iOS开发全面解析多线程

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一.多线程基本概念

1. 进程 
进程是指在系统中正在运行的一个应用程序。每个进程之间是独立的，每个进程均运行在其专用且受保护的内存空间内。

2. 线程 
基本概念 
1个进程要想执行任务，必须得有线程（每1个进程至少要有1条线程），线程是进程的基本执行单元，一个进程（程序）的所有任务都在线程中执行。

线程的串行 
1个线程中任务的执行是串行的，如果要在1个线程中执行多个任务，那么只能一个一个地按顺序执行这些任务。也就是说，在同一时间内，1个线程只能执行1个任务。

3.多线程 
基本概念 
即1个进程中可以开启多条线程，每条线程可以并行（同时）执行不同的任务

线程的并行 
并行即同时执行。比如同时开启3条线程分别下载3个文件（分别是文件A、文件B、文件C。

多线程并发执行的原理 
在同一时间里，CPU只能处理1条线程，只有1条线程在工作（执行）。多线程并发（同时）执行，其实是CPU快速地在多条线程之间调度（切换），如果CPU调度线程的时间足够快，就造成了多线程并发执行的假象

4.多线程优缺点 
优点 
1）能适当提高程序的执行效率。 
2）能适当提高资源利用率（CPU、内存利用率） 
缺点 
1）开启线程需要占用一定的内存空间（默认情况下，主线程占用1M，子线程占用512KB），如果开启大量的线程，会占用大量的内存空间，降低程序的性能。 
2）线程越多，CPU在调度线程上的开销就越大。 
3）程序设计更加复杂：比如线程之间的通信、多线程的数据共享

5.注意事项： 
(1)不要同时开太多的线程（1~3条线程即可，不要超过5条） 
(2) 主线程 ： UI线程，显示、刷新UI界面，处理UI控件的事件 
(3) 子线程 ： 后台线程，异步线程 
(4)不要把耗时的操作放在主线程，要放在子线程中执行

二、NSThread

1.创建和启动线程的3种方式 
(1)先创建，后启动

// 创建 
NSThread *thread = [[NSThread alloc] initWithTarget:self 
selector:@selector(download:) object:nil]; 
// 启动 [thread start];

(2)创建完自动启动

[NSThread detachNewThreadSelector:@selector(download:) toTarget:self 
withObject:nil];

(3) 隐式创建（自动启动）

[self performSelectorInBackground:@selector(download:) 
withObject:nil];

2.常见方法 
(1) 获得当前线程

<code class="hljs diff has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-addition" style="box-sizing: border-box; background-color: rgb(186, 238, 186);">+ (NSThread *)currentThread;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

(2) 获得主线程 
+ (NSThread *)mainThread;

(3) 睡眠（暂停）线程

+(void)sleepUntilDate:(NSDate *)date; 
+(void)sleepForTimeInterval:(NSTimeInterval)ti;

(4)设置线程的名字

-(void)setName:(NSString *)n; 
-(NSString *)name;

线程同步 
1.实质：为了防止多个线程抢夺同一个资源造成的数据安全问题

2.实现：给代码加一个互斥锁（同步锁） 
@synchronized(self) { 
// 被锁住的代码 
}

三、GCD

1.队列和任务 
(1) 任务 ：需要执行什么操作 
用block来封装任务

(2) 队列 ：存放任务 
全局的并发队列 ： 可以让任务并发执行

dispatch_queue_t queue = 
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

自己创建的串行队列 ： 让任务一个接着一个执行

dispatch_queue_t queue = dispatch_queue_create(“cn.heima.queue”, 
NULL);

主队列 ： 让任务在主线程执行

dispatch_queue_t queue = dispatch_get_main_queue();

