// 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.Runtime.ConstrainedExecution;
using System.Runtime.InteropServices;

namespace System
{
    /// <summary>
    /// Schedules a callback roughly every gen 2 GC (you may see a Gen 0 an Gen 1 but only once)
    /// (We can fix this by capturing the Gen 2 count at startup and testing, but I mostly don't care)
    /// </summary>
    internal sealed class Gen2GcCallback : CriticalFinalizerObject
    {
        private Gen2GcCallback()
            : base()
        {
        }

        /// <summary>
        /// Schedule 'callback' to be called in the next GC.  If the callback returns true it is 
        /// rescheduled for the next Gen 2 GC.  Otherwise the callbacks stop. 
        /// 
        /// NOTE: This callback will be kept alive until either the callback function returns false,
        /// or the target object dies.
        /// </summary>
        public static void Register(Func<object, bool> callback, object targetObj)
        {
            // Create a unreachable object that remembers the callback function and target object.
            Gen2GcCallback gcCallback = new Gen2GcCallback();
            gcCallback.Setup(callback, targetObj);
        }

        private Func<object, bool> _callback;
        private GCHandle _weakTargetObj;

        private void Setup(Func<object, bool> callback, object targetObj)
        {
            _callback = callback;
            _weakTargetObj = GCHandle.Alloc(targetObj, GCHandleType.Weak);
        }

        ~Gen2GcCallback()
        {
            // Check to see if the target object is still alive.
            object targetObj = _weakTargetObj.Target;
            if (targetObj == null)
            {
                // The target object is dead, so this callback object is no longer needed.
                _weakTargetObj.Free();
                return;
            }

            // Execute the callback method.
            try
            {
                if (!_callback(targetObj))
                {
                    // If the callback returns false, this callback object is no longer needed.
                    return;
                }
            }
            catch
            {
                // Ensure that we still get a chance to resurrect this object, even if the callback throws an exception.
#if DEBUG
                // Except in DEBUG, as we really shouldn't be hitting any exceptions here.
                throw;
#endif
            }

            // Resurrect ourselves by re-registering for finalization.
            if (!Environment.HasShutdownStarted)
            {
                GC.ReRegisterForFinalize(this);
            }
        }
    }
}