为了实现上述的需求，最好写一个UISearchBar的子类就叫LSSearchBar吧

LSSearchBar.h如下：

#import <UIKit/UIKit.h>

@interface LSSearchBar : UISearchBar

@end

LSSearchBar.m如下：

#import "LSSearchBar.h"

@implementation LSSearchBar

- (void)layoutSubviews {

    [super layoutSubviews];

    //通过遍历self.subviews找到searchField
    UITextField *searchField;
    NSUInteger numViews = [self.subviews count];
    for(int i = 0; i < numViews; i++) {
        if([[self.subviews objectAtIndex:i] isKindOfClass:[UITextField class]]) {
            searchField = [self.subviews objectAtIndex:i];
        }
    }

    //如果上述方法找不到searchField，那就试试下面的方法吧

    if (searchField ==  nil) {
        NSArray *arraySub = [self subviews];
        UIView *viewSelf = [arraySub objectAtIndex:0];
        NSArray *arrayView = [viewSelf subviews];
        for(int i = 0; i < arrayView.count; i++) {
            if([[arrayView objectAtIndex:i] isKindOfClass:[UITextField class]]) {
                searchField = [arrayView objectAtIndex:i];
            }
        }
    }


    if(!(searchField == nil)) {
        //设置颜色
        searchField.textColor = [UIColor whiteColor];

        //设置背景颜色
        [searchField setBackground: [UIImage imageNamed:@"searchbar"] ];
        [searchField setBorderStyle:UITextBorderStyleNone];

        //设置placeholder的颜色
        [searchField setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];

        //设置searchField上的照片
        UIImage *image = [UIImage imageNamed:@"search"];
        UIImageView *iView = [[UIImageView alloc] initWithImage:image];
        iView.frame = CGRectMake(0, 0, 15, 15);
        searchField.leftView = iView;
    }

}

@end