block解析-形參變量

2019-11-14 20:10:22

字體：大中小

來源：轉載

供稿：網友

block形參

之前漏了一篇block形參的介紹，這里給補上。

block形參就是定義block帶的參數，和函數的參數使用一樣，我們可以在block隨意使用修改block形參。

我們來看個例子：

我們聲明了兩個NSString 指針_p1 _p2、int 型_p3、可變數組_p4,并把這些參數傳入block，在block內修改。

-(void )test3{    NSString *_p1=[NSString stringWithFormat:@"hello %@",@"world11"];    NSString *_p2=[NSString stringWithFormat:@"hello %@",@"world12"];    int _p3=1;    NSMutableArray *_p4=[[NSMutableArray alloc]init];    //初始值    NSLog(@"init p1:%@,%p,%p",_p1,_p1,&_p1);    NSLog(@"init p2:%@,%p,%p",_p2,_p2,&_p2);    NSLog(@"init p3:%d,%p",_p3,&_p3);    NSLog(@"init p4:%@,%p,%p",_p4,_p4,&_p4);    void (^myBlock)(NSString *,NSString **,int,NSMutableArray *) = ^(NSString *p1,NSString **p2,int p3,NSMutableArray * p4) {        //block內賦值        p1=@"21";        *p2=@"22";        p3=23;        [p4 addObject:@"23"];        NSLog(@"excuteing p1:%@,%p,%p",p1,p1,&p1);        NSLog(@"excuteing p2:%@,%p,%p,%p",*p2,*p2,p2,&p2);        NSLog(@"excuteing p3:%d,%p",p3,&p3);        NSLog(@"excuteing p4:%@,%p,%p",p4,p4,&p4);    };    myBlock(_p1,&_p2,_p3,_p4);    //block執行后    NSLog(@"excuteafter p1:%@,%p,%p",_p1,_p1,&_p1);    NSLog(@"excuteafter p2:%@,%p,%p",_p2,_p2,&_p2);    NSLog(@"excuteafter p3:%d,%p",_p3,&_p3);    NSLog(@"excuteafter p4:%@,%p,%p",_p4,_p4,&_p4);}

輸出日志：

2014-07-31 14:17:06.774 Test[3655:60b] init p1:hello world11,0x16537b20,0x27df89902014-07-31 14:17:06.776 Test[3655:60b] init p2:hello world12,0x1652d760,0x27df898c2014-07-31 14:17:06.778 Test[3655:60b] init p3:1,0x27df89882014-07-31 14:17:06.779 Test[3655:60b] init p4:(),0x1652b6d0,0x27df89842014-07-31 14:17:06.781 Test[3655:60b] excuteing p1:21,0x11a08,0x27df893c2014-07-31 14:17:06.782 Test[3655:60b] excuteing p2:22,0x11a18,0x27df897c,0x27df89382014-07-31 14:17:06.783 Test[3655:60b] excuteing p3:23,0x27df89342014-07-31 14:17:06.784 Test[3655:60b] excuteing p4:(    23),0x1652b6d0,0x27df89302014-07-31 14:17:06.785 Test[3655:60b] excuteafter p1:hello world11,0x16537b20,0x27df89902014-07-31 14:17:06.786 Test[3655:60b] excuteafter p2:22,0x11a18,0x27df898c2014-07-31 14:17:06.787 Test[3655:60b] excuteafter p3:1,0x27df89882014-07-31 14:17:06.788 Test[3655:60b] excuteafter p4:(    23),0x1652b6d0,0x27df8984

從日志可以看出：

我們把指針_p1（NSString *）傳入block，傳參數的過程其實可以理解為用新的指針指向 _p1所指向的內存地址。這時候在block內部修改指針指向，只是修改了block形參的指針的指向，所以block內部p1的內容和block外部_p1的不一樣。
我們把指針_p4（NSMutableArray *）傳入block，傳參數的過程和上面一樣，用了一個新的指針指向參數所指向的內存地址，在block內部對形參指針指向的內存進行修改，即操作的是同一片內存，所以能同步到block外部。
我們把指針_p2（NSString **）傳到了block內部，傳遞的是指針的地址，我們在block內部修改*p2，其實是修改指針p2 指向的指針指向的內存地址，也能同步到block外部。
因為_p3是值傳遞，所以block內部修改不會影響外部，這個和函數一樣。

我們來看看block對行參變量的處理：

這是block結構體，并沒有生成成員對形參進行引用

struct __KDBlockTest__test3_block_impl_0 {  struct __block_impl impl;  struct __KDBlockTest__test3_block_desc_0* Desc;  __KDBlockTest__test3_block_impl_0(void *fp, struct __KDBlockTest__test3_block_desc_0 *desc, int flags=0) {    impl.isa = &_NSConcreteStackBlock;    impl.Flags = flags;    impl.FuncPtr = fp;    Desc = desc;  }};

我們看block實現的函數，對變量的訪問，都是通過形參來實現的，并沒有通過結構體的成員

static void __KDBlockTest__test3_block_func_0(struct __KDBlockTest__test3_block_impl_0 *__cself, NSString *p1, NSString **p2, int p3, NSMutableArray *p4) {        p1=(NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_9;        *p2=(NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_10;        p3=23;        ((void (*)(id, SEL, id))(void *)objc_msgSend)((id)p4, sel_registerName("addObject:"), (id)(NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_11);        NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_12,p1,p1,&p1);        NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_13,*p2,*p2,p2,&p2);        NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_14,p3,&p3);        NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_15,p4,p4,&p4);    }

下面是我們的測試函數，看紅色部分，是通過函數參數傳遞，和函數調用一樣，和block內使用局部變量、靜態變量、成員變量還是有很大區別的，并沒有截獲變量。

static void _I_KDBlockTest_test3(KDBlockTest * self, SEL _cmd) {    NSString *_p1=((NSString *(*)(id, SEL, NSString *, ...))(void *)objc_msgSend)((id)objc_getClass("NSString"), sel_registerName("stringWithFormat:"), (NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_1, (NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_2);    NSString *_p2=((NSString *(*)(id, SEL, NSString *, ...))(void *)objc_msgSend)((id)objc_getClass("NSString"), sel_registerName("stringWithFormat:"), (NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_3, (NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_4);    int _p3=1;    NSMutableArray *_p4=((id (*)(id, SEL))(void *)objc_msgSend)((id)((id (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSMutableArray"), sel_registerName("alloc")), sel_registerName("init"));    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_5,_p1,_p1,&_p1);    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_6,_p2,_p2,&_p2);    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_7,_p3,&_p3);    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_8,_p4,_p4,&_p4);    void (*myBlock)(NSString *,NSString **,int,NSMutableArray *) = (void (*)(NSString *, NSString **, int, NSMutableArray *))&__KDBlockTest__test3_block_impl_0((void *)__KDBlockTest__test3_block_func_0, &__KDBlockTest__test3_block_desc_0_DATA);    ((void (*)(__block_impl *, NSString *, NSString **, int, NSMutableArray *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock, _p1, &_p2, _p3, _p4);//block 執行    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_16,_p1,_p1,&_p1);    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_17,_p2,_p2,&_p2);    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_18,_p3,&_p3);    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_654672_mi_19,_p4,_p4,&_p4);}