Ios-multi-thread Nsthread Detailed

Last Update:2016-03-02 Source: Internet

Author: User

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Objectivea thread is used to perform a task, and the thread finishes executing task A to perform task B. Multithreading is produced in order to perform two tasks at the same time. I opened a video software, I opened a thread a to let it perform the download task, I opened a thread B, used to play the Video. After I have opened two threads, these two tasks can be executed simultaneously, if I open a thread, only the download task is completed, to perform the playback Task. the thread is equivalent to the waiter in the shop, a waiter can only do one thing at the same time, more than a few waiters, you can do a lot of things at the same time. Process1, the process is an instance of application execution , in short, is running in the operating system Program. For example, I only open QQ and the two software on the phone, there will be two processes in the System. 2. The process cannot perform the task3. The resources created by the process at run time die as the process Terminates. Threads1, the process itself cannot perform a task, process want to perform a task must be wired , Thread is a separate execution unit within the process, You can only perform one task at a time, which is equivalent to a subroutine. Threads are divided into two types of , main thread ( user interface thread and child threads ( worker thread or called background thread ( operating system ) opened an orange product experience shop process There are a lot of staff, there is a shop long to help me decorate the façade ( main thread , Consulting Staff ( sub-thread ( sub-thread 2, the thread execution will be Destroyed. 3. The main thread ( also called the parent thread ) is created and started automatically when the application starts, and is typically used to process the User's input and respond to various events and Messages. The termination of the main thread also means the end of the Program. 4. Child thread: created by the main thread to help the main thread execute the program's background processing Task. If you create a child thread B in Sub-thread a, after it is created, the two are independent of each other and can be executed concurrently between multiple child threads. 5. There can be multiple threads in a process, and all threads are in the virtual address space of the process and can use the global variables and system resources of the PROCESS. 6.Thread state: Five states of threadMultithreading1, most of the current apps, need to connect to the server, and access to the server may be fast or slow. If the operation of an app access server does not operate on a child thread, the software cannot respond to the User's actions during the App's access to the server, only after the app has been accessed, theapp in order to respond to user actions, which leads to thread blocking, which is a common lag phenomenon. A thread can only perform one task at a time , but a process may have multiple threads. Multiple threads can be opened to perform different tasks , thus improving the execution efficiency of the program and avoiding thread blocking. 2, the operating system will be based on the priority of the thread ( thread priority can be set manually ) to schedule the CPU time, High priority threads, the chance of priority calls will be greater, peer, look at the thread execution successively. 3, at the same time, theCPU can only process one thread, only one thread is Working. Multithreading parallel execution, in fact, each thread is constantly switching, because the time to perform the switchover quickly, resulting in the illusion of simultaneous execution, the principle of such as a,B two threads; A execution to a certain time period to switch, but a task is not completed, the system will be a current execution of the location and data in the way to save it in a stackthen the b thread executes,b Execution time is up, and its position status is saved to the stack of b . 4, the system automatically find a stack, will be saved before a data recovery, but also from a before the state of disconnection to continue to execute, so loop, the system each open a thread has a relatively large overhead. If the thread is too open, it will not only take up a lot of memory and make the program more complex, but also aggravate the burden of the CPU , such software will make your mobile phone in the winter into a warm hand treasure. Why ( How to use multithreading ) Improve program execution efficiency and avoid lag caused by thread blocking. can appropriately improve resource utilization (CPU, memory ). non-abusive multithreading:Open threads need to occupy a certain amount of memory (by default, The main thread occupies 1M, sub-threads occupy 512KB, can set their own memory size, but must be a multiple of 4 , if you open a large number of threads, will consume a lot of memory space, reduce the performance of the program The more threads, the greater the overhead of the CPU on the dispatch thread programming is more complex: such as communication between threads, multi-threaded data sharingSummarize thread-to-process Relationshipsa thread is the basic unit of CPU execution, a process can have multiple threads, but can only perform one task at a Time. the process is the running software, is Dynamic. an operating system can correspond to multiple processes , a process can have multiple threads , but at least one thread threads in the same process share resources in a process Nsthread Common Methods <ul> <ul> <li>There are two ways to use Nsthread to open threads:</li> </ul> </ul> <ol> <li><li>Create a manual open mode<pre>1 Object:@ "thread"]; 2 // Open Thread 3 [thread start];</pre></li></li> <li><li>Create and Auto-open mode<pre>1 /* 2 Create and Auto-open mode 3 */ 4 [nsthread detachnewthreadselector: @selector (thread1:) totarget:self withobject:@ "thread1 "];</pre></li></li> </ol>Thread classification1> main thread : UI thread, display, Refresh UI interface, handle events for UI controls 2> Child Threads : background threads, asynchronous Threads Note: do not place time-consuming operations