Easy way to detect a retain cycle in a view controller

⋅ 2 min read ⋅ Xcode Debugging

Table of Contents

A view controller is one component where memory leak usually takes place since it the one that glues many pieces of code together. One mistake, and you end up leaking.

Apple offers a sophisticated tool for debugging memory leaks like Instrument, but I have to admit that not what I use. When I suspect that there is a leak in my view controller, what I use is put a print in deinit method.

deinit {
print("deinit \(self)")

If there is a leak, deinit won't get called, and nothing will be print. Even it's not elegant, but it serves me very well for years.

A better way

Recently I come across a really nice tip from Cédric Luthi (@0xced).

He uses Symbolic Breakpoint to detect view controllers not being deallocated. The same concept as my print, but a lot cleaner. You don't populate your code with print this way.

  1. Go to Breakpoint Navigator (Menu View > Navigators > Show Breakpoint Navigator or ⌘ - command + 8).
Breakpoint Navigator
Breakpoint Navigator
  1. Click + and select Symbolic Breakpoint... or Menu Debug > Breakpoints > Create Symbolic Breakpoint...
New Symbolic Breakpoint
New Symbolic Breakpoint
  1. Set Symbol with value -[UIViewController dealloc].
  1. Click Add Action button and set Sound to pop (or any sound you like).
  1. Add another action by click + next to our sound action.
  2. Set an action to Log Message and set a message to whatever you want to print to the console when a view controller gets deallocated. In my case I set it to --- dealloc @(id)[$arg1 description]@.
  1. Check Automatically continue after evaluating actions option since we don't want a debugger to pause when a view controller gets deallocated.

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Detect a leak

With this breakpoint in place, whenever a view controller dismisses or pops out of a navigation stack, you will hear a popping sound and a log in debug console. That's mean everything is working as expected.

If you pop or dismiss a view controller and don't hear the popping sound, that's means there's a leak.

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Here are the final Symbolic Breakpoint values.

Field Value
Symbol -[UIViewController dealloc]
Module Leave blank or UIKitCore
Action: Sound pop or whatever sound you like
Action: Log Message --- dealloc @(id)[$arg1 description]@ or what ever you want to print in debug console
Options Check Automatically continue after evaluating actions
Symbolic Breakpoint
Symbolic Breakpoint

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