直接呼叫方法

如果你需要從 C 程式碼呼叫 Objective-C 方法，你有兩種方法：使用 objc_msgSend，或者獲取 IMP（方法實現函式指標）並呼叫它。

#import <objc/objc.h>

@implementation Example

- (double)negate:(double)value {
    return -value;
}

- (double)invert:(double)value {
    return 1 / value;
}

@end

// Calls the selector on the object. Expects the method to have one double argument and return a double.
double performSelectorWithMsgSend(id object, SEL selector, double value) {
    // We declare pointer to function and cast `objc_msgSend` to expected signature.
    // WARNING: This step is important! Otherwise you may get unexpected results!
    double (*msgSend)(id, SEL, double) = (typeof(msgSend)) &objc_msgSend;

    // The implicit arguments of self and _cmd need to be passed in addition to any explicit arguments.
    return msgSend(object, selector, value);
}

// Does the same as the above function, but by obtaining the method's IMP.
double performSelectorWithIMP(id object, SEL selector, double value) {
    // Get the method's implementation.
    IMP imp = class_getMethodImplementation([self class], selector);

    // Cast it so the types are known and ARC can work correctly.
    double (*callableImp)(id, SEL, double) = (typeof(callableImp)) imp;

    // Again, you need the explicit arguments.
    return callableImp(object, selector, value);
} 

int main() {
    Example *e = [Example new];

    // Invoke negation, result is -4
    double x = performSelectorWithMsgSend(e, @selector(negate:), 4);

    // Invoke inversion, result is 0.25
    double y = performSelectorWithIMP(e, @selector(invert:), 4);
}

objc_msgSend 通過獲取方法的 IMP 並呼叫它來工作。呼叫最後幾個方法的 IMPs 是快取的，所以如果你在一個非常緊湊的迴圈中傳送一個 Objective-C 訊息，你可以獲得可接受的效能。在某些情況下，手動快取 IMP 可以提供稍好的效能，儘管這是最後的優化。