dispatch_block
最顯著的功能就是可以取消執行的代碼塊
基本函數
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);
函數理解
1.使用dispatch_create創建一個基於GCD的代碼塊,參數flag，指定代碼塊的執行環境，block是代碼執行體。
關於flag的定義
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 保證代碼塊用於原子性，代碼塊的代碼未執行結束前，下一次調用將進入一個FIFO的等待隊列，等待本次代碼塊執行結束，使用較為安全,若不考慮線程安全可使用DISPATCH_BLOCK_DETACHED，其它flag自行查閱文檔。
2.dispatch_block_perform 沒有實際使用過，可能是在當前線程中將閉包的執行體放在指定的flag環境中去執行（待認證）.
3.dispatch_block_cancel 取消執行某個block，只有當block還未執行前執行cancel有效，block正在執行無法取消.
4.dispatch_block_wait 等待block執行，直到timeout后繼續往下執行代碼,如果timeout=DISPATCH_TIME_FOREVER且block永遠不被執行，代碼將永遠等待。
5.dispatch_block_notify(block1,queue,notification_block);注冊一個block1的監聽，當block1已經完成的時候，會在queue里立即執行notification_block.
簡要Demo：
- Demo1.簡單用法
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;
}
}
輸出結果:
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.當block在延時函數中使用
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;
}
}
輸出結果:
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，在函數createBlockIfNeeded函數中，輸出waite BEGIN之后，dispatch_block_waite函數將等待兩秒，再輸出waite END,之后才執行函數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;
}
}
輸出結果:
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
博主已經開通了博客地址: kobeluo，哪里有更豐富的資源，歡迎與我交流。