函数调用仅在包含 MessageBox.Show() 时有效?

问题描述

在我当前的项目中，我有一个自制的音频播放器，它通过我的 musictimer() 函数进行操作.下面是一个子命令，当有人点击图片时，它命令音频播放器转到下一首歌曲.这非常有效.

In my current project I have a self-made audioplayer which is operated trough my musictimer() function. Below is a sub which orders the audioplayer to go to the next song when someone has clicked on a picture. This works perfectly.

Private Sub PictureBox4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles PictureBox4.Click
    If (ListBox1.Items.Count - 1 > songBeingPlayed) Then
        musictimer("next")
    Else
        musictimer("stop")
    End If
End Sub

下面有一个子命令，当一首歌曲播放完毕时，它命令播放器播放下一首歌曲.这个子也有效，但只有当我在那里有 MessageBox.Show("blabla") 行时.否则它只是忽略音乐计时器(下一个").显然，一直有弹出消息很烦人，所以我希望它消失.有谁知道这是怎么回事?我一无所知.

Below there is a sub which orders the player to play the next song when a song is finished playing. This sub also works but only when I have the MessageBox.Show("blabla") line in there. Otherwise it simply ignores the musictimer("next"). Obviously its quite annoying to have popup messages the entire times so I want it gone. Does anyone know whats going on? Im clueless.

Private Sub AxWindowsMediaPlayer1_PlayStateChange(ByVal sender As System.Object, ByVal e As AxWMPLib._WMPOCXEvents_PlayStateChangeEvent) Handles AxWindowsMediaPlayer1.PlayStateChange
    If AxWindowsMediaPlayer1.playState = WMPLib.WMPPlayState.wmppsStopped Then
        musictimer("next")
        MessageBox.Show("blabla")
    End If
End Sub

我非常凌乱的音乐定时器功能.

My very messy musictimer function.

Function musictimer(ByVal action)
    If action Is "initial" Then
        TextBox1.Text = "0:00"
        Timer1.Stop()
        secondsCounter = 1
        doubledigitsecondCounter = 0
        minuteCounter = 0
    End If

    If action Is "reset" Then
        TextBox1.Text = "0:00"
        Timer1.Stop()
        secondsCounter = 1
        doubledigitsecondCounter = 0
        minuteCounter = 0
        Me.AxWindowsMediaPlayer1.URL = ""
        changePlayButton("play")
    End If

    If action Is "start" Then
        If (ListBox1.Items.Count > 0) Then
            Me.AxWindowsMediaPlayer1.URL = directoryPath + listboxpl(songBeingPlayed)
            AxWindowsMediaPlayer1.Ctlcontrols.play()
            Timer1.Start()
            changePlayButton("pause")
        End If
    End If

    If action Is "pause" Then
        Timer1.Stop()
        AxWindowsMediaPlayer1.Ctlcontrols.pause()
        changePlayButton("play")
    End If

    If action Is "next" Then
        If (ListBox1.Items.Count - 1 > songBeingPlayed) Then
            songBeingPlayed += 1
            musictimer("reset")
            musictimer("start")
            changePlayButton("pause")
        Else
            musictimer("pause")
        End If
    End If

    If action Is "previous" Then
        If (songBeingPlayed > 0) Then
            songBeingPlayed -= 1
            musictimer("reset")
            musictimer("start")
        End If
    End If
End Function

推荐答案

PlayStateChanged 事件非常臭名昭著.这实际上只是为了更新显示状态的 UI 元素.在那个事件中与玩家做任何事情都是非常麻烦的.对 MessagBox 的调用会产生影响，因为它会产生一个消息循环，这对于 ActiveX 控件来说总是一个大问题.

The PlayStateChanged event is quite notorious. It was really meant to just update a UI element that shows the state. Doing anything with the player in that event is very troublesome. A call to MessagBox can have an affect because it pumps a message loop, always a big deal for ActiveX controls.

避免麻烦的最好方法是延迟你的代码，让它在事件被触发并且玩家回到静止状态之后运行.通过使用 Control.BeginInvoke() 方法优雅地完成.像这样:

The best way to stay out of trouble is by delaying your code, making it run after the event was fired and the player is back into a quiescent state. Elegantly done by using the Control.BeginInvoke() method. Like this:

Private Sub AxWindowsMediaPlayer1_PlayStateChange(ByVal sender As System.Object, ByVal e As AxWMPLib._WMPOCXEvents_PlayStateChangeEvent) Handles AxWindowsMediaPlayer1.PlayStateChange
    If e.newState = WMPLib.WMPPlayState.wmppsStopped Then
        Me.BeginInvoke(New Action(AddressOf NextSong))
    End If
End Sub

Private Sub NextSong()
    musictimer("next")
End Sub

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