介绍
本文介绍objc公开接口message.h中的所有API, 涉及测试代码在这里
消息发送
msgSend
- 汇编实现, 不同CPU架构有对应版本。该组函数拥有不同返回类型，这么多应该是为了优化。
// 普通的应该是返回int和指针类型
void objc_msgSend(void /* id self, SEL op, ... */ )
void objc_msgSendSuper(void /* struct objc_super *super, SEL op, ... */ )
/*
* On some architectures, use objc_msgSend_stret for some struct return types.
* On some architectures, use objc_msgSend_fpret for some float return types.
* On some architectures, use objc_msgSend_fp2ret for some float return types.
*/
方法定义
// These functions must be cast to an appropriate function pointer type before being called
#if !OBJC_OLD_DISPATCH_PROTOTYPES
typedef void (*IMP)(void /* id, SEL, ... */ ); ✅
#else
typedef id _Nullable (*IMP)(id _Nonnull, SEL _Nonnull, ...);
#endif
测试代码如下，objc_msgSend参数和返回值只是占位用的，实际要转成对应的方法具体类型
- (void)testMsgSend {
Human *instance = [[Human alloc] init];
NSString *result = ((NSString *(*)(id, SEL, NSString *))objc_msgSend)(instance, @selector(say:), @"Hello");
XCTAssertTrue([result isEqualToString:@"Hello"]);
}
动态添加
@interface HumanAddMethod: NSObject
- (NSString *)say:(NSString *)content;
@end
NSString * say(id self, SEL selector, NSString *msg){
return [NSString stringWithFormat:@"%@", msg];
}
@implementation HumanAddMethod
- (instancetype)init{
if(self = [super init]){
const char *types = [[NSString stringWithFormat:@"%s%s%s%s", @encode(NSString *), @encode(id), @encode(SEL), @encode(NSString *)] UTF8String];
NSMethodSignature *sig = [NSMethodSignature signatureWithObjCTypes:types];
NSLog(@"💚 types=%s args= %lu rlength= %s rtype=%lu isOneway=%@", types, (unsigned long)sig.numberOfArguments, sig.methodReturnType, (unsigned long)sig.methodReturnLength, sig.isOneway ? @"YES": @"NO");
for (int i = 0; i < sig.numberOfArguments; i++) {
NSLog(@" - %d arg type = %s", i, [sig getArgumentTypeAtIndex:i]);
}
class_addMethod([self class], @selector(say:), (IMP)say, types);
}
return self;
}
@end
消息转发
void _objc_msgForward(void /* id receiver, SEL sel, ... */ )
若消息发送失败，这进入消息转发，流程如下
- Dynamic Method Resolution：
resolveClassMethod:和resolveInstanceMethod, 若返回YES同时运行时状态有新函数注册，则直接调用实现，完成消息发送. 否则 - Message Forwarding:
forwardingTargetForSelector:若返回不是nil和self，则完成消息发送，否者 - Message Forwarding:
methodSignatureForSelector:若返回不为空，则发送消息给forwardInvocation:由Invocation完成, 否则 - 抛出异常 : 调用
doesNotRecognizeSelector:抛出异常
💚 1. resolveInstanceMethod called say:
💚 2. forwardingTargetForSelector called say:
💚 3. methodSignatureForSelector called say:
💚 1. resolveInstanceMethod called say:
💚 1. resolveInstanceMethod called _forwardStackInvocation:
💚 3. forwardInvocation called <NSInvocation: 0x6000018bc480>
动态解析
- (NSString *)say:(NSString *)content;
@end
@implementation HumanResolve
+ (BOOL)resolveInstanceMethod:(SEL)sel{
if (sel == @selector(say:)) {
const char *types = [[NSString stringWithFormat:@"%s%s%s%s", @encode(NSString *), @encode(id), @encode(SEL), @encode(NSString *)] cStringUsingEncoding:NSUTF8StringEncoding];
class_addMethod([self class], sel, (IMP)say, types);
return YES;
}
return [super resolveInstanceMethod:sel];
}
@end
转发1
@interface HumanForwardTarget: NSObject{
Dog *_surrogate;
}
- (NSString *)say:(NSString *)content;
@end
- (instancetype)init {
if (self) {
_surrogate = [[Dog alloc] init];
}
return self;
}
+ (BOOL)resolveInstanceMethod:(SEL)sel{
NSLog(@"💚 1. resolveInstanceMethod called %@", NSStringFromSelector(sel));
return [super resolveInstanceMethod:sel];
}
- (id)forwardingTargetForSelector:(SEL)aSelector{
