修复 bug 占用我们日常开发的大部分时间，熟练的使用调试工具可以给我们节约大部分的时间。
LLDB 的常用命令
expression
expresion 是一个非常常用的命令，我们可以通过这个命令来执行一些表达式，这样我们就不需要重写运行工程了，例如：
(lldb) expression -- self.view.backgroundColor = [UIColor greenColor]
(lldb) expression -- (void)[CATransaction flush] // 用于刷新页面，我们也可以使用 expression 来输出我们关注的信息
(lldb) expression -O -- self.view //这时候 就会输出对象 UIView，注意 -O 代表输出的是一个对象 ，这里有另外一种简写方式 po.
<UIView: 0x7d2974a0; frame = (0 0; 375 667); autoresize = W+H; layer = <CALayer: 0x7d294813>>有时候我们自定义的类并没有重写 description 方法，如果我们直接输出这个对象的话可能只会显示这个类名和地址,例如
(lldb) expression -O -- self
<JChatAboutMeViewController: 0x7d2974a0>这地址并不是我们想要的，我们想要的是这个对象内部信息，这里我推荐一个插件 chisel ,我们可以使用chisel 提供的命令来打印这个对象, 例如
(lldb) pinternals 0x7d2974a0
(JChatAboutMeViewController) $4 = {
UIViewController = {
UIResponder = {
NSObject = {
isa = JChatAboutMeViewController
}
_hasOverrideClient = ' '
_hasOverrideHost = ' '
_hasInputAssistantItem = ' '
}
_overrideTransitioningDelegate = nil
_view = nil
_tabBarItem = nil
_navigationItem = 0x7d4abf40
_toolbarItems = nil
_title = nil
_nibName = nil
_nibBundle = nil
_parentViewController = 0x7cb84c00
_childModalViewController = nil
_parentModalViewController = nil
_previousRootViewController = nil
_modalTransitionView = nil
_modalPreservedFirstResponder = nil
_dimmingView = nil
_dropShadowView = nil
_currentAction = nil
_storyboard = nil
_externalObjectsTableForViewLoading = nil
_topLevelObjectsToKeepAliveFromStoryboard = nil
_savedHeaderSuperview = nil
_savedFooterSuperview = nil
_editButtonItem = nil
_searchDisplayController = nil
_strongSearchDisplayController = nil
_modalTransitionStyle = 0
_modalPresentationStyle = 0
_lastKnownInterfaceOrientation = 0
_popoverController = nil
_containerViewInSheet = nil
_recordedContentScrollView = nil
_afterAppearance = nil
_explicitAppearanceTransitionLevel = 0
_interfaceBuilderKeyCommands = nil
_addedKeyCommands = nil
_overrideTraitCollections = nil
_previewSourceViews = nil
_retainCount = 0
_ignoreAppSupportedOrientations = ' '
_viewHostsLayoutEngine = ' '
_storyboardIdentifier = nil
_transitioningDelegate = nil
_frozenTraitCollection = nil
overrideUseCustomPresentation = ' '
_modalPresentationCapturesStatusBarAppearance = ' '
_disablesAutomaticKeyboardDismissal = ' '
_ignoresParentMargins = ' '
_childViewControllers = nil
_customNavigationInteractiveTransitionDuration = 0
_customNavigationInteractiveTransitionPercentComplete = 0
_customTransitioningView = nil
_lastNotifiedTraitCollection = nil
_presentationController = nil
_preferredFocusedItem = nil
_navigationControllerContentOffsetAdjustment = 0
_contentMargin = 16
_topLayoutGuide = nil
_bottomLayoutGuide = nil
_topBarInsetGuideConstraint = nil
_bottomBarInsetGuideConstraint = nil
_storyboardSegueTemplates = nil
_segueResponsibleForModalPresentation = nil
_sourceViewControllerIfPresentedViaPopoverSegue = nil
_modalSourceViewController = nil
_presentedStatusBarViewController = nil
_edgesForExtendedLayout = 15
__childControllerToIgnoreWhileLookingForTransitionCoordinator = nil
_presentingFocusedItem = nil
_storyboardPreviewSegueTemplates = nil
_storyboardCommitSegueTemplates = nil
_storyboardPreviewingRegistrants = nil
__embeddedView = 0xffffffff
__embeddingView = 0x78b909c0
__embeddedDelegate = 0x00007faa
_originalPresentationController = 0x78af3630