2.执行任务的函数 
(1) 同步执行 : 不具备开启新线程的能力 
dispatch_sync…

(2) 异步执行 : 具备开启新线程的能力 
dispatch_async…

3.常见的组合

(1) 异步函数+并发队列：开启多条线程，并发执行任务 
(2) 异步函数+串行队列：开启一条线程，串行执行任务 
(3) 同步函数+并发队列：不开线程，串行执行任务 
(4) 同步函数+串行队列：不开线程，串行执行任务 
(5) 异步函数+主队列：不开线程，在主线程中串行执行任务 
(6) 同步函数+主队列：不开线程，串行执行任务（注意死锁发生） 
(7) 注意同步函数和异步函数在执行顺序上面的差异

注意事项: 同步函数往当前的串行队列中添加任务,会卡住当前的线程,形成死锁。

4.线程间的通信

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 }); });

5.GCD的常用方法 
（1）延迟执行 
<1> performSelector 
需求: 3秒后自动回到当前线程调用self的download:方法，并且传递参数：@”http://555.jpg”

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">[<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span> <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">performSelector:</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@selector</span>(<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">download:</span>) <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">withObject:</span>@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"http://555.jpg"</span> <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">afterDelay:</span><span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>];</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

<2> dispatch_after

// 任务放到哪个队列中执行 dispatch_queue_t queue = 
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); double 
delay = 3; 
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 
(int64_t)(delay * NSEC_PER_SEC)), queue, ^{ 
// 3秒后需要执行的任务 
});

(2)一次性代码

static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ 
// 这里面的代码，在程序运行过程中，永远只会执行1次 });

(3)栅栏函数（控制任务的执行顺序）

dispatch_barrier_async(queue, ^{
    NSLog(@"--dispatch_barrier_async-");
});

(4)快速迭代（开多个线程并发完成迭代操作）

dispatch_apply(subpaths.count, queue, ^(size_t index) { 
});

(5)队列组（同栅栏函数）

//创建队列组 
dispatch_group_t group = dispatch_group_create(); 
//队列组中的任务执行完毕之后，执行该函数 
dispatch_group_notify(dispatch_group_t group, 
dispatch_queue_t queue, 
dispatch_block_t block);

四、NSOperation和NSOperationQueue

1.基本概念： 
NSOperation是对GCD的包装, 两个核心概念【队列+操作】 
2.基本使用 
(1) NSOperation本身是抽象类，只能只有它的子类 
(2)三个子类分别是：NSBlockOperation、NSInvocationOperation以及自定义继承自NSOperation的类 
(3) NSOperation和NSOperationQueue结合使用实现多线程并发

3.队列的类型 
(1) 主队列

• [NSOperationQueue mainQueue]
• 添加到”主队列”中的操作，都会放到主线程中执行

(2) 非主队列

• [[NSOperationQueue alloc] init]
• 添加到”非主队列”中的操作，都会放到子线程中执行

4.队列添加任务

• -(void)addOperation:(NSOperation *)op;

• -(void)addOperationWithBlock:(void (^)(void))block;

5.常见用法 
(1) 设置最大并发数

<code class="hljs erlang has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-pp" style="box-sizing: border-box;">- <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-variable" style="box-sizing: border-box;">NSInteger</span>)</span>maxConcurrentOperationCount;
- <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(void)</span>setMaxConcurrentOperationCount:<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-variable" style="box-sizing: border-box;">NSInteger</span>)</span>cnt;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

(2) 队列的其他操作

• 取消所有的操作 
  -(void)cancelAllOperations;

• 暂停所有的操作 
  [queue setSuspended:YES];

• 恢复所有的操作 
  [queue setSuspended:NO];

6.操作之间的依赖（面试题）

• NSOperation之间可以设置依赖来保证执行顺序
• [operationB addDependency:operationA]; 
  // 操作B依赖于操作A，等操作A执行完毕后，才会执行操作B
• 注意：不能相互依赖，比如A依赖B，B依赖A
• 可以在不同queue的NSOperation之间创建依赖关系

7.线程之间的通信

NSOperationQueue *queue = [[NSOperationQueue alloc] init]; [queue 
addOperationWithBlock:^{ 
// 1.执行一些比较耗时的操作 
// 2.回到主线程 
[[NSOperationQueue mainQueue] addOperationWithBlock:^{ 
}]; 
}];

