GCD系列:代码块(dispatch_block)

dispatch_block

最显著的功能就是可以取消执行的代码块

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基本函数

dispatch_block_t dispatch_block_create(dispatch_block_flags_t flags, dispatch_block_t block);void dispatch_block_perform(dispatch_block_flags_t flags,DISPATCH_NOESCAPE dispatch_block_t block);void dispatch_block_cancel(dispatch_block_t block); long dispatch_block_wait(dispatch_block_t block, dispatch_time_t timeout);  void dispatch_block_notify(dispatch_block_t block, dispatch_queue_t queue,        dispatch_block_t notification_block);long dispatch_block_testcancel(dispatch_block_t block);

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函数理解

1.使用dispatch_create创建一个基于GCD的代码块,参数flag，指定代码块的执行环境，block是代码执行体。

关于flag的定义

DISPATCH_ENUM_AVAILABLE_STARTING(__MAC_10_10, __IPHONE_8_0)DISPATCH_ENUM(dispatch_block_flags, unsigned long    DISPATCH_BLOCK_BARRIER = 0x01,    DISPATCH_BLOCK_DETACHED = 0x02,    DISPATCH_BLOCK_ASSIGN_CURRENT = 0x04,    DISPATCH_BLOCK_NO_QOS_CLASS = 0x08,    DISPATCH_BLOCK_INHERIT_QOS_CLASS = 0x10,    DISPATCH_BLOCK_ENFORCE_QOS_CLASS = 0x20,);

• DISPATCH_BLOCK_BARRIER 保证代码块用于原子性，代码块的代码未执行结束前，下一次调用将进入一个FIFO的等待队列，等待本次代码块执行结束，使用较为安全,若不考虑线程安全可使用DISPATCH_BLOCK_DETACHED，其它flag自行查阅文档。

2.dispatch_block_perform 没有实际使用过，可能是在当前线程中将闭包的执行体放在指定的flag环境中去执行（待认证）.

3.dispatch_block_cancel 取消执行某个block，只有当block还未执行前执行cancel有效，block正在执行无法取消.
4.dispatch_block_wait 等待block执行，直到timeout后继续往下执行代码,如果timeout=DISPATCH_TIME_FOREVER且block永远不被执行，代码将永远等待。
5.dispatch_block_notify(block1,queue,notification_block);注册一个block1的监听，当block1已经完成的时候，会在queue里立即执行notification_block.

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简要Demo：

• Demo1.简单用法

dispatch_block_t _dblock;void testDispatchBlock() {    while (_index < 10) {        createBlockIfNeeded();        executingBlockOrCancel();    }}NSInteger _index;void createBlockIfNeeded() {    if (!_dblock) {        _dblock = dispatch_block_create(DISPATCH_BLOCK_BARRIER, ^{            NSLog(@"index -> %lu",_index);        });    }}void executingBlockOrCancel() {    if ( ++ _index % 2 ) {        _dblock();    }else {        dispatch_block_cancel(_dblock);        _dblock = nil;    }}

输出结果:
2017-02-26 23:15:32.362374 dispatch_block_oc[60994:11193423] index -> 1
2017-02-26 23:15:32.362444 dispatch_block_oc[60994:11193423] index -> 3
2017-02-26 23:15:32.362471 dispatch_block_oc[60994:11193423] index -> 5
2017-02-26 23:15:32.362491 dispatch_block_oc[60994:11193423] index -> 7
2017-02-26 23:15:32.362511 dispatch_block_oc[60994:11193423] index -> 9


• Demo2.当block在延时函数中使用

void delayExecutingTask() {    createBlockIfNeeded();    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 5 * NSEC_PER_SEC), dispatch_get_main_queue(), _dblock);    //sometimes ,you need cancel the block,use dispatch_block_cancel     dispatch_block_cancel(_dblock);}

• Demo3.dispatch_block_waite

dispatch_block_t _dblock;void testDispatchBlock() {    createBlockIfNeeded();    executingBlockOrCancel();}NSInteger _index;void createBlockIfNeeded() {    if (!_dblock) {        _dblock = dispatch_block_create(DISPATCH_BLOCK_BARRIER, ^{            [NSThread sleepForTimeInterval:5];            NSLog(@"index -> %lu",_index);        });        NSLog(@"wait BEGIN ");        dispatch_block_wait(_dblock, dispatch_time(DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC));        NSLog(@"wait END ");    }}void executingBlockOrCancel() {    if ( ++ _index % 2 ) {        _dblock();    }else {        dispatch_block_cancel(_dblock);        _dblock = nil;    }}

输出结果:
2017-02-26 23:27:27.484894 dispatch_block_oc[61126:11221388] wait BEGIN
2017-02-26 23:27:29.485858 dispatch_block_oc[61126:11221388] wait END
2017-02-26 23:27:34.487206 dispatch_block_oc[61126:11221388] index -> 1

demo中，在函数createBlockIfNeeded函数中，输出waite BEGIN之后，dispatch_block_waite函数将等待两秒，再输出waite END,之后才执行函数executingBlockOrCancel();

• Demo4.dispatch_block_notify

dispatch_block_t _dblock;void testDispatchBlock() {    NSLog(@"BEGIN");    createBlockIfNeeded();    _dblock();}NSInteger _index;void createBlockIfNeeded() {    if (!_dblock) {        _dblock = dispatch_block_create(DISPATCH_BLOCK_BARRIER, ^{            [NSThread sleepForTimeInterval:5];            NSLog(@"index -> %lu",_index);        });        dispatch_block_notify(_dblock, dispatch_get_main_queue(), ^{            NSLog(@"BLOCK EXECUT COMPLETED");        });    }}void executingBlockOrCancel() {    if ( ++ _index % 2 ) {        _dblock();    }else {        dispatch_block_cancel(_dblock);        _dblock = nil;    }}

输出结果:
2017-02-26 23:36:30.105075 dispatch_block_oc[61245:11242889] BEGIN
2017-02-26 23:36:35.106363 dispatch_block_oc[61245:11242889] index -> 0
2017-02-26 23:36:35.122696 dispatch_block_oc[61245:11242889] BLOCK EXECUT COMPLETED


目前，使用了一些Dispatch_block的基本用法，更复杂的在多线程中的用法会陆续记载。