Chisel

Chisel is a collection of LLDB commands to assist in the debugging of iOS apps.

[Installation • Commands • Custom Commands • Development Workflow Contributing • License]

For a comprehensive overview of LLDB, and how Chisel compliments it, read Ari Grant's Dancing in the Debugger — A Waltz with LLDB in issue 19 of objc.io.

Installation

brew update
brew install chisel

Then follow the instructions that Homebrew displays to add chisel to your ~/.lldbinit.

Alternatively, download chisel and add the following line to your ~/.lldbinit file. If it doesn't exist, create it.

# ~/.lldbinit
...
command script import /path/to/fblldb.py

The commands will be available the next time Xcode starts.

Commands

There are many commands; here's a few: (Compatibility with iOS/Mac indicated at right)

Command	Description	iOS	OS X
pviews	Print the recursive view description for the key window.	Yes	Yes
pvc	Print the recursive view controller description for the key window.	Yes	No
visualize	Open a UIImage, CGImageRef, UIView, or CALayer in Preview.app on your Mac.	Yes	No
fv	Find a view in the hierarchy whose class name matches the provided regex.	Yes	No
fvc	Find a view controller in the hierarchy whose class name matches the provided regex.	Yes	No
show/hide	Show or hide the given view or layer. You don't even have to continue the process to see the changes!	Yes	Yes
mask/unmask	Overlay a view or layer with a transparent rectangle to visualize where it is.	Yes	No
border/unborder	Add a border to a view or layer to visualize where it is.	Yes	Yes
caflush	Flush the render server (equivalent to a "repaint" if no animations are in-flight).)	Yes	Yes
bmessage	Set a symbolic breakpoint on the method of a class or the method of an instance without worrying which class in the hierarchy actually implements the method.	Yes	Yes
wivar	Set a watchpoint on an instance variable of an object.	Yes	Yes
presponder	Print the responder chain starting from the given object.	Yes	Yes
...	... and many more!

To see the list of all of the commands execute the help command in LLDB.

(lldb) help
The following is a list of built-in, permanent debugger commands:
...

The following is a list of your current user-defined commands:
...

The bottom of the list will contain all of the commands sourced from Chisel.

You can also inspect a specific command by passing its name as an argument to the help command (as with all other LLDB commands).

(lldb) help border
Draws a border around <viewOrLayer>. Color and width can be optionally provided.

Arguments:
  <viewOrLayer>; Type: UIView*; The view to border.

Options:
  --color/-c <color>; Type: string; A color name such as 'red', 'green', 'magenta', etc.
  --width/-w <width>; Type: CGFloat; Desired width of border.

Syntax: border [--color=color] [--width=width] <viewOrLayer>

All of the commands provided by Chisel come with verbose help. Be sure to read it when in doubt!

Custom Commands

You can add local, custom commands. Here's a contrived example.

#!/usr/bin/python
# Example file with custom commands, located at /magical/commands/example.py

import lldb
import fblldbbase as fb

def lldbcommands():
  return [ PrintKeyWindowLevel() ]
  
class PrintKeyWindowLevel(fb.FBCommand):
  def name(self):
    return 'pkeywinlevel'
    
  def description(self):
    return 'An incredibly contrived command that prints the window level of the key window.'
    
  def run(self, arguments, options):
    # It's a good habit to explicitly cast the type of all return
    # values and arguments. LLDB can't always find them on its own.
    lldb.debugger.HandleCommand('p (CGFloat)[(id)[(id)[UIApplication sharedApplication] keyWindow] windowLevel]')

Then all that's left is to source the commands in lldbinit. Chisel has a python function just for this, loadCommandsInDirectory in the fblldb.py module.

# ~/.lldbinit
...
command script import /path/to/fblldb.py
script fblldb.loadCommandsInDirectory('/magical/commands/')

There's also builtin support to make it super easy to specify the arguments and options that a command takes. See the border and pinvocation commands for example use.

Development Workflow

Developing commands, whether for local use or contributing to Chisel directly, both follow the same workflow. Create a command as described in the Custom Commands section and then

1. Start LLDB
2. Reach a breakpoint (or simply pause execution via the pause button in Xcode's debug bar or process interrupt if attached directly)
3. Execute command source ~/.lldbinit in LLDB to source the commands
4. Run the command you are working on
5. Modify the command
6. Optionally run script reload(modulename)
7. Repeat steps 3-6 until the command becomes a source of happiness

Contributing

Please contribute any generic commands that you make. If it helps you then it will likely help many others! :D See CONTRIBUTING.md to learn how to contribute.

License

Chisel is BSD-licensed. See LICENSE.