五、单例模式

1.ARC实现单例模式思路 
(1) 在类的内部提供一个static修饰的全局变量 
(2) 提供一个类方法，方便外界访问 
(3) 重写+allocWithZone方法，保证永远都只为单例对象分配一次内存空间 
(4) 严谨起见，重写-copyWithZone方法和-MutableCopyWithZone方法

(1) 创建一个类工厂方法

@interface DataTool : NSObject 
+ (instancetype)sharedDataTool; @end

@implementation DataTool

// 用来保存唯一的单例对象 
static id _instace; 
// 重写allocWithZone方法 
+ (id)allocWithZone:(struct _NSZone *)zone { 
static dispatch_once_t onceToken; 
dispatch_once(&onceToken, ^{ 
_instace = [super allocWithZone:zone]; 
}); 
return _instace; }

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// sharedDataTool方法的实现</span>
+ (instancetype)sharedDataTool {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">dispatch_once_t</span> onceToken;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">dispatch_once</span>(&onceToken, ^{
        _instace = [[<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span> alloc] init];
    });
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> _instace; }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 重写copyWithZone方法</span>- (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">id</span>)copyWithZone:(NSZone *)zone{    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> _instace;}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@end</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

2.MRC实现单例的思路 
(1) 在类的内部提供一个static修饰的全局变量 
(2) 提供一个类方法，方便外界访问 
(3) 重写+allocWithZone方法，保证永远都只为单例对象分配一次内存空间 
(4) 严谨起见，重写-copyWithZone方法和-MutableCopyWithZone方法 
(5) 重写release方法 
(6) 重写retain方法 
(7) 建议在retainCount方法中返回一个最大值

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 声明一个类工厂方法</span>
<span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">DataTool</span> : <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSObject</span></span>
+ (instancetype)sharedDataTool;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@end</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

@implementation HMDataTool

// 用来保存唯一的单例对象 
static id _instace; 
+ (id)allocWithZone:(struct _NSZone *)zone { 
static dispatch_once_t onceToken; 
dispatch_once(&onceToken, ^{ 
_instace = [super allocWithZone:zone]; 
}); 
return _instace; }

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"> + (instancetype)sharedDataTool
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">dispatch_once_t</span> onceToken;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">dispatch_once</span>(&onceToken, ^{
        _instace = [[<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span> alloc] init];
    });
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> _instace;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

// 重写copyWithZone方法 
- (id)copyWithZone:(NSZone *)zone { 
return _instace; }

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 重写release</span>
- (oneway <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>)release {   
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 重写retain</span>- (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">id</span>)retain {    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span>;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

<code class="hljs haml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">-<span class="ruby" style="box-sizing: border-box;"> (<span class="hljs-constant" style="box-sizing: border-box;">NSUInteger</span>)retainCount {</span>    return MAXFLOAT;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 重写autorelease</span>- (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">id</span>)autorelease {    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span>;}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">@end</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

3.判断编译器的环境：ARC还是MRC的方法

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">if</span> __has_feature(objc_arc)</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 当前的编译器环境是ARC</span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">else</span></span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 当前的编译器环境是MRC</span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">endif</span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

六、从其他线程回到主线程的方式

1.performSelectorOnMainThread

<code class="hljs smalltalk has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">[<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span> <span class="hljs-method" style="box-sizing: border-box;">performSelectorOnMainThread:</span><<span class="hljs-array" style="box-sizing: border-box;">#(SEL)</span>#> <span class="hljs-method" style="box-sizing: border-box;">withObject:</span><<span class="hljs-array" style="box-sizing: border-box;">#(id)</span>#> <span class="hljs-method" style="box-sizing: border-box;">waitUntilDone:</span><<span class="hljs-array" style="box-sizing: border-box;">#(BOOL)</span>#>];</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

2.GCD

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;">dispatch_async(<span class="hljs-function" style="box-sizing: border-box;">dispatch_get_main_queue()</span>, ^{

