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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
using System.Collections.Specialized; // NameValueCollection
using System.Configuration; // ConfigurationManager
using System.IO;
using System.Windows;
using System.Text;
using MS.Internal;
namespace MS.Win32
{
// Implementation note:
//
// The theme state is kept in three variables: a bool _isActive and two
// strings _themeName and _themeColor (the latter two are treated equally
// in this discussion, so we'll mention only _themeName for conciseness).
// There are three interesting states:
// I (Inactive): _isActive=false, _themeName=null
// AU (Active/Uninitialized): _isActive=true, _themeName=null
// AI (Active/Initialized): _isActive=true, _themeName=non-null
//
// The original implementation handled OS theme-change notifications by
// transitioning to state I or AU, depending on the result of calling
// the OS function IsThemeActive (and the HighContrast state).
// In the AU case, the next query to ThemeName moved to state AI
// using the result of another OS function GetCurrentThemeName.
//
// A previous change exposed a race condition that occurs when there are two
// windows running in different threads. An OS change to an "active"
// theme sends notifications to both threads, and they both try to change
// state: AI -> AU -> AI. If thread B's first transition occurs in the
// middle of thread A's second transition, thread A's get_ThemeName can
// return null, which crashes the app.
//
// [The opportunities for this race to end badly increased as of Win8.
// Reconnecting to an RDP session now sends WM_THEMECHANGED to all windows.]
//
// The current implementation avoids the race condition without using locks
// (thus dodging questions of deadlock or re-entrancy), while keeping the
// same pattern of OS calls (thus dodging questions of performance or use
// of OS services). The key points are:
// 1. Update the state atomically. The state is stored as a read-only
// class with three members (rather than as three variables), and
// updated by installing a new instance of the class.
// 2. Use the state atomically. Set a local variable to refer to the
// state, and use that reference (rather than re-fetching the static
// member, which another thread might change).
// 3. Avoid the pattern AI -> AU -> AI. If we had initialized the name
// of the old theme, there will almost certainly be a request for
// the name of the new theme, so obtaining the name during the
// theme change (rather than waiting for the ThemeName request)
// entails no wasted effort. This avoids the problematic intermediate
// state where _themeName is null, except when switching between
// active and inactive themes.
// 4. When two threads try to install a new state simultaneously, ensure
// that only one actually does so. Usually this happens when both
// threads are reacting to the same OS theme change and thus are trying
// to make the same transition. In that case the losing thread can
// simply use the state installed by the winning thread.
// 5. In theory, there's a more complicated race condition that might occur,
// as illustrated by this (hypothetical) timeline:
// OS Thread A Thread B
// change to theme X
// broadcast WM_THEMECHANGE(1)
// OnThemeChanged(1)
// get data from OS
// install new state
// change to theme Y
// broadcast WM_THEMECHANGE(2)
// OnThemeChanged(2)
// get data from OS
// install new state
// OnThemeChanged(1)
// get data from OS
// change to theme Z
// broadcast WM_THEMECHANGE(3)
// OnThemeChanged(3)
// get data from OS
// install new state
// try install new state
//
// Thread A knows that thread B won the race, but it doesn't know whether
// the installed state is valid. In this picture it's not - thread
// B installed a state referring to theme Y. But in the same picture
// with the last two lines reversed, thread B is the loser but the
// state installed by the winner is valid - thread A's state refers to
// theme Z.
//
// This situation is unlikely, perhaps even impossible (limited attempts
// failed to cause it). And the original implementation has the same
// problem. Yet ignoring it is pretty bad - in the above picture thread
// B will eventually get the two pending WM_THEMECHANGEs and change the
// state to "theme Z", but until it does thread A will render everything
// according to theme Y, and will never have any reason to change that.
//
// The current implementation handles this situation by restarting
// thread A's update process from the beginning, as if it got a fresh
// OnThemeChanged. This guarantees that thread A always uses the latest
// theme data, but it does raise the possibility of starvation - thread
// A can get stuck in the update loop if other threads always sneak
// their own updates into the window between A's attempts to install
// its state. This would only happen if the OS were raising a continuous
// stream of WM_THEMECHANGEs (real ones, that actually change the theme),
// in which case starvation of thread A is the least of your worries.
