WheelPickerView.m

/*

Copyright (c) 2011 Dmitriy Kalinin

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

*/

#import "WheelPickerView.h"

@interface WheelPickerView ()
@property (nonatomic, retain) UIScrollView *scrollView;

@property (nonatomic, assign) int numberOfCells;
@property (nonatomic, assign) int cellHeight;

@property (nonatomic, assign) BOOL draggingEnded;
@property (nonatomic, assign) float lastYOffset;
@property (nonatomic, assign) double lastTime;

- (void)initialize;
- (void)populate;
@end


@implementation WheelPickerView

@synthesize 
    dataSource = dataSource_,
    scrollView = scrollView_, 
    numberOfCells = numberOfCells_, 
    cellHeight = cellHeight_,
    draggingEnded = draggingEnded_,
    lastYOffset = lastYOffset_,
    lastTime = lastTime_;

- (id)initWithCoder:(NSCoder *)coder {
    if ((self = [super initWithCoder:coder])) {
        [self initialize];
    }
    return self;
}

- (id)initWithFrame:(CGRect)frame {
    if ((self = [super initWithFrame:frame])){
        [self initialize];
    }
    return self;
}

- (void)initialize {
    self.scrollView = [[[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)] autorelease];
    self.scrollView.delegate = self;
    
    self.scrollView.showsHorizontalScrollIndicator = NO;
    self.scrollView.showsVerticalScrollIndicator = NO;
    self.scrollView.scrollsToTop = NO;
    
    [self addSubview:self.scrollView];
}

- (void)dealloc {
    self.scrollView = nil;
    [super dealloc];
}

- (void)setDataSource:(id<WheelPickerViewDataSource>)dataSource {
    dataSource_ = dataSource;
    [self populate];
}

- (void)populate {
    numberOfCells_ = [self.dataSource numberOfCellsForWheelPickerView:self];
    cellHeight_ = [self.dataSource heightForCellForWheelPickerView:self];
    
    int contentSizeHeight = (numberOfCells_-1)*cellHeight_ + self.frame.size.height;
    self.scrollView.contentSize = CGSizeMake(self.frame.size.width, contentSizeHeight);
    
    // first cell will be centered on the screen
    int firstCellYOffset = self.frame.size.height/2 - cellHeight_/2;
    
    for (int i=0; i<numberOfCells_; ++i) {
        UITableViewCell *cell = [self.dataSource wheelPickerView:self cellForRowAtIndex:i];
        cell.frame = CGRectMake(0, i*cellHeight_ + firstCellYOffset, self.frame.size.width, cellHeight_);
        [self.scrollView addSubview:cell];
    }
}

- (int)desiredYOffset:(int)offset {
    // if scrolled more than half way into a next cell, scroll the remaining distance;
    // otherwise snap back to previous cell
    int remainingDistanceUntilNextCell = offset % cellHeight_;
    
    if (remainingDistanceUntilNextCell > cellHeight_/2) {
        return offset + (cellHeight_ - remainingDistanceUntilNextCell);
    } else {
        return offset - remainingDistanceUntilNextCell;
    }
}

#pragma mark UIScrollViewDelegate
- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView {
    draggingEnded_ = YES;
    lastYOffset_ = 0;
    lastTime_ = 0;
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    draggingEnded_ = NO;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    if (draggingEnded_) {
        float currentYOffset = scrollView.contentOffset.y;
        double currentTime = CFAbsoluteTimeGetCurrent();
        
        if (lastTime_ > 0) {
            double velocity = (currentYOffset - lastYOffset_) / (currentTime - lastTime_);
            
            // snap to the nearest cell only if deceleration rate reached a certain value
            // and scroll view is not bouncing (we do not want to interfere with default bouncing)
            if (fabs(velocity) < 100 && currentYOffset > 0 && currentYOffset < (scrollView.contentSize.height - self.frame.size.height)) {
                int desiredOffset = [self desiredYOffset:currentYOffset];
                [scrollView setContentOffset:CGPointMake(0, desiredOffset) animated:YES];
            }
        }
        
        lastYOffset_ = currentYOffset;
        lastTime_ = currentTime;
    }
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    if (!decelerate) {
        int desiredOffset = [self desiredYOffset:scrollView.contentOffset.y];
        [scrollView setContentOffset:CGPointMake(0, desiredOffset) animated:YES];
    }
}

@end