断点的条件判断表达式的写法的心得

关于条件判断的断点的表达式写法的心得：

要用正确的函数

条件判断语句：要用正确的函数
- 举例
  - objc_alloc_init
    - 其定义是
      - id objc_alloc_init(Class cls)
    - 传入参数是Class（而不是Instance）
    - 条件判断的写法
      - 所以只能用，应该用：NSStringFromClass
        
        定义：NSString * NSStringFromClass(Class aClass);
      - 而不应该用：isKindOfClass
        
        定义：是针对NSObject的 - (BOOL)isKindOfClass:(Class)aClass;
        
        因为此时$x0是个Class，不是Instance=Object
    - 所以结论是：objc_alloc_init的断点的条件判断
      - 正确写法：
        (bool)[NSStringFromClass($x0) isEqualToString: @"AADeviceInfo"]
      - 错误写法：
        (bool)[$x0 isKindOfClass: (Class)objc_getClass("AADeviceInfo")]
  - objc_alloc_init函数中的+44行的objc_msgSend
    - 背景
      - 代码是
        
        0x19cbd3c68 <+44>: b 0x19cbcd000 ; objc_msgSend
      - 此时：x0是AADeviceInfo的Instance=obj
    - 条件判断的写法
      - 所以不能用：NSStringFromClass
      - 只能用isKindOfClass
      - 结论
        
        objc_alloc_init函数中的+44行的objc_msgSend的断点的条件判断
        
        正确写法：
        (bool)[$x0 isKindOfClass: (Class)objc_getClass("AADeviceInfo")]
        
        错误写法：
        (bool)[NSStringFromClass($x0) isEqualToString: @"AADeviceInfo"]

最前面加bool类型转换

条件判断语句：往往最前面还要加个布尔类型强制转换（才能起到判断作用）

无效的写法：

[NSStringFromClass($x0) isEqualToString: @"AADeviceInfo"]
[$x0 isKindOfClass: (Class)objc_getClass("AADeviceInfo")]

有效的写法：

(bool)[NSStringFromClass($x0) isEqualToString: @"AADeviceInfo"]
(bool)[$x0 isKindOfClass: (Class)objc_getClass("AADeviceInfo")]

借助于lldb确认表达式语法无误

条件判断语句：当不确定代码是否正确时，可以借助于lldb命令行的po去执行，看看是否有语法错误

objc_alloc_init函数的条件判断表达式写法

当，不确定自己的写法，比如：

objc_alloc_init 的断点的条件判断

[$x0 isKindOfClass: objc_getClass("AADeviceInfo")]

是否有误，可以去用po试试：

(lldb) po [$x0 isKindOfClass: objc_getClass("AADeviceInfo")]
error: expression failed to parse:
warning: <user expression 25>:1:2: receiver type 'unsigned long' is not 'id' or interface pointer, consider casting it to 'id'
[$x0 isKindOfClass: objc_getClass("AADeviceInfo")]
 ^~~
error: <user expression 25>:1:21: 'objc_getClass' has unknown return type; cast the call to its declared return type
[$x0 isKindOfClass: objc_getClass("AADeviceInfo")]

结果提示有误，意思是不清楚objc_getClass返回值的类型

所以后来才去加上Class的类型转换的

(bool)[$x0 isKindOfClass: (Class)objc_getClass("AADeviceInfo")]

此处再去执行，至少语法上是对的，不报错了：

(lldb) po [$x0 isKindOfClass: (Class)objc_getClass("AADeviceInfo")]
 nil

所以确定，最后的正确写法是：

(bool)[$x0 isKindOfClass: (Class)objc_getClass("AADeviceInfo")]

objc_alloc_init的+44行汇编代码的条件判断表达式写法

对于：

libobjc.A.dylib`objc_alloc_init:
...
->  0x19cbd3c68 <+44>: b      0x19cbcd000              ; objc_msgSend

的尝试添加条件判断的断点的过程：

(lldb) po (int)strcmp(class_getName($x0),"AADeviceInfo")==0
error: expression failed to parse:
error: <user expression 10>:1:13: 'class_getName' has unknown return type; cast the call to its declared return type
(int)strcmp(class_getName($x0),"AADeviceInfo")==0
            ^~~~~~~~~~~~~~~~~~

无法识别class_getName返回值类型，所以去加上类型转换

(lldb) po (int)strcmp((const char *)class_getName($x0),"AADeviceInfo")==0
error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=1, address=0x4af7fbe24f30).
The process has been returned to the state before expression evaluation.

却又无法执行

(lldb) po class_getName($x0)
error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=1, address=0x4af7fbe24f30).
The process has been returned to the state before expression evaluation.

继续尝试：

(lldb) po class_getName($x0)
error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=1, address=0x4af7fbe24f30).
The process has been returned to the state before expression evaluation.

发现是class_getName函数无法正常执行

后来找到定义是：

const char * class_getName(Class cls);

-》所以此处是函数用法有误：

class_getName参数是Class，而此处是Instance，所以无法正常运行

换别的函数object_getClassName试试

(lldb) po [object_getClassName($x0) isEqualToString: @"AADeviceInfo"]
error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=1, address=0x665444160).
The process has been returned to the state before expression evaluation.

却也无法运行，但另外发现：

(lldb) po object_getClassName($x0)
0x0000000197633c50

object_getClassName函数本身，是可以正常运行的

然后找到了定义：

const char * object_getClassName(id obj);

发现此处是：返回值是char *，而不是NSString *

所以要换用char *的判断逻辑

(lldb) po (char *)object_getClassName($x0)
"AADeviceInfo"

确保可以输出字符串后，然后换用strcmp去对比

(lldb) po strcmp(object_getClassName($x0),"AADeviceInfo")==0
error: expression failed to parse:
error: <user expression 16>:1:8: 'object_getClassName' has unknown return type; cast the call to its declared return type
strcmp(object_getClassName($x0),"AADeviceInfo")==0
       ^~~~~~~~~~~~~~~~~~~~~~~~

仍然报错，所以也要给object_getClassName加上返回值类型转换，才可以：

(lldb) po strcmp((char *)object_getClassName($x0),"AADeviceInfo")==0
error: expression failed to parse:
error: <user expression 17>:1:1: 'strcmp' has unknown return type; cast the call to its declared return type
strcmp((char *)object_getClassName($x0),"AADeviceInfo")==0
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

但是又报错strcmp返回值类型无法识别

所以也要加上类型转换

(lldb) po (int)(strcmp((char *)object_getClassName($x0),"AADeviceInfo")==0)
error: expression failed to parse:
error: <user expression 18>:1:7: 'strcmp' has unknown return type; cast the call to its declared return type
(int)(strcmp((char *)object_getClassName($x0),"AADeviceInfo")==0)
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

发现搞错了，int类型转换是需要针对于strcmp，而不是表达式

strcmp((char *)object_getClassName($x0),"AADeviceInfo")==0

所以int类型转换放在strcmp前面即可：

(lldb) po (int)strcmp((char *)object_getClassName($x0),"AADeviceInfo")==0
true

终于是，通过po辅助而确认了此处的断点的条件判断的正确写法是：

(int)strcmp((char *)object_getClassName($x0),"AADeviceInfo")==0

表达式写法