Factor out wxMaxWidthCalculatorBase from wxDataViewCtrl code.

This will allow reusing it elsewhere, notably in the generic wxListCtrl.

See #10326.

(this is a backport of fa1c3ecfa6 from master)

include/wx/generic/private/widthcalc.h

@@ -0,0 +1,122 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        wx/generic/private/widthcalc.h
+// Purpose:     wxMaxWidthCalculatorBase helper class.
+// Author:      Václav Slavík, Kinaou Hervé
+// Copyright:   (c) 2015 wxWidgets team
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_GENERIC_PRIVATE_WIDTHCALC_H_
+#define _WX_GENERIC_PRIVATE_WIDTHCALC_H_
+
+#include "wx/defs.h"
+
+#if wxUSE_DATAVIEWCTRL
+
+#include "wx/timer.h"
+
+// ----------------------------------------------------------------------------
+// wxMaxWidthCalculatorBase: base class for calculating max column width
+// ----------------------------------------------------------------------------
+
+class wxMaxWidthCalculatorBase
+{
+public:
+    // column of which calculate the width
+    explicit wxMaxWidthCalculatorBase(size_t column)
+        : m_column(column),
+          m_width(0)
+    {
+    }
+
+    void UpdateWithWidth(int width)
+    {
+        m_width = wxMax(m_width, width);
+    }
+
+    // Update the max with for the expected row
+    virtual void UpdateWithRow(int row) = 0;
+
+    int GetMaxWidth() const { return m_width; }
+    size_t GetColumn() const { return m_column; }
+
+    void
+    ComputeBestColumnWidth(size_t count,
+                           size_t first_visible,
+                           size_t last_visible)
+    {
+        // The code below deserves some explanation. For very large controls, we
+        // simply can't afford to calculate sizes for all items, it takes too
+        // long. So the best we can do is to check the first and the last N/2
+        // items in the control for some sufficiently large N and calculate best
+        // sizes from that. That can result in the calculated best width being too
+        // small for some outliers, but it's better to get slightly imperfect
+        // result than to wait several seconds after every update. To avoid highly
+        // visible miscalculations, we also include all currently visible items
+        // no matter what.  Finally, the value of N is determined dynamically by
+        // measuring how much time we spent on the determining item widths so far.
+
+#if wxUSE_STOPWATCH
+        size_t top_part_end = count;
+        static const long CALC_TIMEOUT = 20/*ms*/;
+        // don't call wxStopWatch::Time() too often
+        static const unsigned CALC_CHECK_FREQ = 100;
+        wxStopWatch timer;
+#else
+        // use some hard-coded limit, that's the best we can do without timer
+        size_t top_part_end = wxMin(500, count);
+#endif // wxUSE_STOPWATCH/!wxUSE_STOPWATCH
+
+        size_t row = 0;
+
+        for ( row = 0; row < top_part_end; row++ )
+        {
+#if wxUSE_STOPWATCH
+            if ( row % CALC_CHECK_FREQ == CALC_CHECK_FREQ-1 &&
+                 timer.Time() > CALC_TIMEOUT )
+                break;
+#endif // wxUSE_STOPWATCH
+            UpdateWithRow(row);
+        }
+
+        // row is the first unmeasured item now; that's our value of N/2
+        if ( row < count )
+        {
+            top_part_end = row;
+
+            // add bottom N/2 items now:
+            const size_t bottom_part_start = wxMax(row, count - row);
+            for ( row = bottom_part_start; row < count; row++ )
+            {
+                UpdateWithRow(row);
+            }
+
+            // finally, include currently visible items in the calculation:
+            first_visible = wxMax(first_visible, top_part_end);
+            last_visible = wxMin(bottom_part_start, last_visible);
+
+            for ( row = first_visible; row < last_visible; row++ )
+            {
+                UpdateWithRow(row);
+            }
+
+            wxLogTrace("items container",
+                       "determined best size from %d top, %d bottom "
+                       "plus %d more visible items out of %d total",
+                       top_part_end,
+                       count - bottom_part_start,
+                       wxMax(0, last_visible - first_visible),
+                       count);
+        }
+    }
+
+private:
+    const size_t m_column;
+    int m_width;
+
+    wxDECLARE_NO_COPY_CLASS(wxMaxWidthCalculatorBase);
+};
+
+#endif // wxUSE_DATAVIEWCTRL
+
+#endif // _WX_GENERIC_PRIVATE_WIDTHCALC_H_