假设我将IBAction与xib文件中的按钮连接起来。当应用程序运行时，我点击按钮，同步调用操作，或者在runloop上放置performSelector请求？
答案 0 :(得分：2)
这是同步的。
在iOS应用程序中，回溯看起来像：
frame #0: 0x000025d2 MyApp`-[ViewController myAction:] + 34 at ViewController.m:31
frame #1: 0x010e2705 libobjc.A.dylib`-[NSObject performSelector:withObject:withObject:] + 77
frame #2: 0x00019920 UIKit`-[UIApplication sendAction:to:from:forEvent:] + 96
frame #3: 0x000198b8 UIKit`-[UIApplication sendAction:toTarget:fromSender:forEvent:] + 61
frame #4: 0x000da671 UIKit`-[UIControl sendAction:to:forEvent:] + 66
frame #5: 0x000dabcf UIKit`-[UIControl(Internal) _sendActionsForEvents:withEvent:] + 578
frame #6: 0x000d9d38 UIKit`-[UIControl touchesEnded:withEvent:] + 546
frame #7: 0x0004933f UIKit`-[UIWindow _sendTouchesForEvent:] + 846
frame #8: 0x00049552 UIKit`-[UIWindow sendEvent:] + 273
frame #9: 0x000273aa UIKit`-[UIApplication sendEvent:] + 436
frame #10: 0x00018cf8 UIKit`_UIApplicationHandleEvent + 9874
frame #11: 0x01becdf9 GraphicsServices`_PurpleEventCallback + 339
frame #12: 0x01becad0 GraphicsServices`PurpleEventCallback + 46
frame #13: 0x01c06bf5 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 53
frame #14: 0x01c06962 CoreFoundation`__CFRunLoopDoSource1 + 146
frame #15: 0x01c37bb6 CoreFoundation`__CFRunLoopRun + 2118
frame #16: 0x01c36f44 CoreFoundation`CFRunLoopRunSpecific + 276
frame #17: 0x01c36e1b CoreFoundation`CFRunLoopRunInMode + 123
frame #18: 0x01beb7e3 GraphicsServices`GSEventRunModal + 88
frame #19: 0x01beb668 GraphicsServices`GSEventRun + 104
frame #20: 0x0001665c UIKit`UIApplicationMain + 1211
frame #21: 0x00001f62 MyApp`main + 130 at main.m:16
frame #22: 0x00001e95 MyApp`start + 53
这是直接的同步代码。应用程序接收触摸结束事件，找出发送它的位置（按钮），并向按钮的目标发送消息。
相比之下，通过-[NSObject performSelector:withObject:afterDelay:]异步调用的方法有这样的回溯：
frame #0: 0x00002628 MyApp`-[ViewController myAsyncAction:] + 24 at ViewController.m:37
frame #1: 0x00acce83 Foundation`__NSFireDelayedPerform + 380
frame #2: 0x01c50376 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 22
frame #3: 0x01c4fe06 CoreFoundation`__CFRunLoopDoTimer + 534
frame #4: 0x01c37a82 CoreFoundation`__CFRunLoopRun + 1810
frame #5: 0x01c36f44 CoreFoundation`CFRunLoopRunSpecific + 276
frame #6: 0x01c36e1b CoreFoundation`CFRunLoopRunInMode + 123
frame #7: 0x01beb7e3 GraphicsServices`GSEventRunModal + 88
frame #8: 0x01beb668 GraphicsServices`GSEventRun + 104
frame #9: 0x0001665c UIKit`UIApplicationMain + 1211
frame #10: 0x00001f62 MyApp`main + 130 at main.m:16
frame #11: 0x00001e95 MyApp`start + 53
您可以清楚地看到计时器触发并调用其回调，并触发构建在其上的“延迟执行”。