什么是Method Swizzling
方法交换(Method Swizzling)，顾名思义就是将两个方法的实现交换，即由原来的SEL(A)－IMP(A)、SEL(B)－IMP(B)对应关系变成了SEL(A)－IMP(B)、SEL(B)－IMP(A)，如下图：
Method类型
Method类型是一个objc_method结构体指针，而结构体objc_method有三个成员，方法交换(Method Swizzling)的本质就是更改两个成员method_types和method_imp
/// An opaque type that represents a method in a class definition.
typedef struct objc_method *Method; // 本质是一个结构体
struct objc_method {
SEL method_name; // 方法名称
char *method_types; // 参数和返回类型的描述字串
IMP method_imp; // 方法的具体的实现的指针
}
Method Swizzling 实现方式
比如我们有一个控制器ParentViewController继承于UIViewController，子控制器SubViewController继承于ParentViewController。我们想替换SubViewController的viewDidAppear为swizzle_viewDidAppear, 运行后先显示ParentViewController页面，然后点击一个Button按钮，push到SubViewController页面，代码如下：（页面都是通过StoryBoard来构建的，请自行构建，我比较懒，这里就只贴上代码）
@interface ParentViewController : UIViewController
@end
@implementation ParentViewController
- (void)viewDidAppear:(BOOL)animated{
NSLog(@"%@ %s (IMP = ParentViewController viewDidAppear)",self, _cmd);
[super viewDidAppear:animated];
}
@end
@interface SubViewController : ParentViewController
@end
@implementation SubViewController
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
// 原方法名和替换方法名
SEL originalSelector = @selector(viewDidAppear:);
SEL swizzledSelector = @selector(swizzle_viewDidAppear:);
// 原方法结构体和替换方法结构体
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// 如果当前类没有原方法的实现IMP，先调用class_addMethod来给原方法添加默认的方法实现IMP
BOOL didAddMethod = class_addMethod(class,
originalSelector,
method_getImplementation(swizzledMethod),
method_getTypeEncoding(swizzledMethod));
if (didAddMethod) {// 添加方法实现IMP成功后，修改替换方法结构体内的方法实现IMP和方法类型编码TypeEncoding
class_replaceMethod(class,
swizzledSelector,
method_getImplementation(originalMethod),
method_getTypeEncoding(originalMethod));
} else { // 添加失败，调用交互两个方法的实现
method_exchangeImplementations(originalMethod, swizzledMethod);
}
});
}
- (void)swizzle_viewDidAppear:(BOOL)animated {
NSLog(@"%@ %s (IMP = SubViewController swizzle_viewDidAppear)",self, _cmd);
[self swizzle_viewDidAppear:animated];
}
@end
代码说明：
dispatch_once 保证方法替换只被执行一次
为什么要先调用类添加方法class_addMethod，然后判断添加失败后，再调用方法交换实现方法method_exchangeImplementations？
上面代码中SubViewController是没有Override父类的viewDidAppear。如果我们直接调用method_exchangeImplementations会怎么样？ 在这种情况下，我们试试
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
// 原方法名和替换方法名
SEL originalSelector = @selector(viewDidAppear:);
SEL swizzledSelector = @selector(swizzle_viewDidAppear:);
// 原方法结构体和替换方法结构体
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// 调用交互两个方法的实现
method_exchangeImplementations(originalMethod, swizzledMethod);
});
}
改成上面的代码，然后运行。哎哟，出错了！Let me see see 什么情况
2018-05-07 21:38:56.615884+0800 TestiOS[3469:385923] <ParentViewController: 0x7f8f38c09aa0> viewDidAppear: (IMP = SubViewController swizzle_viewDidAppear)
2018-05-07 21:38:56.616189+0800 TestiOS[3469:385923] -[ParentViewController swizzle_viewDidAppear:]: unrecognized selector sent to instance 0x7f8f38c09aa0
2018-05-07 21:38:56.621157+0800 TestiOS[3469:385923] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ParentViewController swizzle_viewDidAppear:]: unrecognized selector sent to instance 0x7f8f38c09aa0'
*** First throw call stack:
(
0 CoreFoundation 0x000000010e0fd1e6 __exceptionPreprocess + 294
1 libobjc.A.dylib 0x000000010d792031 objc_exception_throw + 48
2 CoreFoundation 0x000000010e17e784 -[NSObject(NSObject) doesNotRecognizeSelector:] + 132
3 UIKit 0x000000010e7a873b -[UIResponder doesNotRecognizeSelector:] + 295
4 CoreFoundation 0x000000010e07f898 ___forwarding___ + 1432
5 CoreFoundation 0x000000010e07f278 _CF_forwarding_prep_0 + 120
6 TestiOS 0x000000010ce903fb -[SubViewController swizzle_viewDidAppear:] + 75
7 UIKit 0x000000010e723ebf -[UIViewController _setViewAppearState:isAnimating:] + 697
8 UIKit 0x000000010e75ac53 -[UINavigationController viewDidAppear:] + 187
9 UIKit 0x000000010e723ebf -[UIViewController _setViewAppearState:isAnimating:] + 697
10 UIKit 0x000000010e726cfb __64-[UIViewController viewDidMoveToWindow:shouldAppearOrDisappear:]_block_invoke + 42
11 UIKit 0x000000010e72503f -[UIViewController _executeAfterAppearanceBlock] + 78
12 UIKit 0x000000010e58564f _runAfterCACommitDeferredBlocks + 634
