turing complete with a stack of 0xdeadbeef
Whether you are starting a new job or joining a new project, getting oriented in a new iOS codebase can be difficult and overwhelming. It is particularly hard if the codebase is very large, and especially challenging if you are early in your career and do not yet have much experience to draw from.
When debugging a large project in Xcode that a large team works on, the console can get quite busy. Logs are everywhere! It can be difficult to sift through the noise, particularly when you have a number of breakpoints configured to log messages, execute debugger commands, and continue after evaluating rather than pause.
In my previous post, I explained how to use symbolic breakpoints to discover when view controllers load their views into memory. Often breakpoints are specific to a project. You’ll create one for a specific class that only exists for that particular app. However, what I discussed in that post would be useful in any project. Unlike regular breakpoints, symbolic breakpoints (at least when set on system frameworks) are more or less universal.
While working on a very large iOS client project, I was investigating the causes for our slow app launch time. We had a hypothesis that part of the problem was that too many view controllers were getting loaded in memory, in particular, ones that were not even being presented to the user during app startup. What could cause view controllers to load too early? How might you discover this happening? And how do you fix it? Let’s find out.
On iOS the operating system employs a watchdog that monitors for and terminates unresponsive apps. If your app is blocking the main thread for too long, the system will kill it. In crash reports, you can identify watchdog terminations via the termination reason code 0x8badf00d (“ate bad food”).
A few weeks ago, I wrote about this bizarre Xcode 13 crash when running tests. I just discovered the root cause for one of the issues I mentioned in that post — I think. At the very least, I have a “fix”. The issue happens when running unit tests. Sometimes the full test suite will complete, sometimes not, and then LLDB will crash. This occurs with all of my projects. It doesn’t seem to matter what I do, the crash always happens. It has been driving me crazy.
As I continue to pursue Mac app development more seriously, I can build on and borrow from my many years of iOS experience. While many aspects of writing Mac apps are very similar to iOS, or at least somewhat familiar, other aspects are quite different. One of the big differences is testing, and deciding how many versions of macOS to support.
In JavaScript, this pattern is called an Immediately Invoked Function Expression (IIFE) or a Self-Executing Anonymous Function. Swift doesn’t have an “official” name for this, but IIFE works as well as “immediately executed anonymous closure” or “self-executing anonymous closure”. (Thanks to folks on Twitter for helping with this.)
While debugging some code the other day, I wanted to verify the behavior of global variables and static members in Swift. I vaguely remembered from the early days of Swift, that static let members and global constants were atomic and computed lazily — one of the many improvements over Objective-C.
For an iOS project that I am currently working on, I am implementing Dark Mode. The codebase is approaching 7 years old, it is mostly Swift with some legacy Objective-C, and it currently supports iOS 11 and above. Aside from the tedium of ensuring the updated colors are being used throughout the codebase, I expected this task to be straight-forward. However, there were some unanticipated issues.
I recently discovered that unit tests and UI tests for a macOS Xcode project will fail with obscure error messages if the hardened runtime is enabled. It took me awhile to realize what the actual source of the problem was, because the error messages led me in the wrong direction. Hopefully this will save you some time.
Xcode has a great UI for setting and editing breakpoints. I use breakpoints all the time while working and debugging, but I want to share another, unconventional way that I use them.
Continuing from my previous post, there is a bug in NSStatusItem that I forgot to mention. Thanks to Michael Tsai (tweet) for pointing out this workaround.
The other day I was debugging a crash in a UI test for an open pull request at work. The bug turned out to be extremely subtle and difficult to notice. I spent way too much time staring at the changes, trying to understand what was wrong. Let’s see if you can spot the error.