//
// Note that no amount of locking would help this situation. You'd need
// to lock around the OS actions of changing the theme data and broadcasting
// WM_THEMECHANGE. The OS does not provide that kind of locking.
/// <summary>
/// Wrapper class for loading UxTheme system theme data
/// </summary>
internal static class UxThemeWrapper
{
static UxThemeWrapper()
{
// When in high contrast we want to force the WPF theme to be Classic
// because that is the only WPF theme that has full fidelity support for the
// high contrast color scheme. Prior to Win8 the OS automatically switched
// to Classic theme when in high contrast, but Win8 onwards apps that claim
// Win8 OS support via the app.manifest will receive the AeroLite as the theme
// when in high contrast. The reason being the OS team wants to give high
// contrast a face lift starting Win8. However, WPF isnt setup to support this
// currently. Thus we fallback to Classic in this situation.
_themeState = new ThemeState(!SystemParameters.HighContrast && SafeNativeMethods.IsUxThemeActive(), null, null);
}
internal static bool IsActive
{
get
{
return IsActiveCompatWrapper;
}
}
internal static string ThemeName
{
get
{
return ThemeNameCompatWrapper;
}
}
internal static string ThemeColor
{
get
{
return ThemeColorCompatWrapper;
}
}
internal static string ThemedResourceName
{
get
{
return ThemedResourceNameCompatWrapper;
}
}
private static ThemeState EnsureThemeState(bool themeChanged)
{
ThemeState themeState = _themeState; // capture latest state
bool needName = !themeChanged;
string themeName, themeColor;
bool needUpdate = true;
while (needUpdate)
{
// compute the desired new state
ThemeState newState;
if (themeChanged)
{
// when called in response to a ThemeChange, reevaluate
// the full state
// Please refer to elaborate comment about high
// contrast in the static constructor
bool isActive = !SystemParameters.HighContrast && SafeNativeMethods.IsUxThemeActive();
if (isActive && (needName || themeState.ThemeName != null))
{
// avoid AI -> AU -> AI transition; get theme name now
// (see implementation note 3)
GetThemeNameAndColor(out themeName, out themeColor);
}
else
{
themeName = themeColor = null;
}
newState = new ThemeState(isActive, themeName, themeColor);
}
else
{
// when called outside of ThemeChange, only update if we
// need the name of an active theme
if (themeState.IsActive && themeState.ThemeName == null)
{
GetThemeNameAndColor(out themeName, out themeColor);
newState = new ThemeState(themeState.IsActive, themeName, themeColor);
}
else
{
newState = themeState;
needUpdate = false;
}
}
if (needUpdate)
{
// try to update the state
ThemeState currentState = System.Threading.Interlocked.CompareExchange(ref _themeState, newState, themeState);
if (currentState == themeState)
{
// the update worked, we're done
themeState = newState;
needUpdate = false;
}
else if (currentState.IsActive == newState.IsActive &&
(!newState.IsActive ||
newState.ThemeName == null ||
currentState.ThemeName != null)
)
{
// another thread updated the state while we were in this
// method, but installed a state that's as good or better
// than the one we wanted. Accept the other update.
themeState = currentState;
needUpdate = false;
}
else
{
// another thread updated the state to something different
// from what we wanted. This implies the other thread
// was responding to a ThemeChange, but it might have been
// an older ThemeChange, in which case the state isn't
// valid. To be safe, start over as if responding
// to a ThemeChange.
