下拉刷新是一种利用手势刷新用户界面的功能,虽然已经被Twitter申请为专利,但依然不能阻止广大的App开发者在自己的应用中加入该功能。苹果公司甚至在iOS6的sdk中加入了UIRefreshControl,从而实现了系统级的下拉刷新。但是UIRefreshControl是绑定在UITableViewController上的,所以灵活性不高。
如果在网上搜下拉刷新的实现,讨论最多的恐怕是EGOTableViewPullRefresh了,这是一个比较有历史的开源库了,github上最近一次提交都是两年前的事情了。EGOTableViewPullRefresh虽然很好的实现了下拉刷新功能,但是一些OC的新特性比如arc、block没有得到支持,导致加个下拉刷新要写不少代码。
于是乎EGOTableViewPullRefresh的替代者出来了,就是SVPullToRefresh。支持arc,支持block,而且简洁到只需一行就能实现下拉刷新和上拉加载。
SVPullToRefresh的下拉刷新用法相当简单:
1、将下拉刷新控件放在顶部
- [tableView addPullToRefreshWithActionHandler:^{
-
-
- }];
2、将下拉刷新控件放在底部
- [tableView addPullToRefreshWithActionHandler:^{
-
-
- } position:SVPullToRefreshPositionBottom];
3、程序自动调用下拉刷新
- [tableView triggerPullToRefresh];
4、临时性禁用下拉刷新
- tableView.showsPullToRefresh = NO;
SVPullToRefresh的UI支持自定义
下拉刷新对应的view名叫pullToRefreshView,有如下属性和方法修改它的显示。
- @property (nonatomic, strong) UIColor *arrowColor;
- @property (nonatomic, strong) UIColor *textColor;
- @property (nonatomic, readwrite) UIActivityIndicatorViewStyle activityIndicatorViewStyle;
-
- - (void)setTitle:(NSString *)title forState:(SVPullToRefreshState)state;
- - (void)setSubtitle:(NSString *)subtitle forState:(SVPullToRefreshState)state;
- - (void)setCustomView:(UIView *)view forState:(SVPullToRefreshState)state;
简单用法,比如下面一行代码就修改了下拉箭头的颜色。
- tableView.pullToRefreshView.arrowColor = [UIColor whiteColor];
上面介绍了下拉刷新的基本用法,下拉加载的用法也差不多,就不再介绍了,可以参考官方资料https://github.com/samvermette/SVPullToRefresh
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下面探讨一下SVPullToRefresh背后的原理,SVPullToRefresh能把接口设计的如此简单,肯定有一些过人之处,我们以后在设计控件的时候或许也能从中得到一些启发。
SVPullToRefresh的库其实也就两个类,分别对应下拉刷新和上拉加载,因为两者原理差不多,所以只需要看下拉刷新的实现即可。
- @interface UIScrollView (SVPullToRefresh)
-
- typedef NS_ENUM(NSUInteger, SVPullToRefreshPosition) {
- SVPullToRefreshPositionTop = 0,
- SVPullToRefreshPositionBottom,
- };
-
- - (void)addPullToRefreshWithActionHandler:(void (^)(void))actionHandler;
- - (void)addPullToRefreshWithActionHandler:(void (^)(void))actionHandler position:(SVPullToRefreshPosition)position;
- - (void)triggerPullToRefresh;
-
- @property (nonatomic, strong, readonly) SVPullToRefreshView *pullToRefreshView;
- @property (nonatomic, assign) BOOL showsPullToRefresh;
-
- @end
- @interface SVPullToRefreshView : UIView
-
- @property (nonatomic, strong) UIColor *arrowColor;
- @property (nonatomic, strong) UIColor *textColor;
- @property (nonatomic, strong, readonly) UILabel *titleLabel;
- @property (nonatomic, strong, readonly) UILabel *subtitleLabel;
- @property (nonatomic, strong, readwrite) UIColor *activityIndicatorViewColor NS_AVAILABLE_IOS(5_0);
- @property (nonatomic, readwrite) UIActivityIndicatorViewStyle activityIndicatorViewStyle;
-
- @property (nonatomic, readonly) SVPullToRefreshState state;
- @property (nonatomic, readonly) SVPullToRefreshPosition position;
-
- - (void)setTitle:(NSString *)title forState:(SVPullToRefreshState)state;
- - (void)setSubtitle:(NSString *)subtitle forState:(SVPullToRefreshState)state;
- - (void)setCustomView:(UIView *)view forState:(SVPullToRefreshState)state;
-
- - (void)startAnimating;
- - (void)stopAnimating;
-
-
- @property (nonatomic, strong, readonly) UILabel *dateLabel DEPRECATED_ATTRIBUTE;
- @property (nonatomic, strong) NSDate *lastUpdatedDate DEPRECATED_ATTRIBUTE;
- @property (nonatomic, strong) NSDateFormatter *dateFormatter DEPRECATED_ATTRIBUTE;
