MJRefresh项目地址 https://github.com/CoderMJLee/MJRefresh
下载下来后我们打开项目可以看到下面的目录
MJ项目结构
- MJRefresh目录下就是下拉刷新的实现，其中
Base：是实现刷新的核心代码，里面实现了刷新的基础控件(Header/Footer)
Custom：是一些自定义的刷新控件，比如自动刷新、Gif动画刷新等
MJRefreshConst：常量定义
MJRefresh.bundle：多语言处理 - Classes目录下是MJ官方文档中示例的实现，我们阅读源码可以忽略它
虽然MJRefresh里面有很多的类，咋一看好像很复杂，其实它实现的核心只有一个类，其它的都是对它进行一层一层的封装，我们可以用官网的一张图来表示MJ的结构
MJ结构图.png
从上图可以看出，最基础的类就是MJRefreshComonent，我们就来看一下里面都做了什么
MJRefreshComonent继承自UIView，它就是刷新时展示的自定义View
- (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{}
- (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 = -_scrollView.mj_insetL;
// 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.mj_inset;
// 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;
}
}
代码注释原作者已经写的很详细了，总结一下就是
- 设置view的基本属性(自动布局)autoresizingMask、背景(backgroundColor)
- 重新定义了初始化相关的接口
- (void)prepare和- (void)layoutSubviews它们分别是初始化函数和开始加载UI的函数，子类继承时需要实现它们 - 弱引用父视图，并设置对父视图的监听(这里有个细节是
- (void)willMoveToSuperview:(UIView *)newSuperview函数在view添加和移除时都会调用，所以只要调用该函数，就移除一次监听，然后再添加监听，这样就不会出现忘记移除监听而出现的Crash)
再来看一下监听
- (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];
}
}
- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change{}
- (void)scrollViewContentSizeDidChange:(NSDictionary *)change{}
- (void)scrollViewPanStateDidChange:(NSDictionary *)change{}
从上面可以看出一共有3个监听
- ContentOffset：监听scrollview的滑动情况
- ContentSize：监听scrollview的contentSize的大小变换
- PanGesture：监听scrollview上pan手势的状态变化
监听的方法都没有具体的实现，说明它是需要子类去实现，所以MJRefreshComonent是一个抽象类，直接使用它是没有意义的，我们需要创建子类来继承它，下面再来看一下继承它的子类
- MJRefreshHeader，下面是该类的核心函数
- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change
{
[super scrollViewContentOffsetDidChange:change];
// 在刷新的refreshing状态
if (self.state == MJRefreshStateRefreshing) {
// 暂时保留
if (self.window == nil) return;
// sectionheader停留解决
CGFloat insetT = - self.scrollView.mj_offsetY > _scrollViewOriginalInset.top ? - self.scrollView.mj_offsetY : _scrollViewOriginalInset.top;
insetT = insetT > self.mj_h + _scrollViewOriginalInset.top ? self.mj_h + _scrollViewOriginalInset.top : insetT;
self.scrollView.mj_insetT = insetT;
self.insetTDelta = _scrollViewOriginalInset.top - insetT;
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) {
// 转为即将刷新状态
self.state = MJRefreshStatePulling;
} else if (self.state == MJRefreshStatePulling && offsetY >= normal2pullingOffsetY) {
// 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;
}
}
- (void)setState:(MJRefreshState)state
{
MJRefreshCheckState
// 根据状态做事情
if (state == MJRefreshStateIdle) {
if (oldState != MJRefreshStateRefreshing) return;
// 保存刷新时间
[[NSUserDefaults standardUserDefaults] setObject:[NSDate date] forKey:self.lastUpdatedTimeKey];
[[NSUserDefaults standardUserDefaults] synchronize];
// 恢复inset和offset
[UIView animateWithDuration:MJRefreshSlowAnimationDuration animations:^{
self.scrollView.mj_insetT += self.insetTDelta;
// 自动调整透明度
if (self.isAutomaticallyChangeAlpha) self.alpha = 0.0;
} completion:^(BOOL finished) {
self.pullingPercent = 0.0;
if (self.endRefreshingCompletionBlock) {
self.endRefreshingCompletionBlock();
}
}];
} else if (state == MJRefreshStateRefreshing) {
dispatch_async(dispatch_get_main_queue(), ^{
[UIView animateWithDuration:MJRefreshFastAnimationDuration animations:^{
CGFloat top = self.scrollViewOriginalInset.top + self.mj_h;
// 增加滚动区域top
self.scrollView.mj_insetT = top;
// 设置滚动位置
CGPoint offset = self.scrollView.contentOffset;
offset.y = -top;
[self.scrollView setContentOffset:offset animated:NO];
} completion:^(BOOL finished) {
[self executeRefreshingCallback];
}];
});
}
}
MJRefreshHeader通过继承实现父类- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change函数，在该函数中做了一件事就是，当scrollView滑动时，判断当前scrollView应该处于什么状态，然后再通过- (void)setState:(MJRefreshState)state函数来更新UI，这样一个简单的下拉刷新就实现了
- MJRefreshAutoFooter、MJRefreshBackFooter(auto和back两个模式区别是，一个自适应尾部刷新控件位置，一个刷新控件位置始终在底部)
auto的核心函数
- (void)scrollViewContentSizeDidChange:(NSDictionary *)change
{
[super scrollViewContentSizeDidChange:change];
// 设置位置
self.mj_y = self.scrollView.mj_contentH;
}
- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change
{
[super scrollViewContentOffsetDidChange:change];
if (self.state != MJRefreshStateIdle || !self.automaticallyRefresh || self.mj_y == 0) return;
