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NSInvocatio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。NSInvocation与NSMethodSignature配合来完成调用。NSMethodSignature这个类的对象保存了方法的名称、参数和返回值
- SEL
系统NSObject
自带的performSelector
默认只支持一个参数,我们可以给NSObject增加一个category,增加以下方法就可以支持多个参数的调用了
- (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;
}
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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;
}
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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@,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;
}
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NSInvocation.h
@interface NSInvocation : NSObject {
@private
__strong void *_frame;
__strong void *_retdata;
id _signature;
id _container;
uint8_t _retainedArgs;
uint8_t _reserved[15];
}
// 通过NSMethodSignature对象创建NSInvocation对象,NSMethodSignature为方法签名类
+ (NSInvocation *)invocationWithMethodSignature:(NSMethodSignature *)sig;
// 获取NSMethodSignature对象
@property (readonly, retain) NSMethodSignature *methodSignature;
// 保留参数,它会将传入的所有参数以及target都retain一遍
- (void)retainArguments;
// 判断参数是否还存在
// 调用retainArguments之前,值为NO,调用之后值为YES
@property (readonly) BOOL argumentsRetained;
// 设置消息调用者,注意:target最好不要是局部变量
@property (nullable, assign) id target;
// 设置要调用的消息
@property SEL selector;
// 获取消息返回值
- (void)getReturnValue:(void *)retLoc;
// 设置消息返回值
- (void)setReturnValue:(void *)retLoc;
// 获取消息参数
- (void)getArgument:(void *)argumentLocation atIndex:(NSInteger)idx;
// 设置消息参数
- (void)setArgument:(void *)argumentLocation atIndex:(NSInteger)idx;
// 发送消息,即执行方法
- (void)invoke;
// target发送消息,既设置了target同时执行方法。这一步要注意在调用此方法之前必须设置了selector和argument
- (void)invokeWithTarget:(id)target;
@end
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