Sunday, December 18, 2011

Adding a New Control to wxOSX: A Walkthrough.

I've decided to write down my notes about implementing a new control for wxOSX port in the hope that it can help other OS X developers to participate in wxOSX development (but, to be honest, also because I might return to it myself when I have to do this the next time).

So, without further ado, here is the full and unabridged true story [*] of implementing wxDatePickerCtrl in wxOSX/Cocoa.

To start with, you need to set up your wxWidgets development environment. For this you need to get wxWidgets sources, ideally from svn or its git mirror. You then need to build them enabling debug support to make sure you can debug them later. The simplest way to do this is to use the command line, even if you prefer to use Xcode for editing and debugging later. It's strongly recommended to build wxWidgets in a separate directory from the one containing its sources, so, assuming you put the sources in $HOME/src/wx, you should do:

$ mkdir -p ~/build/wx/osx_cocoa-debug
$ cd $_
$ ~/src/wx/configure --with-osx_cocoa --enable-debug

If you prefer to build in 32 bits, add --enable-macosx_arch=i386 switch to configure. Another useful option is -C to make configure cache the results of its checks, this will make it run much faster the subsequent times. In any case, after doing this you just need to do make -s -j8 with "-s" added to avoid huge quantities of output you probably don't care about and "-j8" being adequate for the modern quad-core CPUs. Of course, if you use a less -- or more -- powerful machine, adjust the "8" accordingly. Depending on your machine characteristics the build can take anywhere from a couple of minutes to half an hour or more but it should eventually finish. To test that everything well, and also because we will need it later for testing anyhow, try building and running the "widgets" sample:

$ cd ~/build/wx/osx_cocoa-debug/samples/widgets
$ make -s
$ open ./widgets.app


Next step is to add stubs for OS X-specific version of the control: usually, the control you want to implement will be already available under OS X but its "generic", i.e. platform-independent and hence not really Mac-looking, version would be used. We want to replace it with the native one so let's do it by modifying a few files in wxWidgets sources:

  1. Modify include/wx/datetimectrl.h to include wx/osx/datetimectrl.h if __WXOSX_COCOA__ is defined. Notice that we only implement this control for the Cocoa port version, if you were doing it for both Carbon and Cocoa, you would have tested for just __WXOSX__.

  2. Our example is actually more complicated than a typical control because there are two classes: wxDateTimePickerCtrlBase, which is the common base class for wxDatePickerCtrl and wxTimePickerCtrl and wxDatePickerCtrl itself. So we also need to update include/wx/datectrl.h in a similar way.

  3. Of course, now that we added inclusions of the new files, we actually need to create them so let's do it. Both headers should declare the corresponding classes, i.e. the same as in the common headers from where we include them but without the "Base" suffix. So let's define wxDateTimePickerCtrl, inheriting from wxDateTimePickerCtrlBase in include/wx/osx/datetimectrl.h. and, similarly, wxDatePickerCtrl inheriting from wxDatePickerCtrlBase in include/wx/osx/datectrl.h. You can copy the file structure (i.e. the standard header comment, the include guards, ...) from another OS X header or maybe from the common header that you already modified itself. Don't forget to put your name in the "Author" line!

  4. Usually, the classes we declared are concrete, so they should have the correct constructors and implement base class pure virtual methods. The "Base" classes don't define the constructors so we need to look at an existing implementation, for example the native MSW one in include/wx/datectrl.h, to see which arguments should the constructor take. You can simply copy the constructors (including the default one) and Create() function declaration from there as they must be the same for all ports, it's just their implementation that will be different.


At this stage, it might be a good idea that our new headers compile correctly (of course, nothing is going to work nor even link yet) so you could try to compile some file including them. In my case let me check if "make advdll_datavcmn.o" still works -- it does, so we didn't make any stupid mistakes (yet) and can continue.

Let's add the stubs for the implementation of the new classes too now:

  1. Create src/osx/datetimectrl_osx.cpp and src/osx/cocoa/datetimectrl.mm. The latter is an "Objective-C++" source file, hence the relatively unusual extension. Again, as we actually are implementing two new classes and not one, we also need to create src/osx/datectrl_osx.cpp. But as we suspect that we won't need any Cocoa-specific implementation for this class (as everything will be done in the base one), we don't need any other Objective-C++ files.

