1.NSTimer中方法如：
scheduledTimerWithTimeInterval:target: selector:userInfo:repeats:
注意通过selector调用方法，传递参数时(userInfo)，需要通过[XXX userInfo]来接收。
eg：
timer = [NSTimer scheduledTimerWithTimeInterval:0.2 target:self selector:@selector(test:) userInfo:@3 repeats:NO];
- (void)test:(NSTimer*)number {
NSLog(@"%@",[number userInfo]);
}
2.同时按下两个按钮，限制只触发一个按钮事件。
[myButton setExclusiveTouch:YES];
参考文档：ios - objective c how to set exclusive touch for all UIbuttons on the whole app - Stack Overflow
3.xib布局时，frame的变化。
viewWillAppear:
Notifies the view controller that its view is about to be added to a view hierarchy.
viewDidAppear:
Notifies the view controller that its view was added to a view hierarchy.
As a result the frames of the subviews aren't yet set in theviewWillAppear:
The appropriate method tomodify your UI before the view is presented to the screenis:
viewDidLayoutSubviews
Notifies the view controller that its view just laid out its subviews.
参考文档：
objective c - Unable to set frame correctly before viewDidAppear - Stack Overflow
ios - UIViewController returns invalid frame? - Stack Overflow
4.xcode6创建的工程在ios7下运行，出现显示不全，上下两条黑边的问题，在iOS8能正常显示。
Xcode6中模版工程默认使用LaunchScreen.xib,如果要支持iOS7，还是得使用LaunchImage。
启用过程：在target的general设置中找到App Icons and Launch Images， 然后点击Use Asset Catalog，接下来按提示操作就可以了。
Configuring Your Xcode Project for Distribution
5.button设置image，设置image属性显示实际大小，backgroundImage会随着按钮的size改变。就好比一个view上有一个imageView，修改view的大小，imageview是不会变的。
6.打印fontNames
//打印所有fontName
NSArray*familyNames = [UIFontfamilyNames];
for(NSString*familyNameinfamilyNames )
{
NSArray*fontNames = [UIFontfontNamesForFamilyName:familyName];
for(NSString*fontNameinfontNames )
{
printf("\tFont: %s \n", [fontNameUTF8String] );
}
}
7.计算运行时间（kissGod）
#define TICK NSDate *startTime = [NSDate date]
#define TOCK NSLog(@"Time: %f", -[startTime timeIntervalSinceNow])
在想要查看执行时间的代码的地方进行这么处理
TICK
//do your work here
TOCK
8.icon 有黑边问题
提交方形图片，系统会自动圆角化处理。如果你的图片资源已经切好圆角，会因为不够精确（与系统不是100%一致）而产生黑色的毛边。