iOS multi-thread nsthread use

<span id="Label3"></p><p class="p1"><p class="p1"><span style="font-size: 16px;"><strong>multi-threading Technology in iOS</strong></span></p></p><p class="li1"><p class="li1"><span style="font-size: 16px;"><strong>　　</strong><span style="font-size: 14px;">In our iOS development project, we used multithreaded technology in order to solve the UI operation not being blocked by time-consuming operations. In iOS development, we mainly use three kinds of multithreading technology: NSTHREAD,NSOPERATION,GCD.</span></span></p></p><p class="li1"><p class="li1"><span style="font-size: 16px;"><span style="font-size: 14px;">1.<strong>thread</strong> : Three methods are relatively lightweight, but need to manage the life cycle of the thread, synchronization, lock issues, which can lead to a certain performance cost<br>2.<strong>Cocoa Operations</strong>: Based on OC implementations, Nsoperation encapsulates the actions that need to be performed in an object-oriented manner, without concern for thread management, synchronization, and so On. Nsoperation is an abstract base class, and iOS provides two default implementations: nsinvocationoperation and nsblockoperation, and of course you can customize nsoperation<br>3.<strong>Grand Central Dispatch</strong>(gcd,ios4 only started): provides new features, runtime to support multicore parallel programming, with a higher focus: how to increase efficiency on multiple CPUs</span></span></p></p><p class="li1"><p class="li1"><span style="font-size: 16px;"><span style="font-size: 14px;">This article mainly nsthread the first multithreaded operation Technology.</span></span></p></p><p class="p1"><p class="p1"><span style="font-size: 16px;"><strong>Thread classification</strong></span></p></p><p class="p1"><p class="p1"><span class="s1">1<span class="s3">> <span class="s2">main thread <span class="s3"> <span class="s2">:<span class="s3"> UI <span class="s2">thread, display, Refresh <span class="s3">UI <span class="s2">interface, handle <span class="s3"> <span class="s2">events for UI controls</span> </span> </span> </span> </span> </span> </span></span></span></span></span></p></p><p class="p1"><p class="p1"><span class="s1">2<span class="s3">> <span class="s2">Child Threads <span class="s3"> <span class="s2">:<span class="s3"> <span class="s2">background threads, asynchronous Threads</span> </span> </span> </span> </span> </span></span></p></p><p class="p1"><p class="p1"><span class="s1">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.</span></p></p><p class="li1"><p class="li1"><span style="font-size: 16px;"><strong>Nsthread Common operations</strong></span></p></p> <ul> <ul> <li class="li1">Creating and starting Threads</li> </ul> </ul><pre><pre><span style="color: #0000ff;">Object</span> <span style="color: #000000;">: nil]; [thread start];</span> <span style="color: #008000;">//</span> <span style="color: #008000;">when a thread is started, the Run method of self is executed in threads thread</span></pre></pre> <ul> <ul> <li class="li1">Main thread Correlation method</li> </ul> </ul><pre><pre><span style="color: #008000;">//</span> <span style="color: #008000;">get the main</span> thread <span style="color: #008000;">//</span> is <span style="color: #008000;">the main</span> thread <span style="color: #008000;">//</span> is <span style="color: #008000;">the main</span> thread</pre></pre> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">Get current thread</span></span></li> </ul> </ul><pre><pre>Nsthread *current = [nsthread currentthread];</pre></pre> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">Scheduling Priority for Threads</span></span></li> </ul> </ul><pre><pre>+ (<span style="color: #0000ff;">double</span><span style="color: #000000;">) threadpriority;</span> + (BOOL) setthreadpriority: (<span style="color: #0000ff;">double</span><span style="color: #000000;">) p;</span> -(<span style="color: #0000ff;">Double</span><span style="color: #000000;">) threadpriority;</span> -(BOOL) setthreadpriority: (<span style="color: #0000ff;">double</span>) p;</pre></pre><p class="p4"><p class="p4"><span class="s6">The value range of the scheduling priority <span class="s2">is <span class="s7">0.0 ~ 1.0<span class="s2">, the default <span class="s7">0.5, the higher<span class="s2">the value, the higher the priority level</span> </span> </span> </span> </span></span></p></p><p class="p3"><p class="p3"><span class="s2"></span></p></p> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">The name of the thread</span></span></li> </ul> </ul><pre><pre>-(<span style="color: #0000ff;">void</span>) setName: (nsstring *<span style="color: #000000;">) n;</span> -(nsstring *) name;</pre></pre> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">Automatically start a thread after creating a thread</span></span></li> </ul> </ul><pre><pre>[nsthread detachnewthreadselector: @selector (run) totarget:self withobject:nil];</pre></pre> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">Implicitly