Block的那些事
前言：
本来想写原创来着，因为为了印象深刻都是手动从头整理一遍资料，但是奈何宇神写的太详细，要是写原创他一定会把我拉出去重打八十大板，还让我卖个萌什么的，得不偿失啊。（向晨宇的博客-Blcok）
这篇文章主要是用来学习Block的原理和一些Block的用法，相信有很多小伙伴每天都在用Block，但是对于它在MRC和ARC中的运行模式，包括Block在内存中的位置并不是很清楚，所以咱还是赶紧看我摘取过来的资料吧。
全文的七段文字：
1.Block入门
2.Block原理
3.利用Block实现一对多
4.Delegate、Notification、Block 的对比
5.测试
6.Demo下载
7.参考资料
正题：
一、Block入门
1.概念
Block是iOS4.0+ 和Mac OS X 10.6+ 引进的对C语言的扩展，用来实现匿名函数的特性
闭包是一个能够访问其他函数内部变量的函数.
2.基本用法
每次写block都很绕，语法很麻烦。好吧，再此记录一下。
a.作为方法时
- (void )testGlobalBlock:(NSString*) url
success:(void (^)(BOOL isSuccess))success
failure:(void (^)(BOOL isSuccess))failure;
b.作为成员变量进行声明时
typedef void (^SUCCESSBLOCK)(BOOL isSuccess); typedef void (^FAILEDBLOCK)(BOOL isSuccess); @interface XXEngine() @property(nonatomic,copy) SUCCESSBLOCK successBlock; @property(nonatomic,copy) FAILEDBLOCK failedBlock; @end
c.写在函数内部
void (^_block)() = ^{
_a = 10;
};
3.内存的五个区
这里先普及一个基本的5个概念，内存的5个区。否则在具体讲堆栈的时候怕大家会不理解。
4.使用场景
二、Block原理
1.Block的三种类型
2.类型判断
如何判断在哪个区,我们分ARC和MRC两种情况
int i = 10;
void (^block)() = ^{printf("%d",i);} ;
__weak void (^weakBlock)() = ^{printf("%d",i);}; //被weak修饰之后还是stack
void (^globalStack)() = ^{};
NSLog(@"%@", ^{printf("%d",i);});
NSLog(@"%@", block);
NSLog(@"%@", weakBlock);
NSLog(@"%@", globalStack);
testARC[76639:507728] <__NSStackBlock__: 0x7fff54c9fbc8> testARC[76639:507728] <__NSMallocBlock__: 0x7fdaaae161c0> testARC[76639:507728] <__NSStackBlock__: 0x7fff54c9fbf8> testARC[76639:507728] <__NSGlobalBlock__: 0x10af60230>
2.MRC下
testARC[76639:507728] <__NSStackBlock__: 0x7fff54c9fba0> testARC[76639:507728] <__NSStackBlock__: 0x7fff54c9fc00> testARC[76639:507728] <__NSStackBlock__: 0x7fff54c9fbd0> testARC[76639:507728] <__NSGlobalBlock__: 0x10af60130>
3.判断原则
a.ARC下判断原则
b.MRC下判断原则
3.ARC与MRC区别
ARC引用外部对象会自动copy，MRC则不会。所以MRC要将栈对象变成堆对象只要执行一次copy即可。
3.循环引用
a.造成循环引用的实例
@implementation Person
{
int _a;
void (^_block)();
}
- (void)test
{
void (^_block)() = ^{
_a = 10;
};
}
@end此时_a在用clang编译后可以很明显看到有会self的身影，所以此时我们可以将其理解成self.a（便于记忆),此时Person持有block,block持有self，造成了循环引用. b. 解决方式
- (void)test
{
__weak typeof(self) weakSelf = self;
void (^_block)() = ^{
weakSelf->a = 10;
};
} 3.系统Block
1.经验谈之GCD/UIView的系统动画的Block中，为何可以写self？
因为self并没有对其进行持有，循环引用的原理是两个对象之间相互有持有关系，现在仅仅是GCD持有self，但是self并没有持有GCD，所以是没有问题的。（当GCD对象为其成员变量时才具有持有的关系）
4._block原理
1.我们先看如下代码
void test()
{
__block int a;
^{
a = 10;
};
} 2.用clang -re-write testBlock.c命令后
看关键代码如下:
struct __Block_byref_a_0 {
void *__isa;
__Block_byref_a_0 *__forwarding;
int __flags;
int __size;
int a;
};
struct __test_block_impl_0 {
struct __block_impl impl;
struct __test_block_desc_0* Desc;
