我遇到了一个问题，我无法在 FormClosing 事件中等待异步函数，该函数将确定表单关闭是否应该继续。我创建了一个简单的示例，如果您关闭而不保存（很像使用记事本或 Microsoft Word），它会提示您保存未保存的更改。我遇到的问题是，当我等待异步保存功能时，它会在保存功能完成之前关闭表单，然后在完成后返回关闭功能并尝试继续。我唯一的解决方案是在调用 SaveAsync 之前取消关闭事件，然后如果保存成功，它将调用 form.Close() 函数。我希望有一种更清洁的方法来处理这种情况。
要复制该场景，请创建一个带有文本框 (txtValue)、复选框 (cbFail) 和按钮 (btnSave) 的表单。这是表单的代码。
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace TestZ
{
public partial class Form1 : Form
{
string cleanValue = "";
public Form1()
{
InitializeComponent();
}
public bool HasChanges()
{
return (txtValue.Text != cleanValue);
}
public void ResetChangeState()
{
cleanValue = txtValue.Text;
}
private async void btnSave_Click(object sender, EventArgs e)
{
//Save without immediate concern of the result
await SaveAsync();
}
private async Task<bool> SaveAsync()
{
this.Cursor = Cursors.WaitCursor;
btnSave.Enabled = false;
txtValue.Enabled = false;
cbFail.Enabled = false;
Task<bool> work = Task<bool>.Factory.StartNew(() =>
{
//Work to do on a background thread
System.Threading.Thread.Sleep(3000); //Pretend to work hard.
if (cbFail.Checked)
{
MessageBox.Show("Save Failed.");
return false;
}
else
{
//The value is saved into the database, mark current form state as "clean"
MessageBox.Show("Save Succeeded.");
ResetChangeState();
return true;
}
});
bool retval = await work;
btnSave.Enabled = true;
txtValue.Enabled = true;
cbFail.Enabled = true;
this.Cursor = Cursors.Default;
return retval;
}
private async void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
if (HasChanges())
{
DialogResult result = MessageBox.Show("There are unsaved changes. Do you want to save before closing?", "Unsaved Changes", MessageBoxButtons.YesNoCancel, MessageBoxIcon.Question);
if (result == System.Windows.Forms.DialogResult.Yes)
{
//This is how I want to handle it - But it closes the form while it should be waiting for the Save() to complete.
//bool SaveSuccessful = await Save();
//if (!SaveSuccessful)
//{
// e.Cancel = true;
//}
//This is how I have to handle it:
e.Cancel = true;
bool SaveSuccessful = await SaveAsync();
if (SaveSuccessful)
{
this.Close();
}
}
else if (result == System.Windows.Forms.DialogResult.Cancel)
{
e.Cancel = true;
}
//If they hit "No", just close the form.
}
}
}
}
编辑 2013 年 5 月 23 日
人们会问我为什么要这样做，这是可以理解的。我们库中的数据类通常具有设计为异步运行的 Save、Load、New、Delete 函数（参见 SaveAsync 作为示例）。我实际上并不太关心在 FormClosing 事件中异步运行该函数。但是如果用户想在关闭表单之前保存，我需要它等待，看看保存是否成功。如果保存失败，那么我希望它取消表单关闭事件。我只是在寻找最干净的方法来处理这个问题。