在iOS开发中,Block是常用的数据类型,Block的源码是开放的,对于Blcok的其他探究可以查看这篇文章深入研究Block捕获外部变量和__block实现原理.先来简单介绍一般MethodSignature的获取和配合Invocation的使用.
方法签名
以Person类为例,该类只有一个类方法和对象方法:
@interface Person : NSObject
- (void)say:(NSString *)name;
+ (void)setAge:(int)age;
@end
@implementation Person
- (void)say:(NSString *)name
{
NSLog(@"say : %@",name);
}
+ (void)setAge:(int)age{
}
@end
一般我们获取对象方法和类方法的签名可以这样:
/// 根据方法签名获取TypeEncoding
NSString *getMethodSignatureTypeEncoding(NSMethodSignature *methodSignature){
NSMutableString *str = @"".mutableCopy;
const char *rtvType = methodSignature.methodReturnType;
[str appendString:[NSString stringWithUTF8String:rtvType]];
for (int i = 0; i < methodSignature.numberOfArguments; i ++) {
const char *type = [methodSignature getArgumentTypeAtIndex:i];
[str appendString:[NSString stringWithUTF8String:type]];
}
return [str copy];
}
int main(int argc, const char * argv[]) {
@autoreleasepool {
// 对象方法签名
NSMethodSignature *instanceMethodSignature = [Person instanceMethodSignatureForSelector:@selector(say:)];
// 类方法签名
NSMethodSignature *classMethodSignature = [Person methodSignatureForSelector:@selector(setAge:)];
NSLog(@"instanceMethodSignature : %@ \n classMethodSignature : %@",getMethodSignatureTypeEncoding(instanceMethodSignature),getMethodSignatureTypeEncoding (classMethodSignature));
}
return 0;
}
/*
打印结果:
instanceMethodSignature : v@:@
classMethodSignature : v@:i
*/
方法签名记录着一个方法的返回值类型编码(TypeEncoding)、形参个数、每一个形参的类型编码.有了方法签名之后,可以通过类型编码来反解出真实类型,类型的映射关系可以查看官方资料.
根据Person的say:方法获得的方法签名结果为v@:@,v代表void类型,@代表对象类型,对应的是隐藏参数self,:代表SEL类型,对应的是隐藏参数_cmd,@对应的是第一个形参.每一个OC的类方法或对象方法都会包含两个隐藏参数self和_cmd,并且它们的位置是固定的,在invocation对象中的索引分别是0和1,返回值是不占索引的,需要额外获取.
配合NSInvocation使用
有了方法签名之后,我们可以组装一个NSInvocation对象并调用对应方法,这里就简单直接设置了索引2为name了,一般都会动态循环设置设置,例如JSPatch.
Person *p = [Person new];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:instanceMethodSignature];
invocation.selector = @selector(say:);
NSString *name = @"abc";
[invocation setArgument:&name atIndex:2];
[invocation invokeWithTarget:p];
/*
打印结果: say : abc
*/
如果不是block对象,invocation必须要设置selector,原因跟类的方法结构有关,关于类的方法结构<<深入解析 ObjC 中方法的结构>>这篇文章分析的非常深入.Aspects也对NSInvocation做了分类来获取方法签名并获取对应实参.
