获取文本写入的像素宽度

https://opensource.apple.com/source/WebCore/WebCore-955.66/platform/wx/wxcode/win/fontprops.cpp.auto.html

Windows的方案：

GetTextExtentPoint32
GetTextExtentPoint
GetTextExtentExPoint
DrawText
DrawTextEx
GetTextExtent
GetTextMetrics
GetCharWidth
LpkUseGDIWidthCache
GetTabbedTextExtent
LpkGetTextExtentExPoint
GetTextExtentExPointWPri
GetCharABCWidths
GetCharABCWidthsFloat 

GetOutlineTextMetrics

void GetTextExtent( const wxFont& font, const wxString& str, wxCoord *width, wxCoord *height,
                            wxCoord *descent, wxCoord *externalLeading )
{
    HDC dc = GetDC(0);
    WXHFONT hFont = font.GetHFONT();
    ::SelectObject(dc, hFont);

    HFONT hfontOld;
    if ( font != wxNullFont )
    {
        wxASSERT_MSG( font.Ok(), _T("invalid font in wxDC::GetTextExtent") );

        hfontOld = (HFONT)::SelectObject(dc, hFont);
    }
    else // don't change the font
    {
        hfontOld = 0;
    }

    SIZE sizeRect;
    const size_t len = str.length();
    if ( !::GetTextExtentPoint32(dc, str, len, &sizeRect) )
    {
        wxLogLastError(_T("GetTextExtentPoint32()"));
    }

#if !defined(_WIN32_WCE) || (_WIN32_WCE >= 400)
    // the result computed by GetTextExtentPoint32() may be too small as it
    // accounts for under/overhang of the first/last character while we want
    // just the bounding rect for this string so adjust the width as needed
    // (using API not available in 2002 SDKs of WinCE)
    if ( len > 1 )
    {
        ABC width;
        const wxChar chFirst = *str.begin();
        if ( ::GetCharABCWidths(dc, chFirst, chFirst, &width) )
        {
            if ( width.abcA < 0 )
                sizeRect.cx -= width.abcA;

            if ( len > 1 )
            {
                const wxChar chLast = *str.rbegin();
                ::GetCharABCWidths(dc, chLast, chLast, &width);
            }
            //else: we already have the width of the last character

            if ( width.abcC < 0 )
                sizeRect.cx -= width.abcC;
        }
        //else: GetCharABCWidths() failed, not a TrueType font?
    }
#endif // !defined(_WIN32_WCE) || (_WIN32_WCE >= 400)

    TEXTMETRIC tm;
    ::GetTextMetrics(dc, &tm);

    if (width)
        *width = sizeRect.cx;
    if (height)
        *height = sizeRect.cy;
    if (descent)
        *descent = tm.tmDescent;
    if (externalLeading)
        *externalLeading = tm.tmExternalLeading;

    if ( hfontOld )
    {
        ::SelectObject(dc, hfontOld);
    }
    
    ReleaseDC(0, dc);
}

https://opensource.apple.com/source/WebCore/WebCore-955.66/platform/wx/wxcode/mac/carbon/fontprops.mm.auto.html

Mac平台使用了wxWidgets方案

void GetTextExtent( const wxFont& font, const wxString& str, wxCoord *width, wxCoord *height,
                            wxCoord *descent, wxCoord *externalLeading )
{
    wxGraphicsContext * const gc = wxGraphicsContext::Create();
    
    gc->SetFont(font, *wxBLACK); // colour doesn't matter but must be specified
    struct GCTextExtent
    {
        wxDouble width, height, descent, externalLeading;
    } e;
    gc->GetTextExtent(str, &e.width, &e.height, &e.descent, &e.externalLeading);
    if ( width )
        *width = wxCoord(e.width + .5);
    if ( height )
        *height = wxCoord(e.height + .5);
    if ( descent )
        *descent = wxCoord(e.descent + .5);
    if ( externalLeading )
        *externalLeading = wxCoord(e.externalLeading + .5);

    delete gc;
}

void wxMacCoreGraphicsContext::GetTextExtent( const wxString &str, wxDouble *width, wxDouble *height,
                            wxDouble *descent, wxDouble *externalLeading ) const
{
    wxCHECK_RET( !m_font.IsNull(), wxT("wxMacCoreGraphicsContext::GetTextExtent - no valid font set") );

    if ( width )
        *width = 0;
    if ( height )
        *height = 0;
    if ( descent )
        *descent = 0;
    if ( externalLeading )
        *externalLeading = 0;

    // In wxWidgets (MSW-inspired) API it is possible to call GetTextExtent()
    // with an empty string to get just the descent and the leading of the
    // font, so support this (mis)use.
    wxString strToMeasure(str);
    if (str.empty())
        strToMeasure = wxS(" ");

    wxMacCoreGraphicsFontData* fref = (wxMacCoreGraphicsFontData*)m_font.GetRefData();

    wxCFStringRef text(strToMeasure, wxLocale::GetSystemEncoding() );
    
    wxCFRef<CFAttributedStringRef> attrtext( CFAttributedStringCreate(kCFAllocatorDefault, text, fref->OSXGetCTFontAttributes() ) );
    wxCFRef<CTLineRef> line( CTLineCreateWithAttributedString(attrtext) );