return _surrogate;
}
@end
转发2
@interface HumanForwardInvocation: NSObject{
Cat *_surrogate;
}
- (NSString *)say:(NSString *)content;
@end
@implementation HumanForwardInvocation
- (instancetype)init {
if (self) {
_surrogate = [[Cat alloc] init];
}
return self;
}
+ (BOOL)resolveInstanceMethod:(SEL)sel{
NSLog(@"💚 1. resolveInstanceMethod called %@", NSStringFromSelector(sel));
return [super resolveInstanceMethod:sel];
}
- (id)forwardingTargetForSelector:(SEL)aSelector{
NSLog(@"💚 2. forwardingTargetForSelector called %@", NSStringFromSelector(aSelector));
return [super forwardingTargetForSelector:aSelector];
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
NSLog(@"💚 3. methodSignatureForSelector called %@", NSStringFromSelector(aSelector));
if ([_surrogate respondsToSelector:aSelector]) {
return [_surrogate methodSignatureForSelector:aSelector];
} else {
return [super methodSignatureForSelector:aSelector];
}
}
- (void)forwardInvocation:(NSInvocation *)anInvocation{
NSLog(@"💚 3. forwardInvocation called %@", [anInvocation description]);
if ([_surrogate respondsToSelector: [anInvocation selector]]){
[anInvocation invokeWithTarget:_surrogate];
}
else{
[super forwardInvocation:anInvocation];
}
}
@end
抛出异常
doesNotRecognizeSelector是运行时找不到对应的SEL(方法)最后调用的函数，在NSObject里实现, 虽然注释写着已经改为CF实现，但是不影响我们学习，其内部实现主要是抛出NSInvalidArgumentException异常。这也是iOS常见的崩溃原因之一。
// Replaced by CF (throws an NSException)
+ (void)doesNotRecognizeSelector:(SEL)sel {
_objc_fatal("+[%s %s]: unrecognized selector sent to instance %p",
class_getName(self), sel_getName(sel), self);
}
// Replaced by CF (throws an NSException)
- (void)doesNotRecognizeSelector:(SEL)sel {
_objc_fatal("-[%s %s]: unrecognized selector sent to instance %p",
object_getClassName(self), sel_getName(sel), self);
}
方法调用
如果有两个参数object和method. method_invoke可以调用，从搜索来看，主要用在KVO中
// Using this function to call the implementation of a method is faster than calling method_getImplementation and method_getName.
void method_invoke(void /* id receiver, Method m, ... */ )
没测试出来稳定的速度对比，结果都很快百万次0.3秒，姑且相信文档说的method_invoke更快吧
- (void)testMethodInvoke{
Human *instance = [[Human alloc] init];
Method method = class_getInstanceMethod([instance class], @selector(say:));
uint64_t start = mach_absolute_time();
NSString * result = ((NSString*(*)(id, Method, NSString*))method_invoke)(instance, method, @"Hello");
NSLog(@"invoke = %llul", mach_absolute_time() - start);
XCTAssertTrue([result isEqualToString:@"Hello"]);
[self measureBlock:^{
for (int i = 0; i < self.times; i++) {
NSString *result __attribute__((unused)) = ((NSString*(*)(id, Method, NSString*))method_invoke)(instance, method, @"Hello");
}
}];
}
- (void)testImpAndSel {
Human *instance = [[Human alloc] init];
Method method = class_getInstanceMethod([instance class], @selector(say:));
uint64_t start = mach_absolute_time();
NSString *(*function)(id, SEL, NSString *) = (NSString *(*)(id, SEL, NSString *))method_getImplementation(method);
SEL selecor = method_getName(method);
NSString * result = function(instance, selecor, @"Hello");
NSLog(@"iml&sel = %llul", mach_absolute_time() - start);
XCTAssertTrue([result isEqualToString:@"Hello"]);
[self measureBlock:^{
for (int i = 0; i < self.times; i++) {
NSString *(*function)(id, SEL, NSString *) = (NSString *(*)(id, SEL, NSString *))method_getImplementation(method);
SEL selecor = method_getName(method);
NSString *result __attribute__((unused)) = function(instance, selecor, @"Hello");
}
}];
}