JASidePanelController.m

/*
 Copyright (c) 2012 Jesse Andersen. All rights reserved.
 
 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.
 
 If you happen to meet one of the copyright holders in a bar you are obligated
 to buy them one pint of beer.
 
 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 <QuartzCore/QuartzCore.h>
#import "JASidePanelController.h"

static char ja_kvoContext;

@interface JASidePanelController() {
    CGRect _centerPanelRestingFrame;		
    CGPoint _locationBeforePan;
}

@property (nonatomic, readwrite) JASidePanelState state;
@property (nonatomic, weak) UIViewController *visiblePanel;
@property (nonatomic, strong) UIView *tapView;

// panel containers
@property (nonatomic, strong) UIView *leftPanelContainer;
@property (nonatomic, strong) UIView *rightPanelContainer;
@property (nonatomic, strong) UIView *centerPanelContainer;

@end

@implementation JASidePanelController

@synthesize leftPanelContainer = _leftPanelContainer;
@synthesize rightPanelContainer = _rightPanelContainer;
@synthesize centerPanelContainer = _centerPanelContainer;
@synthesize tapView = _tapView;
@synthesize style = _style;
@synthesize state = _state;
@synthesize leftPanel = _leftPanel;
@synthesize centerPanel = _centerPanel;
@synthesize rightPanel = _rightPanel;
@synthesize leftGapPercentage = _leftGapPercentage;
@synthesize leftFixedWidth = _leftFixedWidth;
@synthesize rightGapPercentage = _rightGapPercentage;
@synthesize rightFixedWidth = _rightFixedWidth;
@synthesize minimumMovePercentage = _minimumMovePercentage;
@synthesize maximumAnimationDuration = _maximumAnimationDuration;
@synthesize bounceDuration = _bounceDuration;
@synthesize bouncePercentage = _bouncePercentage;
@synthesize panningLimitedToTopViewController = _panningLimitedToTopViewController;
@synthesize recognizesPanGesture = _recognizesPanGesture;
@synthesize canUnloadRightPanel = _canUnloadRightPanel;
@synthesize canUnloadLeftPanel = _canUnloadLeftPanel;
@synthesize shouldResizeLeftPanel = _shouldResizeLeftPanel;
@synthesize shouldResizeRightPanel = _shouldResizeRightPanel;
@synthesize allowLeftOverpan = _allowLeftOverpan;
@synthesize allowRightOverpan = _allowRightOverpan;
@synthesize bounceOnSidePanelOpen = _bounceOnSidePanelOpen;
@synthesize bounceOnSidePanelClose = _bounceOnSidePanelClose;
@synthesize visiblePanel = _visiblePanel;
@synthesize shouldDelegateAutorotateToVisiblePanel = _shouldDelegateAutorotateToVisiblePanel;
@synthesize centerPanelHidden = _centerPanelHidden;
@synthesize allowLeftSwipe = _allowLeftSwipe;
@synthesize allowRightSwipe = _allowRightSwipe;

#pragma mark - Icon

+ (UIImage *)defaultImage {
	static UIImage *defaultImage = nil;
	static dispatch_once_t onceToken;
	dispatch_once(&onceToken, ^{
		UIGraphicsBeginImageContextWithOptions(CGSizeMake(20.f, 13.f), NO, 0.0f);
		
		[[UIColor blackColor] setFill];
		[[UIBezierPath bezierPathWithRect:CGRectMake(0, 0, 20, 1)] fill];
		[[UIBezierPath bezierPathWithRect:CGRectMake(0, 5, 20, 1)] fill];
		[[UIBezierPath bezierPathWithRect:CGRectMake(0, 10, 20, 1)] fill];
		
		[[UIColor whiteColor] setFill];
		[[UIBezierPath bezierPathWithRect:CGRectMake(0, 1, 20, 2)] fill];
		[[UIBezierPath bezierPathWithRect:CGRectMake(0, 6,  20, 2)] fill];
		[[UIBezierPath bezierPathWithRect:CGRectMake(0, 11, 20, 2)] fill];   
		
		defaultImage = UIGraphicsGetImageFromCurrentImageContext();
		UIGraphicsEndImageContext();

	});
    return defaultImage;
}

#pragma mark - NSObject

- (void)dealloc {
    if (_centerPanel) {
        [_centerPanel removeObserver:self forKeyPath:@"view"];
        [_centerPanel removeObserver:self forKeyPath:@"viewControllers"];
    }
}

//Support creating from Storyboard
- (id)initWithCoder:(NSCoder *)aDecoder {
    if (self = [super initWithCoder:aDecoder]) {
        [self _baseInit];
    }
    return self;
}

