当数据库连接（可能需要很长时间））运行时，显示启动屏幕 [英] Show a splash screen while a database connection (that might take a long time) runs
问题描述
我正在尝试显示一个启动屏幕，而不是在连接到数据库时冻结应用程序。正常连接（通过ADO到MSSQL）需要大约300毫秒，这不会导致主线程在Windows上显示不响应。
I am trying to show a splash screen, and not freeze up the application, while it connects to a database. Normal connections (to MSSQL via ADO) take about 300 msec, and this does not cause the main thread to show "not responding" on windows.
然而，在（a）网络错误或（b）配置错误（无效的SQL Server主机名/实例），需要60秒才能超时。这不仅使应用程序无响应，而且在冻结时几乎不可能显示任何错误或消息。我可以在开始连接之前弹出一条消息，但当主线程阻塞60秒时，实际上没有解决方案。
However in the case of (a) a network error or (b) a configuration mistake (invalid SQL server hostname/instance), it takes 60 seconds to time out. This not only makes the application non-responsive, but it's almost impossible to show any error or message when it's going to freeze. I could pop up a message before I start the connection but there's really no solution when the main thread blocks for 60 seconds.
解决方案似乎是将连接移动到一个后台线程。这导致以下代码：
The solution seems to be to move the connection to a background thread. This lead to the following code:
-
一个TThread类，使后台连接和一些SyncObj像TEvent一样用于发送一条信号返回到主线程。
a TThread-class that makes the background connection and some SyncObj like a TEvent used to send a signal back to the main thread.
主线程中使用此代码的循环：
A loop in the main thread with this code:
BackgroundThread.StartConnecting;
while not BackgroundThread.IsEventSignalled do begin
Application.ProcessMessages; // keep message pump alive.
end;
// continue startup (reports error if db connection failed)
这是正确的方法吗？我的犹豫涉及上述解决方案的以下元素：
Is this the right way to go? My hesitations involve the following elements of the above solution:
A。我会调用Application.ProcessMessages，我认为这是一个极端的代码气味（这可能是这个规则允许的例外）
A. I would be calling Application.ProcessMessages, which I consider an extreme code smell.(This might be a permissible exception to this rule)
B。我正在引入一个应用程序的启动，我担心引入错误。
B. I'm introducing threads into the startup of an application, and I'm worried about introducing bugs.
如果有人有一个已知是免费的参考实现条件，可以做一个与ADO的背景连接，并且被认为是一种安全的方法，这将是非常有帮助的。否则一般提示或部分示例是好的。
If anyone has a reference implementation that is known to be free of race conditions, that can do a background connection to ADO, and is known to be a safe approach, that would be really helpful. Otherwise general tips or partial examples are good.
推荐答案
由于每个线程必须使用自己的ADO连接的已知限制您不能使用在其他线程中创建的连接对象），我可以想到的唯一选项是创建一个将连接到数据库的线程，并且在连接建立或超时达到后，向主线程发出有关事件的信号并关闭/销毁连接。主线程可以同时显示飞溅或进度，等待来自该线程的消息。所以，你消除了错误的密码或不可达的主机的情况。有一个合理的假设，如果第二个线程可以连接，则主线程将能够在之后连接。
Because of the known limitation that every thread must use it's own ADO connection (ie you cannot use a connection object created in other thread), the only option I can think of is create a thread that will make a connection to database, and, after the connection is established or timeout is reached, signal the main thread about the event and close/destroy the connection. The main thread can in the meantime display splash or progress, waiting for a message from that thread. So, you eliminate the case with wrong password or unreachable host. There is a reasonable assumption, that, if the second thread could connect, the main thread would be able to connect right after.
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