LLDB toolbox: debugging views with private APIs in Swift

Quick overview of some of the view debugging private APIs in the SDK and how to use them with Swift.

• +[UIViewController _printHierarchy] - prints view controller hierarchy:

(lldb) po [UIViewController _printHierarchy]
<UITabBarController 0x7ffb02c98720>, state: appeared, view: <UILayoutContainerView 0x7ffb02d53140>
   | <UINavigationController 0x7ffb0301ac00>, state: appeared, view: <UILayoutContainerView 0x7ffb02cad460>
   |    | <ViewController 0x7ffb02c98af0>, state: appeared, view: <UIView 0x7ffb02ca9870>
   | <UINavigationController 0x7ffb03035a00>, state: disappeared, view:  (view not loaded)
   |    | <SettingsViewController 0x7ffb02d4ccc0>, state: disappeared, view:  (view not loaded)

• -[UIView recursiveDescription] - prints view hierarchy. Note that nowadays it may be more convenient to use Xcode’s visual debugger for going over views structure.
• -[UIView _autolayoutTrace] - prints view hierarchy for the whole app with hints about ambiguous layout
• -[UIView _recursiveAutolayoutTraceAtLevel:] - prints hierarchy with hints only for the specified view. Just use 0 for paramater, it only determines level of indendation.

When breaking inside Swift code, we’re relying on Swift compiler to parse expression. Using Objective-C syntax won’t fly.

(lldb) po [UIViewController _printHierarchy]
error: <EXPR>:1:19: error: expected ',' separator
[UIViewController _printHierarchy]
                  ^

It’s private, so Swift can’t see it as well.

(lldb) po UIViewController._printHierarchy()
error: <EXPR>:1:1: error: type 'UIViewController' has no member '_printHierarchy'
UIViewController._printHierarchy()
^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~

The method is there, but LLDB needs to be informed to use Clang to get to it. That can be achieved with -l/--language switch. po is in fact expression -O --. After adding language information proper way to call _printHierarchy becomes:

expression -l objc++ -O -- [UIViewController _printHierarchy]

Writing expression -l objc++ -O -- each time we want to use SDK’s private API is no fun ¹. To save keystrokes add an alias to ~/.lldbinit file:

command alias spo expr -l objc++ -O --

To reload lldbinit without restarting Xcode execute : command source ~/.lldbinit.

Quick test inside Swift frame confirms that everything works.

(lldb) spo [UIViewController _printHierarchy]
<UITabBarController 0x7febdb4264a0>, state: appeared, view: <UILayoutContainerView 0x7febdb438230>
   | <UINavigationController 0x7febdc015800>, state: appeared, view: <UILayoutContainerView 0x7febdb43acb0>
   |    | <LLDBTesting.SwiftViewController 0x7febdb6073e0>, state: appeared, view: <UIView 0x7febdb5c6710>
   | <UINavigationController 0x7febdc014400>, state: disappeared, view:  (view not loaded)
   |    | <SettingsViewController 0x7febdb429a80>, state: disappeared, view:  (view not loaded)

Happy debugging!