__Block_byref_a_0 *a; // by ref
__test_block_impl_0(void *fp, struct __test_block_desc_0 *desc, __Block_byref_a_0 *_a, int flags=0) : a(_a->__forwarding) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __test_block_func_0(struct __test_block_impl_0 *__cself) {
__Block_byref_a_0 *a = __cself->a; // bound by ref
(a->__forwarding->a) = 10;
}
static void __test_block_copy_0(struct __test_block_impl_0*dst, struct __test_block_impl_0*src) {_Block_object_assign((void*)&dst->a, (void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}
static void __test_block_dispose_0(struct __test_block_impl_0*src) {_Block_object_dispose((void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}
static struct __test_block_desc_0 {
size_t reserved;
size_t Block_size;
void (*copy)(struct __test_block_impl_0*, struct __test_block_impl_0*);
void (*dispose)(struct __test_block_impl_0*);
} __test_block_desc_0_DATA = { 0, sizeof(struct __test_block_impl_0), __test_block_copy_0, __test_block_dispose_0};
void test()
{
__attribute__((__blocks__(byref))) __Block_byref_a_0 a = {(void*)0,(__Block_byref_a_0 *)&a, 0, sizeof(__Block_byref_a_0)};
;
((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA, (__Block_byref_a_0 *)&a, 570425344));
}3.结论
我们观察Block_byref_a_0结构体，这个结构体中含有isa指针，所以也是一个对象，它是用来包装局部变量a的。当block被copy到堆中时，Persontest_block_impl_0的拷贝辅助函数Persontest_block_copy_0会将Block_byref_a_0拷贝至堆中，所以即使局部变量所在堆被销毁，block依然能对堆中的局部变量进行操作。其中Block_byref_a_0成员指针forwarding用来指向它在堆中的拷贝.
三、利用Block实现一对多
1.直接block
这里我直接copy一段AFNetworking的代码，这个库写的太好了，大家有空可以参考一下
- (NSURLSessionDataTask *)dataTaskWithHTTPMethod:(NSString *)method
URLString:(NSString *)URLString
parameters:(id)parameters
uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgress
downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgress
success:(void (^)(NSURLSessionDataTask *, id))success
failure:(void (^)(NSURLSessionDataTask *, NSError *))failure
{
NSError *serializationError = nil;
NSMutableURLRequest *request = [self.requestSerializer requestWithMethod:method URLString:[[NSURL URLWithString:URLString relativeToURL:self.baseURL] absoluteString] parameters:parameters error:&serializationError];
if (serializationError) {
if (failure) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wgnu"
dispatch_async(self.completionQueue ?: dispatch_get_main_queue(), ^{
failure(nil, serializationError);
});
#pragma clang diagnostic pop
}
return nil;
}
__block NSURLSessionDataTask *dataTask = nil;
dataTask = [self dataTaskWithRequest:request
uploadProgress:uploadProgress
downloadProgress:downloadProgress
completionHandler:^(NSURLResponse * __unused response, id responseObject, NSError *error) {
if (error) {
if (failure) {
failure(dataTask, error);
}
} else {
if (success) {
success(dataTask, responseObject);
}
}
}];
return dataTask;
}
2.利用对象做block
现在有RequestObject和XXNetEngine两个文件.
