Objective-C中block实现和技巧学习
LynetteFure
8年前
<p>什么是block?</p> <p>首先,看一个极简的block:</p> <pre> <code class="language-objectivec">int main(int argc, const char * argv[]) { @autoreleasepool { ^{ }; } return 0; } </code></pre> <p>如何声明一个block在 Objective-C ?</p> <ul> <li> <p>As a local variable:</p> <pre> <code class="language-objectivec">returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...}; </code></pre> </li> <li> <p>As a property:</p> <pre> <code class="language-objectivec">@property (nonatomic, copy, nullability) returnType (^blockName)(parameterTypes); </code></pre> </li> <li> <p>As a method parameter:</p> <pre> <code class="language-objectivec">- (void)someMethodThatTakesABlock:(returnType (^nullability)(parameterTypes))blockName; </code></pre> </li> <li> <p>As an argument to a method call:</p> <pre> <code class="language-objectivec">[someObject someMethodThatTakesABlock:^returnType (parameters) {...}]; </code></pre> </li> <li> <p>As a typedef:</p> <pre> <code class="language-objectivec"> typedef returnType (^TypeName)(parameterTypes); TypeName blockName = ^returnType(parameters) {...}; </code></pre> </li> </ul> <h3><strong>block编译转换结构</strong></h3> <pre> <code class="language-objectivec">int myMain() { ^{ } (); ^{ } (); return 0; } </code></pre> <p>对其执行 clang -rewrite-objc 编译转换成C++实现,得到以下代码:</p> <p><img src="https://simg.open-open.com/show/2848af74d920ddbb5f6900a07cac85fd.jpg"></p> <h3><strong>block的实际结构</strong></h3> <p>关于block的数据结构和runtime是开源的,可以在llvm项目看到,或者下载苹果的 <a href="/misc/goto?guid=4959715250410987694" rel="nofollow,noindex">libclosure</a> 库的源码来看。苹果也提供了 <a href="/misc/goto?guid=4959715250499654849" rel="nofollow,noindex">在线的代码查看方式</a> ,其中包含了很多示例和文档说明。</p> <p>接下来观察下Block_private.h文件中对block的相关结构体的真实定义:</p> <p><img src="https://simg.open-open.com/show/3aefe64abfd56116d107cb438c968cc5.jpg"></p> <ul> <li>invoke,同上文的FuncPtr,block执行时调用的函数指针,block定义时内部的执行代码都在这个函数中</li> <li>Block_descriptor,block的详细描述</li> </ul> <p>总体来说,block就是一个里面存储了指向 函数体中包含定义block时的代码块 的函数指针,以及 block外部上下文 变量等信息的结构体。</p> <h2><strong>block的类型</strong></h2> <p>在block runtime中,定义了6种类:</p> <p><img src="https://simg.open-open.com/show/bcfeb19de97f07afed597a6fceef061d.png"></p> <p><img src="https://simg.open-open.com/show/fef09a71cb1b7771050f90db93a1c5fe.png"></p> <ul> <li>_NSConcreteStackBlock 栈上创建的block</li> <li>_NSConcreteMallocBlock 堆上创建的block</li> <li>_NSConcreteGlobalBlock 作为全局变量的block</li> <li>_NSConcreteWeakBlockVariable</li> <li>_NSConcreteAutoBlock</li> <li>_NSConcreteFinalizingBlock</li> </ul> <p>其中我们能接触到的主要是前3种,后三种用于GC,咱们就先不看了。</p> <p>block的常见类型有3种:</p> <ul> <li>_NSConcreteGlobalBlock(全局)</li> <li>_NSConcreteStackBlock(栈)</li> <li>_NSConcreteMallocBlock(堆)</li> </ul> <p>APUE(Unix环境高级编程)的进程虚拟内存段分布图:</p> <p style="text-align:center"><img src="https://simg.open-open.com/show/6c3218a21d4de16dc7e5dd5f9403f931.png"></p> <p>其中前2种在Block.h种声明,后1种在Block_private.h中声明。</p> <h3><strong>NSConcreteGlobalBlock和NSConcreteStackBlock</strong></h3> <p>首先,根据前面两种类型,编写以下代码:</p> <pre> <code class="language-objectivec">void (^globalBlock)() = ^{ }; int block_type_Main() { void (^stackBlock1)() = ^{ }; stackBlock1(); globalBlock(); return 0; } </code></pre> <p>对其进行编译转换后得到以下代码:</p> <p><img src="https://simg.open-open.com/show/847c62d7a299b27b9044536b2bd96874.jpg"></p> <p>可以看出globalBlock的isa指向了_NSConcreteGlobalBlock,即在全局区域创建,编译时就已经确定了,位于上图中的代码段;stackBlock的isa指向了_NSConcreteStackBlock,即在栈区创建。