AppKit, the GUI framework, is not thread safe. In order for things to work properly, you (almost) always need to update GUI classes from the main thread

–Dave Dribin (slightly edited)

I’ve run into UI + threading problem before, but I’d just never seen this limitation of AppKit spelled out.

Dave’s article explains how to call code on the main thread better then I can.

Obviously, for editing a workflow to be useful, you need to be able to add actions to it. And to do that, you need to be able to browse and search actions. But the current Automator.framework has no official support for doing this.

But there are undocumented private APIs, which Automator.app (link launches it) uses…

How to Find Them

class-dump is an amazingly useful command-line tool that lets you generate headers from a compiled Objective-C binary. Download it, and point it at /System/Library/Frameworks/Automator.framework/Automator, to see everything Automator.framework really lets you do.

Instant Library Panel

The most useful definition I found was,
@interface NSApplication (AMLibraryPanel)
- (void)orderFrontAutomatorLibraryPanel:(id)sender;
@end


Calling [NSApp orderFrontAutomatorLibraryPanel:nil]; will show a panel with the same action-library view Automator has.

This is what I’m using right now in IMLocation to let people find actions for workflows. I don’t like the idea of using private APIs. But I like the idea of implementing my own AMLibraryView replacement even less. It would be a lot of work to make it as good as the real thing. Any differences give users a fractured experience — one way to do something in Automator, another in my program. And I would get dismal ROI, because I expect Apple to expose this functionality in the (near) future.

(I also wouldn’t be so bullish on using private APIs if I didn’t have workarounds in place right now. The only way orderFrontAutomatorLibraryPanel: gets called is if a user presses a “Show Actions” button on the toolbar. That same toolbar has an “Open in Automator” button that opens the worklfow in Automator, where they can edit it without bugs. So even if orderFrontAutomatorLibraryPanel: stopped working tomorrow, users could still do everything they could before — albeit less elegantly).

Looking Just Like Automator

I don’t recommend doing this, but I’ve been able to embed an AMLibraryView in an NSSplitView next to an AMWorkflowView, to get a more Automator.app-like appearance.

Here’s how I did it, given the IBOutlets workflowViewAndLibrarySplitView which is an NSSplit view with an AMWorkflowView in one side, and the other side’s view connected to the outlet workflowLibraryView,

[workflowViewAndLibrarySplitView replaceSubview:workflowLibraryView with:[[AMLibraryPanel sharedLibraryPanel] _libraryView]];

Will put the library view inside your split view.

Of course this screws up orderFrontAutomatorLibraryPanel:. And I would not expect it to work in two windows. And it’s using an underscore-private method of an already private and undocumented API. That’s just to too risky for me; so I stick with the panel. It gets the job done with one line of code, and that’s good enough for me.

Accidently disconnected outlets in shipping Cocoa apps are legend.

– Jonathan “Wolf” Rentzsch

From what I can tell assert() is slowly going the way of the goto in the programming world. Exceptions, unit-tests, and other modern software engineering practices, seem to have a better answer for testing something at runtime, and ensuring that you never enter a bad state.

But there is one case where I think you should put an assert() (and not an NSAssert()) in your code: in awakeFromNib assert() that every IBOutlet is connected.

It’s surprisingly easy to accidentally disconnect something in Interface Builder, or rename something in Xcode. I’ve done it before. More then once. assert()-ing IBOutlets has saved me a lot of debugging time.

Hard to Find In Code

An IBOutlet that is not connected is nil. Because Objective-C quietly ignores messages sent to nil, it’s very easy not to notice the problem for a while; then spend a long time debugging a side effect of the issue.

Hard to Find By Eye

A widget in a nib/xib file that is not connected will never change it’s state. But it typically it has a reasonable initial state. This makes it difficult to detect disconnected IBOutlets by eye, because things will look right until they are supposed have changed, but the eye is drawn to change, and de-emphasizes unchanging things.

Why assert()?

As Wolf says, the worst-case-scenario is that right before release you make a trivial change in Interface Builder (“We can’t ship with that window saying “Claculator”), an IBOutlet gets disconnected, and nobody notices in time.

But an assert() failing will be noticed with casual testing, and triggered if the nib is loaded at all, even if the series of interactions needed to cause the widget to change state are not performed.

An assert() is very light-weight and easy to do. It’s exactly one line of code, and you don’t have to add a unit testing framework to your project to do it. And that makes it perfect for test projects. I’m a believer in getting a feature working in a test project first, then copying it into your real product. (Honestly I don’t do it as much as I should, and I almost always kick myself for it.) It lets you test and learn without hacking-up your product. Plus, new things are often easier to try without the weight of a big code base.