1.RequestObject
#import <Foundation/Foundation.h> typedef void (^ReqSuccessBlock)(BOOL isSuccess); typedef void (^FailedBlock)(BOOL isSuccess); @interface RequestObject : NSObject @property(nonatomic,copy) ReqSuccessBlock successBlock; @property(nonatomic,copy) FailedBlock failedBlock; @property(nonatomic,assign) NSInteger CMD; @property(nonatomic,assign) NSInteger seq; @end
2.XXNetEngine
#import "XXNetEngine.h"
#import "RequestObject.h"
@interface XXNetEngine()
@property (nonatomic, strong) NSMutableDictionary *requestDict;
@end
@implementation XXNetEngine
-(instancetype)init
{
if(self = [super init]){
self.requestDict = [NSMutableDictionary dictionary];
}
return self;
}
+(instancetype) sharedInstance
{
static XXNetEngine* instance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
instance = [[XXNetEngine alloc] init];
});
return instance;
}
-(void)requestData:(RequestObject*) reqObject
successBlock:(void (^)(BOOL isSuccess))successBlock
failureBlock:(void (^)(BOOL isSuccess))failureBlock
{
if ( reqObject && successBlock && failureBlock ) {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
//requestSeq是一个随机数
int requestSeq = arc4random() % 100;;
//网络操作
reqObject.successBlock = successBlock;
reqObject.failedBlock = failureBlock;
reqObject.seq = requestSeq;
[self addRequestItem:reqObject seq:requestSeq];
//模拟一个2秒的网络操作
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
[self didReceiveData:reqObject.seq];
});
});
}
}
- (void) didReceiveData:(NSInteger) seq
{
RequestObject *request = [self getRequestItem:seq];
if ( request ) {
[self removeRequestItem:seq];
dispatch_async(dispatch_get_main_queue(), ^{
if ( NULL != request.successBlock ) {
request.successBlock(YES);
}
});
}
}
//用命令字做参数传递
- (RequestObject *) getRequestItem:(NSInteger)seq
{
RequestObject *request = nil;
@synchronized(self) {
NSNumber *numSeq = [NSNumber numberWithInteger:seq];
request = [self.requestDict objectForKey:numSeq];
}
return request;
}
- (void) addRequestItem:(RequestObject *)request seq:(NSInteger)seq
{
@synchronized(self) {
if ( request && (0!=seq) ) {
NSNumber *numSeq = [NSNumber numberWithInteger:seq];
[self.requestDict setObject:request forKey:numSeq];
}
}
}
- (void) removeRequestItem:(NSInteger)seq
{
@synchronized(self) {
NSNumber *numSeq = [NSNumber numberWithInteger:seq];
[self.requestDict removeObjectForKey:numSeq];
}
}
@end3.输出结果
2016-02-02 00:14:59.824 testARC[1232:45931] XXNetEngine successBlock
四、Delegate、Notification、Block 的对比
1.优缺点
2.宇神的个人见解
如果方法过多，一般大于3个的时候，用delegate，因为此时用block，代码将很难维护。比如UITableViewDelegate等等UI组建，都是用的delegate。其他方式用block。
Notification能不用的时候尽量不用，缺点太多明显，增加调试难度。在工程大的情况下，极其难以维护。当然有些情况下也是必不可少的，比如观察者模式.