</p> <h3><strong>NSConcreteMallocBlock</strong></h3> <p>堆中的block无法直接创建,其需要由_NSConcreteStackBlock类型的block拷贝而来(也就是说block需要执行copy之后才能存放到堆中)。由于block的拷贝最终都会调用_Block_copy_internal函数,所以观察这个函数就可以知道堆中block是如何被创建的了:</p> <p><img src="https://simg.open-open.com/show/520a31339e6ba5fe185e7c2f7886f2f5.jpg"></p> <h3><strong>三种类型block测试(MRC)</strong></h3> <pre> <code class="language-objectivec">#import "block_in_MRC.h" typedef long (^BlkSum)(int, int); @implementation block_in_MRC + (void)main { BlkSum blk1 = ^ long (int a, int b) { return a + b; }; NSLog(@"blk1 = %@", blk1);// blk1 = <__NSGlobalBlock__: 0x47d0> int base = 100; BlkSum blk2 = ^ long (int a, int b) { return base + a + b; }; NSLog(@"blk2 = %@", blk2); // blk2 = <__NSStackBlock__: 0xbfffddf8> BlkSum blk3 = [[blk2 copy] autorelease]; NSLog(@"blk3 = %@", blk3); // blk3 = <__NSMallocBlock__: 0x902fda0> } @end </code></pre> <h2><strong>捕捉变量对block结构的影响</strong></h2> <p>编译转换捕捉不同变量类型的block,以对比它们的区别。</p> <h3><strong>局部变量</strong></h3> <p>代码</p> <pre> <code class="language-objectivec">// 局部变量 int capture_var_effect_block_Main() { int a; ^{a;}; // 报错 var is not assignable(missing __block type specifier) // ^{a = 10;}; return 0; } </code></pre> <p>对其进行编译转换后得到以下代码(注释不会被编译):</p> <p><img src="https://simg.open-open.com/show/c1662c7f737ef7a54ae22e281aab6450.jpg"></p> <p>我们通过指针传递</p> <pre> <code class="language-objectivec">int test() { int a = 0; // 利用指针p存储a的地址 int *p = &a; ^{ // 通过a的地址设置a的值 *p = 10; }(); return 0; } </code></pre> <p>变量a的生命周期是和方法test的栈相关联的,当test运行结束,栈随之销毁,那么变量a就会被销毁,p也就成为了野指针。如果block是作为参数或者返回值,这些类型都是跨栈的,也就是说再次调用会造成野指针错误。</p> <h3><strong>全局变量</strong></h3> <p>代码</p> <pre> <code class="language-objectivec">// 全局静态 static int a; // 全局 int b; int capture_global_var_effect_block() { ^{ a = 10; b = 10; }(); return 0; } </code></pre> <p>编译转换后</p> <p><img src="https://simg.open-open.com/show/873a448723a27533fdffa64b35fd4f22.jpg"></p> <p>直接使用了a,b变量;</p> <h3><strong>局部静态变量</strong></h3> <p>代码</p> <pre> <code class="language-objectivec">int capture_local_var_effect_block() { static int a; // 静态局部变量是存储在静态数据存储区域的,也就是和程序拥有一样的生命周期,也就是说在程序运行时,都能够保证block访问到一个有效的变量。但是其作用范围还是局限于定义它的函数中,所以只能在block通过静态局部变量的地址来进行访问。 ^{ a = 10; }(); return 0; } </code></pre> <p>编译转换后</p> <p><img src="https://simg.open-open.com/show/864e4089f6e70feb6abd8e0700f61083.jpg"></p> <h3><strong>__block修饰的变量</strong></h3> <p>代码</p> <pre> <code class="language-objectivec">int __block_modify_var() { __block int a; ^{ a = 10; }(); return 0; } </code></pre> <p>编译转换后</p> <p><img src="https://simg.open-open.com/show/8b96b8cf6856cefa9ce495b1324dfd8b.jpg"></p> <p>runtime.c _Block_byref_assign_copy 方法</p> <p><img src="https://simg.open-open.com/show/3e582ccdc459fec6f6db2b8118a6b936.jpg"></p> <h3><strong>self隐式循环引用</strong></h3> <p>代码</p> <pre> <code class="language-objectivec">@implementation self_hidden_retain_cycle { int _a; void (^_block)(); } - (void)test { void (^_block)() = ^{ _a = 10; }; } @end </code></pre> <p>编译转换后</p> <p><img src="https://simg.open-open.com/show/52a0181f9093e2c837ea486910465e34.jpg"></p> <h3><strong>ObjC对象(MRC)</strong></h3> <p>代码</p> <pre> <code