  2. We need to update wxWidgets build system to take the new files into account. For this, we need to edit build/bakefiles/files.bkl, locate ADVANCED_OSX_COCOA_SRC definition in it and add the new files paths to it. Also do the same for ADVANCED_OSX_COCOA_HDR.

  3. If you don't have it yet, you need to install bakefile for the next step. Just grab the DMG from download page and install it as usual.

  4. Go to build/bakefiles subdirectory and run bakefile_gen -b wx.bkl to update all make and project files for wxWidgets.

  5. Finally, re-run configure or just recreate the makefile using the command ~/src/wx/regen Makefile from the build directory.

  6. And redo make again as a sanity check. I got undefined symbol error from the linker about wxDatePickerCtrl::GetClassInfo() when doing this, which made me realize that I forgot to put wxIMPLEMENT_DYNAMIC_CLASS() in src/osx/datectrl_osx.cpp. After adding it, everything linked correctly. Of course, nothing works yet but we're going to change this soon.



So, finally, here is the interesting part: actually implementing the control using Cocoa API. The details of doing this depend on the exact control used, of course, e.g. we are going to base it on NSDatePicker in this case but another subclass of NSControl would need to be used for another control. But some things need to be done for all of them in more or less the same way because all wxOSX classes are similar and use pImpl-idiom: the wxControl-derived class itself just forwards all of its methods to its "peer", which is a pointer to an internal object of a class deriving from wxWidgetImpl implemented differently for Carbon and Cocoa ports. In more details:

  1. As we need some additional methods in our "impl", we're going to define a new class for it. Notice that sometimes this can be avoided as wxWidgetImpl already has quite a few standard methods which are enough for many common controls. In our case, however, they are not. So we add a new include/wx/osx/core/private/datetimectrl.h header and define wxDateTimeWidgetImpl class with the Set/GetDate() and Set/GetDateRange() methods corresponding to wxDatePickerCtrl methods we need to implement in it. Notice that this is a private header, not installed when wxWidgets is installed, so there is no need to add it to build/bakefiles/files.bkl unlike the public headers we had created before.

  2. We also a static CreateDateTimePicker() method that we'll use for creating the native Cocoa control to this class as well.

  3. The implementation will be the same as for the other Cocoa widgets, i.e. it will first create a subclassed NSDatePicker which is only necessary in order to call wxOSXCocoaClassAddWXMethods() (it would definitely be nice to have some way to avoid duplicating this code, even if it's quite trivial for all classes, but I don't know how to do it -- contributions from Cocoa/Objective-C experts would be welcome) and associate it with the peer object mentioned above.

  4. We also need to implement wxDateTimeWidgetImpl implementing all its pure virtual methods we added above and implement them using the corresponding NSDatePicker methods in the straightforward way. We'll call our derived class wxDateTimeWidgetCocoaImpl for consistency with the similar classes.

  5. Finally we also need to add code for generating events for our control. In this case it is very simple as there is only one event and it's the main/default action of the Cocoa control so it's enough to just override controlAction() method in wxDateTimeWidgetCocoaImpl.



By now we have a minimally working implementation of the control! It's definitely not perfect as we don't handle almost any styles of wxDatePickerCtrl, notably wxDP_ALLOWNONE, but as there doesn't seem to be any simple way to implement it with the native Cocoa control we're just going to leave it unimplemented for now. We do need to mention this to warn the users so let's update interface/wx/datectrl.h to add a note about this.
Otherwise no new documentation needs to be written as it already existed so we are almost done.

The last thing is to submit the changes for the inclusion to wxWidgets. This is normally done by submitting a patch to our Trac but as this patch is rather extensive, it's a good idea to request a review for it first. Remember to clean up your patch before submitting for the review, just as I did before submitting this patch.

While this post turned out to be quite long, it's mostly because I tried to describe all the steps, including the very first ones, in details. It also involved a rather atypical case with 2 different classes being involved when usually only one of them is. Adding new controls to wxOSX is really not that difficult and I hope this walk-through will encourage more people to do it. And if you do follow it and find any inexactitudes (this word sounds so much better than "stupid errors"), please let me know and I'll correct them here.

Happy hacking!




* For some values of "unabridged" and "true". I did simplify a few things, mainly because I implemented both wxDatePickerCtrl and wxTimePickerCtrl at the same time.
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