CocoaAsyncSocket源码分析---Connect （一）

正文：

首先我们来看看框架：

整个库就这么两个类，一个基于TCP，一个基于UDP。其中基于TCP的GCDAsyncSocket，大概8000多行代码。而GCDAsyncUdpSocket稍微少一点，也有5000多行。
所以单纯从代码量上来看，这个库还是做了很多事的。

顺便提一下，之前这个框架还有一个runloop版的，不过因为功能重叠和其它种种原因，后续版本便废弃了，现在仅有GCD版本。

本系列我们将重点来讲GCDAsyncSocket这个类。

我们先来看看这个类的属性:

//层级调用
- (id)init

{
return [self initWithDelegate:nil delegateQueue:NULL socketQueue:NULL];

}

- (id)initWithSocketQueue:(dispatch_queue_t)sq

{
return [self initWithDelegate:nil delegateQueue:NULL socketQueue:sq];

}

- (id)initWithDelegate:(id)aDelegate delegateQueue:(dispatch_queue_t)dq

{
return [self initWithDelegate:aDelegate delegateQueue:dq socketQueue:NULL];

}

- (id)initWithDelegate:(id)aDelegate delegateQueue:(dispatch_queue_t)dq socketQueue:(dispatch_queue_t)sq

{
if((self = [super init]))

     {

          delegate = aDelegate;

          delegateQueue = dq;

//这个宏是在sdk6.0之后才有的,如果是之前的,则OS_OBJECT_USE_OBJC为0，!0即执行if语句
//对6.0的适配，如果是6.0以下，则去retain release，6.0之后ARC也管理了GCD
#if !OS_OBJECT_USE_OBJC

if (dq) dispatch_retain(dq);
#endif

//创建socket，先都置为 -1
//本机的ipv4

          socket4FD = SOCKET_NULL;
//ipv6

          socket6FD = SOCKET_NULL;
//应该是UnixSocket

          socketUN = SOCKET_NULL;
//url

          socketUrl = nil;
//状态

          stateIndex = 0;

if (sq)

          {
//如果scoketQueue是global的,则报错。断言必须要一个非并行queue。
NSAssert(sq != dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0),
@"The given socketQueue parameter must not be a concurrent queue.");
NSAssert(sq != dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),
@"The given socketQueue parameter must not be a concurrent queue.");
NSAssert(sq != dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
@"The given socketQueue parameter must not be a concurrent queue.");
//拿到scoketQueue

               socketQueue = sq;
//iOS6之下retain
#if !OS_OBJECT_USE_OBJC
dispatch_retain(sq);
#endif

          }
else

          {
//没有的话创建一个，  名字为：GCDAsyncSocket,串行

               socketQueue = dispatch_queue_create([GCDAsyncSocketQueueName UTF8String], NULL);

          }

// The dispatch_queue_set_specific() and dispatch_get_specific() functions take a "void *key" parameter.
// From the documentation:
//
// > Keys are only compared as pointers and are never dereferenced.
// > Thus, you can use a pointer to a static variable for a specific subsystem or
// > any other value that allows you to identify the value uniquely.
//
// We're just going to use the memory address of an ivar.
// Specifically an ivar that is explicitly named for our purpose to make the code more readable.
//
// However, it feels tedious (and less readable) to include the "&" all the time:
// dispatch_get_specific(&IsOnSocketQueueOrTargetQueueKey)
//
// So we're going to make it so it doesn't matter if we use the '&' or not,
// by assigning the value of the ivar to the address of the ivar.
// Thus: IsOnSocketQueueOrTargetQueueKey == &IsOnSocketQueueOrTargetQueueKey;

//比如原来为   0X123 -> NULL 变成  0X222->0X123->NULL
//自己的指针等于自己原来的指针，成二级指针了  看了注释是为了以后省略&,让代码更可读？

          IsOnSocketQueueOrTargetQueueKey = &IsOnSocketQueueOrTargetQueueKey;

void *nonNullUnusedPointer = (__bridge void *)self;

//dispatch_queue_set_specific给当前队里加一个标识 dispatch_get_specific当前线程取出这个标识，判断是不是在这个队列
//这个key的值其实就是一个一级指针的地址  ，第三个参数把自己传过去了，上下文对象？第4个参数，为销毁的时候用的，可以指定一个函数
dispatch_queue_set_specific(socketQueue, IsOnSocketQueueOrTargetQueueKey, nonNullUnusedPointer, NULL);
//读的数组 限制为5

          readQueue = [[NSMutableArray alloc] initWithCapacity:5];

          currentRead = nil;

//写的数组，限制5

          writeQueue = [[NSMutableArray alloc] initWithCapacity:5];

          currentWrite = nil;

//设置大小为 4kb

          preBuffer = [[GCDAsyncSocketPreBuffer alloc] initWithCapacity:(1024 * 4)];

#pragma mark alternateAddressDelay??
//交替地址延时？？ wtf

        alternateAddressDelay = 0.3;

     }
return self;

}