如何“刷新”TListBox？

Question

我正在创建一个显示账单号码的应用程序，就像你在麦当劳看到的一样。POS机系统将账单号码发送到我的应用程序中，这些号码显示在一个名为"ListBoxPrep“的TListBox中。然后，当POS机系统向我的应用程序发送要删除的账单编号时，我的应用程序会从"ListBoxPrep“中删除它，并将其添加到"ListBoxReady”中。POS和我的应用程序之间的每一次通信都是通过TCP连接完成的，我对此没有任何问题。

我面临的问题是，即使在通过“pItem->ListBoxPrep()；”将其删除后，我仍然看到数字保留在"ListBoxPrep“中。"pItem“是TListBoxItem的指针。我希望一旦我的应用程序从POS机接收到“删除信号”，这些数字就会消失，特别是没有用户的干预，比如点击面板等。我想使用TTimer，但我不知道如何让"ListBoxPrep“刷新。你有这样做的想法吗？任何建议都将不胜感激。我使用的是RAD Studio 10.4。

在我的应用程序收到来自POS的“删除信号”后，我仍然在右侧看到数字。他们应该消失了。

​

我一点击"ListBoxPrep"，数字就消失了。

​

void __fastcall TForm1::IdTCPServerExecute(TIdContext *AContext)
{
    //We receive data: POS --> Screen(PC)
    String sentDataFromPOS = AContext->Connection->Socket->ReadLn();

    if(sentDataFromPOS .IsEmpty())
    {
        ShowMessage("Data sent from POS is empty!");
        return;
    }

    // 1. Find an order number to move to the right (prep -> ready)
    int indexOrderToRemove = ListBoxPrep->Items->IndexOf(sentDataFromPOS);

    // 2. Add the order number to the "Ready list"
    addNumberToReady(sentDataFromPOS);

    // 3. Remove the order from the "Prep list"
    ListBoxPrep->BeginUpdate();
    TListBoxItem* pItem = ListBoxPrep->ItemByIndex(indexOrderToRemove);
    pItem->Free(); // HERE I have a problem

    // test: To refresh the screen
    LayoutLeft->Visible = false;
    LayoutLeft->Visible = true;

    /*
    ListBoxPrep->Enabled = false;
    ListBoxPrep->Visible = false;
    ListBoxPrep->Enabled = true;
    ListBoxPrep->Visible = true;
    ListBoxPrep->Repaint();
    */
    ListBoxPrep->EndUpdate();
}

Answer 1

TIdTCPServer是一个多线程组件。它的OnExecute事件在工作线程的上下文中调用。因此，在访问UI控件时，它必须与主UI线程同步(这也适用于ShowMessage()，BTW)。为此，可以使用RTL的TThread::Synchronize() (同步)或TThread::Queue() (异步)方法。

另外，你不应该直接Free() TListBoxItem对象，你有了所需项目的索引，你可以使用ListBoxPrep->Items->Delete()。

如果您使用的是其中一种clang-based compilers，请尝试更多类似以下内容的方法：

void __fastcall TForm1::IdTCPServerExecute(TIdContext *AContext)
{
    //We receive data: POS --> Screen(PC)
    String sentDataFromPOS = AContext->Connection->Socket->ReadLn();

    if (sentDataFromPOS.IsEmpty())
    {
        TThread::Synchronize(nullptr,
            [](){ ShowMessage("Data sent from POS is empty!"); }
        );
        return;
    }

    TThread::Queue(nullptr, // or Synchronize(), your choice...
        [=, this](){ this->orderIsReady(sentDataFromPOS); }
    );
}

void __fastcall TForm1::orderIsReady(String orderNumber)
{
    // 1. Find an order number to move to the right (prep -> ready)
    int indexOrderToRemove = ListBoxPrep->Items->IndexOf(orderNumber);

    // 2. Add the order number to the "Ready list"
    addNumberToReady(orderNumber);

    // 3. Remove the order from the "Prep list"
    if (indexOrderToRemove != -1)
        ListBoxPrep->Items->Delete(indexOrderToRemove);
}

