一 方法調(diào)用的本質(zhì)

1.runtime 的使用方法 跪者，首先導(dǎo)入objc/message.h ,在build setting 里面輸入msg 設(shè)置為NO 因?yàn)閺膞code5開(kāi)始蘋(píng)果不推薦使用底層的東西了擦！！ runtime是運(yùn)行時(shí)的機(jī)制，也就是說(shuō)所有方法的調(diào)用在編譯command+b的時(shí)候不會(huì)真正檢測(cè)到底調(diào)用誰(shuí)，只會(huì)在運(yùn)行時(shí)候（動(dòng)態(tài)）調(diào)用方法 猖腕，方法調(diào)用的本質(zhì)順序; e.g.

[p eat] ---> [p performselector:@selector:eat] -> objc_msgsend(p,@selector:eat);
[Person eat] --> 先轉(zhuǎn)化成類(lèi)對(duì)象 Class personClass = [Person Class]; --> [personClass eat] --> [personClass performselector:@selector:eat]; ---> objc_msgsend([Person Class],@selector:eat);

無(wú)論是類(lèi)調(diào)用放還是類(lèi)對(duì)象調(diào)用方法本質(zhì)都是 轉(zhuǎn)化成objc_msgsend

- (void)text1 {
    
    person *p = [[person alloc] init];
    
    // xcode5 之后使用運(yùn)行時(shí)方法
    
    // 蘋(píng)果不推薦調(diào)用底層的方法
    // 在 buildsettings 搜索msg 設(shè)置為NO
    
    // 讓p 發(fā)送消息
    objc_msgSend(p, @selector(eat));
    
    //
    objc_msgSend([person class], @selector(eat));  // 類(lèi)對(duì)象
    
    // 類(lèi)名調(diào)用類(lèi)方法 其實(shí)就是用類(lèi)對(duì)象調(diào)用方法
    [person eat];
    
    
    // 獲取類(lèi)對(duì)象
    
    Class personClass = [person class];
    [personClass performSelector:@selector(eat)];
    
    [personClass eat];

}

二 交換方法的實(shí)現(xiàn)

本質(zhì)先得到兩個(gè)方法 然后交換兩個(gè)方法的實(shí)現(xiàn)請(qǐng)見(jiàn)官方文檔

開(kāi)發(fā)時(shí)候的作用很大比如更改文字的大小顏色等等替換官方的方法實(shí)現(xiàn)換成自己的但是記得回調(diào)官方的方法哦！恨闪！ 這樣可以替換所有的方法如果用分類(lèi)的方法只能替換將來(lái)的已經(jīng)使用的方法就沒(méi)辦法了一勞永逸Ｌ雀小！咙咽！ 裝逼利器老玛！

/** 
 * Sets the implementation of a method.
 * 
 * @param m The method for which to set an implementation.
 * @param imp The implemention to set to this method.
 * 
 * @return The previous implementation of the method.
 */
OBJC_EXPORT IMP method_setImplementation(Method m, IMP imp) 
     __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);

/** 
 * Exchanges the implementations of two methods.
 * 
 * @param m1 Method to exchange with second method.
 * @param m2 Method to exchange with first method.
 * 
 * @note This is an atomic version of the following:
 *  \code 
 *  IMP imp1 = method_getImplementation(m1);
 *  IMP imp2 = method_getImplementation(m2);
 *  method_setImplementation(m1, imp2);
 *  method_setImplementation(m2, imp1);
 *  \endcode
 */
OBJC_EXPORT void method_exchangeImplementations(Method m1, Method m2) 
     __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);

#import "UIImage+myImage.h"
#import <objc/message.h>
@implementation UIImage (myImage)

+ (void)load {

      // 方法和屬性等都是在類(lèi)中所以用class_
    
    Method m1 = class_getClassMethod([UIImage class], @selector(imageNamed:));
    
    Method m2 = class_getClassMethod([UIImage class], @selector(stimageNamed:));
    
    method_exchangeImplementations(m1 , m2 );
}


+ (UIImage *)stimageNamed:(NSString *)imageName {
    UIImage *image = [UIImage stimageNamed:imageName];
    
    if (image ==nil) {
    
        NSLog(@"圖片為nil");
    }
    
    return image;
    
    
}

三 動(dòng)態(tài)的為某個(gè)類(lèi)添加方法的實(shí)現(xiàn)

    class_addMethod(self , @selector(eat:), (IMP)eatsssss, "v@:@");

