這個問題找了將近一天，baidu、google了很久，竟然沒有人遇到同樣問題？

performSelector:withObject:afterDelay:和[NSTimer timerWithTimeInterval:invocation:repeats:]都存在此問題：
呼叫後，延遲後（延遲0也一樣）沒有進入指定的@selecter()中。
反覆驗證發現，這種現象出現在子執行緒中，主執行緒中可正常使用。因此必須保證呼叫performSelector:withObject:afterDelay:是在主執行緒中。

但沒有測試在子執行緒中再建立個NSTimer用來延遲，應該也可以，反正直接用

[NSTimer timerWithTimeInterval:invocation:repeats:]是不行的。

特實現

- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay

方法，保證在主執行緒中呼叫成功

//  NSObject_performSelectorEx.h

@interface NSObject (performSelectorEx)

- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval

@end

//  NSObject_performSelectorEx.m

#import "NSObject_performSelectorEx.h"

@implementation NSObject (performSelectorEx)

- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay

{

    NSMethodSignature *sig = [self methodSignatureForSelector

    if (!sig)

    {

        return;

    }

NSInvocation *invo = [NSInvocationinvocationWithMethodSignature:sig];

    [invo setTarget:self];

    [invo setSelector:aSelector];

    [invo setArgument:&anArgument atIndex:2];

    [invo retainArguments];

    NSMethodSignature *sigMT = [invo methodSignatureForSelector:@selector(performSelector:withObject:afterDelay:)];

NSInvocation *invoMT = [NSInvocationinvocationWithMethodSignature:sigMT];

    [invoMT setTarget:invo];

    [invoMT setSelector:@selector(performSelector:withObject:afterDelay:)];

    SEL arg1 = @selector(invoke);

    void *arg2 = nil;

    NSTimeInterval arg3 = delay;

    [invoMT setArgument:&arg1 atIndex:2];

    [invoMT setArgument:&arg2 atIndex:3];

    [invoMT setArgument:&arg3 atIndex:4];

    [invoMT retainArguments];

    [invoMT performSelectorOnMainThread:@selector(invoke) withObject:nilwaitUntilDone:NO];

}

@end