themeChanged = true;
themeState = currentState;
}
}
}
return themeState;
}
private static void GetThemeNameAndColor(out string themeName, out string themeColor)
{
StringBuilder themeNameSB = new StringBuilder(Win32.NativeMethods.MAX_PATH);
StringBuilder themeColorSB = new StringBuilder(Win32.NativeMethods.MAX_PATH);
if (UnsafeNativeMethods.GetCurrentThemeName(themeNameSB, themeNameSB.Capacity,
themeColorSB, themeColorSB.Capacity,
null, 0) == 0)
{
// Success
themeName = themeNameSB.ToString();
themeName = Path.GetFileNameWithoutExtension(themeName);
if(string.Equals(themeName, "aero", StringComparison.OrdinalIgnoreCase) && Utilities.IsOSWindows8OrNewer)
{
themeName = "Aero2";
}
#if DEBUG
// for debugging, config file can override the theme name
NameValueCollection appSettings = null;
try
{
appSettings = ConfigurationManager.AppSettings;
}
catch (ConfigurationErrorsException)
{
}
if (appSettings != null)
{
string s = appSettings["ThemeNameOverride"];
if (!String.IsNullOrEmpty(s))
{
themeName = s;
}
}
#endif
themeColor = themeColorSB.ToString();
}
else
{
// Failed to retrieve the name
themeName = themeColor = String.Empty;
}
}
internal static void OnThemeChanged()
{
RestoreSupportedState();
EnsureThemeState(themeChanged: true);
}
private static ThemeState _themeState;
private class ThemeState
{
public ThemeState(bool isActive, string name, string color)
{
_isActive = isActive;
_themeName = name;
_themeColor = color;
}
public bool IsActive { get { return _isActive; } }
public string ThemeName { get { return _themeName; } }
public string ThemeColor { get { return _themeColor; } }
private bool _isActive;
private string _themeName;
private string _themeColor;
}
#region Compatibility
// There are apps that override the system theme with one of their own
// choosing, and intercept (and discard) WM_THEMECHANGED messages to
// keep their theme in place even when the end-user selects a different
// theme. They do this using private reflection to assign values to
// the three state variables.
//
// This state (i.e. where the three variables have values that differ from
// the ones we choose) is unsupported. So is the technique for getting
// into that state (i.e. private reflection). Nevertheless, .Net wants
// to preserve some level of compatibility - at the very least, avoid
// crashing. [The apps use the result of GetField("_isActive") without
// checking for null.]
//
// We do this in three steps:
// 1) preserve the three fields; this fixes the crashes.
// 2) if the app overrides the values, use the overridden values
// in preference to ours.
// 3) during WM_THEMECHANGED, restore the preference for our values.
// If the app overrides them again, step (2) will kick in.
// Note that step (3) will never happen if the app is intercepting
// WM_THEMECHANGED.
private static bool _isActive;
private static string _themeName;
private static string _themeColor;
private static bool IsAppSupported
{
// we never set _themeName to non-null, so it can only be non-null
// if the app has set it by private reflection => unsupported
get { return (_themeName == null); }
}
private static bool IsActiveCompatWrapper
{
get { return IsAppSupported ? _themeState.IsActive : _isActive; }
}
private static string ThemeNameCompatWrapper
{
get
{
if (IsAppSupported)
{
ThemeState themeState = EnsureThemeState(themeChanged:false);
if (themeState.IsActive)
{
return themeState.ThemeName;
}
else
{
return "classic";
}
}
else
{
return _themeName;
}
}
}
private static string ThemeColorCompatWrapper
{
get
{
if (IsAppSupported)
{
ThemeState themeState = EnsureThemeState(themeChanged:false);
Debug.Assert(themeState.IsActive, "Queried ThemeColor while UxTheme is not active.");
return themeState.ThemeColor;
}
else
{
return _themeColor;
}
}
}
private static string ThemedResourceNameCompatWrapper
{
get
{
if (IsAppSupported)
{
ThemeState themeState = EnsureThemeState(themeChanged:false);
if (themeState.IsActive)
{
return $"themes/{themeState.ThemeName.ToLowerInvariant()}.{themeState.ThemeColor.ToLowerInvariant()}";
}
else
{
return SystemResources.ClassicResourceName;
}
}
else
{
if (_isActive)
{
return $"themes/{_themeName.ToLowerInvariant()}.{_themeColor.ToLowerInvariant()}";
}
else
{
return SystemResources.ClassicResourceName;
}
}
}
}
private static void RestoreSupportedState()
{
_isActive = false;
_themeName = null;
_themeColor = null;
}
#endregion Compatibility
}
}
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