runtime那些事兒（消息機制）
一、關于runtime
之前在項目中有遇到過用runtime解決改變全局字體的問題，所以再一次感受到了runtime黑魔法的強大，趁現(xiàn)在有機會分享一下對runtime的一些理解。在對象調用方法是Objective-C中經(jīng)常使用的功能，也就是消息的傳遞，而Objective-C是C的超集，所以和C不同的是，Objective-C使用的是動態(tài)綁定，也就是runtime。Objective-C的消息傳遞和消息機制也就不多說了，今天主要說的是動態(tài)方法，也就是函數(shù)的調用。
二、相關的幾個函數(shù)
下面一張圖詳細的概括了每個函數(shù)調用的先后以及執(zhí)行的前提
消息傳遞函數(shù)的調用
1.對象在收到無法解讀的消息后，首先會調用所屬類的
- + (BOOL)resolveInstanceMethod:(SEL)sel
這個方法在運行時，沒有找到SEL的IML時就會執(zhí)行。這個函數(shù)是給類利用class_addMethod添加函數(shù)的機會。根據(jù)文檔，如果實現(xiàn)了添加函數(shù)代碼則返回YES，未實現(xiàn)返回NO。舉個例子，新建了一個工程，首先我在ViewController這個類中執(zhí)行doSomething1這個方法，代碼如下
- //
- // ViewController.m
- // RuntimeTest1
- //
- // Created by HenryCheng on 15/12/24.
- // Copyright ?（版權符號） 2015年 www.igancao.com All rights reserved.
- //
- #import "ViewController.h"
- @interface ViewController ()
- @end
- @implementation ViewController
- - (void)viewDidLoad {
- [super viewDidLoad];
- [self performSelector:@selector(doSomething)];
- }
- - (void)didReceiveMemoryWarning {
- [super didReceiveMemoryWarning];
- // Dispose of any resources that can be recreated.
- }
- @end
運行結果
- **2015-12-24 10:35:37.726 RuntimeTest1[1877:337842] -[ViewController doSomething]: unrecognized selector sent to instance 0x7fe9f3736680**
- **2015-12-24 10:35:37.729 RuntimeTest1[1877:337842] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController doSomething]: unrecognized selector sent to instance 0x7fe9f3736680'**
- ***** First throw call stack:**
不出意外，程序崩潰，因為沒有找到doSomething這個方法，下面我們在里面實現(xiàn) + (BOOL)resolveInstanceMethod:(SEL)sel這個方法，并且判斷如果SEL 是doSomething那就輸出add method here
- //
- // ViewController.m
- // RuntimeTest1
- //
- // Created by HenryCheng on 15/12/24.
- // Copyright ?（版權符號） 2015年 www.igancao.com All rights reserved.
- //
- #import "ViewController.h"
- @interface ViewController ()
- @end
- @implementation ViewController
- - (void)viewDidLoad {
- [super viewDidLoad];
- [self performSelector:@selector(doSomething)];
- }
- + (BOOL)resolveInstanceMethod:(SEL)sel {
- if (sel == @selector(doSomething)) {
- NSLog(@"add method here");
- return YES;
- }
- return [super resolveInstanceMethod:sel];
- }
- - (void)didReceiveMemoryWarning {
- [super didReceiveMemoryWarning];
- // Dispose of any resources that can be recreated.
- }
- @end
繼續(xù)運行，然后看到log
- **2015-12-24 10:47:24.687 RuntimeTest1[2007:382077] add method here**
- **2015-12-24 10:47:24.687 RuntimeTest1[2007:382077] -[ViewController doSomething]: unrecognized selector sent to instance 0x7f9568c331f0**
- **2015-12-24 10:47:24.690 RuntimeTest1[2007:382077] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController doSomething]: unrecognized selector sent to instance 0x7f9568c331f0'**
- ***** First throw call stack:**
可以看到程序依然是崩潰了，但是我們可以看到輸出了add method here，這說明我們 + (BOOL)resolveInstanceMethod:(SEL)sel這個方法執(zhí)行了，并進入了判斷，所以，在這兒，我們可以做一下操作，使這個方法得到相應，不至于走到***- (void)doesNotRecognizeSelector:(SEL)aSelector這個方法中而崩掉了，接下來，我么繼續(xù)操作，如下
- //
- // ViewController.m
- // RuntimeTest1
- //
- // Created by HenryCheng on 15/12/24.
