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For Developers‎ > ‎How-Tos‎ > ‎

Leak GDI Objects in Windows

Tools

  • Windows Task Manager
    • Add "GDI Objects" column by [View]-[Select Columns...]
    • You may also want to add "Handle", "Thread" and "USER Objects"
  • GDIView (works Win7 32bit and 64bit)

Anti Patterns

DeleteObject with HICON

DeleteObject WIN32 API works all GDI objects except for icon (97559) 
We must use DestroyIcon.

In tabe_view.cc(237)
810    HICON empty_icon =
811        IconUtil::CreateHICONFromSkBitmap(canvas.ExtractBitmap());
812    ImageList_AddIcon(image_list, empty_icon);
813    ImageList_AddIcon(image_list, empty_icon);
814    DeleteObject(empty_icon);

Leak Screen DC aka GetDC(NULL)

GetDC(NULL) returns newly created screen DC. So, you must release it by using ReleaseDC. You may want to use ScopedGetDC (http://codesearch.google.com/#OAMlx_jo-ck/src/base/win/scoped_hdc.h)

I would like to draw in bitmap (http://crbug.com/98523)

In print_web_view_helper_win.cc(237)

235 // Page used alpha blend, but printer doesn't support it. Rewrite the
236      // metafile and flatten out the transparency.

237      HDC bitmap_dc = CreateCompatibleDC(GetDC(NULL));
238      if (!bitmap_dc)
Easy way to know screen DPI:
In chrome_content_utility_cline.cc(299)

299  int screen_dpi = GetDeviceCaps(GetDC(NULL), LOGPIXELSX);
300  xform.eM11 = xform.eM22 =
301      static_cast<float>(screen_dpi) / static_cast<float>(render_dpi);

Play Enhanced Metafile Record More Than Once

Oops, some devices don't support alpha blending. We should do alpha blending by ourselves by using bitmap DC (http://crbug.com/98523)
Since, metafile records having GDI object creation command and stores into lpHandleTable of second parameter of EnhMetaFileProc.

In crhome/renderer/print_web_view_helper_win.cc(40-69)

40 // Play this command to the bitmap DC.

41 PlayEnhMetaFileRecord(*bitmap_dc, handle_table, record, num_objects);
 ...
68 // Play this command to the metafile DC.

69 PlayEnhMetaFileRecord(dc, handle_table, record, num_objects);

References

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