XCode的一些調(diào)試技巧
po 命令：為 print object 的縮寫，顯示對象的文本描述（顯示從對象的 description 消息獲得的字符串信息）。
比如：
上圖中，我使用 po 命令顯示一個 NSDictionary 的內(nèi)容。注意在左側(cè)我們可以看到 dict 的一些信息：3 key/value pairs，顯示該 dict 包含的數(shù)據(jù)量，而展開的信息顯示 isa 層次體系（即class 和 metaclass結(jié)構(gòu)關(guān)系）。我們可以右擊左側(cè)的 dict，選中“Print Description of "dict"”，則可以在控制臺輸出 dict 的詳細(xì)信息：
view plaincopyprint?
- Printing description of dict:
- <CFBasicHash 0x1001149e0 [0x7fff7e27ff40]>{type = immutable dict, count = 3,
- entries =>
- 0 : <CFString 0x100002458 [0x7fff7e27ff40]>{contents = "first"} = <CFString 0x100002438 [0x7fff7e27ff40]>{contents = "one"}
- 1 : <CFString 0x100002498 [0x7fff7e27ff40]>{contents = "second"} = <CFString 0x100002478 [0x7fff7e27ff40]>{contents = "two"}
- 2 : <CFString 0x1000024d8 [0x7fff7e27ff40]>{contents = "third"} = <CFString 0x1000024b8 [0x7fff7e27ff40]>{contents = "three"}
- }
- (gdb)
print 命令：有點類似于格式化輸出，可以輸出對象的不同信息：
如：
[cpp] view plaincopyprint?
- (gdb) print (char *)[[dict description] cStringUsingEncoding:4]
- $1 = 0x1001159c0 "{\n first = one;\n second = two;\n third = three;\n}"
- (gdb) print (int)[dict retainCount]
- $2 = 1
- (gdb)
注：4是 NSUTF8StringEncoding 的值。
info 命令：我們可以查看內(nèi)存地址所在信息
比如 "info symbol 內(nèi)存地址" 可以獲取內(nèi)存地址所在的 symbol 相關(guān)信息：
[cpp] view plaincopyprint?
- (gdb) info symbol 0x00000001000017f7
- main + 343 in section LC_SEGMENT.__TEXT.__text of /Users/LuoZhaohui/Library/Developer/Xcode/DerivedData/RunTimeSystem-anzdlhiwvlbizpfureuvenvmatnp/Build/Products/Debug/RunTimeSystem
比如 "info line *內(nèi)存地址" 可以獲取內(nèi)存地址所在的代碼行相關(guān)信息：
[cpp] view plaincopyprint?
- (gdb) info line *0x00000001000017f7
- Line 62 of "/Users/LuoZhaohui/Documents/Study/RunTimeSystem/RunTimeSystem/main.m" starts at address 0x1000017f7 <main+343> and ends at 0x10000180a <main+362>.
show 命令：顯示 GDB 相關(guān)的信息。如：show version 顯示GDB版本信息
[cpp] view plaincopyprint?
- (gdb) show version
- GNU gdb 6.3.50-20050815 (Apple version gdb-1708) (Mon Aug 8 20:32:45 UTC 2011)
- Copyright 2004 Free Software Foundation, Inc.
- GDB is free software, covered by the GNU General Public License, and you are
- welcome to change it and/or distribute copies of it under certain conditions.
- Type "show copying" to see the conditions.
- There is absolutely no warranty for GDB. Type "show warranty" for details.
- This GDB was configured as "x86_64-apple-darwin".
help 命令：如果忘記某條命令的語法了，可以使用 help 命令名 來獲取幫助信息。如：help info 顯示 info 命令的用法。
[cpp] view plaincopyprint?
- (gdb) help info
- Generic command for showing things about the program being debugged.
- List of info subcommands:
- info address -- Describe where symbol SYM is stored
- info all-registers -- List of all registers and their contents
- info args -- Argument variables of current stack frame
- info auxv -- Display the inferior's auxiliary vector
- info breakpoints -- Status of user-settable breakpoints
- info catch -- Exceptions that can be caught in the current stack frame
- info checkpoints -- Help
- info classes -- All Objective-C classes
- ......
- Type "help info" followed by info subcommand name for full documentation.
- Command name abbreviations are allowed if unambiguous.
- (gdb)
在系統(tǒng)拋出異常處設(shè)置斷點
有時候我們的程序不知道跑到哪個地方就 crash 了，而 crash 又很難重現(xiàn)。保守的做法是在系統(tǒng)拋出異常之前設(shè)置斷點，具體來說是在 objc_exception_throw處設(shè)置斷點。設(shè)置步驟為：首先在 XCode 按 CMD + 6，進入斷點管理窗口；然后點擊右下方的 +，增加新的 Symbolic Breakpoint，在 Symbol 一欄輸入：objc_exception_throw，然后點擊 done，完成。 這樣在 Debug 模式下，如果程序即將拋出異常，就能在拋出異常處中斷了。比如在前面的代碼中，我讓 [firstObjctcrashTest]; 拋出異常。在 objc_exception_throw 處設(shè)置斷點之后，程序就能在該代碼處中斷了，我們從而知道代碼在什么地方出問題了。