Speaking of unit tests, I don’t have complete faith in them here. That’s because a disconnected IBOutlet in a shipping program is fundamentally an issue with the way the release-build is configured. So anything that’s not testing the the actual release build is not exhaustively testing for this problem. an OCUnit-test can catch a disconnected IBOutlet if it is loaded into the application. But in my experience, such tests are cumbersome. Running them involves launching your full application, and programatically manipulating it. This can take a while to run since all the UI will be displayed and animated. And you have to be careful about state since each test will be changing the state of your program.

Ultimately, assert() just seems to be optimal, in dependability and simplicity, for catching a disconnected IBOutlet.

But I suspect there’s a better way I don’t know.

How do you catch IBOutlets?

The application had to install a component, which involved displaying authentication dialogs and such. Because the component was necessary for the application to work correctly, I thought it would be safest to do this as early as possible. But displaying a dialog in code called from awakeFromNib ended up being the cause of the double Help Menu issue.

Waiting until applicationWillFinishLaunching: or applicationDidFinishLaunching: to interact with the user fixed the problem. (In my case, it was safe to defer the installation until then.)

I am not aware of any other issues from putting up a window “too early” … however, it seems to me that doing it is asking for trouble. The AppKit/Cocoa environment obviously isn’t 100% ready at that point. Why risk running your code with half-baked libraries if you don’t have to?

Whenever possible, I will defer “first run” behavior until applicationWillFinishLaunching: or applicationDidFinishLaunching:.

Let me say this again in slow motion: NEVER type in ANY English string without typing NSLocalizedString() around it! This will save you SO MUCH HASSLE later on when your app is popular. Remember that enterprising polyglots can localize your code from just the binary you ship if you follow a few rules of localization, so you may wake up one day and find that someone from across the world has mailed you a your app in another language. It’s a fuzzy feeling and it gets you instant market-share.

–Wil Shipley

Problem

If you send a setHidden:YES message to an NSTextView, and it’s text disappears, but the view itself (white box) stays visible here’s the problem, and the solution.

It turns out that if you created the NSTextView by dragging it off the pallet in Interface Builder, then it’s not an NSTextView. It’s an NSTextView wrapped inside an NSClipView inside an NSScrollView. The NSScrollView is what puts up the scroll-bars if the NSTextView gets really big; the NSClipView helps make the scrolling work.

So if text is your IBOutlet to your NSTextView, then when you say [text setHidden:YES];, the NSTextView is hidden, but the the total package won’t disappear, unless you hide the NSScrollView as well.

Solutions

You can send the message to NSScrollView containing text, like so:
   [[text enclosingScrollView] setHidden:YES];.
This will hide everything inside the NSScrollView, including text.

Another solution is to create just an NSTextView in Interface Builder. To do this, put an NSView in your interface (it’s called a “Custom View”,in the Interface Builder objects pallet). Then select it, bring up the object inspector (cmd-shift-i), choose the “custom class” from the category menu at the top, and select NSTextView from the list of subclasses. This puts an NSTextView in your interface, without the surrounding clip and scroll views. Unfortunately, it also means you can’t configure it in Interface Builder, beyond resizing it. That’s why I’m not partial to this approach, although I have used it.

Thanks to ZachR for suggesting enclosingScrollView.

Problem:
When using an NSAlert to display a sheet in a multi-threaded application, unexpected badness can happen.

I was using
beginSheetModalForWindow:modalDelegate:didEndSelector:contextInfo:
To display an NSAlert as a sheet.

But when the sheet appeared, the window it was attached to disappeared and got into some weird broken state where it would appear iff the application was not frontmost.

Fortunately, I remembered having encountered weirdness with NSAlert sheets before. The symptoms were different (previously the alert didn’t have focus), but the same solution still worked.

Solution: make sure the message to display the sheet is sent by the main thread. To do this, put the call to beginSheetModalForWindow:modalDelegate:didEndSelector:contextInfo: inside another method, showMyAlert, then use performSelectorOnMainThread:withObject:waitUntilDone: to make sure showMyAlert is called on the main thread.

Work around use runModal to display the alert as a modal dialog instead of a sheet. runModal Does not appear to have any problems when called from other threads.

Just like last time:

The whole incident feels funny to me. I suspect there may be some deeper issue at work that I am not aware of. When I have time to investigate further I shall update this post. Unfortunately I don’t have time to look into ‘solved’ bugs today.

UPDATED 2008-12-26: in general, all AppKit code should be called on the main thread.