class="language-objectivec">@interface MyClass : NSObject { NSObject* _instanceObj; } @end @implementation MyClass NSObject* __globalObj = nil; - (id) init { if (self = [super init]) { _instanceObj = [[NSObject alloc] init]; } return self; } - (void) test { static NSObject* __staticObj = nil; __globalObj = [[NSObject alloc] init]; __staticObj = [[NSObject alloc] init]; NSObject* localObj = [[NSObject alloc] init]; __block NSObject* blockObj = [[NSObject alloc] init]; typedef void (^MyBlock)(void) ; MyBlock aBlock = ^{ NSLog(@"%@", __globalObj); NSLog(@"%@", __staticObj); NSLog(@"%@", _instanceObj); NSLog(@"%@", localObj); NSLog(@"%@", blockObj); }; aBlock = [[aBlock copy] autorelease]; aBlock(); NSLog(@"%d", [__globalObj retainCount]); NSLog(@"%d", [__staticObj retainCount]); NSLog(@"%d", [_instanceObj retainCount]); NSLog(@"%d", [localObj retainCount]); NSLog(@"%d", [blockObj retainCount]); } @end </code></pre> <p>执行结果为1 1 1 2 1。</p> <p>globalObj和staticObj在内存中的位置是确定的,所以Block copy时不会retain对象。</p> <p>_instanceObj在Block copy时也没有直接retain _instanceObj对象本身,但会retain self。所以在Block中可以直接读写_instanceObj变量。</p> <p>localObj在Block copy时,系统自动retain对象,增加其引用计数。</p> <p>blockObj在Block copy时也不会retain。</p> <p>非ObjC对象,如GCD队列dispatch_queue_t。Block copy时并不会自动增加他的引用计数。</p> <h3><strong>Block中使用的ObjC对象的行为</strong></h3> <pre> <code class="language-objectivec">@property (nonatomic, copy) void(^myBlock)(void); block_in_MRC* obj = [[[block_in_MRC alloc] init] autorelease]; self.myBlock = ^ { // obj doSomething }; </code></pre> <p>对象obj在Block被copy到堆上的时候自动retain了一次。因为Block不知道obj什么时候被释放,为了不在Block使用obj前被释放,Block retain了obj一次,在Block被释放的时候,obj被release一次。</p> <h2><strong>不同类型block的复制</strong></h2> <p>block的复制代码在_Block_copy_internal函数中。</p> <p><img src="https://simg.open-open.com/show/60c75852e65ab069b7a74f966d023d03.jpg"></p> <h3><strong>Block的copy、retain、release操作(MRC)</strong></h3> <pre> <code class="language-objectivec">+ (void)test { int base = 100; BlkSum blk2 = ^ long (int a, int b) { return base + a + b; }; NSLog(@"blk2 = %@", blk2); // blk2 = <__NSStackBlock__: 0xbfffddf8> BlkSum blk3 = [[[[[blk2 copy] copy] copy] copy] copy]; NSLog(@"blk3 = %@", blk3); // blk3 = <__NSMallocBlock__: 0x902fda0> NSLog(@"blk3 retainCount = %@", @([blk3 retainCount]));// blk3 retainCount = 1 BlkSum blk4 = [blk2 copy]; [blk4 retain]; NSLog(@"blk4 retainCount = %@", @([blk4 retainCount]));// blk4 retainCount = 1 [blk4 release]; NSLog(@"blk4 retainCount = %@", @([blk4 retainCount]));// blk4 retainCount = 1 } </code></pre> <p>Block_release in runtime.c</p> <p><img src="https://simg.open-open.com/show/e2f06c54959d679f2d090733832b371f.jpg"></p> <ul> <li>对Block不管是retain、copy、release都不会改变引用计数retainCount,retainCount始终是1;</li> <li>NSGlobalBlock:retain、copy、release操作都无效;</li> <li>NSStackBlock:retain、release操作无效,必须注意的是,NSStackBlock在函数返回后,Block内存将被回收。即使retain也没用。容易犯的错误是[[mutableAarry addObject:stackBlock],在函数出栈后,从mutableAarry中取到的stackBlock已经被回收,变成了野指针。正确的做法是先将stackBlock copy到堆上,然后加入数组:[mutableAarry addObject:[[stackBlock copy] autorelease]]。支持copy,copy之后生成新的NSMallocBlock类型对象。</li> <li>NSMallocBlock支持retain、release,虽然retainCount始终是1,但内存管理器中仍然会增加、减少计数。