一、為什么蘋果在iOS5添加了addChildViewController阁苞？

蘋果新的API增加了addChildViewController方法差购，并且希望我們?cè)谑褂胊ddSubview時(shí)亭罪，同時(shí)調(diào)用[self addChildViewController:child]方法將sub view對(duì)應(yīng)的viewController也加到當(dāng)前ViewController的管理中复哆。
對(duì)于那些當(dāng)前暫時(shí)不需要顯示的subview欣喧，只通過(guò)addChildViewController把subViewController加進(jìn)去；需要顯示時(shí)再調(diào)用transitionFromViewController方法梯找。將其添加進(jìn)入底層的ViewController中。
這樣做的好處：
1.無(wú)疑益涧，對(duì)頁(yè)面中的邏輯更加分明了锈锤。相應(yīng)的View對(duì)應(yīng)相應(yīng)的ViewController。
2.當(dāng)某個(gè)子View沒(méi)有顯示時(shí)闲询，將不會(huì)被Load久免，減少了內(nèi)存的使用。
3.當(dāng)內(nèi)存緊張時(shí)扭弧，沒(méi)有Load的View將被首先釋放阎姥，優(yōu)化了程序的內(nèi)存釋放機(jī)制。
在iOS5中鸽捻，ViewController中新添加了下面幾個(gè)方法：
* addChildViewController:
* removeFromParentViewController
* transitionFromViewController:toViewController:duration:options:animations:completion:
* willMoveToParentViewController:
* didMoveToParentViewController:

能想到的其它的一些使用場(chǎng)景或者說(shuō)是優(yōu)勢(shì)吧

• 不以navVC管理的呼巴，transition需要?jiǎng)赢嫷?（例如某些login模塊相關(guān)有注冊(cè)，忘記密碼御蒲，修改密碼等）
• 容器vc
• 左右聯(lián)動(dòng)衣赶，或上下聯(lián)動(dòng)
• 條件顯示，一個(gè)vc不是一直顯示的view厚满，且該view比較復(fù)雜府瞄，業(yè)務(wù)功能比較多

不管怎樣，開發(fā)過(guò)程中還是看具體使用場(chǎng)景碘箍，addChildViewController本質(zhì)優(yōu)點(diǎn)還是清晰易管理遵馆，但是濫用的話也會(huì)造成vc耦合度太高的問(wèn)題

二、該demo代碼實(shí)現(xiàn)的功能

1.addChildViewController使用
2.選中下劃線標(biāo)識(shí)（兩種實(shí)現(xiàn)方案）
3.切換標(biāo)簽下劃線動(dòng)畫
4.頭部標(biāo)簽headscrollview抽離封裝
5.childViewController里子view布局時(shí)機(jī)注意事項(xiàng)

三丰榴、addChildViewController使用（代碼有詳細(xì)的注釋和注意點(diǎn)）

主容器VC--MainViewController

//
//  MainViewController.m
//  測(cè)試addChildViewController
//  1货邓、選中下劃線標(biāo)識(shí)（兩種實(shí)現(xiàn)方案） 2、切換標(biāo)簽下劃線動(dòng)畫 3多艇、headscrollview抽離封裝
//  Created by caohx on 2021/1/13.
//

#import "MainViewController.h"
#import "FirstViewController.h"
#import "SecondViewController.h"
#import "ThirdViewController.h"
#import "HeadTitlesScrollView.h"

static CGFloat headHeight = 40;

@interface MainViewController ()
@property (nonatomic, strong) HeadTitlesScrollView *headScrollView;
@property (nonatomic, strong) UIViewController *currentVC;
@property (nonatomic, strong) FirstViewController *firstVC;
@property (nonatomic, strong) SecondViewController *secondVC;
@property (nonatomic, strong) ThirdViewController *thirdVC;
@end

@implementation MainViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor grayColor];
    self.automaticallyAdjustsScrollViewInsets = NO;
    
    [self setupSubViews];
    [self requestData];
}

/// 請(qǐng)求網(wǎng)絡(luò)數(shù)據(jù)
- (void)requestData {
    //nothing
}

/// 初始化并布局子view
- (void)setupSubViews {
    NSArray *headTitles = @[@"第一個(gè)",@"第二個(gè)",@"第三個(gè)",@"第一個(gè)",@"第二個(gè)",@"第三個(gè)"];
    //初始換headTitleview
    self.headScrollView = [[HeadTitlesScrollView alloc] initWithFrame:CGRectMake(0, 0, ScreenW, 40) titles:headTitles];
    self.headScrollView.backgroundColor = [UIColor blueColor];
    [self.view addSubview:self.headScrollView];
    //點(diǎn)擊頭部標(biāo)簽回調(diào)事件
    __weak typeof(self) weakSelf = self;
    self.headScrollView.blockClickHeadTitle = ^(NSInteger btnTag) {
        [weakSelf clickHeadBtn:btnTag];
    };
    