_temporaryPresentationController = 0x00007faa
}
}这样我们就能看到这个对象的内部信息了。
chisel 还提供一些便捷的功能，比如打印 pviews 递归打印层级，不过我更喜欢使用 xcode 自带的debug view hierarchy，这样更加直观。
我们可以使用 pvc 递归输出试图控制器的层级关系，例如
(lldb) pvc
<JChatSwift.JChatMainTabViewController 0x7d49d560>, state: appeared, view: <UILayoutContainerView 0x7d479860>
| <UINavigationController 0x7c3afc00>, state: appeared, view: <UILayoutContainerView 0x7be8e9a0>
| | <JChatConversationListViewController 0x7d49d2b0>, state: disappeared, view: <UIView 0x7d375320> not in the window
| | <JChatChattingViewController 0x7bfbdde0>, state: appeared, view: <UIView 0x7d373b30>
| <UINavigationController 0x7eb7a000>, state: disappeared, view: <UILayoutContainerView 0x7d4a2c20> not in the window
| | <JChatContactsViewController 0x7bea3d70>, state: disappeared, view: <UILayoutContainerView 0x7d4a1d40> not in the window
| <UINavigationController 0x7cb84c00>, state: disappeared, view: <UILayoutContainerView 0x7bf87590> not in the window
| | <JChatAboutMeViewController 0x7d2974a0>, state: disappeared, view: (view not loaded)通过 pvc 和 pinternals 这样我们就可以在任何地方了解我们所有界面状态了。
在 xcode8 以后，我们也可以通过debug memory graph 来查看程序运行的内存状态。
thread
我们可以使用 thread backtrace 来输出线程的堆栈信息，例如
(lldb) thread backtrace // 这个命令可以简写为bt
* thread #1: tid = 0x14169a, 0x001273e5 JChatSwift`JChatChattingViewController.viewDidLayoutSubviews(self=0x7b6bc560) -> () + 21 at JChatChattingViewController.swift:48, queue = 'com.apple.main-thread', stop reason = breakpoint 7.1
* frame #0: 0x001273e5 JChatSwift`JChatChattingViewController.viewDidLayoutSubviews(self=0x7b6bc560) -> () + 21 at JChatChattingViewController.swift:48
frame #1: 0x00127502 JChatSwift`@objc JChatChattingViewController.viewDidLayoutSubviews() -> () + 34 at JChatChattingViewController.swift:0
frame #2: 0x039dc811 UIKit`-[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1598
frame #3: 0x0305c1b9 libobjc.A.dylib`-[NSObject performSelector:withObject:] + 59
frame #4: 0x03791769 QuartzCore`-[CALayer layoutSublayers] + 141
frame #5: 0x03784a47 QuartzCore`CA::Layer::layout_if_needed(CA::Transaction*) + 401
frame #6: 0x0378489d QuartzCore`CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 21
frame #7: 0x0370e49f QuartzCore`CA::Context::commit_transaction(CA::Transaction*) + 339
frame #8: 0x0373d290 QuartzCore`CA::Transaction::commit() + 498
frame #9: 0x0373eda0 QuartzCore`CA::Transaction::flush_transaction() + 38
frame #10: 0x0393685c UIKit`_afterCACommitHandler + 375
frame #11: 0x022f676e CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 30
frame #12: 0x022f66c7 CoreFoundation`__CFRunLoopDoObservers + 391
frame #13: 0x022da3a6 CoreFoundation`__CFRunLoopRun + 1414
frame #14: 0x022d9bab CoreFoundation`CFRunLoopRunSpecific + 395
frame #15: 0x022d9a0b CoreFoundation`CFRunLoopRunInMode + 123
frame #16: 0x06a1ab4c GraphicsServices`GSEventRunModal + 177
frame #17: 0x06a1a9c7 GraphicsServices`GSEventRun + 80
frame #18: 0x039077fb UIKit`UIApplicationMain + 148
frame #19: 0x001a7ce1 JChatSwift`main + 145 at AppDelegate.swift:15
frame #20: 0x06037799 libdyld.dylib`start + 1我们可以看到程序停在JChatChattingViewController.swift:48 这一行
watchpoint
监视某个变量的改变，有时候我们想知道一个对象在什么时候被修改了，我们可以使用 watchpoint set var,当var 改变的时候程序就就会停在改变的地方， 如果用 chisel
的话，我们可以使用 wivar 命令来监听值的变化,例如
(lldb) wivar self name