#import "person.h"
#import <objc/message.h>
@implementation person


// 調(diào)用了一個(gè)沒(méi)有實(shí)現(xiàn)的方法就會(huì)調(diào)用這個(gè)方法 可以知道哪些方法沒(méi)有實(shí)現(xiàn)  self _cmd //隱私參數(shù)

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    
    // 動(dòng)態(tài)添加方法
    if ([NSStringFromSelector(sel) isEqualToString:@"eat:"]) {
        
        /**
         
         cls: 給哪個(gè)類(lèi)添加方法
         SEL:添加方法的方法編號(hào)
         IMP:函數(shù)名 方法實(shí)現(xiàn)  函數(shù)入口
         types : 方法類(lèi)型
         
         */
        
        
        
        class_addMethod(self , @selector(eat:), (IMP)eatsssss, "v@:@");
    }

    return [super resolveInstanceMethod:sel];
}


// 定義函數(shù)
void eatsssss (id self ,SEL _cmd,id parames)
{
    NSLog(@"hahha%@",parames);

}

官方文檔

Dynamic Method Resolution

There are situations where you might want to provide an implementation of a method dynamically. For example, the Objective-C declared properties feature (see Declared Properties in The Objective-C Programming Language) includes the @dynamic directive:

@dynamic propertyName;
which tells the compiler that the methods associated with the property will be provided dynamically.

You can implement the methods resolveInstanceMethod: and resolveClassMethod: to dynamically provide an implementation for a given selector for an instance and class method respectively.

An Objective-C method is simply a C function that take at least two arguments—self and _cmd. You can add a function to a class as a method using the function class_addMethod. Therefore, given the following function:

void dynamicMethodIMP(id self, SEL _cmd) {
    // implementation ....
}
you can dynamically add it to a class as a method (called resolveThisMethodDynamically) using resolveInstanceMethod: like this:

@implementation MyClass
+ (BOOL)resolveInstanceMethod:(SEL)aSEL
{
    if (aSEL == @selector(resolveThisMethodDynamically)) {
          class_addMethod([self class], aSEL, (IMP) dynamicMethodIMP, "v@:");
          return YES;
    }
    return [super resolveInstanceMethod:aSEL];
}
@end
Forwarding methods (as described in Message Forwarding) and dynamic method resolution are, largely, orthogonal. A class has the opportunity to dynamically resolve a method before the forwarding mechanism kicks in. If respondsToSelector: or instancesRespondToSelector: is invoked, the dynamic method resolver is given the opportunity to provide an IMP for the selector first. If you implement resolveInstanceMethod: but want particular selectors to actually be forwarded via the forwarding mechanism, you return NO for those selectors.

Dynamic Loading

An Objective-C program can load and link new classes and categories while it’s running. The new code is incorporated into the program and treated identically to classes and categories loaded at the start.

Dynamic loading can be used to do a lot of different things. For example, the various modules in the System Preferences application are dynamically loaded.

In the Cocoa environment, dynamic loading is commonly used to allow applications to be customized. Others can write modules that your program loads at runtime—much as Interface Builder loads custom palettes and the OS X System Preferences application loads custom preference modules. The loadable modules extend what your application can do. They contribute to it in ways that you permit but could not have anticipated or defined yourself. You provide the framework, but others provide the code.

Although there is a runtime function that performs dynamic loading of Objective-C modules in Mach-O files (objc_loadModules, defined in objc/objc-load.h), Cocoa’s NSBundle class provides a significantly more convenient interface for dynamic loading—one that’s object-oriented and integrated with related services. See the NSBundle class specification in the Foundation framework reference for information on the NSBundle class and its use. See OS X ABI Mach-O File Format Reference for information on Mach-O files.

四 動(dòng)態(tài)的為某個(gè)類(lèi)添加屬性

#import "NSObject+name.h"
#import <objc/message.h>
// @property在分類(lèi)里面只會(huì)生成方法的set get方法的生明不會(huì)生成實(shí)現(xiàn)和_成員變量
@implementation NSObject (name)

- (void)setName:(NSString *)name {

    // 添加屬性一般也給對(duì)象添加的 objc
    
    /***
     objc 給哪個(gè)對(duì)象添加屬性
     
     key 屬性名 根據(jù)key 去獲取關(guān)聯(lián)的對(duì)象  void * == id
     value 關(guān)聯(lián)的值賦給的值name
     policy weak retain copy
     
     */
    objc_setAssociatedObject(self, @"name", name, OBJC_ASSOCIATION_COPY);
    
}


- (NSString *)name {

    return objc_getAssociatedObject(self, @"name");
}