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IOS开发（67）
dispatch_once可以保证代码被执行一次
+(NSDateFormatter*)getDBDateFormat
static NSDateFormatter* format;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
format = [[LKDateFormatter alloc]init];
format.dateFormat = @&yyyy-MM-dd HH:mm:ss&;
return format;
dispatch_once_t的描述是typedef long dispatch_once_t;
Description A predicate for use with the dispatch_once function.
dispatch_once展开是
_dispatch_once(dispatch_once_t *predicate, dispatch_block_t block)
if (DISPATCH_EXPECT(*predicate, ~0l) != ~0l) {
dispatch_once(predicate, block);
~0l 是 long 的0 取反也就是 一大堆1
我们再展开DISPATCH_EXPECT, 是__builtin_expect((x), (v))
__builtin_expect是GCC(version&=2.9)引进的宏，其作用就是帮助编译器判断条件跳转的预期值，避免跳转造成时间乱费。并没有改变其对真值的判断。
所以呢dispatch_once可以看成
+(NSDateFormatter*)getDBDateFormat
static NSDateFormatter* format;
static long onceToken = 0;
if (onceToken != 0){
format = [[LKDateFormatter alloc]init];
format.dateFormat = @&yyyy-MM-dd HH:mm:ss&;
return format;
我们可以猜测在下面的2...里的代码是修改了 onceToken的值
输出查看一下,
+(NSDateFormatter *) dateFormatter{
static NSDateFormatter* format;
static dispatch_once_t onceToken;
NSLogD(@&%ld&, onceToken);
dispatch_once(&onceToken, ^{
NSLogD(@&%ld&, onceToken);
format = [[NSDateFormatter alloc] init];
format.dateFormat = @&yyyy-MM-dd HH:mm:ss&;
NSLogD(@&%ld&, onceToken);
return format;
发现在1里改变了一次
然后在2里改成了-1
这样我们就不难理解dispatch_once的逻辑了
参考知识库
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相关词典网站：Don’t forget the things you once you owned. Treasure the things you can’t get. D...
on't give up the things that belong to you and keep those lost thi 的翻译是：别忘了东西你一旦您拥有。珍惜你不能得到的东西。D...不要放弃的东西，属于你的让那些丢失的陈氏 中文翻译英文意思，翻译英语
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Don’t forget the things you once you owned. Treasure the things you can’t get. D...
on't give up the things that belong to you and keep those lost thi
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罗马尼亚语
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不要忘记你，一旦你拥有的东西。珍惜的东西，你不能得到。 D. .. on't放弃属于你的东西，让那些失去THI
不要忘记你，一旦你拥有的东西。珍惜的东西，你不能得到。 D. .. on't放弃属于你的东西，让那些失去THI
别忘了东西你一旦您拥有。珍惜你不能得到的东西。D...不要放弃的东西，属于你的让那些丢失的陈氏
不要忘了你一旦你拥有的东西。 珍惜的事情,你不能得到。 D.. O不可以放弃的事情,属於你,把那些失去氏
一旦您拥有了，不要忘记事您。 珍惜您不可能得到的事。 D… on't放弃属于您并且保留那些失去的thi的事
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