五、测试
1.测试一
快来看看大家现在对Block的理解吧
Block测试题, 从唐巧大神博客看到的转载链接
2.测试二
现在ViewController中有一个XXEngine对象
1.XXEngine代码如下
typedef void (^SUCCESSBLOCK)(BOOL isSuccess);
typedef void (^FAILEDBLOCK)(BOOL isSuccess);
@interface XXEngine()
@property(nonatomic,copy) SUCCESSBLOCK successBlock;
@property(nonatomic,copy) FAILEDBLOCK failedBlock;
@end
@implementation XXEngine
-(instancetype)init
{
if (self = [super init]) {
NSLog(@"XXEngine init");
}
return self;
}
-(void)dealloc
{
NSLog(@"XXEngine dealloc");
}
- (void )testGlobalBlock:(NSString*) url
success:(void (^)(BOOL isSuccess))success
failure:(void (^)(BOOL isSuccess))failure
{
//设置时间
double delayInSeconds = 2.0;
dispatch_time_t delayInNanoSeconds =
dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_after(delayInNanoSeconds, concurrentQueue, ^(void){
NSLog(@"网络操作完成");
if(success){
NSLog(@"%@",success);
success(YES);
}
});
}
- (void )testMallocBlock:(NSString*) url
success:(void (^)(BOOL isSuccess))success
failure:(void (^)(BOOL isSuccess))failure
{
self.successBlock = success;
self.failedBlock = failure;
//设置时间
double delayInSeconds = 2.0;
dispatch_time_t delayInNanoSeconds =
dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_after(delayInNanoSeconds, concurrentQueue, ^(void){
NSLog(@"网络操作完成");
if(self.successBlock){
NSLog(@"%@",self.successBlock);
self.successBlock(YES);
}
});
}2.我们在ViewController中做如下处理(1)
- (IBAction)testXXEngine:(UIButton *)sender {
XXEngine *xxEngine = [[XXEngine alloc] init];
[xxEngine testGlobalBlock:nil success:^(BOOL isSuccess) {
NSLog(@"2秒已到");
} failure:^(BOOL isSuccess) {
NSLog(@"2秒结束");
}];
}此时的输出结果大家能想到吗？这里公布答案：
2016-02-01 23:10:24.100 testARC[699:16506] XXEngine init 2016-02-01 23:10:24.101 testARC[699:16506] XXEngine dealloc 2016-02-01 23:10:26.101 testARC[699:16615] 网络操作完成 2016-02-01 23:10:26.102 testARC[699:16615] <__NSGlobalBlock__: 0x102bbd110> 2016-02-01 23:10:26.102 testARC[699:16615] 2秒已到
- (IBAction)testXXEngineMalloc:(id)sender {
XXEngine *xxEngine = [[XXEngine alloc] init];
[xxEngine testMallocBlock:nil success:^(BOOL isSuccess) {
NSLog(@"2秒已到");
} failure:^(BOOL isSuccess) {
NSLog(@"2秒结束");
}];
}此时的输出结果大家能想到吗？这里公布答案:2016-02-01 23:11:16.416 testARC[699:16506] XXEngine init 2016-02-01 23:11:18.417 testARC[699:17079] 网络操作完成 2016-02-01 23:11:18.417 testARC[699:17079] <__NSGlobalBlock__: 0x102bbd190> 2016-02-01 23:11:18.417 testARC[699:17079] 2秒已到 2016-02-01 23:11:18.417 testARC[699:17079] XXEngine dealloc
4.结论
在（1）中，success是一个NSGlobalBlock对象，dispatch对success做了持有操作，GCD我们可以认为是一个系统单例对象，此时XXEngine虽然被释放了，但是success被GCD持有了一份，也就是引用计数+1，所以success这个block不会被释放。GCD持有的对象会被拷贝到堆中，现在GCD执行完成，堆中对象要进行释放，所以知道success完成后，GCD释放。Block可以回调的。
在（2）中，GCD对XXEngine进行了持有，也就是引用计数+1，此时ViewController执行完对XXEngine引用计数-1，但是还是无法释放XXEngine，等到GCD结束之后，对XXEngine引用计数-1，此时会进行XXEngine的dealloc
六、Demo下载
testBlock（可以看下呦）
七、参考资料
1.Delegate,Notification,Block
2.AFNetworking 3.0迁移指南
3.Block技巧与底层解析
4.谈Objective-C Block的实现
最后：终于在回家过年前整理完了，虽然是拿别人现成的资料，整理起来也是很费劲的，Block天天在用，你确定真的已经了解他了么~ 希望这篇文章对你有用。
感谢观看，学以致用更感谢.