fix 'No enumerators available' - on Windows 10

screeninfo/enumerators/windows.py

@@ -1,46 +1,86 @@
+
 import typing as T
+import ctypes
+import ctypes.wintypes
+from ctypes.wintypes import (
+    HMONITOR,
+    HDC,
+    LPARAM,
+    LPRECT,
+    WCHAR,
+    DWORD,
+    RECT,
+    BOOL, HWND,
+)
 
 from screeninfo.common import Monitor
 
 
 def enumerate_monitors() -> T.Iterable[Monitor]:
-    import ctypes
-    import ctypes.wintypes
 
     CCHDEVICENAME = 32
     # gdi32.GetDeviceCaps keys for monitor size in mm
     HORZSIZE = 4
     VERTSIZE = 6
 
-    MonitorEnumProc = ctypes.WINFUNCTYPE(
-        ctypes.c_int,
-        ctypes.c_ulong,
-        ctypes.c_ulong,
-        ctypes.POINTER(ctypes.wintypes.RECT),
-        ctypes.c_double,
-    )
-
+    # https://docs.microsoft.com/en-gb/windows/win32/api/winuser/ns-winuser-monitorinfoexw
     class MONITORINFOEXW(ctypes.Structure):
         _fields_ = [
-            ("cbSize", ctypes.wintypes.DWORD),
-            ("rcMonitor", ctypes.wintypes.RECT),
-            ("rcWork", ctypes.wintypes.RECT),
-            ("dwFlags", ctypes.wintypes.DWORD),
-            ("szDevice", ctypes.wintypes.WCHAR * CCHDEVICENAME),
+            ("cbSize", DWORD),
+            ("rcMonitor", RECT),
+            ("rcWork", RECT),
+            ("dwFlags", DWORD),
+            ("szDevice", WCHAR * CCHDEVICENAME),
         ]
 
+    LPMONITORINFO = ctypes.POINTER(MONITORINFOEXW)
+    # https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getmonitorinfow
+    GetMonitorInfoW = ctypes.windll.user32.GetMonitorInfoW
+    GetMonitorInfoW.restype = BOOL
+    GetMonitorInfoW.argtypes = (HMONITOR, LPMONITORINFO)
+
+    # https://docs.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-getdevicecaps
+    GetDeviceCaps = ctypes.windll.gdi32.GetDeviceCaps
+    GetDeviceCaps.restype = ctypes.c_int
+    GetDeviceCaps.argtype = (HDC, ctypes.c_int)
+
+    # https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getdc
+    GetDC = ctypes.windll.user32.GetDC
+    GetDC.restype = HDC
+    GetDC.argtype = (HWND, )
+
+    # https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-releasedc
+    ReleaseDC = ctypes.windll.user32.ReleaseDC
+    ReleaseDC.restype = ctypes.c_int
+    ReleaseDC.argtype = (HWND, HDC)
+
+    # https://docs.microsoft.com/en-gb/windows/win32/api/winuser/nc-winuser-monitorenumproc
+    MonitorEnumProc = ctypes.WINFUNCTYPE(
+        BOOL,  # resType
+        HMONITOR,  # monitor
+        HDC,  # dc
+        LPRECT,  # rect
+        LPARAM,  # data
+    )
+
+    LPCRECT = ctypes.POINTER(RECT)
+    # https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-enumdisplaymonitors
+    EnumDisplayMonitors = ctypes.windll.user32.EnumDisplayMonitors
+    EnumDisplayMonitors.restype = BOOL
+    EnumDisplayMonitors.argtype = (HDC, LPCRECT, MonitorEnumProc, LPARAM)
+
     monitors = []
 
-    def callback(monitor: T.Any, dc: T.Any, rect: T.Any, data: T.Any) -> int:
+    def callback(monitor: int, dc: int, rect: LPRECT, data: int) -> bool:
         info = MONITORINFOEXW()
         info.cbSize = ctypes.sizeof(MONITORINFOEXW)
-        if ctypes.windll.user32.GetMonitorInfoW(monitor, ctypes.byref(info)):
+        if GetMonitorInfoW(monitor, ctypes.pointer(info)):
             name = info.szDevice
         else:
             name = None
 
-        h_size = ctypes.windll.gdi32.GetDeviceCaps(dc, HORZSIZE)
-        v_size = ctypes.windll.gdi32.GetDeviceCaps(dc, VERTSIZE)
+        h_size = GetDeviceCaps(dc, HORZSIZE)
+        v_size = GetDeviceCaps(dc, VERTSIZE)
 
         rct = rect.contents
         monitors.append(
@@ -54,7 +94,7 @@ def callback(monitor: T.Any, dc: T.Any, rect: T.Any, data: T.Any) -> int:
                 name=name,
             )
         )
-        return 1
+        return True
 
     # Make the process DPI aware so it will detect the actual
     # resolution and not a virtualized resolution reported by
@@ -64,28 +104,45 @@ def callback(monitor: T.Any, dc: T.Any, rect: T.Any, data: T.Any) -> int:
     # multiple monitors have different DPIs.
     ctypes.windll.shcore.SetProcessDpiAwareness(2)
 
+    hwnd = HWND(None)
     # On Python 3.8.X GetDC randomly fails returning an invalid DC.
     # To workaround this request a number of DCs until a valid DC is returned.
     for retry in range(100):
         # Create a Device Context for the full virtual desktop.
-        dc_full = ctypes.windll.user32.GetDC(None)
-        if dc_full > 0:
+        # On failure, GetDC returns NULL
+        dc_full: HDC = GetDC(hwnd)
+        if dc_full is not None:
             # Got a valid DC, break.
             break
-        ctypes.windll.user32.ReleaseDC(dc_full)
+        ReleaseDC(hwnd, dc_full)
     else:
         # Fallback to device context 0 that is the whole
         # desktop. This allows fetching resolutions
         # but monitor specific device contexts are not
         # passed to the callback which means that physical
         # sizes can't be read.
-        dc_full = 0
+        dc_full = HDC(None)
+
+    # Make sure you keep references to CFUNCTYPE() or WINFUNCTYPE() objects as long as they are
+    # used from C code. ctypes doesn’t, and if you don’t, they may be garbage collected,
+    # crashing your program when a callback is made.
+    monEnumProc = MonitorEnumProc(callback)
+    monEnumProc.restype = BOOL
+    monEnumProc.argtypes = [
+        HMONITOR,  # monitor
+        HDC,  # dc
+        LPRECT,  # rect
+        LPARAM,  # data
+    ]
     # Call EnumDisplayMonitors with the non-NULL DC
     # so that non-NULL DCs are passed onto the callback.
     # We want monitor specific DCs in the callback.
-    ctypes.windll.user32.EnumDisplayMonitors(
-        dc_full, None, MonitorEnumProc(callback), 0
+    EnumDisplayMonitors(
+        dc_full,
+        LPCRECT(),
+        monEnumProc,
+        LPARAM(0)
     )
-    ctypes.windll.user32.ReleaseDC(dc_full)
+    ReleaseDC(hwnd, dc_full)
 
     yield from monitors