實現方法，自定義UISearchBar：

//  placeholder 和icon 和 間隙的整體寬度
// @property (nonatomic, assign) CGFloat placeholderWidth;

//  在layoutSubviews裡預設先居中
if (@available(iOS 11.0, *)) {
        [self setPositionAdjustment:UIOffsetMake((field.frame.size.width-self.placeholderWidth)/2, 0) forSearchBarIcon:UISearchBarIconSearch];
 } 

// 實現textfield的代理方法
 

// 開始編輯的時候重置為靠左
-(BOOL)textFieldShouldBeginEditing:(UITextField *)textField {
// 繼續傳遞代理方法
     if ([self.delegate respondsToSelector:@selector(searchBarShouldBeginEditing:)]) {
        [self.delegate searchBarShouldBeginEditing:self];
    }
    if (@available(iOS 11.0, *)) {
        [self setPositionAdjustment:UIOffsetZero forSearchBarIcon:UISearchBarIconSearch];
    }
    return
 
 YES;
}

// 結束編輯的時候設定為居中
-(BOOL)textFieldShouldEndEditing:(UITextField *)textField {
     if ([self.delegate respondsToSelector:@selector(searchBarShouldEndEditing:)]     {
        [self.delegate searchBarShouldEndEditing:self];
    }
    if (@available(iOS 11.0, *)) {
        [self setPositionAdjustment:UIOffsetMake((textField.frame
 
.size.width-self.placeholderWidth)/2, 0) forSearchBarIcon:UISearchBarIconSearch];
    }
    return YES;
}