qt 5中的信号槽问题

Question

I have one c++ class and have one signal in it and want to connect that signal with slot another C++ class. Here is my code

class Data : public QObject
{
Q_OBJECT

public:
static Data *instance(void);
 signals:
 void sendUserID(const QString& userId);

 private:
static Data *s_instance;
};

here is my Slot in another class

void DataDetails::seTUserID(const QString user_id)
{
 QAndroidJniObject user_id = QAndroidJniObject::fromString(user_id);
 QAndroidJniObject::callStaticMethod<void>("com/user/data/userActivity",
                                          "setUserID",
                                           "(Ljava/lang/String;)V",
                                           user_id.object<jstring>());
}

The idea is to access the value of user_id from Data class to DataDetails class

The connection is trying is

 QObject::connect(&s_instance, SIGNAL(sendUserID(uid), this, SIGNAL(setUserID(uid))

any other id to get uid to other class is also fine ..

Answer 1

Generally speaking, when you encounter an issue with QObject::connect ...

Make sure you have your declarations in order:

• Both classes need the Q_OBJECT macro in their declaration.
• Make sure your slot is actually declared as a slot (ie is part of a public slot: section).

Because connecting signals and slots just uses character strings evaluated at run-time, it's possible to write absolute nonsense and have it compile. In my experience, errors caused by typos are pretty common.

• Always test in an environment where you can see your application's console output. Failed connect calls will usually trigger an error message printed to stderr.
• Double-check your method names and signatures. Code will still compile even if you've made a typo!
• For debugging, use assertions (eg bool c = connect(...); Q_ASSERT(c); ) to catch missed connections early.
• Alternatively, you can use the QMetaMethod-based version of QObject::connect , introduced in Qt 4.8, to avoid some of these issues.

---

In your particular case:

1. You've got a typo in the function declaration: it's called seTUserID but you're using setUserID in the connect call.
2. You're using variable names, not function signatures, in your signal and slot names. Qt expects to see QObject::connect(&s_instance, SIGNAL(sendUserID(const QString), this, SLOT(setUserID(const QString))
3. You've got a signal connected to another signal , which is valid but doesn't do what you want (it's usually used to chain stuff like this: SomeChildWidget's signal -> MyClass1's signal -> MyClass2's slot).

Answer 2

Check that seTUserID definition is marked as slot. You are probably calling connect in some Data method (because you are using private member directly). Are you trying to use a ref to a pointer to s_instance O_o? Write sendUserID(const Qstring) (the signature of a signal) rather then sendUserID(uid). The same situation with setUserID. You are trying to connect to this pointer and want to send info to another class!? Use new style of Qt connecting signals and slots (pointers to members), with was introduced in Qt 5. Check for setUserID has written right.