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NSString有多个compare相关方法：

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- (NSComparisonResult)compare:(NSString *)string;
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask;
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask range:(NSRange)compareRange;
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask range:(NSRange)compareRange locale:(id)locale;

NSComparisonResult 是定义的一个枚举，定义如下：

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typedef NS_ENUM(NSInteger, NSComparisonResult) {NSOrderedAscending = -1L, NSOrderedSame, NSOrderedDescending};

其中，NSOrderedSame 表示比较的两个字符串完全一致, 同时，在这个枚举中，它的值是 0.

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注册监听键盘事件的通知

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[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardWillShow:)
                                             name:UIKeyboardWillShowNotification
                                           object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardShow:)
                                             name:UIKeyboardDidShowNotification
                                           object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardWillHide:)
                                             name:UIKeyboardWillHideNotification
                                           object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardHide:)
                                             name:UIKeyboardDidHideNotification
                                           object:nil];

在键盘将要出现和隐藏的回调中，加入动画

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- (void)keyboardWillShow:(NSNotification *)notif {
    if (self.hidden == YES) {
        return;
    }
    CGRect rect = [[notif.userInfo objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];
    CGFloat y = rect.origin.y;
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:0.25];
    NSArray *subviews = [self subviews];
    for (UIView *sub in subviews) {
        CGFloat maxY = CGRectGetMaxY(sub.frame);
        if (maxY > y - 2) {
            sub.center = CGPointMake(CGRectGetWidth(self.frame)/2.0, sub.center.y - maxY + y - 2);
        }
    }
    [UIView commitAnimations];
}
- (void)keyboardShow:(NSNotification *)notif {
    if (self.hidden == YES) {
        return;
    }
}
- (void)keyboardWillHide:(NSNotification *)notif {
    if (self.hidden == YES) {
        return;
    }
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:0.25];
    NSArray *subviews = [self subviews];
    for (UIView *sub in subviews) {
        if (sub.center.y < CGRectGetHeight(self.frame)/2.0) {
            sub.center = CGPointMake(CGRectGetWidth(self.frame)/2.0, CGRectGetHeight(self.frame)/2.0);
        }
    }
    [UIView commitAnimations];
}
- (void)keyboardHide:(NSNotification *)notif {
    if (self.hidden == YES) {
        return;
    }
}

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判断iOS设备是否是iPad

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#define IS_IPAD ([[UIDevice currentDevice] respondsToSelector:@selector(userInterfaceIdiom)] && [[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPad)

iOS中URL编码

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NSString* escapedURLString = [unescapedString
stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]

或

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NSString * encodedString = (NSString *)CFURLCreateStringByAddingPercentEscapes
(NULL, (CFStringRef)yourtext, NULL,
(CFStringRef)@”!*’();:@&=+$,/?%#[]“, kCFStringEncodingUTF8);

iOS中不定参数（可变参数）的方法

常见于NSArray初始化方法中，比如：

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@interface NSArray (NSArrayCreation)
+ (id)arrayWithObjects:(id)firstObj, ... NS_REQUIRES_NIL_TERMINATION;
- (id)initWithObjects:(id)firstObj, ... NS_REQUIRES_NIL_TERMINATION;
//...
@end

NS_REQUIRES_NIL_TERMINATION 是一个宏，用于编译时非nil结尾的检查。自定义不定参数的方法与C/C++一样，示例如下：

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- (id)initWithColumns: (NSString*)firstColumnName, ... {
    if (self = [self init]) {
        NSMutableArray* arrays = [NSMutableArray array];
        va_list argList;
        if (firstColumnName) {
            [arrays addObject:firstColumnName];
            va_start(argList, firstColumnName);
            id arg;
            while ((arg = va_arg(argList, id))) {
                [arrays addObject:arg];
            }
        }
        self.columnNames = [NSArray arrayWithArray:arrays];
    }
    return self;
}

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前言

iOS开发中NSDateFormatter是一个很常用的类，用于格式化NSDate对象，支持本地化的信息。与时间相关的功能还可能会用到NSDateComponents类和NSCalendar类等。本文主要列出NSDateFormatter常见用法。
NSDate对象包含两个部分，日期（Date）和时间（Time）。格式化的时间字符串主要也是针对日期和时间的。[以下代码中开启了ARC，所以没有release。]

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