前言
在消息转发中提到过NSInvocation这个类，这里说一下我所理解的NSInvocation。NSInvocation 是命令模式的一种实现，它包含选择器、方法签名、相应的参数以及目标对象。所谓的方法签名，即方法所对应的返回值类型和参数类型。当NSInvocation被调用，它会在运行时通过目标对象去寻找对应的方法，从而确保唯一性，可以用[receiver message]来解释。实际开发过程中直接创建NSInvocation的情况不多见，这些事情通常交给系统来做。比如bang的JSPatch中arm64方法替换的实现就是利用runtime消息转发最后一步中的NSInvocation实现的。
正文
基于这种命令模式，可以利用NSInvocation调用任意SEL甚至block。
SEL
- (id)performSelector:(SEL)aSelector withArguments:(NSArray *)arguments {
if (aSelector == nil) return nil;
NSMethodSignature *signature = [[self class] instanceMethodSignatureForSelector:aSelector];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
invocation.target = self;
invocation.selector = aSelector;
// invocation 有2个隐藏参数，所以 argument 从2开始
if ([arguments isKindOfClass:[NSArray class]]) {
NSInteger count = MIN(arguments.count, signature.numberOfArguments - 2);
for (int i = 0; i < count; i++) {
const char *type = [signature getArgumentTypeAtIndex:2 + i];
// 需要做参数类型判断然后解析成对应类型，这里默认所有参数均为OC对象
if (strcmp(type, "@") == 0) {
id argument = arguments[i];
[invocation setArgument:&argument atIndex:2 + i];
}
}
}
[invocation invoke];
id returnVal;
if (strcmp(signature.methodReturnType, "@") == 0) {
[invocation getReturnValue:&returnVal];
}
// 需要做返回类型判断。比如返回值为常量需要包装成对象，这里仅以最简单的`@`为例
return returnVal;
}
运行结果
与NSObject中的performSelector相比，没有了参数个数限制。
invocation
signature
block
static id invokeBlock(id block ,NSArray *arguments) {
if (block == nil) return nil;
id target = [block copy];
const char *_Block_signature(void *);
const char *signature = _Block_signature((__bridge void *)target);
NSMethodSignature *methodSignature = [NSMethodSignature signatureWithObjCTypes:signature];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
invocation.target = target;
// invocation 有1个隐藏参数，所以 argument 从1开始
if ([arguments isKindOfClass:[NSArray class]]) {
NSInteger count = MIN(arguments.count, methodSignature.numberOfArguments - 1);
for (int i = 0; i < count; i++) {
const char *type = [methodSignature getArgumentTypeAtIndex:1 + i];
NSString *typeStr = [NSString stringWithUTF8String:type];
if ([typeStr containsString:@"\""]) {
type = [typeStr substringToIndex:1].UTF8String;
}
// 需要做参数类型判断然后解析成对应类型，这里默认所有参数均为OC对象
if (strcmp(type, "@") == 0) {
id argument = arguments[i];
[invocation setArgument:&argument atIndex:1 + i];
}
}
}
[invocation invoke];
id returnVal;
const char *type = methodSignature.methodReturnType;
NSString *returnType = [NSString stringWithUTF8String:type];
if ([returnType containsString:@"\""]) {
type = [returnType substringToIndex:1].UTF8String;
}
if (strcmp(type, "@") == 0) {
[invocation getReturnValue:&returnVal];
}
// 需要做返回类型判断。比如返回值为常量需要包装成对象，这里仅以最简单的`@`为例
return returnVal;
}
运行结果
invocation
signature
SEL与block比较
- invocation
SEL既有target也有selector，block只有target - signature
SEL有两个隐藏参数，类型均为类型为@@和:，分别对应target和selector。block有一个隐藏参数，类型为@?，对应target且block的target为他本身 - type
以OC对象为例：SEL的type为@，block的type会跟上具体类型，如@"NSString"
再谈block
在block的invocation中有这样的代码
const char *_Block_signature(void *);
const char *signature = _Block_signature((__bridge void *)target);
_Block_signature其实是JavaScriptCore/ObjcRuntimeExtras.h中的私有API(这个头文件并没有公开可以戳这里查看)
既然苹果把API封了，那就自己实现咯，万能的github早有答案CTObjectiveCRuntimeAdditions
把CTBlockDescription.h和CTBlockDescription.m拖到项目中，代码这样写
static id invokeBlock(id block ,NSArray *arguments) {
if (block == nil) return nil;
id target = [block copy];
CTBlockDescription *ct = [[CTBlockDescription alloc] initWithBlock:target];
NSMethodSignature *methodSignature = ct.blockSignature;
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
invocation.target = target;
// invocation 有1个隐藏参数，所以 argument 从1开始
if ([arguments isKindOfClass:[NSArray class]]) {
NSInteger count = MIN(arguments.count, methodSignature.numberOfArguments - 1);
for (int i = 0; i < count; i++) {
const char *type = [methodSignature getArgumentTypeAtIndex:1 + i];
NSString *typeStr = [NSString stringWithUTF8String:type];
if ([typeStr containsString:@"\""]) {
type = [typeStr substringToIndex:1].UTF8String;
}
// 需要做参数类型判断然后解析成对应类型，这里默认所有参数均为OC对象
if (strcmp(type, "@") == 0) {
id argument = arguments[i];
[invocation setArgument:&argument atIndex:1 + i];
}
}
}
[invocation invoke];
id returnVal;
const char *type = methodSignature.methodReturnType;
NSString *returnType = [NSString stringWithUTF8String:type];
if ([returnType containsString:@"\""]) {
type = [returnType substringToIndex:1].UTF8String;
}
if (strcmp(type, "@") == 0) {
[invocation getReturnValue:&returnVal];
}
// 需要做返回类型判断。比如返回值为常量需要包装成对象，这里仅以最简单的`@`为例
return returnVal;
}
运行结果