另一方面，如果您使用的是"classic" Borland compiler，请尝试执行以下操作：

struct orderHelper
{
    String orderNumber;

    orderHelper(const String &orderNumber)
        : orderNumber(orderNumber)
    {
    }

    void __fastcall orderIsReady()
    {
        Form1->orderIsReady(orderNumber);
    }
};

void __fastcall TForm1::orderIsEmpty()
{
    ShowMessage("Data sent from POS is empty!");
}

void __fastcall TForm1::IdTCPServerExecute(TIdContext *AContext)
{
    //We receive data: POS --> Screen(PC)

    String sentDataFromPOS = AContext->Connection->Socket->ReadLn();

    if (sentDataFromPOS.IsEmpty())
    {
        TThread::Synchronize(NULL, &orderIsEmpty);
        return;
    }

    orderHelper helper(sentDataFromPOS);
    TThread::Synchronize(NULL, &(helper.orderIsReady));
}

void __fastcall TForm1::orderIsReady(String orderNumber)
{
    // 1. Find an order number to move to the right (prep -> ready)
    int indexOrderToRemove = ListBoxPrep->Items->IndexOf(orderNumber);

    // 2. Add the order number to the "Ready list"
    addNumberToReady(orderNumber);

    // 3. Remove the order from the "Prep list"
    if (indexOrderToRemove != -1)
        ListBoxPrep->Items->Delete(indexOrderToRemove);
}

或者这样：

struct orderHelper
{
    String orderNumber;

    orderHelper(const String &orderNumber)
        : orderNumber(orderNumber)
    {
    }

    void __fastcall orderIsReady()
    {
        try {
            Form1->orderIsReady(orderNumber);
        } __finally {
            delete this;
        }
    }
};

void __fastcall TForm1::orderIsEmpty()
{
    ShowMessage("Data sent from POS is empty!");
}

void __fastcall TForm1::IdTCPServerExecute(TIdContext *AContext)
{
    //We receive data: POS --> Screen(PC)

    String sentDataFromPOS = AContext->Connection->Socket->ReadLn();

    if (sentDataFromPOS.IsEmpty())
    {
        TThread::Synchronize(NULL, &orderIsEmpty);
        return;
    }

    orderHelper *helper = new orderHelper(sentDataFromPOS);
    TThread::Queue(NULL, &(helper->orderIsReady));
}

void __fastcall TForm1::orderIsReady(String orderNumber)
{
    // 1. Find an order number to move to the right (prep -> ready)
    int indexOrderToRemove = ListBoxPrep->Items->IndexOf(orderNumber);

    // 2. Add the order number to the "Ready list"
    addNumberToReady(orderNumber);

    // 3. Remove the order from the "Prep list"
    if (indexOrderToRemove != -1)
        ListBoxPrep->Items->Delete(indexOrderToRemove);
}

Answer 2

我修改了IdTCPServerExecute中的一些代码，以消除一些编译错误。下面是运行良好的代码。再次感谢你，Remy Lebeau！

void __fastcall TForm1::IdTCPServerExecute(TIdContext *AContext)
{
    //We receive data: POS --> Screen(PC)
    String sentDataFromPos = AContext->Connection->Socket->ReadLn();

    // test
    //sentDataFromPos = "";

    if(sentDataFromPos.IsEmpty())
    {
        TThread::Synchronize(nullptr,
            [=](){ ShowMessage("Data sent from POS is empty!"); }
        );

        return;
    }

    TThread::Synchronize(nullptr, 
// Queue doesn't make the numbers disappear. It doesn't display them at right side either .
        [&, this]()
        {
            this->orderIsReady(sentDataFromPos);
        }
    );
}
//---------------------------------------------------------------------------
void __fastcall TForm1::orderIsReady(String orderNumber)
{
    // 1. Find an order to move to the right (prep -> ready)
    int indexOrderToRemove = ListBoxPrep->Items->IndexOf(orderNumber);

    // 2. Add an order of the same order number to the "Ready list"
    addNumberToReady(orderNumber);

    // 3. Remove the order from the "Prep list"
    if(indexOrderToRemove != -1)
    {
        ListBoxPrep->Items->Delete(indexOrderToRemove);
    }
}