MJRefreshBackFooter

重寫MJRefreshComponent處理監(jiān)聽的方法
根據(jù)滾動的offset算出scrollView的滾動距離

#pragma mark - 初始化
- (void)willMoveToSuperview:(UIView *)newSuperview
{
    [super willMoveToSuperview:newSuperview];
    
    [self scrollViewContentSizeDidChange:nil];
}

#pragma mark - 實(shí)現(xiàn)父類的方法
- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change
{
    [super scrollViewContentOffsetDidChange:change];
    
    // 如果正在刷新趣席，直接返回
    if (self.state == MJRefreshStateRefreshing) return;
    
    _scrollViewOriginalInset = self.scrollView.contentInset;
    
    // 當(dāng)前的contentOffset
    CGFloat currentOffsetY = self.scrollView.mj_offsetY;
    // 尾部控件剛好出現(xiàn)的offsetY
    CGFloat happenOffsetY = [self happenOffsetY];
    // 如果是向下滾動到看不見尾部控件，直接返回
    if (currentOffsetY <= happenOffsetY) return;
    
    CGFloat pullingPercent = (currentOffsetY - happenOffsetY) / self.mj_h;
    
    // 如果已全部加載，僅設(shè)置pullingPercent帆调，然后返回
    if (self.state == MJRefreshStateNoMoreData) {
        self.pullingPercent = pullingPercent;
        return;
    }
    
    if (self.scrollView.isDragging) {
        self.pullingPercent = pullingPercent;
        // 普通 和 即將刷新 的臨界點(diǎn)
        CGFloat normal2pullingOffsetY = happenOffsetY + self.mj_h;
        
        if (self.state == MJRefreshStateIdle && currentOffsetY > normal2pullingOffsetY) {
            // 轉(zhuǎn)為即將刷新狀態(tài)
            self.state = MJRefreshStatePulling;
        } else if (self.state == MJRefreshStatePulling && currentOffsetY <= normal2pullingOffsetY) {
            // 轉(zhuǎn)為普通狀態(tài)
            self.state = MJRefreshStateIdle;
        }
    } else if (self.state == MJRefreshStatePulling) {// 即將刷新 && 手松開
        // 開始刷新
        [self beginRefreshing];
    } else if (pullingPercent < 1) {
        self.pullingPercent = pullingPercent;
    }
}

處理設(shè)置footer的frame的y犯助，如果顯示內(nèi)容超過顯示區(qū)域的高度則y在顯示內(nèi)容下面，如果顯示內(nèi)容不足以顯示占滿顯示區(qū)域，footer的y在顯示區(qū)域的下面

- (void)scrollViewContentSizeDidChange:(NSDictionary *)change
{
    [super scrollViewContentSizeDidChange:change];
    
    // 顯示內(nèi)容的高度
    CGFloat contentHeight = self.scrollView.mj_contentH + self.ignoredScrollViewContentInsetBottom;
    // 表格的高度牲证，可顯示區(qū)域的高度
    CGFloat scrollHeight = self.scrollView.mj_h - self.scrollViewOriginalInset.top - self.scrollViewOriginalInset.bottom + self.ignoredScrollViewContentInsetBottom;
    // 設(shè)置位置和尺寸
// 如果顯示內(nèi)容超過顯示區(qū)域的高度則y在顯示內(nèi)容下面研叫，如果顯示內(nèi)容不足以顯示占滿顯示區(qū)域锤窑，footer的y在顯示區(qū)域的下面
    self.mj_y = MAX(contentHeight, scrollHeight);
}

根據(jù)設(shè)置的狀體進(jìn)行相應(yīng)的操作

- (void)setState:(MJRefreshState)state
{
    MJRefreshCheckState
    
    // 根據(jù)狀態(tài)來設(shè)置屬性
    if (state == MJRefreshStateNoMoreData || state == MJRefreshStateIdle) {
        // 刷新完畢
        if (MJRefreshStateRefreshing == oldState) {
            [UIView animateWithDuration:MJRefreshSlowAnimationDuration animations:^{
                self.scrollView.mj_insetB -= self.lastBottomDelta;
                
                // 自動調(diào)整透明度
                if (self.isAutomaticallyChangeAlpha) self.alpha = 0.0;
            } completion:^(BOOL finished) {
                self.pullingPercent = 0.0;
                
                if (self.endRefreshingCompletionBlock) {
                    self.endRefreshingCompletionBlock();
                }
            }];
        }
        
        CGFloat deltaH = [self heightForContentBreakView];
        // 剛刷新完畢
        if (MJRefreshStateRefreshing == oldState && deltaH > 0 && self.scrollView.mj_totalDataCount != self.lastRefreshCount) {
            self.scrollView.mj_offsetY = self.scrollView.mj_offsetY;
        }
    } else if (state == MJRefreshStateRefreshing) {
        // 記錄刷新前的數(shù)量
        self.lastRefreshCount = self.scrollView.mj_totalDataCount;
        
        [UIView animateWithDuration:MJRefreshFastAnimationDuration animations:^{
            CGFloat bottom = self.mj_h + self.scrollViewOriginalInset.bottom;
            CGFloat deltaH = [self heightForContentBreakView];
            if (deltaH < 0) { // 如果內(nèi)容高度小于view的高度
                bottom -= deltaH;
            }
            self.lastBottomDelta = bottom - self.scrollView.mj_insetB;
            self.scrollView.mj_insetB = bottom;
            self.scrollView.mj_offsetY = [self happenOffsetY] + self.mj_h;
        } completion:^(BOOL finished) {
            [self executeRefreshingCallback];
        }];
    }
}

總結(jié)：
1、對offset進(jìn)行判斷嚷炉，并且判斷isDragging
2渊啰、隨著contentSize變化調(diào)整footer的y
3、修改scrollView.contentInset
狀態(tài)間的變化申屹，對scrollView的inset進(jìn)行修改绘证，是的能看見footer或恢復(fù)成原狀，這個(gè)很特別