How to be notified when a solution has been loaded for a VSPackage?

I know the answer, but I DON"t KNOW how to put class code in this

Add a async package call VSPackageEvents, and put this code in file, and and replace dte with your package dte

using System;
using System.ComponentModel.Design;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.Runtime.InteropServices;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.VisualStudio;
using Microsoft.VisualStudio.OLE.Interop;
using Microsoft.VisualStudio.Shell;
using Microsoft.VisualStudio.Shell.Interop;
using Microsoft.Win32;
using Task = System.Threading.Tasks.Task;

namespace VSIXProject
{
    /// <summary>
    /// This is the class that implements the package exposed by this assembly.
    /// </summary>
    /// <remarks>
    /// <para>
    /// The minimum requirement for a class to be considered a valid package for Visual Studio
    /// is to implement the IVsPackage interface and register itself with the shell.
    /// This package uses the helper classes defined inside the Managed Package Framework (MPF)
    /// to do it: it derives from the Package class that provides the implementation of the
    /// IVsPackage interface and uses the registration attributes defined in the framework to
    /// register itself and its components with the shell. These attributes tell the pkgdef creation
    /// utility what data to put into .pkgdef file.
    /// </para>
    /// <para>
    /// To get loaded into VS, the package must be referred by &lt;Asset Type="Microsoft.VisualStudio.VsPackage" ...&gt; in .vsixmanifest file.
    /// </para>
    /// </remarks>
    [PackageRegistration(UseManagedResourcesOnly = true, AllowsBackgroundLoading = true)]
    [InstalledProductRegistration("#1110", "#1112", "1.0", IconResourceID = 1400)] // Info on this package for Help/About
    [Guid(VSPackageEvents.PackageGuidString)]
    [SuppressMessage("StyleCop.CSharp.DocumentationRules", "SA1650:ElementDocumentationMustBeSpelledCorrectly", Justification = "pkgdef, VS and vsixmanifest are valid VS terms")]
#pragma warning disable VSSDK004 // Use BackgroundLoad flag in ProvideAutoLoad attribute for asynchronous auto load.
    [ProvideAutoLoad(UIContextGuids80.SolutionExists)]
#pragma warning restore VSSDK004 // Use BackgroundLoad flag in ProvideAutoLoad attribute for asynchronous auto load.
    public sealed class VSPackageEvents : AsyncPackage
    {
        EnvDTE.DTE dte = null;
        EnvDTE.Events events = null;
        EnvDTE.SolutionEvents solutionEvents = null;
        /// <summary>
        /// VSPackageEvents GUID string. Replace this Guid
        /// </summary>
        public const string PackageGuidString = "12135331-70d8-48bb-abc7-5e5ffc65e041";

        /// <summary>
        /// Initializes a new instance of the <see cref="VSPackageEvents"/> class.
        /// </summary>
        public VSPackageEvents()
        {
            // Inside this method you can place any initialization code that does not require
            // any Visual Studio service because at this point the package object is created but
            // not sited yet inside Visual Studio environment. The place to do all the other
            // initialization is the Initialize method.
        }
        private void SolutionEvents_Opened()
        {
// put your code here after opened event
        }
        private void SolutionEvents_AfterClosing()
        {
// put your code here after closed event
        }

        /// <summary>
        /// Initialization of the package; this method is called right after the package is sited, so this is the place
        /// where you can put all the initialization code that rely on services provided by VisualStudio.
        /// </summary>
        /// <param name="cancellationToken">A cancellation token to monitor for initialization cancellation, which can occur when VS is shutting down.</param>
        /// <param name="progress">A provider for progress updates.</param>
        /// <returns>A task representing the async work of package initialization, or an already completed task if there is none. Do not return null from this method.</returns>
        protected override async Task InitializeAsync(CancellationToken cancellationToken, IProgress<ServiceProgressData> progress)
        {
            // When initialized asynchronously, the current thread may be a background thread at this point.
            // Do any initialization that requires the UI thread after switching to the UI thread.
            await this.JoinableTaskFactory.SwitchToMainThreadAsync(cancellationToken);
            // replace dte from next line with your dte package
            // dte = Utilities.Utility.GetEnvDTE(this);
            events = dte.Events;       
            solutionEvents = events.SolutionEvents;
            solutionEvents.Opened += SolutionEvents_Opened;
            solutionEvents.AfterClosing += SolutionEvents_AfterClosing;
        }    

    }
}