    CGFloat a, d, l, w;
    w = CTLineGetTypographicBounds(line, &a, &d, &l);

    if ( !str.empty() )
    {
        if ( width )
            *width = w;
        if ( height )
            *height = a+d+l;
    }
    if ( descent )
        *descent = d;
    if ( externalLeading )
        *externalLeading = l;

    CheckInvariants();    
}

void wxMacCoreGraphicsContext::GetPartialTextExtents(const wxString& text, wxArrayDouble& widths) const
{
    widths.clear();

    wxCHECK_RET( !m_font.IsNull(), wxT("wxMacCoreGraphicsContext::DrawText - no valid font set") );

    if (text.empty())
        return;

    wxMacCoreGraphicsFontData* fref = (wxMacCoreGraphicsFontData*)m_font.GetRefData();

    wxCFStringRef t(text, wxLocale::GetSystemEncoding() );
    wxCFRef<CFAttributedStringRef> attrtext( CFAttributedStringCreate(kCFAllocatorDefault, t, fref->OSXGetCTFontAttributes()) );
    wxCFRef<CTLineRef> line( CTLineCreateWithAttributedString(attrtext) );

    widths.reserve(text.length());
    CFIndex u16index = 1;
    for ( wxString::const_iterator iter = text.begin(); iter != text.end(); ++iter, ++u16index )
    {
        // Take care of surrogate pairs: they take two, not one, of UTF-16 code
        // units used by CoreText.
        if ( *iter >= 0x10000 )
        {
            ++u16index;
        }
        widths.push_back( CTLineGetOffsetForStringIndex( line, u16index, NULL ) );
    }

    CheckInvariants();
}

因为wxWidgets也是跨平台的：

https://opensource.apple.com/source/WebCore/WebCore-955.66/platform/wx/wxcode/gtk/fontprops.cpp.auto.html

Linux平台使用了Pango：

void GetTextExtent( const wxFont& font, const wxString& str, wxCoord *width, wxCoord *height,
                            wxCoord *descent, wxCoord *externalLeading )
{
    if ( width )
        *width = 0;
    if ( height )
        *height = 0;
    if ( descent )
        *descent = 0;
    if ( externalLeading )
        *externalLeading = 0;

    if (str.empty())
        return;

// FIXME: Doesn't support height, descent or external leading, though we don't need this for WebKit
// it will need to be implemented before merging into wx unless we craft a new API.
#if USE(WXGC)
    PangoFont* pangoFont = WebCore::createPangoFontForFont(&font);
    PangoContext* pangoContext = pango_cairo_font_map_create_context(PANGO_CAIRO_FONT_MAP(WebCore::pangoFontMap()));
    PangoGlyph pangoGlyph = WebCore::pango_font_get_glyph(pangoFont, pangoContext, (gunichar)g_utf8_get_char(str.ToUTF8()));
    cairo_glyph_t cglyph = { pangoGlyph, 0, 0 };
    cairo_text_extents_t extents;
    cairo_scaled_font_t* scaled_font = WebCore::createScaledFontForFont(&font);
    cairo_scaled_font_glyph_extents(scaled_font, &cglyph, 1, &extents);

    if (cairo_scaled_font_status(scaled_font) == CAIRO_STATUS_SUCCESS && extents.x_advance != 0)
        *width = (wxCoord)extents.x_advance;

    cairo_scaled_font_destroy(scaled_font);
    g_object_unref(pangoContext);
    g_object_unref(pangoFont);
#else
    PangoContext* context = gdk_pango_context_get_for_screen( gdk_screen_get_default() );
    PangoLayout* m_layout = pango_layout_new(context);
    // and use it if it's valid
    if ( font && font->IsOk() )
    {
        pango_layout_set_font_description
        (
            m_layout,
            font->GetNativeFontInfo()->description
        );
    }

    // Set layout's text
    const wxCharBuffer dataUTF8 = wxConvUTF8.cWX2MB(str);
    if ( !dataUTF8 )
    {
        // hardly ideal, but what else can we do if conversion failed?
        return;
    }

    pango_layout_set_text( m_layout, dataUTF8, strlen(dataUTF8) );

    int h = 0;
    pango_layout_get_pixel_size( m_layout, width, &h );

    if (descent)
    {
        PangoLayoutIter *iter = pango_layout_get_iter(m_layout);
        int baseline = pango_layout_iter_get_baseline(iter);
        pango_layout_iter_free(iter);
        *descent = h - PANGO_PIXELS(baseline);
    }

    if (height)
        *height = (wxCoord) h;
#endif
}