DLL Injection with slots... ?!?!?

[tbscope]

I'm assuming you didn't create a plugin. This means a library exporting certain symbols that you use directly in the application you want to inject.

I assume that you can not change the application code itself directly. If that's not the case, then see the examples and documentation of creating plugins or using libraries.

Consider the following schema:

+-----------------------------------------------------------------------------------------+
| Application                                                                             |
|                                                                                         |
|    +---------------+               +-----------------------------------------+          |
|   +---------------+|               | Application code, contains objects.     |          |
|  +---------------+||               |                                         |          |
| +---------------+|||               | mainWindow (a QMainWindow)              |          |
| | Linked        |||+               |      |                                  |          |
| | libraries     ||+                |      +-- button (a QPushButton)         |          |
| |               |+                 |      +-- label (a QLabel)               |          |
| +---------------+                  +-----------------------------------------+          |
|        |                                                                                |
|        v                                                                                |
| +-------------------------------------------------------------------------------------+ |
| | Your injected DLL                                                                   | |
| |                                                                                     | |
| | +--------------------------+                                                        | |
| | | Class based on QObject   |                                                        | |
| | | inside DLL               |                                                        | |
| | |                          |                                                        | |
| | | Contains signals and     |                                                        | |
| | | slots                    |                                                        | |
| | +--------------------------+                                                        | |
| |              |                                                                      | |
| |              v                                                                      | |
| |      +-------------------------------------------------------------------------+    | |
| |      | Create an object:                       | Do this from within a context |    | |
| |      |                                         | where the application event   |    | |
| |      | MyClass *myclass = new MyClass;         | loop is running.              |    | |
| |      +-------------------------------------------------------------------------+    | |
| |                            |                                                        | |
| |                            v                                                        | |
| |                +------------------------------------------------+                   | |
| |                | Example:                                       |                   | |
| |                | Suppose you have installed an application      |                   | |
| |                | event filter.                                  |                   | |
| |                |                                                |                   | |
| |                | Also, suppose you intercept the mainWindow     |                   | |
| |                | show event.                                    |                   | |
| |                |                                                |                   | |
| |                | From this event, you have a pointer to         |                   | |
| |                | mainWindow, let's call it pMainWindow          |                   | |
| |                |                                                |                   | |
| |                | Then you can write:                            |                   | |
| |                |                                                |                   | |
| |                | connect(pMainWindow->button, SIGNAL(...),      |                   | | 
| |                | myclass, SLOT(...));                           |                   | |
| |                |                                                |                   | |
| |                |                                                |                   | |
| |                | In pseudocode:                                 |                   | |
| |                | ---------------------------------------------- |                   | |
| |                | when application started                       |                   | |
| |                |     install eventfilter                        |                   | |
| |                |                                                |                   | |
| |                | when eventfilter gets called                   |                   | |
| |                |     check the event and the target object      |                   | |
| |                |     if event = show and object = mainWindow    |                   | |
| |                |         Create a new MyClass object if none    |                   | |
| |                |         already exists.                        |                   | |
| |                |         Connect signals and slots              |                   | |
| |                |                                                |                   | |
| |                +------------------------------------------------+                   | |
| |                                                                                     | |
| +-------------------------------------------------------------------------------------+ |
|                                                                                         |
+-----------------------------------------------------------------------------------------+

+-----------------------------------------------------------------------------------------+
| Application                                                                             |
|                                                                                         |
|    +---------------+               +-----------------------------------------+          |
|   +---------------+|               | Application code, contains objects.     |          |
|  +---------------+||               |                                         |          |
| +---------------+|||               | mainWindow (a QMainWindow)              |          |
| | Linked        |||+               |      |                                  |          |
| | libraries     ||+                |      +-- button (a QPushButton)         |          |
| |               |+                 |      +-- label (a QLabel)               |          |
| +---------------+                  +-----------------------------------------+          |
|        |                                                                                |
|        v                                                                                |
| +-------------------------------------------------------------------------------------+ |
| | Your injected DLL                                                                   | |
| |                                                                                     | |
| | +--------------------------+                                                        | |
| | | Class based on QObject   |                                                        | |
| | | inside DLL               |                                                        | |
| | |                          |                                                        | |
| | | Contains signals and     |                                                        | |
| | | slots                    |                                                        | |
| | +--------------------------+                                                        | |
| |              |                                                                      | |
| |              v                                                                      | |
| |      +-------------------------------------------------------------------------+    | |
| |      | Create an object:                       | Do this from within a context |    | |
| |      |                                         | where the application event   |    | |
| |      | MyClass *myclass = new MyClass;         | loop is running.              |    | |
| |      +-------------------------------------------------------------------------+    | |
| |                            |                                                        | |
| |                            v                                                        | |
| |                +------------------------------------------------+                   | |
| |                | Example:                                       |                   | |
| |                | Suppose you have installed an application      |                   | |
| |                | event filter.                                  |                   | |
| |                |                                                |                   | |
| |                | Also, suppose you intercept the mainWindow     |                   | |
| |                | show event.                                    |                   | |
| |                |                                                |                   | |
| |                | From this event, you have a pointer to         |                   | |
| |                | mainWindow, let's call it pMainWindow          |                   | |
| |                |                                                |                   | |
| |                | Then you can write:                            |                   | |
| |                |                                                |                   | |
| |                | connect(pMainWindow->button, SIGNAL(...),      |                   | | 
| |                | myclass, SLOT(...));                           |                   | |
| |                |                                                |                   | |
| |                |                                                |                   | |
| |                | In pseudocode:                                 |                   | |
| |                | ---------------------------------------------- |                   | |
| |                | when application started                       |                   | |
| |                |     install eventfilter                        |                   | |
| |                |                                                |                   | |
| |                | when eventfilter gets called                   |                   | |
| |                |     check the event and the target object      |                   | |
| |                |     if event = show and object = mainWindow    |                   | |
| |                |         Create a new MyClass object if none    |                   | |
| |                |         already exists.                        |                   | |
| |                |         Connect signals and slots              |                   | |
| |                |                                                |                   | |
| |                +------------------------------------------------+                   | |
| |                                                                                     | |
| +-------------------------------------------------------------------------------------+ |
|                                                                                         |
+-----------------------------------------------------------------------------------------+

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You create a dll that reimplements the application event function to install an event filter.
In that event filter, you intercept the show event of a widget (for example). While intercepting that event, create a new QObject based object that contains signals and slots. Connect the slots of that object to the signals of the widget (or a member of that widget).

Make sure that when you build your library, the definitions of all the classes are known (include the correct headers). Also make sure that your code is processed by MOC.

Then inject the library in the program.

How this is exactly done on Windows, I don't know. Maybe you don't need to reimplement the event function of the application in order to install an event filter. That would make it a little bit easier.

EDIT: this is just a brainstorm from me. I do not assume that everything above is 100% correct.