Reader beware: this post is part of the older "Objective-C era" on Cocoa with Love. I don't keep these articles up-to-date so the code may be broken or superceded by newer APIs. There's some good information but there's also some opinions I no longer endorse – keep a skeptical mind. Read "A new era for Cocoa with Love" for more.
In this post I will recreate code that is normally concealed between the
NSApplicationMain call (invoked in a Cocoa application's
main function) and the
sendEvent: method (which distributes the events to windows and views from the main run loop). By recreating this code for you, I hope to explain the steps that occur between program startup and the dispatch of events to your code — so you can gain greater understanding of what
NSApplication does on your behalf.
The default Cocoa Application template provided by Xcode contains just one line of code, in one function:
Somehow, this one line of code is enough to create a menubar, put a window onscreen and then handle user events (mouse clicks and menu selections) until the user quits the program, at which point the application gracefully exits.
What we know
Scattered throughout the Cocoa documentation are hints about what must happen in
- Read from the "Info.plist" file to determine:
- The name of the "MainMenu" NIB file to load automatically on startup
- The name of the application's principal class (the class of the application object)
- Construct the application object
- Load the "MainMenu" NIB file
- Begin the "Run Loop" (which handles the rest of the program)
I will recreate all of these steps by implementing my own version of
NSApplicationMain — named
MyApplicationMain — and by creating my own
NSApplication subclass that implements the
run method for itself.
Create the application object
The first step in the
MyApplicationMain function is to read the name of the "Principal class" and construct the application object from this class.
infoDictionary method returns the "Info.plist" for a bundle (in this case, our application's bundle) converted into an
NSDictionary for easy access. All we need to do is get the "NSPrincipalClass" string and fetch the class identified by that string.
Since the application is a singleton, we construct it by calling the
sharedApplication method on the class. If we try to construct the object using standard
init calls, the
NSApp singleton instance won't be set correctly and an exception will be thrown at a later point when a second application is created.
For more information on singletons, you can see my earlier post about singletons or visit Apple's page on Creating a Singleton Instance.
Load the contents of the MainMenu NIB file
Loading the "MainMenu" NIB file is a similar task: get the name from the
infoDictionary and then load it.
I briefly considered implementing the
NSNib code too, to show how that works for loading objects in a NIB file, but since the format of a NIB file is not publicly declared, it wasn't possible. Suffice it to say that the
instantiateNibWithOwner:topLevelObjects method performs the following steps:
- Allocates all the objects in the NIB file and initializes them with one of the following methods:
initWithCoder:(used for most other objects in the Interface Builder library)
init(used for all other objects)
- Sets the
IBOutletpointers on objects as specified in the NIB file and establishes all bindings
awakeFromNibon all objects that implement this method
Start the run loop
It is extremely important that the application's main loop runs on the main thread. For this reason, we don't directly invoke
run but instead make sure it is performed on the main thread.
I also check that the
applicationObject will respond to the method before trying to invoke it, just in case the "Principal Class" from the Info.plist file is not a valid
Implementing our own run loop
The remaining task required so that you can trace the application's execution from startup to dispatch of events to your code is the implementation of the run loop.
Despite Apple's comment in the
NSApplication documentation that
run is one of the "Methods to Override", my brief search failed to uncover anyone who had actually done it. My suspicion is this: it is not a method to override and that document is woefully out-of-date. The only reason to implement your own run method now is curiosity, not functionality.
In that spirit, here's a reimplementation of
run and a corresponding
It's as simple as you might expect: take the next event out of the queue for your application and route it appropriately using
sendEvent: which handles the task of determining which window wants the event and invokes
sendEvent: on the window to further route the event to a specific view.
For those interested: I discovered all the methods that need to be invoked in the
run method in the most naive way: by searching the
NSApplication documentation for "loop" to find all methods that claimed they were invoked in the main run loop.
The specific function of
finishLaunching method is a bit of a mystery. All I know is that menus, menu updates and a host of other features won't work if it isn't invoked. It is possible that it sets up the responder chain for some actions. It is possible it adds observers to the run loop. I don't really know.
nextEventMatchingMask:untilDate:inMode:dequeue: is the biggest reason why I chose, when "reimplementing"
NSApplication for this post, to make my implementation a subclass of
NSApplication instead of a completely separate class. This method (or something it contains) converts notifications from the operating system and hardware drivers into
NSEvent objects. A principal class is not required to be a subclass of
NSApplication but I wouldn't know where to begin reimplementing this method, so I had to use
NSApplication to do it.
You can download an implementation of the default Cocoa Application Xcode 3.1 project that uses this code: RecreatingNSApplication.zip (60kB)
This recreation of
run does not do everything that the real implementations do (I've deliberately kept it simple for clarity) but I think it shows that the key steps involved are straightforward and easy to understand.
There are a few steps that I haven't reimplemented: loading NIB files, retrieving events and routing events. Unfortunately, these remain black boxes to me, so I can't shed further light on them. You can find out some things by setting breakpoints (in your constructors for objects created in NIBs or in your event methods) and reading the names of Apple functions invoked to bring these commands to you.
Beyond this, you can also read how GNUStep implements the same methods. Apple doesn't necessarily do the same thing but GNUStep at least represents an observable solution to the same problems.
Multiple row selection and editing in a UITableView