简单介绍下iOS的RunLoop
who
runloop 是线程的基础设施，是线程的一部分，有一定的监听的功能
what
Each NSThread object, including the application’s main thread, has an NSRunLoop object automatically created for it as needed.
why
An NSRunLoopobject processes input sources such as mouse and keyboard events from the window system,NSTimer events, NSPort objects, and NSConnection objects.
how
do{} while()
mode : NSDefaultRunLoopMode、NSRunLoopCommonModes、NSEventTrackingRunLoopMode
例子 : 定时器add到defaultMode上，scroll会导致mode切到trackingMode，这样定时器就暂停了
NSURLConnection 下载的时候，要开启runLoop
CFRunLoop 是线程安全的，NSRunLoop 是线程不安全的
