iOS/Xcode Debugging

1. Stop when exception

1.1 Open Breakpoint navigator (Cmd + 8)
1.2 Add Exception Breakpoint

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1.3 Keep default values

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1.4 Move breakpoint to User level

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2. Edit breakpoint

Edit Breakpoint

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Add Debugger Command:

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We will see the following output:

7.89903245455977
7.890427382096
7.87502883137136
7.87004694731339

or Add Log Message

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ScreensFromBottom: @screensFromBottom@ Threshold: @screensFromBottomToLoadMoreCats@

We will see the logs in output window:

ScreensFromBottom: 8.5084718344868104 Threshold: 2.5
ScreensFromBottom: 8.5034899504288379 Threshold: 2.5
ScreensFromBottom: 8.499866762023041 Threshold: 2.5
ScreensFromBottom: 8.4966964721679688 Threshold: 2.5
ScreensFromBottom: 8.4948848779650703 Threshold: 2.5
ScreensFromBottom: 8.4921674866607226 Threshold: 2.5
ScreensFromBottom: 8.4899029939070996 Threshold: 2.5
ScreensFromBottom: 8.4880913997042011 Threshold: 2.5

3. Symbolic breakpoint - Condition

For example, check if some specified UIViewController is released properly.

Add Symbolic Breakpoint...

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(Optional) Add Condition:

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Check if it is UIViewController:

(BOOL)[$arg1 isKindOfClass: isKindOfClass: [UIViewController class]]

Check if it is user's custom CatDetailViewController:

(BOOL)[$arg1 isKindOfClass: (id)NSClassFromString(@"Catstagram.CatDetailViewController")]

3. Symbolic breakpoint - Action

Print the class name when UIView released:

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We will see the following output:

<_UIVisualEffectSubview: 0x7f9e6e41aea0; frame = (0 0; 414 64); autoresize = W+H; userInteractionEnabled = NO; layer = >
>
>
>
>
>
>

4. LLDB

LLDB is Apple’s “from the ground up” replacement for GDB, developed in close coordination with the LLVM compilers to bring you state-of-the-art debugging with extensive capabilities in flow control and data inspection. Starting with Xcode 5, all new and preexisting development projects are automatically reconfigured to use LLDB.

reference: http://lldb.llvm.org/lldb-gdb.html

4.1 p/po

4.2 frame variable(fr v)

frame variable is only for printing the contents of variables, no side effect to variable

(lldb) frame var global
(int32_t) global = 5

example:

(lldb) p screensFromBottom
(CGFloat) $R1 = 9.3780370518781133
(lldb) po screensFromBottom
9.37803705187811

(lldb) frame variable screensFromBottom
(CGFloat) screensFromBottom = 9.3780370518781133
(lldb) fr v screensFromBottom
(CGFloat) screensFromBottom = 9.3780370518781133
(lldb)

4.3 Variable out of scope

Create a variable out of debug session scope by using $:

(lldb) p let $label = cell.titleLabel
(lldb) p print($label.text!)

5. Changing UI when debugging

run CATransaction.flush(), flush pending changing to the UI:

(lldb) p titleLabel.text = "Hello lldb"
(lldb) p CATransaction.flush()

6. Get memory address of return value of a function:

(lldb) register read $rax
rax = 0x000062401232eb99a

Print out the memory in LLDB:

(lldb) po unsafeBitCast(0x000062401232eb99a, to UIImage.self)
, {40, 40}

(lldb)