    //初始化子vc
    CGFloat originY = headHeight;
    CGFloat childVCHeight = ScreenH - headHeight;
    self.firstVC = [FirstViewController new];
    self.firstVC.view.frame = CGRectMake(0, originY, ScreenW, childVCHeight);
    self.secondVC = [SecondViewController new];
    self.secondVC.view.frame = CGRectMake(0, originY, ScreenW-100, childVCHeight-100);
    self.thirdVC = [ThirdViewController new];
    self.thirdVC.view.frame = CGRectMake(0, originY, ScreenW, childVCHeight);
    
    //默認(rèn)添加第一個(gè)子vc
    [self.view addSubview:self.firstVC.view]; //這個(gè)不能刪除
    [self addChildViewController:self.firstVC];
    self.currentVC = self.firstVC;
}


/// 處理點(diǎn)擊某個(gè)頭部標(biāo)簽逻恐，展示對(duì)應(yīng)的vc
/// @param btnTag 正在被點(diǎn)擊按鈕的tag
- (void)clickHeadBtn:(NSInteger)btnTag {
    //點(diǎn)擊的是同一個(gè)則返回
    if (((self.currentVC == self.firstVC) && btnTag == 100) ||
        ((self.currentVC == self.secondVC) && btnTag == 101) ||
        ((self.currentVC == self.thirdVC) && btnTag == 102)) {
        return;
    }
    
    //改變子vc
    switch (btnTag) {
        case 100:
            //顯示第一個(gè)vc
            [self replaceOldVC:self.currentVC toNewVC:self.firstVC];
            break;
        case 101:
            //顯示第二個(gè)vc
            [self replaceOldVC:self.currentVC toNewVC:self.secondVC];
            break;
        case 102:
            //顯示第三個(gè)vc
            [self replaceOldVC:self.currentVC toNewVC:self.thirdVC];
            break;
        default:
            break;
    }
}

- (void)replaceOldVC:(UIViewController*)oldVC toNewVC:(UIViewController*)newVC {
    [self addChildViewController:newVC];
    [self transitionFromViewController:self.currentVC toViewController:newVC duration:1 options:UIViewAnimationOptionTransitionCrossDissolve animations:nil completion:^(BOOL finished) {
        if (finished) {
            //下面兩句刪除不會(huì)影響
            [self.view addSubview:newVC.view];
            [newVC didMoveToParentViewController:self];
            //移除舊的的vc
            [oldVC willMoveToParentViewController:nil];
            [oldVC removeFromParentViewController];
            self.currentVC = newVC;
        }else {
            self.currentVC = oldVC;
        }
    }];
}

@end

抽離封裝的HeadTitlesScrollView

#import "HeadTitlesScrollView.h"

//以下都可以暴露給外面，動(dòng)態(tài)設(shè)置
static CGFloat headScrollViewHeight = 40;
static CGFloat headScrollViewBtnWidth = 100;
static CGFloat lineWidth = 50.f;
//按鈕的樣式：顏色，字體复隆，背景色等
//底部線的樣式：顏色拨匆，高度，寬度等

@interface HeadTitlesScrollView () <UIScrollViewDelegate>
@property (nonatomic, strong) UIScrollView *mainScrollView;
@property (nonatomic, strong) UILabel *bLine;
@property (nonatomic, strong) NSArray *arrTitles;
@property (nonatomic, strong) NSMutableArray *mutArrBtns;
@property (nonatomic, strong) NSMutableArray *mutArrBottomLines;
@end

@implementation HeadTitlesScrollView

#pragma mark- 初始化
- (instancetype)initWithFrame:(CGRect)frame titles:(NSArray*)titles {
    self = [super initWithFrame:frame];
    if (self) {
        self.backgroundColor = [UIColor whiteColor];
        self.arrTitles = titles.copy;
        self.mutArrBtns = [NSMutableArray array];
        self.mutArrBottomLines = [NSMutableArray array];
        headScrollViewHeight = self.frame.size.height;
        [self setupSubViews];
    }
    
    return self;
}

- (instancetype)initWithCoder:(NSCoder *)coder {
    NSAssert(NO, @"請(qǐng)使用initWithFrame:titles:初始化");
    return [self initWithFrame:CGRectZero titles:@[]];
}

- (instancetype)initWithFrame:(CGRect)frame {
    return [self initWithFrame:frame titles:@[]];
}

#pragma mark- 設(shè)置并布局子view
- (void)setupSubViews {
    //主scrollview
    self.mainScrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, ScreenW, headScrollViewHeight)];
    self.mainScrollView.backgroundColor = [UIColor blueColor];
    CGFloat contentSizeWidth = self.arrTitles.count*headScrollViewBtnWidth;
    self.mainScrollView.contentSize = CGSizeMake(contentSizeWidth, 0);
//    self.mainScrollView.pagingEnabled = YES;
    self.mainScrollView.scrollEnabled = YES;
    self.mainScrollView.bounces = NO;
    self.mainScrollView.showsHorizontalScrollIndicator = NO;
    [self addSubview:self.mainScrollView];
    
