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关于runtime的 class_addMethod 和 method_exchangeImplementations方法

////  UIBarButtonItem+Public.m//  RuntimeDemo////  Created by huokr on 2/27/14.//  Copyright (c) 2014 huokr. All rights reserved.//#import "UIBarButtonItem+Public.h"#import 
   
    @implementation UIBarButtonItem (Public)/**/- (id)initWithCustomTitle:(NSString *)title                    style:(UIBarButtonItemStyle)style                   target:(id)target                   action:(SEL)action{    ___start_log___    self = [super init];    if (self) {        UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 44, 44)];        CGRect frame = button.frame;        if (title) {            CGSize size = [title sizeWithFont:[UIFont boldSystemFontOfSize:16.0f]                            constrainedToSize:CGSizeMake(MAXFLOAT, 16.0f)                                lineBreakMode:(NSLineBreakByCharWrapping)];            [button setTitle:title                    forState:(UIControlStateNormal)];            [button setTitleColor:[UIColor purpleColor]                         forState:(UIControlStateNormal)];            button.titleLabel.font = [UIFont systemFontOfSize:16.0f];            if (size.width > 40) {                frame.size.width = size.width + 10;                button.frame = frame;            }        }        button.backgroundColor = [UIColor clearColor];        [button addTarget:target                   action:action         forControlEvents:(UIControlEventTouchUpInside)];        self = [[UIBarButtonItem alloc] initWithCustomView:button];    }    return self;}+ (void)load{    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        Class class = [self class];//系统的原方法//        - (instancetype)initWithTitle:(NSString *)title style:(UIBarButtonItemStyle)style target:(id)target action:(SEL)action;        SEL originalSelector = @selector(initWithTitle:style:target:action:);        SEL swizzledSelector = @selector(initWithCustomTitle:style:target:action:);        Method originalMethod =        class_getInstanceMethod(class,                                @selector(initWithTitle:style:target:action:));        Method swizzledMethod =        class_getInstanceMethod(class,                                @selector(initWithCustomTitle:style:target:action:));        //class_addMethod 参考 [这里写链接内容](http://longtimenoc.com/archives/ios%E5%9C%A8%E8%BF%90%E8%A1%8C%E6%97%B6%E4%B8%BA%E7%B1%BB%E6%B7%BB%E5%8A%A0%E6%96%B9%E6%B3%95)        BOOL didAddMethod =        class_addMethod(class,                        originalSelector,                        method_getImplementation(originalMethod),                        method_getTypeEncoding(originalMethod));        if (didAddMethod) {            ___start_log___            class_replaceMethod(class,                                swizzledSelector,                                method_getImplementation(originalMethod),                                method_getTypeEncoding(originalMethod));        } else {            //交换两个方法的实现！            method_exchangeImplementations(originalMethod, swizzledMethod);        }    });}/**/@end
   

以上代码重载了load方法，其中利用oc 的 runtime机制把系统的initWithTitle:style:target:action:方法替换为自定义的initWithCustomTitle:style:target:action:

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