- // Copyright ?（版權符號） 2015年 www.igancao.com All rights reserved.
- //
- #import "ViewController.h"
- #import [objc/runtime.h](因識別問題，此處將尖括號改為方括號)
- @interface ViewController ()
- @end
- @implementation ViewController
- - (void)viewDidLoad {
- [super viewDidLoad];
- [self performSelector:@selector(doSomething)];
- }
- + (BOOL)resolveInstanceMethod:(SEL)sel {
- if (sel == @selector(doSomething)) {
- NSLog(@"add method here");
- class_addMethod([self class], sel, (IMP)dynamicMethodIMP, "v@:");
- return YES;
- }
- return [super resolveInstanceMethod:sel];
- }
- void dynamicMethodIMP (id self, SEL _cmd) {
- NSLog(@"doSomething SEL");
- }
- - (void)didReceiveMemoryWarning {
- [super didReceiveMemoryWarning];
- // Dispose of any resources that can be recreated.
- }
- @end
導入了并且在+ (BOOL)resolveInstanceMethod:(SEL)sel中執(zhí)行了class_addMethod這個方法，然后定義了一個void dynamicMethodIMP (id self, SEL _cmd)這個函數(shù)，運行工程，看log
- **2015-12-24 11:45:11.934 RuntimeTest1[2284:478571] add method here**
- **2015-12-24 11:45:11.934 RuntimeTest1[2284:478571] doSomething SEL**
這時候我們發(fā)現(xiàn)，程序并沒有崩潰，而且還輸出了doSomething SEL，這時候就說明我們已經(jīng)通過runtime成功的向我們這個類中添加了一個方法。關于class_addMethod這個方法，是這樣定義的
-
cls 在這個類中添加方法，也就是方法所添加的類
-
name 方法名，這個可以隨便起的
-
imp 實現(xiàn)這個方法的函數(shù)
-
types 定義該數(shù)返回值類型和參數(shù)類型的字符串，這里比如"v@:"，其中v就是void，帶表返回類型就是空，@代表參數(shù)，這里指的是id(self)，這里:指的是方法SEL(_cmd)，比如我再定義一個函數(shù)
- int newMethod (id self, SEL _cmd, NSString *str) {
- return 100;
- }
那么添加這個函數(shù)的方法就應該是ass_addMethod([self class], @selector(newMethod), (IMP)newMethod, "i@:@");
2.如果在+ (BOOL)resolveInstanceMethod:(SEL)sel中沒有找到或者添加方法
消息繼續(xù)往下傳遞到- (id)forwardingTargetForSelector:(SEL)aSelector看看是不是有對象可以執(zhí)行這個方法，我們來重新建個工程，然后新建一個叫SecondViewController的類，里面有一個- (void)secondVCMethod方法，如下
- //
- // SecondViewController.m
- // RuntimeTest2
- //
- // Created by HenryCheng on 15/12/24.
- // Copyright ?（版權符號） 2015年 www.igancao.com All rights reserved.
- //
- #import "SecondViewController.h"
- @interface SecondViewController ()
- @end
- @implementation SecondViewController
- - (void)viewDidLoad {
- [super viewDidLoad];
- // Do any additional setup after loading the view.
- }
- - (void)secondVCMethod {
- NSLog(@"This is secondVC method !");
- }
- - (void)didReceiveMemoryWarning {
- [super didReceiveMemoryWarning];
- // Dispose of any resources that can be recreated.
- }
- /*
- #pragma mark - Navigation
- // In a storyboard-based application, you will often want to do a little preparation before navigation
- - (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
- // Get the new view controller using [segue destinationViewController].
- // Pass the selected object to the new view controller.