creating and starting a thread</span></span></li> </ul> </ul><pre><pre>[self performselectorinbackground: @selector (run) withobject:nil]; <span class="s2"> </span></pre></pre><p class="li1"><p class="li1"><span class="s1"><span class="s2"><span class="s6"> <span class="s2">Pros and cons of the above 2 ways to create threads</span> </span></span></span></p></p> <ul> <ul> <li class="li1"><span class="s7"><span class="s2">Advantages: Simple and fast</span></span></li> <li class="li4"><span class="s7"><span class="s2">Cons: Unable to set the thread in more detail</span></span></li> </ul> </ul><p class="li4"><p class="li4"><span style="font-size: 16px;"><strong>The state of the thread</strong></span></p></p> <ol> <ol> <li class="li4"><strong><span style="font-size: 14px;">Possible state evolution of threads in a program</span></strong></li> </ol> </ol><p class="li4"><p class="li4"><span style="font-size: 16px;"><strong>　　　　</strong><span style="font-size: 14px;">For example, show the state of a thread at different times</span></span></p></p><p class="li4"><p class="li4"><span style="font-size: 16px;"><span style="font-size: 14px;"></span></span></p></p><p class="li4"><p class="li4"></p></p><p class="li4"><p class="li4">　　<strong>2. How to control thread state</strong></p></p> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">Start thread</span></span></li> </ul> </ul><pre><pre>-(<span style="color: #0000ff;">void</span><span style="color: #000000;"></span><span style="color: #008000;">//</span> <span style="color: #008000;"> Enter ready status, run Status.) Automatically enters the death state when the thread task is completed</span></pre></pre> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">Blocking (pausing) Threads</span></span></li> </ul> </ul><pre><pre>+ (<span style="color: #0000ff;">void</span>) sleepuntildate: (nsdate *<span style="color: #000000;">) date;</span> + (<span style="color: #0000ff;">void</span>) sleepfortimeinterval: (nstimeinterval) ti;<br><span class="s2">Enter blocking State</span></pre></pre> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">Force Stop Thread</span></span></li> </ul> </ul><pre><pre>+ (<span style="color: #0000ff;">void</span><span style="color: #000000;">) exit;</span> <span style="color: #008000;">//</span> <span style="color: #008000;">into the state of death</span></pre></pre><p class="p6"><p class="p6"><span class="s2">Note: once the thread has stopped (dead), it cannot be opened Again.</span></p></p><p class="p6"><p class="p6"></p></p><p class="p6"><p class="p6"><span style="font-size: 16px;"><strong>The security hidden trouble of multithreading</strong></span></p></p> <ol> <ol> <li class="li1"><strong><span class="s1"><span class="s2">Resource sharing</span></span></strong></li> </ol> </ol> <ul> <ul> <li class="li1"><span class="s3"><span class="s4">1 <span class="s2">of resources may be shared by multiple threads, that is, <span class="s5">multiple threads may access the same</span> block of resources</span></span></span></li> <li class="li1"><span class="s3"><span class="s2">For example, multiple threads access the same object, the same variable, the same file</span></span></li> <li class="li1"><span style="line-height: 1.5;">When multiple threads access the same piece of resources, it is easy to throw</span> <span class="s5" style="line-height: 1.5;">data confusion and data security <span class="s2">issues</span> </span></li> </ul> </ul><p class="li1"><p class="li1">As shown in the Thread safety vulnerability analysis:</p></p><p class="li1"><p class="li1"></p></p><p class="li1"><p class="li1"></p></p><p class="li1"><p class="li1">To solve the problem, we can use the mutex technique in nsthread, as shown in:</p></p><p class="li1"><p class="li1"></p></p><p class="p1"><p class="p1"><span style="font-size: 16px;"><strong><span class="s1">Security hazard resolution – Mutual exclusion lock</span></strong></span></p></p> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">Mutex use format</span></span></li> </ul> </ul><pre><pre><span style="color: #008000;">//</span> <span style="color: #008000;">code that needs to be locked }</span></pre></pre><p class="p3"><p class="p3"><span class="s2">Note: locking <span class="s7">1 <span class="s2">parts of code with only <span class="s7">1 <span class="s2">locks, with multiple locks is not valid</span> </span> </span> </span></span></p></p><p class="p4"><p class="p4"><span class="s2"></span></p></p> <ul> <ul> <li class="li1"><span class="s8"><span class="s2">Advantages and disadvantages of mutual exclusion lock</span></span></li> </ul> </ul><p class="li1"><p class="li1"><span class="s9"><span class="s2">Advantages: it can effectively prevent the data security problem caused by multi-thread snatch resource</span></span></p></p><p class="li1"><p class="li1"><span class="s9"><span class="s2">Cons: consumes a lot of <span class="s7">CPU <span class="s2">Resources</span> </span></span></span></p></p> <ul> <ul> <li class="li1"><span