copy之后不会生成新的对象,只是增加了一次引用,类似retain;</li> </ul> <h2><strong>ARC中的block</strong></h2> <p><img src="https://simg.open-open.com/show/4efb234089c1928ab8ca98949de16e92.png"></p> <p><a href="/misc/goto?guid=4959674157494650120" rel="nofollow,noindex">苹果文档</a> 提及,在ARC模式下,在栈间传递block时,不需要手动copy栈中的block,即可让block正常工作。主要原因是ARC对栈中的block自动执行了copy,将_NSConcreteStackBlock类型的block转换成了_NSConcreteMallocBlock的block。</p> <h3><strong>block 实验</strong></h3> <pre> <code class="language-objectivec">+ (void)main { int i = 10; void (^block)() = ^{i;}; __weak void (^weakBlock)() = ^{i;}; void (^stackBlock)() = ^{}; // ARC情况下 // 创建时,都会在栈中 // <__NSStackBlock__: 0x7fff5fbff730> NSLog(@"%@", ^{i;}); // 因为stackBlock为strong类型,且捕获了外部变量,所以赋值时,自动进行了copy // <__NSMallocBlock__: 0x100206920> NSLog(@"%@", block); // 如果是weak类型的block,依然不会自动进行copy // <__NSStackBlock__: 0x7fff5fbff728> NSLog(@"%@", weakBlock); // 如果block是strong类型,并且没有捕获外部变量,那么就会转换成__NSGlobalBlock__ // <__NSGlobalBlock__: 0x100001110> NSLog(@"%@", stackBlock); // 在非ARC情况下,产生以下输出 // <__NSStackBlock__: 0x7fff5fbff6d0> // <__NSStackBlock__: 0x7fff5fbff730> // <__NSStackBlock__: 0x7fff5fbff700> // <__NSGlobalBlock__: 0x1000010d0> } </code></pre> <p>可以看出,ARC对类型为strong且捕获了外部变量的block进行了copy。并且当block类型为strong,但是创建时没有捕获外部变量,block最终会变成 <strong>NSGlobalBlock</strong> 类型(这里可能因为block中的代码没有捕获外部变量,所以不需要在栈中开辟变量,也就是说,在编译时,这个block的所有内容已经在代码段中生成了,所以就把block的类型转换为全局类型)</p> <h3><strong>block作为参数传递</strong></h3> <p>在栈中的block需要注意的情况:</p> <pre> <code class="language-objectivec">NSMutableArray *arrayM; void myBlock() { int a = 5; Block block = ^ { NSLog(@"%d", a); }; [arrayM addObject:block]; NSLog(@"%@", block); } + (void)test { arrayM = @[].mutableCopy; myBlock(); Block block = [arrayM firstObject]; // 非ARC这里崩溃 block(); } </code></pre> <p>可以看到,ARC情况下因为自动执行了copy,所以返回类型为 <strong>NSMallocBlock</strong> ,在函数结束后依然可以访问;而非ARC情况下,需要我们手动调用[block copy]来将block拷贝到堆中,否则因为栈中的block生命周期和函数中的栈生命周期关联,当函数退出后,相应的堆被销毁,block也就不存在了。</p> <p>如果把block的以下代码删除:</p> <pre> <code class="language-objectivec">NSLog(@"%d", a); </code></pre> <p>那么block就会变成全局类型,在test中访问也不会出崩溃。</p> <h3><strong>block作为返回值</strong></h3> <p>在非ARC情况下,如果返回值是block,则一般这样操作:</p> <pre> <code class="language-objectivec">return [[block copy] autorelease]; </code></pre> <p>对于外部要使用的block,更趋向于把它拷贝到堆中,使其脱离栈生命周期的约束。</p> <h3><strong>block属性</strong></h3> <p>这里还有一点关于block类型的ARC属性。上文也说明了,ARC会自动帮strong类型且捕获外部变量的block进行copy,所以在定义block类型的属性时也可以使用strong,不一定使用copy。也就是以下代码:</p> <pre> <code class="language-objectivec">/** 假如有栈block赋给以下两个属性 **/ // 这里因为ARC,当栈block中会捕获外部变量时,这个block会被copy进堆中 // 如果没有捕获外部变量,这个block会变为全局类型 // 不管怎么样,它都脱离了栈生命周期的约束 @property (strong, nonatomic) Block *strongBlock; // 这里都会被copy进堆中 @property (copy, nonatomic) Block *copyBlock; </code></pre> <h2><strong>ARC与非ARC(MRC)下的Weak-Strong Dance</strong></h2> <h3>ARC</h3> <p>在使用block过程中,经常会遇到 retain cycle 的问题,例如:</p> <pre> <code class="language-objectivec">- (void)dealloc { [[NSNotificationCenter defaultCenter] removeObserver:_observer]; } - (void)loadView { [super loadView]; _observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"testKey" object:nil queue:nil usingBlock:^(NSNotification *note) { [self dismissModalViewControllerAnimated:YES]; }]; } </code></pre> <p>在block中用到了self,self会被block retain,而_observer会copy一份该block,就是说_observer间接持有self,同时当前的self也会retain _observer,最终导致self持有_observer,_observer持有self,形成 retain cycle 。