13 UIKit 0x000000010e57477e _cleanUpAfterCAFlushAndRunDeferredBlocks + 388
14 UIKit 0x000000010e5942d7 __34-[UIApplication _firstCommitBlock]_block_invoke_2 + 155
15 CoreFoundation 0x000000010e09fb0c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
16 CoreFoundation 0x000000010e0842db __CFRunLoopDoBlocks + 331
17 CoreFoundation 0x000000010e083a84 __CFRunLoopRun + 1284
18 CoreFoundation 0x000000010e08330b CFRunLoopRunSpecific + 635
19 GraphicsServices 0x0000000113267a73 GSEventRunModal + 62
20 UIKit 0x000000010e57a0b7 UIApplicationMain + 159
21 TestiOS 0x000000010ce9047f main + 111
22 libdyld.dylib 0x0000000111b56955 start + 1
)
控制台打印ParentViewController找不到swizzle_viewDidAppear方法。 这是为什么呢？
第一行日志显示，ParentViewController是调用自己的方法viewDidAppear，通过打印_cmd输出为viewDidAppear可知道，但是执行的是SubViewController的swizzle_viewDidAppear的方法实现IMP，在swizzle_viewDidAppear方法实现IMP里又调用了[self swizzle_viewDidAppear:animated]这行代码，但是此时self是ParentViewController实例对象，类方法列表里根本没有swizzle_viewDidAppear方法，所以就导致找不到方法错误。说的我都觉得挺绕口的，千言万语不如一张图来的直观，向下瞅:
现在明白了吧，方法交换（Method Swizzling）在子类没有实现viewDidAppear方法的情况下会交换父类的viewDidAppear的实现IMP，所以在swizzle_viewDidAppear实现IMP中调用swizzle_viewDidAppear方法会触发doesNotRecognizeSelector找不到方法错误
如果我们的子类SubViewController重写Override了父类的viewDidAppear方法会怎么样？
我们在SubViewController中重写viewDidAppear方法
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
// 原方法名和替换方法名
SEL originalSelector = @selector(viewDidAppear:);
SEL swizzledSelector = @selector(swizzle_viewDidAppear:);
// 原方法结构体和替换方法结构体
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// 调用交互两个方法的实现
method_exchangeImplementations(originalMethod, swizzledMethod);
});
}
- (void)viewDidAppear:(BOOL)animated{
NSLog(@"%@ %s (IMP = SubViewController viewDidAppear)",self, _cmd);
[super viewDidAppear:animated];
}
- (void)swizzle_viewDidAppear:(BOOL)animated {
NSLog(@"%@ %s (IMP = SubViewController swizzle_viewDidAppear)",self, _cmd);
[self swizzle_viewDidAppear:animated];
}
改成上面的代码，然后运行。这次竟然成功了
2018-05-07 21:41:58.467190+0800 TestiOS[3556:400520] <ParentViewController: 0x7fd5a060b730> viewWillAppear
2018-05-07 21:41:58.475185+0800 TestiOS[3556:400520] <ParentViewController: 0x7fd5a060b730> viewDidAppear: (IMP = ParentViewController viewDidAppear)
2018-05-07 21:42:08.772307+0800 TestiOS[3556:400520] <SubViewController: 0x7fd5a0405170> viewWillAppear
2018-05-07 21:42:09.312426+0800 TestiOS[3556:400520] <SubViewController: 0x7fd5a0405170> viewDidAppear: (IMP = SubViewController swizzle_viewDidAppear)
2018-05-07 21:42:09.312643+0800 TestiOS[3556:400520] <SubViewController: 0x7fd5a0405170> swizzle_viewDidAppear: (IMP = SubViewController viewDidAppear)
2018-05-07 21:42:09.312792+0800 TestiOS[3556:400520] <SubViewController: 0x7fd5a0405170> viewDidAppear: (IMP = ParentViewController viewDidAppear)
你逗我玩儿呢，怎么子类实现viewDidAppear就好了。那是因为子类在检查到自己有viewDidAppear方法就直接交换自己的viewDidAppear方法实现IMP，直接上图，我不想废话了
这下我们明白了，如果直接调用方法method_exchangeImplementations来交换方法，需要考虑到子类有没有相应的方法，如果没有就要特殊处理，那岂不是太麻烦了。哈哈?不用你瞎操心，苹果有这个方法class_addMethod来帮助我们解决
我们把直接调用method_exchangeImplementations稍微做点修改
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
// 原方法名和替换方法名
SEL originalSelector = @selector(viewDidAppear:);
SEL swizzledSelector = @selector(swizzle_viewDidAppear:);
// 原方法结构体和替换方法结构体
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// 如果当前类没有原方法的实现IMP，先调用class_addMethod来给原方法添加默认的方法实现IMP
BOOL didAddMethod = class_addMethod(class,
originalSelector,
method_getImplementation(swizzledMethod),
method_getTypeEncoding(swizzledMethod));
if (didAddMethod) {// 添加方法实现IMP成功后，修改替换方法结构体内的方法实现IMP和方法类型编码TypeEncoding
class_replaceMethod(class,
swizzledSelector,
method_getImplementation(originalMethod),
method_getTypeEncoding(originalMethod));
} else { // 添加失败，调用交互两个方法的实现
method_exchangeImplementations(originalMethod, swizzledMethod);
}
});
}
这样我们就不用关心子类有没有实现viewDidAppear方法，方法class_addMethod在子类没有实现viewDidAppear方法的时候，为其添加swizzle_viewDidAppear方法实现IMP，原方法swizzle_viewDidAppear指向父类的viewDidAppear方法实现IMP
这就是为什么先要调用class_addMethod方法的原因了，如果子类SubViewController实现了方法viewDidAppear，那么class_addMethod方法会返回NO，意思子类存在viewDidAppear方法实现，就直接走method_exchangeImplementations方法交换
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