    //頭部scrollview添加button 和 button初始化
    for (int i = 0; i<self.arrTitles.count; i++) {
        UIButton *btn = [UIButton buttonWithType:UIButtonTypeSystem];
        btn.frame = CGRectMake(i*headScrollViewBtnWidth, 0, headScrollViewBtnWidth, headScrollViewHeight);
        [btn setTitle:self.arrTitles[i] forState:UIControlStateNormal];
        [btn setTitleColor:[UIColor purpleColor] forState:UIControlStateNormal];
        btn.titleLabel.textAlignment = NSTextAlignmentCenter;
        btn.tag = 100+i;
        [self.mainScrollView addSubview:btn];
        [btn addTarget:self action:@selector(clickHeadBtn:) forControlEvents:UIControlEventTouchUpInside];
        
//        //選中標(biāo)示線方案一:每一個(gè)btn對(duì)應(yīng)一個(gè)下劃線挽拂，通過(guò)hidden來(lái)控制
//        UILabel *bottomLine = [UILabel new];
//        bottomLine.backgroundColor = [UIColor blackColor];
//        CGFloat lineOriginY = headScrollViewBtnWidth*i + (headScrollViewBtnWidth - lineWidth)/2.0;
//        bottomLine.frame = CGRectMake(lineOriginY, headScrollViewHeight-2, lineWidth, 2);
//        [self.mainScrollView addSubview:bottomLine];
//        //默認(rèn)第一個(gè)選中惭每，顯示下劃線標(biāo)識(shí)線
//        bottomLine.hidden = !(i == 0);
//        [self.mutArrBottomLines addObject:bottomLine];
    }
    
    //選中標(biāo)示線方案二：一個(gè)line 通過(guò)改變offset.x來(lái)改變選中標(biāo)識(shí)
    UILabel *bottomLine = [UILabel new];
    bottomLine.backgroundColor = [UIColor blackColor];
    CGFloat lineOriginX = headScrollViewBtnWidth*0 + (headScrollViewBtnWidth - lineWidth)/2.0;
    bottomLine.frame = CGRectMake(lineOriginX, headScrollViewHeight-2, lineWidth, 2);
    [self.mainScrollView addSubview:bottomLine];
    self.bLine = bottomLine;
}


/// 回調(diào)點(diǎn)擊標(biāo)簽事件
/// @param currClickedBtn 當(dāng)前被點(diǎn)擊的btn
- (void)clickHeadBtn:(UIButton*)currClickedBtn {
    NSInteger currIndex = currClickedBtn.tag -100;
//    //改變選中下劃線 + 方案一
//    [self.mutArrBottomLines enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
//        UILabel *currLine = obj;
//        currLine.hidden = !(idx == currIndex);
//    }];
    
    //改變選中下劃線 + 方案二
    CGFloat lineOriginX = headScrollViewBtnWidth*currIndex + (headScrollViewBtnWidth - lineWidth)/2.0;
    [UIView animateWithDuration:0.5 delay:0 options:UIViewAnimationOptionCurveEaseInOut animations:^{
        self.bLine.frame = CGRectMake(lineOriginX, headScrollViewHeight-2, lineWidth, 2);
    } completion:nil];
    
    
    if (self.blockClickHeadTitle) {
        self.blockClickHeadTitle(currClickedBtn.tag);
    }
}

#pragma mark- UIScrollViewDelegate
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    //監(jiān)聽(tīng)contentoffset 改變標(biāo)識(shí)下劃線位置
}
@end

childViewController里子view布局時(shí)機(jī)注意

#import "SecondViewController.h"

@interface SecondViewController () <UITableViewDataSource,UITableViewDelegate>
@property (nonatomic, strong) UITableView *mainTb;
@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor purpleColor];
    
//    //不能在這布局；該vc的view此時(shí)還沒(méi)布局完成亏栈，取的會(huì)是主容器view的frame
//    [self.view addSubview:self.mainTb];
}

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];
    
    //子view正確布局時(shí)機(jī)
    [self.view addSubview:self.mainTb];
}

#pragma mark- UITableViewDatasource
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return 20;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"cellid"];
    cell.textLabel.text = @"test";
    return cell;
}

#pragma mark- getter
- (UITableView *)mainTb {
    if (!_mainTb) {
        _mainTb = [[UITableView alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height) style:UITableViewStylePlain];
        _mainTb.delegate = self;
        _mainTb.dataSource = self;
        _mainTb.rowHeight = 100;
        [_mainTb registerClass:[UITableViewCell class] forCellReuseIdentifier:@"cellid"];
    }
    return _mainTb;
}

- (void)dealloc
{
    NSLog(@"%s",__func__);
}

@end

效果圖

效果圖台腥，抱歉不會(huì)上傳mp4視頻

結(jié)束語(yǔ)

addChildViewController參考鏈接
本來(lái)想放demo地址的，但是覺(jué)得還是自己敲一遍比較好绒北，主要代碼都在上面黎侈，能力有限 有什么不對(duì)的地方，歡迎指正[Salute]