bounding box to you, not the caret position.
IDWriteTextLayout::HitTestPoint's isInside flag is true if it is over the text itself, not just the layout bounds.

Answer 2

You can get the layout's bounding rectangle via IDWriteTextLayout::GetMetrics.
DWRITE_TEXT_METRICS textMetrics;
textLayout->GetMetrics(&textMetrics);
Your rectangle is
D2D1::RectF( textMetrics.left,
textMetrics.top,
textMetrics.left + textMetrics.width,
textMetrics.top + textMetrics.height );
You can then draw the caret along the right boundary line.