</p> <p>对于在block中的 retain cycle ,在2011 WWDC Session #322 (Objective-C Advancements in Depth)有一个解决方案 weak-strong dance ,很漂亮的名字。其实现如下:</p> <pre> <code class="language-objectivec">- (void)dealloc { [[NSNotificationCenter defaultCenter] removeObserver:_observer]; } - (void)loadView { [super loadView]; __weak TestViewController *wself = self; _observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"testKey" object:nil queue:nil usingBlock:^(NSNotification *note) { __strong TestViewController *sself = wself; [sself dismissModalViewControllerAnimated:YES]; }]; } </code></pre> <p>在block中使用self之前先用一个 __weak 变量引用self,导致block不会retain self,打破retain cycle,然后在block中使用wself之前先用 __strong 类型变量引用wself,以确保使用过程中不会dealloc。简而言之就是推迟对self的retain,在使用时才进行retain。这有点像lazy loading的意思。</p> <p>注:iOS5以下没有 __weak ,则需使用 __unsafe_unretained 。</p> <h3><strong>非ARC(MRC)</strong></h3> <p>在非ARC环境中,显然之前的使用的 __weak 或 __unsafe_unretained 将会是无效的,那么我们需使用另外一种方法来代替,这里就需要用到 __block 。</p> <p>__block 在ARC和非ARC中有点细微的差别( <a href="/misc/goto?guid=4959518189778229840" rel="nofollow,noindex">Automatic Reference Counting : Blocks</a> ):</p> <ul> <li> <p>在ARC中, __block 会自动进行retain</p> <pre> <code class="language-objectivec">// ARC 中 `__block`会自动进行retain 实验 + (void)test__Block { // You can use CFGetRetainCount with Objective-C objects, even under ARC: NSObject *objc = [[NSObject alloc] init]; NSLog(@"test__Block-- objc Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)objc)); __block NSObject *objcNew = objc; NSLog(@"test__Block-- objc Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)objc)); } // 输出 // test__Block-- objc Retain count is 1 // test__Block-- objc Retain count is 2 </code></pre> </li> <li> <p>在非ARC中, __block 不会自动进行retain</p> <pre> <code class="language-objectivec">// 在MRC中 __block不会自动进行retain + (void)test__Block { // You can use CFGetRetainCount with Objective-C objects, even under ARC: NSObject *objc = [[NSObject alloc] init]; NSLog(@"test__Block-- objc Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)objc)); __block NSObject *objcNew = objc; NSLog(@"test__Block-- objc Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)objc)); } // 输出 // test__Block-- objc Retain count is 1 // test__Block-- objc Retain count is 1 </code></pre> </li> </ul> <p>因此首先要注意的一点就是用 __block 打破 retain cycle 的方法仅在非ARC下有效,下面是非ARC的 weak-strong dance :</p> <pre> <code class="language-objectivec">- (void)dealloc { [[NSNotificationCenter defaultCenter] removeObserver:_observer]; [_observer release]; [super dealloc]; } - (void)loadView { [super loadView]; __block TestViewController *bself = self; _observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"testKey" object:nil queue:nil ngBlock:^(NSNotification *note) { [bself retain]; [bself dismissModalViewControllerAnimated:YES]; [bself release]; }]; } </code></pre> <p>将self赋值为 __block 类型变量,在非ARC中 __block 类型变量不会进行retain,从而打破retain cycle,然后在使用bself前进行retain,以确保在使用过程中不会dealloc 。</p> <h2><strong>总结</strong></h2> <p>打破循环引用:</p> <ul> <li>ARC下: __week</li> <li>非ARC(MRC)下:__block</li> </ul> <p>__block的作用:</p> <p>非ARC(MRC)下</p> <ol> <li>说明变量可改</li> <li>说明指针指向的对象不做隐式retain操作。</li> </ol> <p>ARC下只有1。</p> <p> </p> <p>来自:http://www.lijianfei.cn/2016/07/21/objective-block-learning/</p> <p> </p>