- }
- */
- @end
工程結構應該是這樣的
工程目錄圖
現(xiàn)在我想在ViewController中調用- (void)secondVCMethod這個方法，我們知道ViewController和SecondViewController并無繼承關系，按照正常的步驟去做程序肯定會因為在ViewController找不到- (void)secondVCMethod這個方法而直接崩潰的
- //
- // ViewController.m
- // RuntimeTest2
- //
- // Created by HenryCheng on 15/12/24.
- // Copyright ?（版權符號） 2015年 www.igancao.com All rights reserved.
- //
- #import "ViewController.h"
- #import @interface ViewController ()
- @end
- @implementation ViewController
- - (void)viewDidLoad {
- [super viewDidLoad];
- [self performSelector:@selector(secondVCMethod)];
- }
- - (void)didReceiveMemoryWarning {
- [super didReceiveMemoryWarning];
- // Dispose of any resources that can be recreated.
- }
- @end
運行結果
- **2015-12-24 13:54:44.314 RuntimeTest2[3164:835814] -[ViewController secondVCMethod]: unrecognized selector sent to instance 0x7fc3a8535c10**
- **2015-12-24 13:54:44.317 RuntimeTest2[3164:835814] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController secondVCMethod]: unrecognized selector sent to instance 0x7fc3a8535c10'**
- ***** First throw call stack:**
現(xiàn)在我們來處理一下這個消息，如下
- //
- // ViewController.m
- // RuntimeTest2
- //
- // Created by HenryCheng on 15/12/24.
- // Copyright ?（版權符號） 2015年 www.igancao.com All rights reserved.
- //
- #import "ViewController.h"
- #import @interface ViewController ()
- @end
- @implementation ViewController
- - (void)viewDidLoad {
- [super viewDidLoad];
- [self performSelector:@selector(secondVCMethod)];
- }
- - (id)forwardingTargetForSelector:(SEL)aSelector {
- Class class = NSClassFromString(@"SecondViewController");
- UIViewController *vc = class.new;
- if (aSelector == NSSelectorFromString(@"secondVCMethod")) {
- NSLog(@"secondVC do this !");
- return vc;
- }
- return nil;
- }
- + (BOOL)resolveInstanceMethod:(SEL)sel {
- return [super resolveInstanceMethod:sel];
- }
- - (void)didReceiveMemoryWarning {
- [super didReceiveMemoryWarning];
- // Dispose of any resources that can be recreated.
- }
- @end
運行結果
- **2015-12-24 14:00:34.168 RuntimeTest2[3284:870957] secondVC do this !**
- **2015-12-24 14:00:34.169 RuntimeTest2[3284:870957] This is secondVC method !**
我們會發(fā)現(xiàn)- (void)secondVCMethod這個方法執(zhí)行了，程序也并沒有崩潰，原因就是在這一步
- - (id)forwardingTargetForSelector:(SEL)aSelector {
- Class class = NSClassFromString(@"SecondViewController");
- UIViewController *vc = class.new;
- if (aSelector == NSSelectorFromString(@"secondVCMethod")) {
- NSLog(@"secondVC do this !");
- return vc;
- }
- return nil;
- }
在沒有找到- (void)secondVCMethod這個方法的時候，消息繼續(xù)傳遞，直到- (id)forwardingTargetForSelector:(SEL)aSelector，然后我在里面創(chuàng)建了一個SecondViewController的對象，并且判斷如過有這個方法，就返回SecondViewController的對象。這個函數(shù)就是消息的轉發(fā)，在這兒我們成功的把消息傳給了SecondViewController，然后讓它來執(zhí)行，所以就執(zhí)行了那個方法。同時，也相當于完成了一個多繼承！
三、***一點
當然，還有好幾個函數(shù)，在上面那張圖里面已經(jīng)清晰的表達了，有興趣的可以自己試試，看看消息的傳遞順序到底是怎么樣的。上面提到的這些知識runtime的冰山一角，runtime黑魔法的強大遠不止于此，比如方法的調配（Method Swizzling）等，在項目實戰(zhàn)中還是很有用的，后面有時間會再介紹.
參考