if (_scrollView.mj_insetT + _scrollView.mj_contentH > _scrollView.mj_h) { // 内容超过一个屏幕
// 这里的_scrollView.mj_contentH替换掉self.mj_y更为合理
if (_scrollView.mj_offsetY >= _scrollView.mj_contentH - _scrollView.mj_h + self.mj_h * self.triggerAutomaticallyRefreshPercent + _scrollView.mj_insetB - self.mj_h) {
// 防止手松开时连续调用
CGPoint old = [change[@"old"] CGPointValue];
CGPoint new = [change[@"new"] CGPointValue];
if (new.y <= old.y) return;
// 当底部刷新控件完全出现时，才刷新
[self beginRefreshing];
}
}
}
- (void)scrollViewPanStateDidChange:(NSDictionary *)change
{
[super scrollViewPanStateDidChange:change];
if (self.state != MJRefreshStateIdle) return;
UIGestureRecognizerState panState = _scrollView.panGestureRecognizer.state;
if (panState == UIGestureRecognizerStateEnded) {// 手松开
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 beginRefreshing];
}
} else { // 超出一个屏幕
if (_scrollView.mj_offsetY >= _scrollView.mj_contentH + _scrollView.mj_insetB - _scrollView.mj_h) {
[self beginRefreshing];
}
}
} else if (panState == UIGestureRecognizerStateBegan) {
self.oneNewPan = YES;
}
}
- (void)setState:(MJRefreshState)state
{
MJRefreshCheckState
if (state == MJRefreshStateRefreshing) {
[self executeRefreshingCallback];
} else if (state == MJRefreshStateNoMoreData || state == MJRefreshStateIdle) {
if (MJRefreshStateRefreshing == oldState) {
if (self.endRefreshingCompletionBlock) {
self.endRefreshingCompletionBlock();
}
}
}
}
back的核心函数
- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change
{
[super scrollViewContentOffsetDidChange:change];
// 如果正在刷新，直接返回
if (self.state == MJRefreshStateRefreshing) return;
_scrollViewOriginalInset = self.scrollView.mj_inset;
// 当前的contentOffset
CGFloat currentOffsetY = self.scrollView.mj_offsetY;
// 尾部控件刚好出现的offsetY
CGFloat happenOffsetY = [self happenOffsetY];
// 如果是向下滚动到看不见尾部控件，直接返回
if (currentOffsetY <= happenOffsetY) return;
CGFloat pullingPercent = (currentOffsetY - happenOffsetY) / self.mj_h;
// 如果已全部加载，仅设置pullingPercent，然后返回
if (self.state == MJRefreshStateNoMoreData) {
self.pullingPercent = pullingPercent;
return;
}
if (self.scrollView.isDragging) {
self.pullingPercent = pullingPercent;
// 普通 和 即将刷新 的临界点
CGFloat normal2pullingOffsetY = happenOffsetY + self.mj_h;
if (self.state == MJRefreshStateIdle && currentOffsetY > normal2pullingOffsetY) {
// 转为即将刷新状态
self.state = MJRefreshStatePulling;
} else if (self.state == MJRefreshStatePulling && currentOffsetY <= normal2pullingOffsetY) {
// 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;
}
}
- (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);
}
- (void)setState:(MJRefreshState)state
{
MJRefreshCheckState
// 根据状态来设置属性
if (state == MJRefreshStateNoMoreData || state == MJRefreshStateIdle) {
// 刷新完毕
if (MJRefreshStateRefreshing == oldState) {
[UIView animateWithDuration:MJRefreshSlowAnimationDuration animations:^{
self.scrollView.mj_insetB -= self.lastBottomDelta;
// 自动调整透明度
if (self.isAutomaticallyChangeAlpha) self.alpha = 0.0;
} completion:^(BOOL finished) {
self.pullingPercent = 0.0;
if (self.endRefreshingCompletionBlock) {
self.endRefreshingCompletionBlock();
}
}];
}
CGFloat deltaH = [self heightForContentBreakView];
// 刚刷新完毕
if (MJRefreshStateRefreshing == oldState && deltaH > 0 && self.scrollView.mj_totalDataCount != self.lastRefreshCount) {
self.scrollView.mj_offsetY = self.scrollView.mj_offsetY;
}
} else if (state == MJRefreshStateRefreshing) {
// 记录刷新前的数量
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) { // 如果内容高度小于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];
}];
}
}
Footer相对于Header来说要相对复杂一些
MJRefreshAutoFooter通过- (void)scrollViewContentSizeDidChange:(NSDictionary *)change监听到contentSize改变来改变Footer的y值，以达到自适应位置，通过- (void)scrollViewContentOffsetDidChange:(NSDictionary *)change和- (void)scrollViewPanStateDidChange:(NSDictionary *)change监听来改变刷新状态，通过- (void)setState:(MJRefreshState)state来更新UI，MJRefreshBackFooter也一样，不过不同的是MJRefreshBackFooter的footer的y值最小是scrollView的高度
总结：我们参照MJ实现下拉刷新大概需要以下步骤
- 自定义一个View
- 将view加载到scrollView上，并在此时对scrollView的offset、contentSize、panGesture.state进行监听，在移除view时，需要移除监听
- 通过上面的监听来修改view的位置、动画等自定的内容(这一步也是自定义刷新的难点，然而像这种对UI的操作，项目的需求不同差别很大，我们去阅读对我们的参考价值也不大)
