AwfulPostsView.m

//  AwfulPostsView.m
//
//  Copyright 2014 Awful Contributors. CC BY-NC-SA 3.0 US https://github.com/Awful/Awful.app

#import "AwfulPostsView.h"
#import "AwfulFrameworkCategories.h"

typedef enum : NSInteger {
    TopBarHidden,
    TopBarPartiallyVisible,
    TopBarVisible,
} TopBarState;

@interface AwfulPostsView () <UIScrollViewDelegate>

@property (strong, nonatomic) UIWebView *webView;

@property (assign, nonatomic) CGFloat exposedTopBarSlice;
@property (assign, nonatomic) CGPoint lastContentOffset;
@property (readonly, assign, nonatomic) TopBarState topBarState;
@property (assign, nonatomic) BOOL ignoreScrollViewDidScroll;
@property (assign, nonatomic) BOOL maintainTopBarState;
@property (assign, nonatomic) BOOL topBarAlwaysVisible;

@end

@implementation AwfulPostsView

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

- (id)initWithFrame:(CGRect)frame
{
    if ((self = [super initWithFrame:frame])) {
        _webView = [UIWebView awful_nativeFeelingWebView];
        _webView.backgroundColor = nil;
        [self addSubview:_webView];
        _webView.scrollView.delegate = self;
        
        _topBar = [AwfulPostsViewTopBar new];
        [self addSubview:_topBar];
        
        self.maintainTopBarState = YES;
        
        [self updateForVoiceOverAnimated:NO];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(voiceOverStatusDidChange:) name:UIAccessibilityVoiceOverStatusChanged object:nil];
    }
    return self;
}

- (void)layoutSubviews
{
    CGRect topBarFrame = self.topBar.bounds;
    topBarFrame.origin.y = self.exposedTopBarSlice - CGRectGetHeight(topBarFrame);
    topBarFrame.size.width = CGRectGetWidth(self.bounds);
    self.topBar.frame = topBarFrame;
    
    self.webView.frame = CGRectMake(0, CGRectGetMaxY(topBarFrame), CGRectGetWidth(self.bounds), CGRectGetHeight(self.bounds) - self.exposedTopBarSlice);
}

- (void)updateForVoiceOverAnimated:(BOOL)animated
{
    self.topBarAlwaysVisible = UIAccessibilityIsVoiceOverRunning();
    if (self.topBarAlwaysVisible) {
        self.exposedTopBarSlice = CGRectGetHeight(self.topBar.bounds);
        [UIView animateWithDuration:(animated ? 0.2 : 0) animations:^{
            [self layoutIfNeeded];
        }];
    }
}

- (void)voiceOverStatusDidChange:(NSNotification *)notification
{
    [self updateForVoiceOverAnimated:YES];
}

- (void)furtherExposeTopBarSlice:(CGFloat)delta
{
    CGFloat oldExposedSlice = self.exposedTopBarSlice;
    #define CLAMP(low, x, high) MIN(MAX(low, x), high)
    self.exposedTopBarSlice = CLAMP(0, self.exposedTopBarSlice + delta, CGRectGetHeight(self.topBar.bounds));
    CGFloat exposedSliceDelta = self.exposedTopBarSlice - oldExposedSlice;
    
    self.ignoreScrollViewDidScroll = YES;
    CGPoint contentOffset = self.webView.scrollView.contentOffset;
    contentOffset.y = MAX(contentOffset.y + exposedSliceDelta, 0);
    self.webView.scrollView.contentOffset = contentOffset;
    self.ignoreScrollViewDidScroll = NO;
}

- (TopBarState)topBarState
{
    if (self.exposedTopBarSlice == 0) {
        return TopBarHidden;
    } else if (self.exposedTopBarSlice >= CGRectGetHeight(self.topBar.bounds)) {
        return TopBarVisible;
    } else {
        return TopBarPartiallyVisible;
    }
}

- (void)setExposedTopBarSlice:(CGFloat)exposedTopBarSlice
{
    if (exposedTopBarSlice != _exposedTopBarSlice) {
        _exposedTopBarSlice = exposedTopBarSlice;
        [self setNeedsLayout];
    }
}

#pragma mark - UIScrollViewDelegate

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
    self.lastContentOffset = scrollView.contentOffset;
    self.maintainTopBarState = NO;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    if (self.ignoreScrollViewDidScroll || self.topBarAlwaysVisible) return;
    
    CGFloat scrollDistance = scrollView.contentOffset.y - self.lastContentOffset.y;
    if (scrollDistance == 0) return;
    
    switch (self.topBarState) {
        case TopBarHidden: {
            
            // Don't start showing a hidden topbar after bouncing.
            if (self.maintainTopBarState) break;
            
            // Only moving the content down can expose the topbar.
            if (scrollDistance < 0) {
                
                // Only start showing the topbar if we're scrolling past the bottom of the scrollview's contents. Otherwise we can briefly trap ourselves at the bottom, exposing some topbar causing the scrollview to bounce back.
                if (CGRectGetMaxY(scrollView.bounds) - scrollView.contentInset.bottom - scrollDistance <= scrollView.contentSize.height) {
                    [self furtherExposeTopBarSlice:-scrollDistance];
                }
            }
            break;
        }
            
        case TopBarPartiallyVisible: {
            [self furtherExposeTopBarSlice:-scrollDistance];
            break;
        }
            
        case TopBarVisible: {
            
            // Don't start hiding a visible topbar after bouncing.
            if (self.maintainTopBarState) break;
            
            // Only start hiding the topbar if we're scrolling past the top of the scrollview's contents. Otherwise we can briefly trap ourselves at the top, hiding some topbar causing the scrollview to bounce back.
            if (scrollView.contentOffset.y < 0) break;
            
            // Only moving the content up can hide the topbar.
            if (scrollDistance > 0) {
                [self furtherExposeTopBarSlice:-scrollDistance];
            }
            break;
        }
    }
    
    self.lastContentOffset = scrollView.contentOffset;
}

- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView
{
    self.maintainTopBarState = YES;
}

- (BOOL)scrollViewShouldScrollToTop:(UIScrollView *)scrollView
{
    self.maintainTopBarState = YES;
    return YES;
}

@end