MJRefresh源码浅析

最近项目需要添加刷新交互，想着参考一下别人的源码再封装，然后就找了MJRefresh，然后记录下。

原理：

在UIScrollView可滚动区域的顶部上方或底部下方加一个UIView，下拉时候监听偏移量，改变UIView的内容显示。
利用UIScrollView的contentInset属性，在上下拉刷新时提供了显示UIView的额外位置。

先看个例子：

tableView.mj_header = [MJRefreshNormalHeader headerWithRefreshingBlock:^{
    NSLog(@"下拉刷新");
}];

源码

首先创建分类使用runtime关联属性：

@implementation UIScrollView (MJRefresh)

static const char MJRefreshHeaderKey = '\0';
- (void)setMj_header:(MJRefreshHeader *)mj_header
{
    if (mj_header != self.mj_header) {
        // 删除旧的，添加新的
        [self.mj_header removeFromSuperview];
        
        if (mj_header) {
            [self insertSubview:mj_header atIndex:0];
        }
        // 存储新的
        objc_setAssociatedObject(self, &MJRefreshHeaderKey,
                                 mj_header, OBJC_ASSOCIATION_RETAIN);//关联属性
    }
}

- (MJRefreshHeader *)mj_header
{
    return objc_getAssociatedObject(self, &MJRefreshHeaderKey);//关联属性
}

然后初始化刷新控件：

@implementation MJRefreshHeader

+ (instancetype)headerWithRefreshingBlock:(MJRefreshComponentAction)refreshingBlock
{
    MJRefreshHeader *cmp = [[self alloc] init];
    cmp.refreshingBlock = refreshingBlock;
    return cmp;
}
+ (instancetype)headerWithRefreshingTarget:(id)target refreshingAction:(SEL)action
{
    MJRefreshHeader *cmp = [[self alloc] init];
    [cmp setRefreshingTarget:target refreshingAction:action];
    return cmp;
}

控件通过继承来拓展功能：

@interface MJRefreshNormalHeader : MJRefreshStateHeader

@interface MJRefreshStateHeader : MJRefreshHeader

@interface MJRefreshHeader : MJRefreshComponent

@interface MJRefreshComponent : UIView

下面是继承结构与作用：

核心主要看KVO操作，当被添加进UIScrollView时添加监听：

@implementation MJRefreshComponent

- (void)willMoveToSuperview:(UIView *)newSuperview
{
    [super willMoveToSuperview:newSuperview];
    
    // 如果不是UIScrollView，不做任何事情
    if (newSuperview && ![newSuperview isKindOfClass:[UIScrollView class]]) return;
    
    // 旧的父控件移除监听
    [self removeObservers];
    
    if (newSuperview) { // 新的父控件
        ...
        // 添加监听
        [self addObservers];
    }
}
/*
NSString *const MJRefreshKeyPathContentOffset = @"contentOffset";
NSString *const MJRefreshKeyPathContentSize = @"contentSize";
NSString *const MJRefreshKeyPathPanState = @"state";
*/
- (void)addObservers
{
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.scrollView addObserver:self forKeyPath:MJRefreshKeyPathContentOffset options:options context:nil];
    [self.scrollView addObserver:self forKeyPath:MJRefreshKeyPathContentSize options:options context:nil];
    self.pan = self.scrollView.panGestureRecognizer;
    [self.pan addObserver:self forKeyPath:MJRefreshKeyPathPanState options:options context:nil];
    
    [NSNotificationCenter.defaultCenter addObserver:self selector:@selector(i18nDidChange) name:MJRefreshDidChangeLanguageNotification object:MJRefreshConfig.defaultConfig];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    // 遇到这些情况就直接返回
    if (!self.userInteractionEnabled) return;
    
    // 这个就算看不见也需要处理
    if ([keyPath isEqualToString:MJRefreshKeyPathContentSize]) {
        [self scrollViewContentSizeDidChange:change];
    }
    
    // 看不见
    if (self.hidden) return;
    if ([keyPath isEqualToString:MJRefreshKeyPathContentOffset]) {
        [self scrollViewContentOffsetDidChange:change];
    } else if ([keyPath isEqualToString:MJRefreshKeyPathPanState]) {
        [self scrollViewPanStateDidChange:change];
    }
}
//监听调用的方法由子类重写
- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change{}
- (void)scrollViewContentSizeDidChange:(NSDictionary *)change{}
- (void)scrollViewPanStateDidChange:(NSDictionary *)change{}

