IOS Multithreading (GCD nsthread)

1.NSThread

①. Several ways of the thread

* First created, then started

Turn on thread    nsthread *thread =[[nsthread alloc]initwithtarget:self selector: @selector (run) Object:nil];    Start    [thread start];        Direct start    [nsthread detachnewthreadselector: @selector (< #selector #>) totarget:self Withobject:nil];

②. Other uses

    Gets the current thread    nsthread *current =[nsthread CurrentThread];        Get the main thread    + (nsthread*) mainthread;    Inter-thread communication (back to the main thread from the child threads)
    -(void) performselectoronmainthread:<# (SEL) #> withobject:<# (ID) #> waituntildone:<# (BOOL) #>

CGD (Key)

1. Types of queues

* Concurrent queues: Multiple tasks can be performed concurrently

Get global Concurrent queue: Dispatch_get_global_queue (< #long Identifier#>, < #unsigned long flags#>)

* Serial queue: One task is completed before the next task is executed

A. Create yourself: dispatch_queue_create

B. Home row: Dispatch_get_main_queue

2. Method types for performing tasks

* Synchronization (Sync) Execution: Task can only be performed on the current thread

* Asynchronous (Async) Execution: Multiple threads can perform tasks (the system will automatically give you the open, you just write what you want to do)

3. Learn how to use queues in conjunction with

* Asynchronous Concurrent Queue

Gets the global concurrent queue    //The first parameter represents the priority    dispatch_queue_t queue= Dispatch_get_global_queue (dispatch_queue_priority_ DEFAULT, 0);    2. Add task to queue, perform task    dispatch_async (queue, ^{        NSLog (@ "------Download picture 1-------%@", [Nsthread CurrentThread]);    });    Dispatch_async (queue, ^{             NSLog (@ "------Download picture 1-------%@", [Nsthread CurrentThread]);    });    Dispatch_async (queue, ^{        NSLog (@ "------Download picture 1-------%@", [Nsthread CurrentThread]);    });    Summary: 3 threads Open at the same time

* Asynchronous Serial Queue

  1. Create serial queue dispatch_queue_t queue=   dispatch_queue_create ("Com.itheima.queue", NULL);    Dispatch_async (queue, ^{        NSLog (@ "------Download picture 1-------%@", [Nsthread CurrentThread]);    });    Dispatch_async (queue, ^{        NSLog (@ "------Download picture 1-------%@", [Nsthread CurrentThread]);    });    Dispatch_async (queue, ^{        NSLog (@ "------Download picture 1-------%@", [Nsthread CurrentThread]);    });    Summary: Only one thread will be opened;

* Synchronous serial

Create serial queue    dispatch_queue_t queue=dispatch_queue_create ("Wangbinbin.queue", NULL);    2. Add task to queue, perform task    dispatch_sync (queue, ^{        NSLog (@ "------Download picture 1-------%@", [Nsthread CurrentThread]);    });    Dispatch_sync (queue, ^{                NSLog (@ "------Download picture 2-------%@", [Nsthread CurrentThread]);    });    Dispatch_sync (queue, ^{                NSLog (@ "------Download picture 3-------%@", [Nsthread CurrentThread]);    });        Summary: The thread will not be opened only on the current thread (the current thread is the main course, Dispatch_sync is the main path)

* Synchronizing concurrent queues

Gets the global concurrent queue    //The first parameter represents the priority    dispatch_queue_t Queue=dispatch_get_global_queue (dispatch_queue_priority_ DEFAULT, 0);        2. Add task to queue, perform task    dispatch_sync (queue, ^{                NSLog (@ "------Download picture 1-------%@", [Nsthread CurrentThread]);    });    Dispatch_sync (queue, ^{                NSLog (@ "------Download picture 2-------%@", [Nsthread CurrentThread]);    });    Dispatch_sync (queue, ^{        NSLog (@ "------Download picture 2-------%@", [Nsthread CurrentThread]);    });        Summary: New threads are not opened and concurrent queues lose concurrency

GCD Other common methods

1. From the child thread back to the main thread

NSLog (@ "%@", [Nsthread CurrentThread]);    Asynchronous concurrent Queue    Dispatch_async (dispatch_get_global_queue (dispatch_queue_priority_default, 0), ^{        NSLog (@ "------ %@ ", [Nsthread CurrentThread]);                Request Picture        Nsurl *url =[nsurl urlwithstring:@ "Http://img.taopic.com/uploads/allimg/110812/1820-110Q20K01549.jpg"] ;        NSData *data =[nsdata Datawithcontentsofurl:url];        UIImage *image=[uiimage Imagewithdata:data];         Back to the main thread display picture        Dispatch_async (Dispatch_get_main_queue (), ^{            NSLog (@ "+++%@", [Nsthread CurrentThread]);            (Self.imgview.image=image;        });            });

2. Disposable code

One-time code  Singleton used to get    static dispatch_once_t Oncetoken;    Dispatch_once (&oncetoken, ^{                NSLog (@ "Execute only once,");    });

3. Delay operation

Delay operation 2 seconds    //calculation time    dispatch_time_t when=dispatch_time (Dispatch_time_now, (int64_t) (2.0 * nsec_per_sec));        Will execute the queue at the time of this point (queues can choose for themselves)    Dispatch_after (when, Dispatch_get_main_queue (), ^{                NSLog (@ "See Me!!!");    });

4. Queue groups

There is such a demand
First: Perform 2 time-consuming operations asynchronously, respectively
Second: Wait for 2 asynchronous operations to complete, then return to the main thread to perform the operation

Create a group of dispatch_group_t groups =dispatch_group_create ();    Open a task download image 1 __block UIImage *image1 =nil; Dispatch_group_async (Group, Dispatch_get_global_queue (Dispatch_queue_priority_default, 0), ^{image1 =[self requestimagewithurl:@ "Http://d.hiphotos.baidu.com/image/pic/item/faedab64034f78f09c7428247d310a55b2191ced.jpg"    ];        });    Open a task download Image 2 __block UIImage *image2 =nil;  Dispatch_group_async (Group, Dispatch_get_global_queue (Dispatch_queue_priority_default, 0), ^{image2 =[self requestimagewithurl:@ "http://b.hiphotos.baidu.com/image/h%3D360/sign=502eb6fad954564efa65e23f83df9cde/    80cb39dbb6fd5266c96cb6fdaf18972bd5073697.jpg "];    }); All the tasks in the group are executed, and the other operations Dispatch_group_notify (group, Dispatch_get_main_queue (), ^{self.        Uiimageview1.image=image1; Self.                Uiimageview2.image=image2;        Merge 2 Image Uigraphicsbeginimagecontextwithoptions (Cgsizemake (+), NO, 0); [Image1 Drawinrect:cgrectmakE (0, 0, 100, 100)];        [Image2 drawinrect:cgrectmake (100, 0, 100, 100)];        Self.biguiimageview.image=uigraphicsgetimagefromcurrentimagecontext ();    Close context Uigraphicsendimagecontext ();     });

IOS Multithreading (GCD nsthread)