『ios』-objc_msgSend + 消息轉(zhuǎn)發(fā) 全面解析（二）

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對(duì)于 NSInvocation 之前的意識(shí)一直很模糊构舟，雖然在消息轉(zhuǎn)發(fā)中用過肝箱，但是缺的就是沉下心來，分析一波扶关。now，let's go.

先來分析一波api

@class NSMethodSignature;

NS_ASSUME_NONNULL_BEGIN

NS_SWIFT_UNAVAILABLE("NSInvocation and related APIs not available")
@interface NSInvocation : NSObject {
@private
    void *_frame;
    void *_retdata;
    id _signature;
    id _container;
    uint8_t _retainedArgs;
    uint8_t _reserved[15];
}

+ (NSInvocation *)invocationWithMethodSignature:(NSMethodSignature *)sig;//初始化方法

@property (readonly, retain) NSMethodSignature *methodSignature;//方法簽名

- (void)retainArguments;//防止參數(shù)釋放掉
@property (readonly) BOOL argumentsRetained;

@property (nullable, assign) id target; //target
@property SEL selector;//方法

- (void)getReturnValue:(void *)retLoc;//獲取返回值
- (void)setReturnValue:(void *)retLoc;//設(shè)置返回值

- (void)getArgument:(void *)argumentLocation atIndex:(NSInteger)idx;//獲取參數(shù)
- (void)setArgument:(void *)argumentLocation atIndex:(NSInteger)idx;//設(shè)置參數(shù)

- (void)invoke; //執(zhí)行
- (void)invokeWithTarget:(id)target;// target發(fā)送消息残吩，即target執(zhí)行方法

@end

從初始化方法+ (NSInvocation *)invocationWithMethodSignature:(NSMethodSignature *)sig;來看生棍，我們需要 NSMethodSignature *這個(gè)對(duì)象。
那么下個(gè)方法簽名

NS_ASSUME_NONNULL_BEGIN

NS_SWIFT_UNAVAILABLE("NSInvocation and related APIs not available")
@interface NSMethodSignature : NSObject {
@private
    void *_private;
    void *_reserved[5];
    unsigned long _flags;
}

+ (nullable NSMethodSignature *)signatureWithObjCTypes:(const char *)types;//初始化方法

@property (readonly) NSUInteger numberOfArguments;//參數(shù)數(shù)量
- (const char *)getArgumentTypeAtIndex:(NSUInteger)idx NS_RETURNS_INNER_POINTER;//獲取參數(shù)類型

@property (readonly) NSUInteger frameLength;

- (BOOL)isOneway;// 是否是單向

@property (readonly) const char *methodReturnType NS_RETURNS_INNER_POINTER;//返回值類型
@property (readonly) NSUInteger methodReturnLength;//返回長(zhǎng)度

@end

api看完了猪钮，我覺得有些東西還是得從方法中來看才能學(xué)到東西。

-(NSString *)doit:(NSInteger)test1 doit2:(NSString *)test2{
    return @"2";
}

- (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個(gè)隱藏參數(shù)胆建，所以 argument 從2開始
    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];
            
            // 需要做參數(shù)類型判斷然后解析成對(duì)應(yīng)類型烤低，這里默認(rèn)所有參數(shù)均為OC對(duì)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];
    }
    // 需要做返回類型判斷。比如返回值為常量需要包裝成對(duì)象笆载，這里僅以最簡(jiǎn)單的`@`為例
    return returnVal;
}

直接用po看打印扑馁。

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從上面我們可以看到，invocation有四個(gè)參數(shù)凉驻，target selector argument argument.
然后分別對(duì)應(yīng)這四個(gè)符號(hào)腻要。@ ： q @.