当上下拉刷新时，监听contentOffset更新状态：

/** 刷新控件的状态 */
typedef NS_ENUM(NSInteger, MJRefreshState) {
    /** 普通闲置状态 */
    MJRefreshStateIdle = 1,
    /** 松开就可以进行刷新的状态 */
    MJRefreshStatePulling,
    /** 正在刷新中的状态 */
    MJRefreshStateRefreshing,
    /** 即将刷新的状态 */
    MJRefreshStateWillRefresh,
    /** 所有数据加载完毕，没有更多的数据了 */
    MJRefreshStateNoMoreData
};

@implementation MJRefreshHeader

- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change
{
    [super scrollViewContentOffsetDidChange:change];
    
    // 在刷新的refreshing状态
    if (self.state == MJRefreshStateRefreshing) {
        [self resetInset];
        return;
    }
    
    // 跳转到下一个控制器时，contentInset可能会变
    _scrollViewOriginalInset = self.scrollView.mj_inset;
    
    // 当前的contentOffset
    CGFloat offsetY = self.scrollView.mj_offsetY;
    // 头部控件刚好出现的offsetY
    CGFloat happenOffsetY = - self.scrollViewOriginalInset.top;
    
    // 如果是向上滚动到看不见头部控件，直接返回
    // >= -> >
    if (offsetY > happenOffsetY) return;
    
    // 普通 和 即将刷新 的临界点
    CGFloat normal2pullingOffsetY = happenOffsetY - self.mj_h;
    CGFloat pullingPercent = (happenOffsetY - offsetY) / self.mj_h;
    
    if (self.scrollView.isDragging) { // 如果正在拖拽
        self.pullingPercent = pullingPercent;
        if (self.state == MJRefreshStateIdle && offsetY < normal2pullingOffsetY) {//往下拉超过临界点
            // 转为即将刷新状态
            self.state = MJRefreshStatePulling;
        } else if (self.state == MJRefreshStatePulling && offsetY >= normal2pullingOffsetY) {//正在往下拉还未超过临界点
            // 转为普通状态
            self.state = MJRefreshStateIdle;
        }
    } else if (self.state == MJRefreshStatePulling) {// 即将刷新 && 手松开
        // 开始刷新
        [self beginRefreshing];
    } else if (pullingPercent < 1) {
        self.pullingPercent = pullingPercent;
    }
}

当往下拉时，状态为MJRefreshStateIdle，一直往下拉超过临界点，状态变更为MJRefreshStatePulling，松手后判断此前状态为MJRefreshStatePulling时，代表要进行刷新操作，状态变为MJRefreshStateRefreshing：

@implementation MJRefreshComponent

- (void)beginRefreshing
{
    ...
    // 只要正在刷新，就完全显示
    if (self.window) {
        self.state = MJRefreshStateRefreshing;
    } else {
        // 预防正在刷新中时，调用本方法使得header inset回置失败
        if (self.state != MJRefreshStateRefreshing) {
            self.state = MJRefreshStateWillRefresh;
            // 刷新(预防从另一个控制器回到这个控制器的情况，回来要重新刷新一下)
            [self setNeedsDisplay];
        }
    }
}

当状态变化时，根据状态做事情：

- (void)setState:(MJRefreshState)state
{
    MJRefreshCheckState
    
    // 根据状态做事情
    if (state == MJRefreshStateIdle) {
        if (oldState != MJRefreshStateRefreshing) return;
        
        [self headerEndingAction];
    } else if (state == MJRefreshStateRefreshing) {
        [self headerRefreshingAction];
    }
}

