We looked at using symbolic breakpoints to inspect how an app works.
For example, how does the navigation in an app work? We can try adding breakpoints at some common navigation locations:
-[UINavigationController pushViewController:animated:]-[UIViewController presentViewController:animated:completion:]-[UIViewController addChildViewController:]
We can combine our knowledge of UIKit with lldb to find good places for symbolic breakpoints.
Remember to import UIKit! You might need to import it in both "Swift mode" and "Objective-C" mode.
expr -l swift -- import UIKit
expr @import UIKit
Swift variables in lldb: get typed variables in LLDB Swift mode:
expr -l Swift -- let $view = unsafeBitCast(0x7df67c50, to: UIView.self)
expr -l Swift -- print($view.frame)
Swift in LLDB: If you like Swift, you can add a command alias to your ~/.lldbinit file:
command alias sc expression -l swift --
Then in lldb, you can use sc import UIKit rather than expr -l swift -- import UIKit. Hat tip to Jorge Ortiz for the alias!
We looked at Chisel, an open-source project from Facebook that adds extra debugging commands to lldb.
Remember, you can replace <obj> below with an instance or a hex address.
Some favorites:
| Command | Notes |
|---|---|
| pviews | Print the recursiveDescription of the key window |
| pvc | Print the recursive view controller hierarchy of the key window |
| pinternals | Print the internals (instance variables, etc.) of the object |
| pclass | Print the class hierarchy of an object |
| mask/unmask | Put a transparent color view on top of the view |
| border/unborder | Draw a border around the view |
| visualize | Render the target to a PNG and open it in Preview.app |
| pdocspath | Print out the Documents directory path |
| pbundlepath | Print out the app bundle directory path |
We took a quick sidebar to talk about FLEX and UIDebuggingInformationOverlay for visual debugging.
FLEX is an amazing debugging tool (not just for view debugging) and it's very easy to integrate into your own apps.
We looked at the memory graph debugger in Xcode to inspect memory usage and detect retain cycles. Note that you can't activate the memory graph debugger if you have the thread sanitizer option turned on.
Next, we looked at the thread sanitizer to help find race conditions and threading issues.
The thread sanitizer only works in the simulator and slows down the app. You can activate it in the scheme editor.
Here's the lovely retain cycle I made special for this session:
😜