iOS Method Swizzle的祕密
什麼是Method Swizzling
方法交換(Method Swizzling)，顧名思義就是將兩個方法的實現交換，即由原來的SEL(A)－IMP(A)、SEL(B)－IMP(B)對應關係變成了SEL(A)－IMP(B)、SEL(B)－IMP(A)，以下圖：app
Method類型
Method類型是一個objc_method結構體指針，而結構體objc_method有三個成員，方法交換(Method Swizzling)的本質就是更改兩個成員method_types和method_impide
runtime.h源碼oop
/// An opaque type that represents a method in a class definition.
typedef struct objc_method *Method; // 本質是一個結構體
struct objc_method {
SEL method_name; // 方法名稱
char *method_types; // 參數和返回類型的描述字串
IMP method_imp; // 方法的具體的實現的指針
}
Method Swizzling 實現方式
好比咱們有一個控制器ParentViewController繼承於UIViewController，子控制器SubViewController繼承於ParentViewController。咱們想替換SubViewController的viewDidAppear爲swizzle_viewDidAppear, 運行後先顯示ParentViewController頁面，而後點擊一個Button按鈕，push到SubViewController頁面，代碼以下：（頁面都是經過StoryBoard來構建的，請自行構建，我比較懶，這裏就只貼上代碼）編碼
@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];
// 原方法名和替換方法名
SEL originalSelector = @selector(viewDidAppear:);
SEL swizzledSelector = @selector(swizzle_viewDidAppear:);
// 原方法結構體和替換方法結構體
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// 若是當前類沒有原方法的實現IMP，先調用class_addMethod來給原方法添加默認的方法實現IMP
BOOL didAddMethod = class_addMethod(class,
originalSelector,
method_getImplementation(swizzledMethod),
method_getTypeEncoding(swizzledMethod));
if (didAddMethod) {// 添加方法實現IMP成功後，修改替換方法結構體內的方法實現IMP和方法類型編碼TypeEncoding
class_replaceMethod(class,
swizzledSelector,
method_getImplementation(originalMethod),
method_getTypeEncoding(originalMethod));
} else { // 添加失敗，調用交互兩個方法的實現
method_exchangeImplementations(originalMethod, swizzledMethod);
}
});
}
- (void)swizzle_viewDidAppear:(BOOL)animated {
NSLog(@"%@ %s (IMP = SubViewController swizzle_viewDidAppear)",self, _cmd);
[self swizzle_viewDidAppear:animated];
}
@end
代碼說明：spa
dispatch_once 保證方法替換隻被執行一次指針
爲何要先調用類添加方法class_addMethod，而後判斷添加失敗後，再調用方法交換實現方法method_exchangeImplementations？日誌
上面代碼中SubViewController是沒有Override父類的viewDidAppear。若是咱們直接調用method_exchangeImplementations會怎麼樣？ 在這種狀況下，咱們試試code
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
// 原方法名和替換方法名
SEL originalSelector = @selector(viewDidAppear:);
SEL swizzledSelector = @selector(swizzle_viewDidAppear:);
// 原方法結構體和替換方法結構體
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// 調用交互兩個方法的實現
method_exchangeImplementations(originalMethod, swizzledMethod);
});
}
改爲上面的代碼，而後運行。哎喲，出錯了！Let me see see 什麼狀況對象
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
)
控制檯打印ParentViewController找不到swizzle_viewDidAppear方法。 這是爲何呢？blog
第一行日誌顯示，ParentViewController是調用本身的方法viewDidAppear，經過打印_cmd輸出爲viewDidAppear可知道，可是執行的是SubViewController的swizzle_viewDidAppear的方法實現IMP，在swizzle_viewDidAppear方法實現IMP裏又調用了[self swizzle_viewDidAppear:animated]這行代碼，可是此時self是ParentViewController實例對象，類方法列表里根本沒有swizzle_viewDidAppear方法，因此就致使找不到方法錯誤。說的我都以爲挺繞口的，千言萬語不如一張圖來的直觀，向下瞅:
如今明白了吧，方法交換（Method Swizzling）在子類沒有實現viewDidAppear方法的狀況下會交換父類的viewDidAppear的實現IMP，因此在swizzle_viewDidAppear實現IMP中調用swizzle_viewDidAppear方法會觸發doesNotRecognizeSelector找不到方法錯誤
若是咱們的子類SubViewController重寫Override了父類的viewDidAppear方法會怎麼樣？
咱們在SubViewController中重寫viewDidAppear方法
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
// 原方法名和替換方法名
SEL originalSelector = @selector(viewDidAppear:);
SEL swizzledSelector = @selector(swizzle_viewDidAppear:);
// 原方法結構體和替換方法結構體
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// 調用交互兩個方法的實現
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];
}
改爲上面的代碼，而後運行。此次居然成功了
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)
你逗我玩兒呢，怎麼子類實現viewDidAppear就行了。那是由於子類在檢查到本身有viewDidAppear方法就直接交換本身的viewDidAppear方法實現IMP，直接上圖，我不想廢話了
這下咱們明白了，若是直接調用方法method_exchangeImplementations來交換方法，須要考慮到子類有沒有相應的方法，若是沒有就要特殊處理，那豈不是太麻煩了。哈哈😁不用你瞎操心，蘋果有這個方法class_addMethod來幫助咱們解決
咱們把直接調用method_exchangeImplementations稍微作點修改
+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
// 原方法名和替換方法名
SEL originalSelector = @selector(viewDidAppear:);
SEL swizzledSelector = @selector(swizzle_viewDidAppear:);
// 原方法結構體和替換方法結構體
Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
// 若是當前類沒有原方法的實現IMP，先調用class_addMethod來給原方法添加默認的方法實現IMP
BOOL didAddMethod = class_addMethod(class,
originalSelector,
method_getImplementation(swizzledMethod),
method_getTypeEncoding(swizzledMethod));
if (didAddMethod) {// 添加方法實現IMP成功後，修改替換方法結構體內的方法實現IMP和方法類型編碼TypeEncoding
class_replaceMethod(class,
swizzledSelector,
method_getImplementation(originalMethod),
method_getTypeEncoding(originalMethod));
} else { // 添加失敗，調用交互兩個方法的實現
method_exchangeImplementations(originalMethod, swizzledMethod);
}
});
}
這樣咱們就不用關心子類有沒有實現viewDidAppear方法，方法class_addMethod在子類沒有實現viewDidAppear方法的時候，爲其添加swizzle_viewDidAppear方法實現IMP，原方法swizzle_viewDidAppear指向父類的viewDidAppear方法實現IMP
這就是爲何先要調用class_addMethod方法的緣由了，若是子類SubViewController實現了方法viewDidAppear，那麼class_addMethod方法會返回NO，意思子類存在viewDidAppear方法實現，就直接走method_exchangeImplementations方法交換
好了，就說到這裏吧，有問題請留言
參考：
- 1. iOS逆向之Method Swizzle
- 2. iOS開發 - method swizzle方式的選擇
- 3. iOS Swizzle
- 4. 淺談 iOS swizzle
- 5. Method Swizzle 打點 UIKit 的問題
- 6. iOS App上線的祕密
- 7. IOS開發-ios runtime-Method Swizzling
- 8. 成長爲 iOS 大 V 的祕密
- 9. iOS開發之 Method Swizzling 深刻淺出
- 10. axios的祕密
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