案例：

UISeachBar的Placeholder上移问题,如下图

修改placeholder上移

分析：

UISeachBar是系统控件未改动过, 所有要修改系统控件.
UISearchBar并未提供相关API来修改位置.
UITextField侧提供一下API供重写:

- (CGRect)placeholderRectForBounds:(CGRect)bounds;

- (void)drawPlaceholderInRect:(CGRect)rect;

解决：

建立UISeachBar子类,从而获取到UISeachBar的子视图UITextField,然后重写

- (CGRect)placeholderRectForBounds:(CGRect)bounds;

建立UITextField的分类,placeHolder偏移接口

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        
        SEL originalSelector = @selector(placeholderRectForBounds:);
        SEL swizzledSelector = @selector(swizzled_placeholderRectForBounds:);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL didAddMethod = class_addMethod(class, swizzledSelector,
                                            method_getImplementation(swizzledMethod),
                                            method_getTypeEncoding(swizzledMethod));
        if (!didAddMethod) {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

- (CGRect)swizzled_placeholderRectForBounds:(CGRect)rect
{
    CGRect newRect = rect;
    newRect.origin.y += self.placeHolderOffsetY;
    return [self swizzled_placeholderRectForBounds:newRect];
}

- (void)setPlaceHolderOffsetY:(float)offsetY {
    objc_setAssociatedObject(self, @selector(placeHolderOffsetY), @(offsetY), OBJC_ASSOCIATION_ASSIGN);
}

- (float)placeHolderOffsetY {
    NSNumber *typeNum = objc_getAssociatedObject(self, _cmd);
    return typeNum.floatValue;
}

在子类(例如LMSearchBar)设置placeHolderOffsetY.

- (void)layoutSubviews {
    
    [self adaptAppearance];
    
    [super layoutSubviews];
}

- (void)adaptAppearance {
    UITextField*searchField = [self valueForKey:@"_searchField"];
    if (searchField) {
        searchField.placeHolderOffsetY = 1;
    }
}

这里有个系统问题,iOS8系统会崩溃,要改成OBJC_ASSOCIATION_RETAIN_NONATOMIC,
验证了浮点型都有这个问题:

- (void)setPlaceHolderOffsetY:(float)offsetY {
    objc_setAssociatedObject(self, @selector(placeHolderOffsetY), @(offsetY),OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (float)placeHolderOffsetY {
    NSNumber *typeNum = objc_getAssociatedObject(self, _cmd);
    return typeNum.floatValue;
}

修改后如图:

修改placeholder上移