如何在Qt / c ++中使用多线程通过Api访问我的数据库？

Question

I have a problem, I'm connecting my program to the data base with an Api, to do this I use the network and webkit webkitwidgets funtions in Qt. I have been able to do this without any problem.

The think it's that I have to call some funtions in the api class from different threads.

Firts I try passing the api object to the Qthread class, but I'm getting this error:

@QObject: Cannot create children for a parent that is in a different thread.
(Parent is SerialPort(0x24a1738), parent's thread is QThread(0x24a1770), current thread is QThread(0x18a8b0)

So I try ussing signals ans slots, it work with the firts function I use, a funtion to create what a call in my data base a log, but when I try to close the log, so I can send the information needed. I get this error:

 QEventLoop::exec: instance xxxxxx has already called exec()

What is the best way to work with Threads to avoid this problems ??

Answer 1

You haven't provided any code, only error messages. Since I have to guess, I'll say that you're creating a QObject -derived worker object and using the QObject::moveToThread method to change its thread affinity. The problem in this case is that you've already given the worker object a parent. You can't use moveToThread on an object that already has a parent. So, don't set the parent of your worker object when you create it and you should be good-to-go.

As others users have commented, you will need to use signals & slots to initiate queries, etc. in your worker object. Qt will handle synchronization issues via the thread's event loop, so you don't have to worry about mutex locking, etc (as long as you use only sig/slot to invoke methods and get output values from the worker object).

Don't forget to call QThread::start to get the thread's event loop started.

NOTE 1: I assume you're using a fairly recent version of Qt (>= 4.8).

NOTE 2: If you're using QSqlDatabase and friends to access your DB, make sure to create the DB connection in the worker object after it is moved to the new thread.