MJRefresh研究一MJRefreshComponent

MJRefreshComponent

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一、主要功能

1、给子类留下准备工作的钩子方法，并设置基本属性

- (instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
        // 准备工作
        [self prepare];
        // 默认是普通状态
        self.state = MJRefreshStateIdle;
    }
    return self;
}

- (void)prepare
{
    // 基本属性
    self.autoresizingMask = UIViewAutoresizingFlexibleWidth;
    self.backgroundColor = [UIColor clearColor];
}
- (void)layoutSubviews
{
    [self placeSubviews];
    
    [super layoutSubviews];
}

- (void)placeSubviews{}

2、在属性的View节将加入到UIScrollView上时，
2.1设置UIScrollView可滚动的弹簧属性
2.2保存要添加上的scrollView为自己的成员变量，并且保存最开始的contentInset
3.3 删除旧的监听、重新添加新的监听，主要用来监听滚动的距离

- (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
        // 设置宽度
        self.mj_w = newSuperview.mj_w;
        // 设置位置
        self.mj_x = 0;
        
        // 记录UIScrollView
        _scrollView = (UIScrollView *)newSuperview;
        // 设置永远支持垂直弹簧效果
        _scrollView.alwaysBounceVertical = YES;
        // 记录UIScrollView最开始的contentInset
        _scrollViewOriginalInset = _scrollView.contentInset;
        
        // 添加监听
        [self addObservers];
    }
}

- (void)drawRect:(CGRect)rect
{
    [super drawRect:rect];
    
    if (self.state == MJRefreshStateWillRefresh) {
        // 预防view还没显示出来就调用了beginRefreshing
        self.state = MJRefreshStateRefreshing;
    }
}

3.3 删除旧的监听、重新添加新的监听，主要用来监听滚动的距离contentOffset、可滚动区域的变化contentSize以及添加在uiscrollview上的滑动手势的的状态监听state

#pragma mark - KVO监听
- (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];
}

- (void)removeObservers
{
    [self.superview removeObserver:self forKeyPath:MJRefreshKeyPathContentOffset];
    [self.superview removeObserver:self forKeyPath:MJRefreshKeyPathContentSize];;
    [self.pan removeObserver:self forKeyPath:MJRefreshKeyPathPanState];
    self.pan = nil;
}

- (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];
    }
}
// 在子类进行具体的实现操作
- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change{}
- (void)scrollViewContentSizeDidChange:(NSDictionary *)change{}
- (void)scrollViewPanStateDidChange:(NSDictionary *)change{}

总结：所用知识点，
1、View的生命周期方法UIView(UIViewHierarchy)
willMoveToSuperview：，这个方法保证能拿到自己的superView
2、UIScrollView的保持垂直或水平的弹簧效果，这样才能滚动看到拉取的效果
alwaysBounceVertical
alwaysBounceHorizontal
3、对scrollView的contentOffset、contentSize、scrollView.panGestureRecognizer的state进行监听
4、添加action-target及callback的执行操作