objective-c 编程总结，第八篇运行时操作 - 消息转发
第七篇中讲动态属性时，提到了resolveInstanceMethod，这个方法不仅在这里用，还用来实现消息的转发。
消息的转发就是向对象发送一个它本身并没有实现的消息，在运行时确定它实际产生的行为。
举个例子来说，一个Person对象，在运行时根据实际情况，决定是否响应fly这样的方法。如果条件具备，则fly被响应。否则，则不具备这样的方法。类似于AoP的做法。
要实现消息转发，需要覆盖三个方法：
1， resolveInstanceMethod（可选），这个方法为你提供了一个机会，在消息被发现本身没有在类中定义时你可以通过class_addMethod将它添加进去。如果你不这样做，不管你最终返回YES还是NO，程序都会jmp到下一个方法。
2， –(NSMethodSignature*)methodSignatureForSelector: (SEL) sel;通过覆盖这个方法，可以将你要转发的消息的签名创建并返回。如果在这里你返回nil，那么就会直接抛出异常。如果返回一个签名，则程序会jmp到第三个方法。这里返回的方法签名，必须满足两个条件之一（方法名相同||输入参数相同).
3， –(void)forwardInvocation:(NSInvocation * )anInvocation;在这里进行实际的方法调用。参考以下代码:
#import <Foundation/Foundation.h>
#import "Human.h"
@interface Plane : NSObject
-(void)fly:(Human*)p;
-(int)fly1;
-(void)fly2:(Human*)p withBird:(NSString*)birdName;
@end
#import "Plane.h"
@implementation Plane
-(void)fly:(Human*)p{
NSLog(@"Fly with a guy whose information is \"%@\"", [p description]);
NSLog(@"fly......");
}
-(int)fly1{
NSLog(@"I can fly in Plane for fly1");
return 0;
}
-(void)fly2:(Human*)p withBird:(NSString*)birdName{
NSLog(@"Fly with a guy whose information is \"%@\"", [p description]);
NSLog(@"fly......bird:'%@'.", birdName);
}
@end
#import <Foundation/Foundation.h>
void dynamicMethod(id self, SEL _cmd, float height);
@interface Human : NSObject{
float _height;
}
@property(retain, nonatomic) NSString * name;
@property(readonly) float weight;
@property float height;
-(id) initWithWeight:(float)weight;
-(NSString*)description;
@end
#import <objc/runtime.h>
#import "Human.h"
#import "Plane.h"
void dynamicMethod(id self, SEL _cmd, float height){
NSLog(@"dynamicMethod:%@", NSStringFromSelector(_cmd));
// here is a question. how to implement the statements to assign the property of id(human.height);
// looks like the dynamic property is just for linkage.
// 当赋值发生时，由于动态函数的全局性，可以访问其他全局变量，对象。所以更像一种链接机制
}
@implementation Human
@synthesize name;
@synthesize weight;
@dynamic height;
-(id)initWithWeight:(float)w{
if(self=[super init]){
weight = w;
}
return self;
}
-(NSString*)description{
return [NSString stringWithFormat:@"The human whose name is \"%@\". weight is [%f]. height is [%f]",
self.name, self.weight, self.height];
}
-(float)height{
return _height;
}
-(void)fly2{
NSLog(@"yes I can fly in 2!");
}
+(BOOL)resolveInstanceMethod:(SEL)sel{
NSString* method = NSStringFromSelector(sel);
if([method isEqualToString:@"setHeight:"]){
class_addMethod([self class], sel, (IMP)dynamicMethod, "v@:f");
}
return [super resolveInstanceMethod:sel];
}
-(NSMethodSignature*)methodSignatureForSelector:(SEL)selector{
NSMethodSignature * signature = [super methodSignatureForSelector:selector];
if(!signature&&[NSStringFromSelector(selector) isEqualToString:@"fly"]){
//signature = [[self class] instanceMethodSignatureForSelector:@selector(fly2)];
SEL newSel = NSSelectorFromString(@"fly1"); //will ok.
// SEL newSel = NSSelectorFromString(@"fly:");//will ok.
// SEL newSel = NSSelectorFromString(@"fly2:withBird");//will wrong.
//这里返回的消息，必须符合以下条件之一：
//方法名相同
//输入参数相同
if([Plane instancesRespondToSelector:newSel]){
signature = [Plane instanceMethodSignatureForSelector:newSel];
}
}
return signature;
}
-(void)forwardInvocation:(NSInvocation *)anInvocation{
NSString * selName = NSStringFromSelector([anInvocation selector]);
NSMethodSignature * sig = [anInvocation methodSignature];
NSLog(@"the signature %@", [NSString stringWithCString:[sig methodReturnType] encoding:NSUTF8StringEncoding]);
if([selName isEqualToString:@"fly"]){
Plane * p = [[Plane alloc] init];
SEL newSel = NSSelectorFromString(@"fly:");
IMP imp = [p methodForSelector:newSel];
imp(p, newSel, self);
[p release];
}
}
-(void)dealloc{
[self setName:nil];
[super dealloc];
}
@end
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import "Human.h"
#import "Plane.h"
int main (int argc, const char * argv[])
{
@autoreleasepool {
// insert code here...
Human * h = [[Human alloc] initWithWeight:17.3];
h.name=@"Chris";
h.height = 18.0;
NSLog(@"%@", [h description]);
[h fly];
[h release];
h=nil;
}}