})</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

3.NSOperationQueue

<code class="hljs css has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-attr_selector" style="color: rgb(0, 136, 0); box-sizing: border-box;">[[NSOperationQueue mainQueue]</span> <span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">addOperationWithBlock</span>:^<span class="hljs-rules" style="box-sizing: border-box;">{

<span class="hljs-rule" style="box-sizing: border-box;">}</span></span>];</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

七、类的初始化方法

1.+(void)load

• 当某个类第一次装载到OC运行时系统（内存）时，就会调用
• 程序一启动就会调用
• 程序运行过程中，只会调用1次

2.+(void)initialize

• 当某个类第一次被使用时（比如调用了类的某个方法），就会调用
• 并非程序一启动就会调用

3.在程序运行过程中：1个类中的某个操作，只想执行1次，那么这个操作放到+(void)load方法中最合适

八、cell的图片下载

1.面试题 
1> 如何防止一个url对应的图片重复下载

• “cell下载图片思路 – 有沙盒缓存”

2> SDWebImage的默认缓存时长是多少？

• 1个星期

3> SDWebImage底层是怎么实现的？

(1)无沙盒缓存 
实现步骤: 
1.先缓存需要的图片 
2.根据图片的url去image中取图片 有图片 直接显示到cell上 
3.图片不存在,显示一个占位图片 
4.根据图片的url查看operations中是否存在下载操作 
5.若下载操作存在,执行正在下载 
6.不存在,创建下载操作放到operation中 
7.下载完成,将操作从operation中移除,将图片放到image中 
8.刷新表格

(2)有沙盒缓存 
1.根据图片的URL去image中取图片 存在 显示到cell上 
2.如果图片不存在 检查沙盒中是否有图片 存在 显示到cell上 
3.如果不存在 显示一张占位图片 
4.根据图片的URL检查operation是否有下载操作 存在 继续下载 
5.如果不存在 创建下载操作 放到operation中 
6.下载完毕 将操作从operation中移除 
7.刷新表格 按行刷新 
8.将图片保存到沙盒中

2.第三方框架SDWebImage的使用 
（1） 常用方法

-(void)sd_setImageWithURL:(NSURL )url placeholderImage:(UIImage )placeholder;

<code class="hljs erlang has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-pp" style="box-sizing: border-box;">- <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(void)</span>sd_setImageWithURL:<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-variable" style="box-sizing: border-box;">NSURL</span> *)</span>url placeholderImage:<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-variable" style="box-sizing: border-box;">UIImage</span> *)</span>placeholder options:<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-variable" style="box-sizing: border-box;">SDWebImageOptions</span>)</span>options;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

<code class="hljs erlang has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-pp" style="box-sizing: border-box;">-<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(void)</span>sd_setImageWithURL:<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-variable" style="box-sizing: border-box;">NSURL</span> *)</span>url placeholderImage:<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-variable" style="box-sizing: border-box;">UIImage</span> *)</span>placeholder completed:<span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(<span class="hljs-variable" style="box-sizing: border-box;">SDWebImageCompletionBlock</span>)</span>completedBlock;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

-(void)sd_setImageWithURL:(NSURL )url placeholderImage:(UIImage )placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock 
completed:(SDWebImageCompletionBlock)completedBlock;

（2） 内存处理：当app接收到内存警告时 
// 当app接受到内存警告

–(void)applicationDidReceiveMemoryWarning:(UIApplication *)application { 
SDWebImageManager *mgr = [SDWebImageManager sharedManager]; 
// 1.取消正在下载的操作 
[mgr cancelAll]; 
// 2.清除内存缓存 
[mgr.imageCache clearMemory]; }

(3) SDWebImageOptions

• SDWebImageRetryFailed : 下载失败后，会自动重新下载
• SDWebImageLowPriority : 当正在进行UI交互时，自动暂停内部的一些下载操作
• SDWebImageRetryFailed | SDWebImageLowPriority : 拥有上面2个功能