on the main thread, but in sub-threads, because the main thread contains the user interface, so it is important to reduce the time-consuming operation of the main thread to avoid challenging the User's Patience.Nsthread Common operations <ul> <ul> <li class="li1">Creating and starting Threads</li> </ul> </ul><pre><pre>Nsthread *thread = [[nsthread alloc] initwithtarget:self selector: @selector (run) object:nil]; [thread Start];//thread starts, It executes the run method of self in Threads thread</pre></pre> <ul> <ul> <li class="li1">Main thread Correlation method</li> </ul> </ul><pre><pre>+ (nsthread *) mainthread; Get the main thread-(BOOL) ismainthread; is the main thread + (BOOL) ismainthread; is the main thread</pre></pre> <ul> <ul> <li class="li1">Get current thread</li> </ul> </ul><pre><pre>Nsthread *current = [nsthread currentthread];</pre></pre> <ul> <ul> <li class="li1">Scheduling Priority for Threads</li> </ul> </ul><pre><pre>+ (double) threadpriority;+ (bool) setthreadpriority: (double) p;-(double) threadpriority;-(bool) setthreadpriority: ( Double) p;</pre></pre>The value range of the scheduling priority is 0.0 ~ 1.0, the default 0.5, the higherthe value, the higher the priority level <ul> <ul> <li class="li1">The name of the thread</li> </ul> </ul><pre><pre>-(void) setName: (nsstring *) n;-(nsstring *) name;</pre></pre> <ul> <ul> <li class="li1">Automatically start a thread after creating a thread</li> </ul> </ul><pre><pre>[nsthread detachnewthreadselector: @selector (run) totarget:self withobject:nil];</pre></pre> <ul> <ul> <li class="li1">Implicitly creating and starting a thread</li> </ul> </ul><pre><pre>[self performselectorinbackground: @selector (run) withobject:nil]; </pre></pre> Pros and cons of the above 2 ways to create threads <ul> <ul> <li class="li1">Advantages: Simple and fast</li> <li class="li4">Cons: Unable to set the thread in more detail</li> </ul> </ul>The state of the thread <ol> <ol> <li class="li4">Possible state evolution of threads in a program</li> </ol> </ol>　　　　For example, show the state of a thread at different times　　2. How to control thread state <ul> <ul> <li class="li1">Start thread</li> </ul> </ul><pre><pre>-(void) start; Go to Ready status, run Status. Automatically enters the death state when the thread task is completed</pre></pre> <ul> <ul> <li class="li1">Blocking (pausing) Threads</li> </ul> </ul><pre><pre>+ (void) sleepuntildate: (nsdate *) date;+ (void) sleepfortimeinterval: (nstimeinterval) ti; Enter blocking State</pre></pre> <ul> <ul> <li class="li1">Force Stop Thread</li> </ul> </ul><pre><pre>+ (void) Exit;//into death state</pre></pre>Note: once the thread has stopped (dead), it cannot be opened Again.The security hidden trouble of multithreading <ol> <ol> <li class="li1">Resource sharing</li> </ol> </ol> <ul> <ul> <li class="li1">1 of resources may be shared by multiple threads, that is, multiple threads may access the same block of resources</li> <li class="li1">For example, multiple threads access the same object, the same variable, the same file</li> <li class="li1">Data confusion and data security issues can easily arise when multiple threads access the same resource</li> </ul> </ul>As shown in the Thread safety vulnerability analysis:To solve the problem, we can use the mutex technique in nsthread, as shown in:Security hazard resolution – Mutual exclusion lock <ul> <ul> <li class="li1">Mutex use format</li> </ul> </ul><pre><pre>@synchronized (lock Object) {//code to be locked }</pre></pre>Note: locking 1 parts of code with only 1 locks, with multiple locks is not valid <ul> <ul> <li class="li1">Advantages and disadvantages of mutual exclusion lock</li> </ul> </ul>Advantages: it can effectively prevent the data security problem caused by multi-thread snatch resourceCons: consumes a lot of CPU Resources <ul> <ul> <li class="li1">Use of mutexes: multiple threads rob the same piece of resources</li> </ul> </ul> <ul> <ul> <li class="li1">Related professional terminology: thread synchronization</li> </ul> </ul>Thread synchronization means that multiple threads are executing on the same line (performing tasks Sequentially)Mutex is the use of thread synchronization technologyAtomic and Non-atomic Properties <ul> <ul> <li class="li1">OC has nonatomic and atomic two choices when defining attributes</li> </ul> </ul>Atomic: Atomic attribute, locking for Setter method (default is atomic) Nonatomic: non-atomic attribute, does not lock the setter method <ul> <ul> <li class="li3">Selection of atomic and Non-atomic properties</li> </ul> </ul> comparison of nonatomic and Atomic <ul> <ul> <li> <ul> <li> <ul> <li class="li2">Atomic: thread-safe, requires a lot of resources</li> <li class="li2">Nonatomic: non-thread-safe, Suitable for small memory mobile devices</li> </ul></li> </ul></li> </ul> </ul>Recommendations for 2.iOS development <ul> <ul> <li> <ul> <li> <ul> <li class="li2">All properties are declared as nonatomic</li> <li class="li2">Try to avoid multiple threads robbing the same piece of resources</li> <li class="li5">As far as possible to locking, resource-grabbing business logic to the Server-side processing, reduce the pressure of mobile clients</li> </ul></li> </ul></li> </ul> </ul>Inter-thread Communication <ul> <ul> <li class="li1">What is called Inter-thread communication</li> </ul> </ul>In 1 processes, threads often do not exist in isolation and require frequent communication between multiple threads <ul> <ul> <li class="li1">The embodiment of inter-thread communication</li> </ul> </ul>1 threads Pass data to another 1 threads After performing a specific task in 1 threads, go to another 1 thread to continue the task <ul> <ul> <li class="li1">Common methods for inter-thread communication</li> </ul> </ul><pre><pre>-(void) performselectoronmainthread: (SEL) aselector withobject: (id) arg waituntildone: (BOOL) wait;-(void) Performselector: (SEL) aselector onthread: (nsthread *) thr withobject: (id) arg waituntildone: (BOOL) wait;</pre></pre> <ul> <ul> <li class="p1">Example of inter-thread communication – image download</li> </ul> </ul>Ios-multi-thread Nsthread Detailed

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