-performSelectorOnMainThread:withObject:waitUntilDone: VS dispatch_async(dispatch_get_main_queue(),
默认情况下,-performSelectorOnMainThread:withObject:waitUntilDone:只当主线程的runloop处于默认模式下,才会调度你指定的selector,当runloop处于其他模式下(如trackingmode)时,selector只有在runloop切换会默认模式下时才会被执行。你可以使用-performSelectorOnMainThread:wit