看到這符號(hào)也許你有點(diǎn)蒙蔽。

NSMethodSignature的初始化方法涝登。

+ (nullable NSMethodSignature *)signatureWithObjCTypes:(const char *)types;

那么上面的這些符號(hào)就是types.就拿上面這個(gè)方法舉例子雄家。

 NSMethodSignature *signature =  [NSMethodSignature signatureWithObjCTypes:@"@@:q@"];
第一個(gè)@是返回值NSString *
第二個(gè)@是target
第三個(gè)：是Selector
第四個(gè)q 是nsintager
第五個(gè)@是nsstring *

對(duì)了好像還沒有展示NSMethodSignature的結(jié)構(gòu)。

image.png

當(dāng)然還有其他兩個(gè)

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");

+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");

說到這其實(shí)對(duì)于NSInvocation應(yīng)該就可以運(yùn)用自如了吧胀滚。
上面的例子中的方法趟济，是為了解決performSelector中的傳值問題的。

image.png

實(shí)際項(xiàng)目中咽笼，我們用的地方就應(yīng)該是消息轉(zhuǎn)發(fā)的過程中顷编。

正好看到一個(gè)挺好的例子。
動(dòng)態(tài)實(shí)現(xiàn)set get方法剑刑。

     data = [[NSMutableDictionary alloc] init];
        [data setObject:@"Tom Sawyer" forKey:@"title"];
        [data setObject:@"Mark Twain" forKey:@"author"];

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
    NSString *sel = NSStringFromSelector(selector);
    if ([sel rangeOfString:@"set"].location == 0) {
        //動(dòng)態(tài)造一個(gè) setter函數(shù)
        return [NSMethodSignature signatureWithObjCTypes:"v@:@"];
    } else {
        //動(dòng)態(tài)造一個(gè) getter函數(shù)
        return [NSMethodSignature signatureWithObjCTypes:"@@:"];
    }
}
 
- (void)forwardInvocation:(NSInvocation *)invocation
{
    //拿到函數(shù)名
    NSString *key = NSStringFromSelector([invocation selector]);
    if ([key rangeOfString:@"set"].location == 0) {
        //setter函數(shù)形如 setXXX: 拆掉 set和冒號(hào) 
        key = [[key substringWithRange:NSMakeRange(3, [key length]-4)] lowercaseString];
        NSString *obj;
        //從參數(shù)列表中找到值
        [invocation getArgument:&obj atIndex:2];
        [data setObject:obj forKey:key];
    } else {
        //getter函數(shù)就相對(duì)簡(jiǎn)單了媳纬，直接把函數(shù)名做 key就好了。
        NSString *obj = [data objectForKey:key];
        [invocation setReturnValue:&obj];
    }
}
 

最后，附上某位大神寫的對(duì)于block钮惠，應(yīng)該怎么應(yīng)用

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];
    CTBlockDescription *ct = [[CTBlockDescription alloc] initWithBlock:target];
    NSMethodSignature *methodSignature = ct.blockSignature;
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
    
    invocation.target = target;
    [invocation retainArguments];
    // invocation 有1個(gè)隱藏參數(shù)杨伙，所以 argument 從1開始
    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;
            }
            
            // 需要做參數(shù)類型判斷然后解析成對(duì)應(yīng)類型，這里默認(rèn)所有參數(shù)均為OC對(duì)象
            if (strcmp(type, "@") == 0) {
                id argument = arguments[I];
                [invocation setArgument:&argument atIndex:1 + I];
            }
        }
    }
    
    [invocation invoke];
    
   __weak  id  returnVal;
    
//        printf("retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(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];
    }
    
    NSString *returnStr = returnVal;
   
    printf("retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(returnVal)));
     printf("retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(returnStr)));
     printf("retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(target)));
    // 需要做返回類型判斷萌腿。比如返回值為常量需要包裝成對(duì)象，這里僅以最簡(jiǎn)單的`@`為例
    return returnStr;
}

上面代碼經(jīng)過測(cè)試抖苦，

image.png

這個(gè)地方如果不改為__weak的話就會(huì)崩潰毁菱。
下面附上相關(guān)打印

image.png

//    const char *_Block_signature(void *);
//    const char *signature = _Block_signature((__bridge void *)target);
//
//    NSMethodSignature *methodSignature = [NSMethodSignature signatureWithObjCTypes:signature];
//    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];

未完待續(xù)窗慎。。卤材。