- (void)headerEndingAction {
    ...
    if (!self.isCollectionViewAnimationBug) {
        // 恢复inset和offset
        [UIView animateWithDuration:self.slowAnimationDuration animations:^{
            self.scrollView.mj_insetT += self.insetTDelta;//修改contentInset，隐藏刷新控件
            ...
        } completion:^(BOOL finished) { ... }];
        
        return;
    }
    ...
}

- (void)headerRefreshingAction {
    if (!self.isCollectionViewAnimationBug) {
        MJRefreshDispatchAsyncOnMainQueue({// 异步主线程执行，不强持有Self
            [UIView animateWithDuration:self.fastAnimationDuration animations:^{
                if (self.scrollView.panGestureRecognizer.state != UIGestureRecognizerStateCancelled) {
                    CGFloat top = self.scrollViewOriginalInset.top + self.mj_h;
                    // 增加滚动区域top
                    self.scrollView.mj_insetT = top;//修改contentInset，显示出刷新控件
                    // 设置滚动位置
                    CGPoint offset = self.scrollView.contentOffset;
                    offset.y = -top;
                    [self.scrollView setContentOffset:offset animated:NO];//滚动到最顶部
                }
            } completion:^(BOOL finished) {
                [self executeRefreshingCallback];//触发回调
            }];
        })
        return;
    }
    ...
}

@implementation UIScrollView (MJExtension)

- (void)setMj_insetT:(CGFloat)mj_insetT
{
    UIEdgeInsets inset = self.contentInset;
    inset.top = mj_insetT;
#ifdef __IPHONE_11_0
    if (respondsToAdjustedContentInset_) {
        inset.top -= (self.adjustedContentInset.top - self.contentInset.top);
    }
#endif
    self.contentInset = inset;
}

@implementation MJRefreshComponent

- (void)executeRefreshingCallback
{
    MJRefreshDispatchAsyncOnMainQueue({
        if (self.refreshingBlock) {
            self.refreshingBlock();
        }
        if ([self.refreshingTarget respondsToSelector:self.refreshingAction]) {
            MJRefreshMsgSend(MJRefreshMsgTarget(self.refreshingTarget), self.refreshingAction, self);
        }
        if (self.beginRefreshingCompletionBlock) {
            self.beginRefreshingCompletionBlock();
        }
    })
}

到目前为止完成了刷新的调用，当刷新结束后，把状态变回MJRefreshStateIdle就完成一个刷新操作了。

但是还有两个监听还没看到，监听contentSize用来更新footer的位置：

@implementation MJRefreshBackFooter

- (void)scrollViewContentSizeDidChange:(NSDictionary *)change
{
    [super scrollViewContentSizeDidChange:change];
    
    // 内容的高度
    CGFloat contentHeight = self.scrollView.mj_contentH + self.ignoredScrollViewContentInsetBottom;
    // 表格的高度
    CGFloat scrollHeight = self.scrollView.mj_h - self.scrollViewOriginalInset.top - self.scrollViewOriginalInset.bottom + self.ignoredScrollViewContentInsetBottom;
    // 设置位置和尺寸
    self.mj_y = MAX(contentHeight, scrollHeight);
}

@implementation UIView (MJExtension)

- (void)setMj_y:(CGFloat)mj_y
{
    CGRect frame = self.frame;
    frame.origin.y = mj_y;
    self.frame = frame;
}

监听scrollView.panGestureRecognizer的state只用来更新MJRefreshAutoFooter这一个类的状态：

@implementation MJRefreshAutoFooter

- (void)scrollViewPanStateDidChange:(NSDictionary *)change
{
    ...
    switch (panState) {
        // 手松开
        case UIGestureRecognizerStateEnded: 
            if (_scrollView.mj_insetT + _scrollView.mj_contentH <= _scrollView.mj_h) {  // 不够一个屏幕
                if (_scrollView.mj_offsetY >= - _scrollView.mj_insetT) { // 向上拽
                    self.triggerByDrag = YES;
                    [self beginRefreshing];
                }
            } else { // 超出一个屏幕
                if (_scrollView.mj_offsetY >= _scrollView.mj_contentH + _scrollView.mj_insetB - _scrollView.mj_h) {
                    self.triggerByDrag = YES;
                    [self beginRefreshing];
                }
            }
            break;
        case UIGestureRecognizerStateBegan: 
            [self resetTriggerTimes];
            break;
        default:
            break;
    }
}

简单来说，要封装刷新控件核心是监听UIScrollView的contentOffset进行更新界面与触发回调，以及监听UIScrollView的contentSize来更新控件位置。

MJRefresh源码浅析

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