class="s9"><span class="s2">Use of mutexes: multiple threads rob the same piece of resources</span></span></li> </ul> </ul> <ul> <ul> <li class="li1"><span class="s9"><span class="s2">Related professional terminology: <span class="s10">thread synchronization</span></span></span></li> </ul> </ul><p class="li1"><p class="li1"><span class="s8"><span class="s2">Thread synchronization means that multiple threads are executing on the same line (performing tasks Sequentially)</span></span></p></p><p class="li6"><p class="li6"><span class="s9"><span class="s2">Mutex is the use of thread synchronization technology</span></span></p></p><p class="p1"><p class="p1"><span style="font-size: 16px;"><strong><span class="s1">Atomic and Non-atomic Properties</span></strong></span></p></p> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">OC <span class="s3">has <span class="s4">nonatomic <span class="s3">and <span class="s4">atomic <span class="s3">two choices</span> </span> </span> </span> when defining attributes</span></span></span></li> </ul> </ul><p class="li2"><p class="li2"><span class="s5"><span class="s6">Atomic<span class="s4">: Atomic attribute, <span class="s7">locking for Setter <span class="s4">method (default is <span class="s6">atomic<span class="s4">)</span> </span> </span> </span> </span></span></span></p></p><p class="li3"><p class="li3"><span class="s8"><span class="s6">Nonatomic<span class="s4">: non-atomic attribute, does <span class="s7">not <span class="s4">lock</span> the setter method</span> </span></span></span></p></p> <ul> <ul> <li class="li3"><p class="p1"><span class="s1">Selection of atomic and Non-atomic properties</span></p></li> </ul> </ul><p class="p1"><p class="p1"><span class="s1"><span class="s2"><span class="s3"> <span class="s2"> <span class="s3">comparison</span> </span> of nonatomic and Atomic</span></span></span></p></p> <ul> <ul> <li style="list-style-type: none;"> <ul> <li style="list-style-type: none;"> <ul> <li class="li2"><span class="s4"><span class="s5">Atomic<span class="s2">: thread-safe, requires a lot of resources</span></span></span></li> <li class="li2"><span class="s4"><span class="s5">Nonatomic<span class="s2">: non-thread-safe, Suitable for small memory mobile devices</span></span></span></li> </ul></li> </ul></li> </ul> </ul><p class="li2"><p class="li2"><span class="s6"><span class="s7"><span class="s2">Recommendations for 2.iOS development</span></span></span></p></p> <ul> <ul> <li style="list-style-type: none;"> <ul> <li style="list-style-type: none;"> <ul> <li class="li2"><span class="s6"><span class="s8">All properties are declared as <span class="s9">nonatomic</span></span></span></li> <li class="li2"><span class="s6"><span class="s2">Try to avoid multiple threads robbing the same piece of resources</span></span></li> <li class="li5"><span class="s6"><span class="s2">As far as possible to locking, resource-grabbing business logic to the Server-side processing, reduce the pressure of mobile clients</span></span></li> </ul></li> </ul></li> </ul> </ul><p class="li5"><p class="li5"></p></p><p class="p1"><p class="p1"><span class="s1">Inter-thread Communication</span></p></p> <ul> <ul> <li class="li1"><span class="s1"><span class="s2">What is called Inter-thread communication</span></span></li> </ul> </ul><p class="li1"><p class="li1"><span class="s3"><span class="s2">In <span class="s4">1 <span class="s2">processes, threads often do not exist in isolation and require frequent communication between multiple threads</span> </span></span></span></p></p> <ul> <ul> <li class="li1"><span class="s3"><span class="s2">The embodiment of inter-thread communication</span></span></li> </ul> </ul><p class="li1"><p class="li1"><span class="s3"><span class="s4">1 <span class="s2">threads Pass data to another <span class="s4">1 <span class="s2">threads</span> </span> </span></span></span></p></p><p class="li1"><p class="li1"><span class="s3"><span class="s2"><span class="s4"> <span class="s2">After performing a specific task in 1 threads, go to another <span class="s4">1 <span class="s2">thread to continue the task</span> </span> </span> </span></span></span></p></p> <ul> <ul> <li class="li1"><span class="s3"><span class="s2">Common methods for inter-thread communication</span></span></li> </ul> </ul><pre><pre>-(<span style="color: #0000ff;">void</span>) performselectoronmainthread: (SEL) aselector withobject: (<span style="color: #0000ff;">ID</span><span style="color: #000000;">) arg waituntildone: ( BOOL) wait;</span> -(<span style="color: #0000ff;">void</span>) performselector: (SEL) aselector onthread: (nsthread *) thr withobject: (<span style="color: #0000ff;">ID</span>) arg Waituntildone: (BOOL) wait;</pre></pre> <ul> <ul> <li class="p1"><span class="s1">Example of inter-thread communication – image download</span></li> </ul> </ul><p class="p1"><p class="p1"><span class="s1"></span></p></p><p class="p3"><p class="p3"></p></p><p class="li5"><p class="li5"></p></p><p><p>iOS multi-thread nsthread use</p></p></span>