Multithreaded development----GCD

Multi-Thread-GCD

• Grand centeral Dispatch (Grand Central Scheduler)
• There are 2 core concepts in GCD
  • Task : What action to take
  • queues : Used to store tasks

• Follow FIFO (first in, out) principle

• Perform tasks

  • Synchronization method: Dispatch_sync
  • Async method: dispatch_async
  • The difference between synchronous and asynchronous
    • Synchronization : Tasks can only be performed in the current thread, without the ability to open new threads
    • Async : Can perform tasks in a new thread, with the ability to open new threads

• Queue

  • Concurrent queues
    • Multiple tasks can be executed concurrently (concurrently) (automatic opening of multiple threads concurrently executing tasks)
    • concurrency function is only valid under asynchronous (Dispatch_async) functions
  • Serial queue
    • Let the task execute one after the other (once a task has finished executing, then the next task is performed)

• Watch out.

  • Primary impact of synchronous and asynchronous: Can I open a new thread
    • Synchronous: Only performs tasks in the current thread, does not have the ability to open new threads
    • Async: Can perform tasks in a new thread, with the ability to open new threads
  • Concurrency and serial primary impact: How tasks are executed
    • Concurrency: Allows multiple tasks to execute concurrently (concurrently)
    • Serial: Once a task is completed, the next task is performed

-GCD all APIs are in Libdispatch.dylib, Xcode will automatically import this library
-Main header file: #import

• Types of queues

1.to create a global concurrent queue(most commonly used),Allows the task to execute concurrentlydispatch_queue_tQueue=Dispatch_get_global_queue(Dispatch_queue_priority_default,0);To add a task to a global queue asynchronous executionDispatch_async(Queue,^{NSLog(@ "-----(0_0)------%@",[NsthreadCurrentThread]);});2.Create a serial queue,let the task execute one after anotherconst char *label: queue name, C-language string//dispatch_queue_attr_t attr: Queue properties dispatch_queue_t queue = dispatch_queue_create ( "Down.queue"  null3.   Create a home column   (  tasks added to the main queue    will automatically be placed in the main thread Line   dispatch_queue_t queue = dispatch_get_main_queue ();     

GCD inter-thread communication

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ // 执行耗时的异步操作... dispatch_async(dispatch_get_main_queue(), ^{ // 回到主线程，执行UI刷新操作 });});

GCD Other uses

• Delay execution
  • Deferred execution is not used sleepForTimeInterval because the current thread is stuck

Non-GCD

// 一旦定制好延时任务后,不会卡主当前线程    [self performSelector:@selector(download) withObject:@"http://abc.jpg" afterDelay:3];

GCD

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{ // 2秒后执行这里的代码...});

• Disposable code
  • Use the Dispatch_once function to ensure that a piece of code 程序运行过程中 is executed only 1 times

static dispatch_once_t onceToken;dispatch_once(&onceToken, ^{ // 只执行1次的代码(这里面默认是线程安全的)});

• Fast Iteration

dispatch_apply(10, dispatch_get_global_queue(0, 0), ^(size_t index){ // 执行10次代码，index顺序不确定});

• Barrier
  • It does not execute until after the execution of the previous task, and the task after it is completed before it executes
  • 不能是全局的并发队列
  • 所有的任务都必须在一个队列中

dispatch_barrier_async(dispatch_queue_t queue, dispatch_block_t block);

• Queue Group

dispatch_group_tGroup=Dispatch_group_create();Dispatch_group_async(Group,Dispatch_get_global_queue(dispatch_queue_priority_default0^{//perform 1 time-consuming asynchronous operations  (groupdispatch_get_global _queue (dispatch_queue_priority_default0^{//perform 1 time-consuming asynchronous operations  (groupdispatch_get_main_ Queue (), ^{//etc. before the asynchronous operation is complete, return to the main thread ...                

Multithreaded Development----GCD