-
-
- - (void)triggerRefresh DEPRECATED_ATTRIBUTE;
-
- @end
SVPullToRefresh的实现用到了很多OC Runtime的特性。以前在用EGOTableViewPullRefresh时,我们需要
为当前的ViewController增加EGORefreshTableHeaderView的成员,还要实现相应protocol,这样ViewController就会显得臃肿。而SVPullToRefresh通过增加UIScrollView的Category,给需要下拉的scrollview增加了pullToRefreshView的属性,使用者不需要显式的创建它,与pullToRefreshView状态相关的逻辑也不用关心,只需要提供一个actionHandler来完成刷新结束后的一些操作。
但是category声明的property并不会自动@synthesize,所以需要手动实现getter和setter方法。通过runtime.h中objc_getAssociatedObject / objc_setAssociatedObject来访问和生成关联对象,从而模拟生成属性。如果要在不修改现有类的情况下增加属性,这是一个很好的办法。
- - (void)setPullToRefreshView:(SVPullToRefreshView *)pullToRefreshView {
- [self willChangeValueForKey:@"SVPullToRefreshView"];
- objc_setAssociatedObject(self, &UIScrollViewPullToRefreshView,
- pullToRefreshView,
- OBJC_ASSOCIATION_ASSIGN);
- [self didChangeValueForKey:@"SVPullToRefreshView"];
- }
-
- - (SVPullToRefreshView *)pullToRefreshView {
- return objc_getAssociatedObject(self, &UIScrollViewPullToRefreshView);
- }
除了动态增加属性,SVPullToRefresh还用到了KVO来观察自身属性的变化从而进行相应的UI调整(用EGOTableViewPullRefresh的话这些代码都由调用方实现,如
UIScrollViewDelegate
)。UIScrollView在滑动时contentOffset,contentSize,frame都是需要监测的对象。
- - (void)setShowsPullToRefresh:(BOOL)showsPullToRefresh {
- self.pullToRefreshView.hidden = !showsPullToRefresh;
-
- if(!showsPullToRefresh) {
- if (self.pullToRefreshView.isObserving) {
- [self removeObserver:self.pullToRefreshView forKeyPath:@"contentOffset"];
- [self removeObserver:self.pullToRefreshView forKeyPath:@"contentSize"];
- [self removeObserver:self.pullToRefreshView forKeyPath:@"frame"];
- [self.pullToRefreshView resetScrollViewContentInset];
- self.pullToRefreshView.isObserving = NO;
- }
- }
- else {
- if (!self.pullToRefreshView.isObserving) {
- [self addObserver:self.pullToRefreshView forKeyPath:@"contentOffset" options:NSKeyValueObservingOptionNew context:nil];
- [self addObserver:self.pullToRefreshView forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:nil];
- [self addObserver:self.pullToRefreshView forKeyPath:@"frame" options:NSKeyValueObservingOptionNew context:nil];
- self.pullToRefreshView.isObserving = YES;
-
- CGFloat yOrigin = 0;
- switch (self.pullToRefreshView.position) {
- case SVPullToRefreshPositionTop:
- yOrigin = -SVPullToRefreshViewHeight;
- break;
- case SVPullToRefreshPositionBottom:
- yOrigin = self.contentSize.height;
- break;
- }
-
- self.pullToRefreshView.frame = CGRectMake(0, yOrigin, self.bounds.size.width, SVPullToRefreshViewHeight);
- }
- }
- }
- - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
- if([keyPath isEqualToString:@"contentOffset"])
- [self scrollViewDidScroll:[[change valueForKey:NSKeyValueChangeNewKey] CGPointValue]];
- else if([keyPath isEqualToString:@"contentSize"]) {
- [self layoutSubviews];
-
- CGFloat yOrigin;
- switch (self.position) {
- case SVPullToRefreshPositionTop:
- yOrigin = -SVPullToRefreshViewHeight;
- break;
- case SVPullToRefreshPositionBottom:
- yOrigin = MAX(self.scrollView.contentSize.height, self.scrollView.bounds.size.height);
- break;
- }
- self.frame = CGRectMake(0, yOrigin, self.bounds.size.width, SVPullToRefreshViewHeight);
- }
- else if([keyPath isEqualToString:@"frame"])
- [self layoutSubviews];
-
- }
总结:SVPullToRefresh把所有的逻辑都由自身实现,对外只提供一个Block接口,方便调用者。原理上和EGOTableViewPullRefresh差不多,但通过Runtime特性和KVO的运用,把原来客户端实现的逻辑放到了自身来实现,最终呈现一个简单的接口。