ZGParallelView/Class/UITableView+ZGParallelView.m

//
//  UITableView+ZGParallelView.m
//  ZGParallelViewForTable
//
//  Created by Kyle Fang on 1/7/13.
//  Copyright (c) 2013 kylefang. All rights reserved.
//

#import "UITableView+ZGParallelView.h"
#import <objc/runtime.h>

@interface ZGScrollView : UIScrollView
@property (nonatomic, weak) UITableView *tableView;
@end



static char UITableViewZGParallelViewDisplayRadio;
static char UITableViewZGParallelViewViewHeight;
static char UITableViewZGParallelViewEmbededScrollView;
static char UITableViewZGParallelViewIsObserving;

@interface UITableView (ZGParallelViewPri)
@property (nonatomic, assign) CGFloat displayRadio;
@property (nonatomic, assign) CGFloat viewHeight;
@property (nonatomic, strong) ZGScrollView *embededScrollView;
@property (nonatomic, assign) BOOL isObserving;
@end


@implementation UITableView (ZGParallelViewPri)
@dynamic displayRadio, viewHeight, embededScrollView, isObserving;

- (void)setDisplayRadio:(CGFloat)displayRadio {
    [self willChangeValueForKey:@"displayRadio"];
    objc_setAssociatedObject(self, &UITableViewZGParallelViewDisplayRadio,
                             [NSNumber numberWithFloat:displayRadio],
                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [self didChangeValueForKey:@"displayRadio"];
}

- (CGFloat)displayRadio {
    NSNumber *number = objc_getAssociatedObject(self, &UITableViewZGParallelViewDisplayRadio);
    return [number floatValue];
}

- (void)setViewHeight:(CGFloat)viewHeight {
    [self willChangeValueForKey:@"viewHeight"];
    objc_setAssociatedObject(self, &UITableViewZGParallelViewViewHeight,
                             [NSNumber numberWithFloat:viewHeight],
                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [self didChangeValueForKey:@"viewHeight"];
}

- (CGFloat)viewHeight {
    NSNumber *number = objc_getAssociatedObject(self, &UITableViewZGParallelViewViewHeight);
    return [number floatValue];
}

- (void)setEmbededScrollView:(ZGScrollView *)embededScrollView {
    [self willChangeValueForKey:@"embededScrollView"];
    objc_setAssociatedObject(self, &UITableViewZGParallelViewEmbededScrollView,
                             embededScrollView,
                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [self didChangeValueForKey:@"embededScrollView"];
}

- (ZGScrollView *)embededScrollView {
    return objc_getAssociatedObject(self, &UITableViewZGParallelViewEmbededScrollView);
}

- (void)setIsObserving:(BOOL)isObserving {
    if (self.isObserving == YES && isObserving == NO) {
        [self removeObserver:self forKeyPath:@"contentOffset"];
    }
    
    [self willChangeValueForKey:@"isObserving"];
    objc_setAssociatedObject(self, &UITableViewZGParallelViewIsObserving,
                             [NSNumber numberWithBool:isObserving],
                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [self didChangeValueForKey:@"isObserving"];
}

- (BOOL)isObserving {
    NSNumber *number = objc_getAssociatedObject(self, &UITableViewZGParallelViewIsObserving);
    if (number == nil) {
        return NO;
    } else {
        return [number boolValue];
    }
}
@end



#define DEFAULT_DISPLAY_RADIO   0.5f
@implementation UITableView (ZGParallelView)
- (void)addParallelViewWithUIView:(UIView *)aViewToAdd {
    NSAssert(aViewToAdd != nil, @"aViewToAdd can not be nil");
    [self addParallelViewWithUIView:aViewToAdd withDisplayRadio:DEFAULT_DISPLAY_RADIO];
}

- (void)addParallelViewWithUIView:(UIView *)aViewToAdd withDisplayRadio:(CGFloat)aDisplayRadio {
    NSAssert(aViewToAdd != nil, @"aViewToAdd can not be nil");
    
    aViewToAdd.frame = CGRectOffset(aViewToAdd.frame, -aViewToAdd.frame.origin.x, -aViewToAdd.frame.origin.y);
    if (aDisplayRadio>1 && aDisplayRadio<0) {
        self.displayRadio = 1;
    } else {
        self.displayRadio = aDisplayRadio;
    }
    self.viewHeight = aViewToAdd.frame.size.height;
    self.embededScrollView = [[ZGScrollView alloc] initWithFrame:CGRectMake(0, 0, self.frame.size.width, self.viewHeight)];
    self.embededScrollView.tableView = self;
    [self.embededScrollView addSubview:aViewToAdd];
    aViewToAdd.frame = CGRectOffset(aViewToAdd.frame, 0, self.viewHeight*(1.f - self.displayRadio)/2.f);
    UIView *headView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, self.frame.size.width, self.viewHeight*self.displayRadio)];
    [headView addSubview:self.embededScrollView];
    self.embededScrollView.frame = CGRectOffset(self.embededScrollView.frame, 0, self.viewHeight*(self.displayRadio-1.f));
    self.tableHeaderView = headView;
    
    if (self.isObserving == NO) {
        [self addObserver:self forKeyPath:@"contentOffset" options:NSKeyValueObservingOptionNew context:nil];
        self.isObserving = YES;
    }
}

- (void)updateParallelView {
    CGFloat yOffset = self.contentOffset.y;
    if (yOffset<0 && yOffset>self.viewHeight*(self.displayRadio-1.f)) {
        self.embededScrollView.contentOffset = CGPointMake(0.f, -yOffset*0.5f);
    }
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if([keyPath isEqualToString:@"contentOffset"]) {
        [self scrollViewDidScroll:[[change valueForKey:NSKeyValueChangeNewKey] CGPointValue]];
    }
}

- (void)scrollViewDidScroll:(CGPoint)contentOffset {
    [self updateParallelView];
}
@end



@implementation ZGScrollView
@synthesize tableView;
- (void)dealloc {
    if ([self.tableView isObserving] == YES) {
        self.tableView.isObserving = NO;//!!Remove KVO Observer
    }
}
@end