runtime方法 - class_addMethod

runtime方法 - class_addMethod

official document 官方文档

/** 
 * Adds a new method to a class with a given name and implementation.
 * 
 * @param cls The class to which to add a method.
 * @param name A selector that specifies the name of the method being added.
 * @param imp A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd.
 * @param types An array of characters that describe the types of the arguments to the method. 
 * 
 * @return YES if the method was added successfully, otherwise NO 
 *  (for example, the class already contains a method implementation with that name).
 *
 * @note class_addMethod will add an override of a superclass's implementation, 
 *  but will not replace an existing implementation in this class. 
 *  To change an existing implementation, use method_setImplementation.
 */
OBJC_EXPORT BOOL class_addMethod(Class _Nullable cls, SEL _Nonnull name, IMP _Nonnull imp, const char * _Nullable types) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);

details that are easy to ignore 容易忽略的细节

1. 这是一个 c 语言函数;
2. 最后一个参数const char * _Nullable types, 官方解释为: An array of characters that describe the types of the arguments to the method, 注意, 这里的method指的同样是 c 语言函数, 不了解这一点, 一定有此疑问 - Objective-C 方法和An array of characters中元素的个数不对应;
3. 同样, imp参数的注释 - A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd中, function同样指的是 C 语言函数;

parameter analysis 参数解析

1. cls: 添加方法的类对象, 获取方式为[<instance> class]或者<instance>.class;
2. name: 用来指定被添加方法名称的选择子, 类型为SEL, 若方法名可以直接获取, 则等同于@selector(方法名称);
3. imp: c 语言函数指针, 类型为IMP, 若函数在作用域内可以获取到, 则
   • (1) C语言写法: (IMP) 方法名;
   • (2) OC写法: class_getMethodImplementation(self, @selector(方法名));
4. types: 代表imp函数返回值和参数的字符串, Type Encodings的详细用法可以参考官方文档, 强烈建议使用如下方法自动生成, 而不是暴力手打;?

/** 
 * Returns a string describing a method's parameter and return types.
 * 
 * @param m The method to inspect.
 * 
 * @return A C string. The string may be \c NULL.
 */
OBJC_EXPORT const char * _Nullable method_getTypeEncoding(Method _Nonnull m) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);

吐槽 ?

如果最后一个参数没有特别的用法, 真的非常鸡肋, 感觉是闲得蛋疼, 不知道为什么要有这么个参数, 写这个方法的童鞋, 你粗来, 咱聊谈谈…