表单即将获得焦点时的事件？

Question

我想编写一个屏幕键盘。

要将密钥发送到另一个应用程序，我需要在我的应用程序即将获得焦点时存储当前前台窗口（=具有焦点的窗口），并且当用户按下我的表单上的按钮以发送密钥时，我将SetForegroundWindow到这个前一个窗口句柄。

这样文本将被发送到先前聚焦的窗口。

但是，我没有找到正确的事件。

Form_Activated 为时已晚。

使用计时器不断检查当前前景窗口似乎有点粗鲁。

在我的应用获得焦点之前是否发生了“官方”事件？

Answer 1

无礼：，我同意 Jimi 和 jmcilhinney 的 cmets，我也同意 相信这不是实现屏幕键盘的正确方法， 但这篇文章只是想帮助你：

• 查找停用窗口的句柄时 此窗口已被激活。

当该窗口被激活时，查找已停用窗口的句柄

您可以使用SetWinEventHook 来监听其他进程的some events，并注册一个WinEventProc 回调方法，以便在事件引发时接收事件。

这里我们对EVENT_SYSTEM_FOREGROUND 感兴趣。每次我们收到这个事件，如果激活的窗口不是我们的窗体，我们跟踪已经被激活的窗口，然后当我们的窗口被激活时，我们查看被跟踪的窗口的值，它现在是之前丢失的窗口重点。

C#

这是我尝试过的代码：

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;
public class MyForm : Form
{
    public const uint EVENT_SYSTEM_FOREGROUND = 0x0003;
    public const uint EVENT_OBJECT_DESTROY = 0x8001;
    public const uint WINEVENT_OUTOFCONTEXT = 0;

    public delegate void WinEventDelegate(IntPtr hWinEventHook, uint eventType, 
        IntPtr hwnd, int idObject, int idChild, 
        uint dwEventThread, uint dwmsEventTime);
    
    [DllImport("user32.dll")]
    public static extern IntPtr SetWinEventHook(uint eventMin, uint eventMax, 
        IntPtr hmodWinEventProc, WinEventDelegate lpfnWinEventProc, 
        uint idProcess, uint idThread, uint dwFlags);
    
    [DllImport("user32.dll")]
    public static extern bool UnhookWinEvent(IntPtr hWinEventHook);

    [DllImport("user32.dll")]
    private static extern IntPtr GetForegroundWindow();

    IntPtr hook = IntPtr.Zero;
    protected override void OnLoad(EventArgs e)
    {
        previous = GetForegroundWindow();
        hook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND, 
        EVENT_SYSTEM_FOREGROUND,
        IntPtr.Zero, new WinEventDelegate(WinEventProc),
        0, 0, WINEVENT_OUTOFCONTEXT);
        base.OnLoad(e);
    }
    protected override void OnFormClosing(FormClosingEventArgs e)
    {
        UnhookWinEvent(hook);
        base.OnFormClosing(e);
    }
    IntPtr? previous = null;
    void WinEventProc(IntPtr hWinEventHook, uint eventType,
        IntPtr hwnd, int idObject, int idChild, uint dwEventThread,
        uint dwmsEventTime)
    {
        if (hwnd != this.Handle)
        {
            previous = hwnd;
        }
        else
        {
            if (previous.HasValue)
                this.Text = $"Previous window: {(int)previous:X}";
            else
                this.Text = $"No idea about previous window.";
        }
    }
}

VB.NET

Imports System.Runtime.InteropServices

Public Class MyForm
    Inherits Form

    Public Const EVENT_SYSTEM_FOREGROUND As UInteger = &H3
    Public Const EVENT_OBJECT_DESTROY As UInteger = &H8001
    Public Const WINEVENT_OUTOFCONTEXT As UInteger = 0
    Public Delegate Sub WinEventDelegate(ByVal hWinEventHook As IntPtr,
                                         ByVal eventType As UInteger,
                                         ByVal hwnd As IntPtr,
                                         ByVal idObject As Integer,
                                         ByVal idChild As Integer,
                                         ByVal dwEventThread As UInteger,
                                         ByVal dwmsEventTime As UInteger)
    <DllImport("user32.dll")>
    Public Shared Function SetWinEventHook(ByVal eventMin As UInteger,
                                           ByVal eventMax As UInteger,
                                           ByVal hmodWinEventProc As IntPtr,
                                           ByVal lpfnWinEventProc As WinEventDelegate,
                                           ByVal idProcess As UInteger,
                                           ByVal idThread As UInteger,
                                           ByVal dwFlags As UInteger) As IntPtr
    End Function
    <DllImport("user32.dll")>
    Public Shared Function UnhookWinEvent(ByVal hWinEventHook As IntPtr) As Boolean
    End Function
    <DllImport("user32.dll")>
    Private Shared Function GetForegroundWindow() As IntPtr
    End Function
    Private hook As IntPtr = IntPtr.Zero
    Protected Overrides Sub OnLoad(ByVal e As EventArgs)
        previous = GetForegroundWindow()
        hook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND,
                               EVENT_SYSTEM_FOREGROUND, IntPtr.Zero,
                               New WinEventDelegate(AddressOf WinEventProc),
                               0, 0, WINEVENT_OUTOFCONTEXT)
        MyBase.OnLoad(e)
    End Sub

    Protected Overrides Sub OnFormClosing(ByVal e As FormClosingEventArgs)
        UnhookWinEvent(hook)
        MyBase.OnFormClosing(e)
    End Sub

    Private previous As IntPtr? = Nothing

    Private Sub WinEventProc(ByVal hWinEventHook As IntPtr,
                             ByVal eventType As UInteger,
                             ByVal hwnd As IntPtr,
                             ByVal idObject As Integer,
                             ByVal idChild As Integer,
                             ByVal dwEventThread As UInteger,
                             ByVal dwmsEventTime As UInteger)
        If hwnd <> Me.Handle Then
            previous = hwnd
        Else

            If previous.HasValue Then
                Me.Text = $"Previous window: {CInt(previous):X}"
            Else
                Me.Text = $"No idea about previous window."
            End If
        End If
    End Sub
End Class