参考：ON IOS 7 UISEARCHBAR LEFT ALIGN PLACEHOLDER TEXT?

控件代码：https://gist.github.com/CoCrash/de56f5f0b70b7eb6d6e3

一个私有方法setCenterPlaceholder，在调用时修改这个布尔值可以实现修改位置的效果。

1 @interface NSCodingSearchBar : UISearchBar
2
3 // Default by the system is YES.
4 // https://github.com/nst/iOS-Runtime-Headers/blob/master/Frameworks/UIKit.framework/UISearchBar.h
5 @property (nonatomic, assign, setter = setHasCentredPlaceholder:) BOOL hasCentredPlaceholder;
6
7 @end

实现文件：

 1 @implementation NSCodingSearchBar
 2
 3
 4 // ------------------------------------------------------------------------------------------
 5 #pragma mark - Initializers
 6 // ------------------------------------------------------------------------------------------
 7 - (instancetype)initWithFrame:(CGRect)frame
 8 {
 9     if ((self = [super initWithFrame:frame]))
10     {
11         self.hasCentredPlaceholder = YES;
12     }
13
14     return self;
15 }
16
17
18 // ------------------------------------------------------------------------------------------
19 #pragma mark - Methods
20 // ------------------------------------------------------------------------------------------
21 - (void)setHasCentredPlaceholder:(BOOL)hasCentredPlaceholder
22 {
23     _hasCentredPlaceholder = hasCentredPlaceholder;
24
25     SEL centerSelector = NSSelectorFromString([NSString stringWithFormat:@"%@%@", @"setCenter", @"Placeholder:"]);
26     if ([self respondsToSelector:centerSelector])
27     {
28         NSMethodSignature *signature = [[UISearchBar class] instanceMethodSignatureForSelector:centerSelector];
29         NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
30         [invocation setTarget:self];
31         [invocation setSelector:centerSelector];
32         [invocation setArgument:&_hasCentredPlaceholder atIndex:2];
33         [invocation invoke];
34     }
35
36 }
37
38
39 @end