Block的方法签名
Aspects 源码中,有一段获取Blcok方法签名的代码比较有意思:
typedef NS_OPTIONS(int, AspectBlockFlags) {
AspectBlockFlagsHasCopyDisposeHelpers = (1 << 25),
AspectBlockFlagsHasSignature = (1 << 30)
};
typedef struct _AspectBlock {
__unused Class isa;
AspectBlockFlags flags;
__unused int reserved;
void (__unused *invoke)(struct _AspectBlock *block, ...);
struct {
unsigned long int reserved;
unsigned long int size;
// requires AspectBlockFlagsHasCopyDisposeHelpers
void (*copy)(void *dst, const void *src);
void (*dispose)(const void *);
// requires AspectBlockFlagsHasSignature
const char *signature;
const char *layout;
} *descriptor;
// imported variables
} *AspectBlockRef;
static NSMethodSignature *aspect_blockMethodSignature(id block, NSError **error) {
AspectBlockRef layout = (__bridge void *)block;
if (!(layout->flags & AspectBlockFlagsHasSignature)) {
NSString *description = [NSString stringWithFormat:@"The block %@ doesn't contain a type signature.", block];
AspectError(AspectErrorMissingBlockSignature, description);
return nil;
}
void *desc = layout->descriptor;
desc += 2 * sizeof(unsigned long int);
if (layout->flags & AspectBlockFlagsHasCopyDisposeHelpers) {
desc += 2 * sizeof(void *);
}
if (!desc) {
NSString *description = [NSString stringWithFormat:@"The block %@ doesn't has a type signature.", block];
AspectError(AspectErrorMissingBlockSignature, description);
return nil;
}
const char *signature = (*(const char **)desc);
return [NSMethodSignature signatureWithObjCTypes:signature];
}
_AspectBlock结构体是不是很熟悉? 对,就是跟Block指向的结构体是基本一样的!为什么说基本一样的呢?因为官方文档里面block是长这样子的:
struct Block_layout {
void *isa;
int flags;
int reserved;
void (*invoke)(void *, ...);
struct Block_descriptor *descriptor;
/* Imported variables. */
};
struct Block_descriptor {
unsigned long int reserved;
unsigned long int size;
void (*copy)(void *dst, void *src);
void (*dispose)(void *);
};
先来分析一下这段代码,AspectBlockFlags枚举是block的标记.aspect_blockMethodSignature方法先强制将Blcok转换成了AspectBlockRef类型指针,然后利用flags判断是否存在方法签名,再获取block的descriptor,descriptor结构体的copy或者signature存在着方法签名.但是由于全局block和堆block存在的位置不同,所以需要判断.如果是全局block,则方法签名存在于signature上,如果是堆block,则存在于copy上.我们可以来验证一下:
int main(int argc, const char * argv[]) {
@autoreleasepool {
// _NSConcreteMallocBlock 堆block
__block int a = 0;
id mallocBlock = ^(int b){
a ++;
NSLog(@"mallocBlock: a = %d ,b = %d",a,b);
};
// _NSConcreteGlobalBlock 全局block
id globalBlock = ^(NSString *name){
NSLog(@"globalBlock : name = %@",name);
};
NSMethodSignature *mallocBlockSignature = aspect_blockMethodSignature(mallocBlock, NULL);
NSMethodSignature *globalBlockSignature = aspect_blockMethodSignature(globalBlock, NULL);
NSLog(@"mallocBlockSignature : %@ \n globalBlockSignature : %@",getMethodSignatureTypeEncoding(mallocBlockSignature),getMethodSignatureTypeEncoding (globalBlockSignature));
}
return 0;
}
/*
打印结果:
mallocBlockSignature : v@?i
globalBlockSignature : v@?@"NSString"
*/
在aspect_blockMethodSignature方法的desc += 2 * sizeof(void *);处打断点,然后发现全局block执行了这句,而堆block没有执行这句.
栈block由于在ARC下无法验证,所以目前不知道.
我们发现block的方法签名和对象的方法签名不一样,第二位和第三位不再是@和:了,而是被@?这个block标记代替了.再看看类型编码,基本数据类型在两者情况下是一样的,但是对象类型在block的方法签名对应的@后面会标明对象类型.
调用block
既然获取到了block的方法签名,这样就可以组装一个NSInvocation对象来调用block.
__block int a = 0;
id mallocBlock = ^(int b){
a ++;
NSLog(@"mallocBlock: a = %d ,b = %d",a,b);
};
NSMethodSignature *methodSignature = aspect_blockMethodSignature(mallocBlock, NULL);
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
int b = 1;
[invocation setArgument:&b atIndex:1];
[invocation invokeWithTarget:mallocBlock];
/*
打印结果: mallocBlock: a = 1 ,b = 1
*/
由于block的方法签名最前面除了返回值以外只有@?,所以所以需要从1开始设置参数.
总结:我们可以利用block来表示一个方法的实现,block的参数类型和个数可以根据实际的方法来动态一一对应.这样做的好处是如果方法的参数个数和类型是不确定的,利用block可以将整个方法的实现替换成一个block,可以达到替换任意方法的目的,这也是Aspects这个AOP实现框架的重要一部分.