- (id)init {
    if (self = [super init]) {
        [self _baseInit];
    }
    return self;
}

- (void)_baseInit {
    self.style = JASidePanelSingleActive;
    self.leftGapPercentage = 0.8f;
    self.rightGapPercentage = 0.8f;
    self.minimumMovePercentage = 0.15f;
    self.maximumAnimationDuration = 0.2f;
    self.bounceDuration = 0.1f;
    self.bouncePercentage = 0.075f;
    self.panningLimitedToTopViewController = YES;
    self.recognizesPanGesture = YES;
    self.allowLeftOverpan = YES;
    self.allowRightOverpan = YES;
    self.bounceOnSidePanelOpen = YES;
    self.shouldDelegateAutorotateToVisiblePanel = YES;
    self.allowRightSwipe = YES;
    self.allowLeftSwipe = YES;
}

#pragma mark - UIViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
    
    self.centerPanelContainer = [[UIView alloc] initWithFrame:self.view.bounds];
    _centerPanelRestingFrame = self.centerPanelContainer.frame;
    _centerPanelHidden = NO;
    
    self.leftPanelContainer = [[UIView alloc] initWithFrame:self.view.bounds];
    self.leftPanelContainer.hidden = YES;
    
    self.rightPanelContainer = [[UIView alloc] initWithFrame:self.view.bounds];
    self.rightPanelContainer.hidden = YES;
    
    [self _configureContainers];
    
    [self.view addSubview:self.centerPanelContainer];
    [self.view addSubview:self.leftPanelContainer];
    [self.view addSubview:self.rightPanelContainer];
    
    self.state = JASidePanelCenterVisible;
    
    [self _swapCenter:nil with:_centerPanel];
    [self.view bringSubviewToFront:self.centerPanelContainer];
}

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    // ensure correct view dimensions
    [self _layoutSideContainers:NO duration:0.0f];
    [self _layoutSidePanels];
    self.centerPanelContainer.frame = [self _adjustCenterFrame];
    [self styleContainer:self.centerPanelContainer animate:NO duration:0.0f];
}

- (void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    [self _adjustCenterFrame]; //Account for possible rotation while view appearing
}

#if __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_6_0

- (void)viewDidUnload {
    [super viewDidUnload];
    self.tapView = nil;
    self.centerPanelContainer = nil;
    self.leftPanelContainer = nil;
    self.rightPanelContainer = nil;
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation {
    __strong UIViewController *visiblePanel = self.visiblePanel;

    if (self.shouldDelegateAutorotateToVisiblePanel) {
        return [visiblePanel shouldAutorotateToInterfaceOrientation:toInterfaceOrientation];
    } else {
        return YES;
    }
}

#else

- (BOOL)shouldAutorotate {
    __strong UIViewController *visiblePanel = self.visiblePanel;

    if (self.shouldDelegateAutorotateToVisiblePanel && [visiblePanel respondsToSelector:@selector(shouldAutorotate)]) {
        return [visiblePanel shouldAutorotate];
    } else {
        return YES;
    }
}


#endif

- (void)willAnimateRotationToInterfaceOrientation:(__unused UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration {
    self.centerPanelContainer.frame = [self _adjustCenterFrame];	
    [self _layoutSideContainers:YES duration:duration];
    [self _layoutSidePanels];
    [self styleContainer:self.centerPanelContainer animate:YES duration:duration];
    if (self.centerPanelHidden) {
        CGRect frame = self.centerPanelContainer.frame;
        frame.origin.x = self.state == JASidePanelLeftVisible ? self.centerPanelContainer.frame.size.width : -self.centerPanelContainer.frame.size.width;
        self.centerPanelContainer.frame = frame;
    }
}

#pragma mark - State

- (void)setState:(JASidePanelState)state {
    if (state != _state) {
        _state = state;
        switch (_state) {
            case JASidePanelCenterVisible: {
                self.visiblePanel = self.centerPanel;
                self.leftPanelContainer.userInteractionEnabled = NO;
                self.rightPanelContainer.userInteractionEnabled = NO;
                break;
			}
            case JASidePanelLeftVisible: {
                self.visiblePanel = self.leftPanel;
                self.leftPanelContainer.userInteractionEnabled = YES;
                break;
			}
            case JASidePanelRightVisible: {
                self.visiblePanel = self.rightPanel;
                self.rightPanelContainer.userInteractionEnabled = YES;
                break;
			}
        }
    }
}

#pragma mark - Style

- (void)setStyle:(JASidePanelStyle)style {
    if (style != _style) {
        _style = style;
        if (self.isViewLoaded) {
            [self _configureContainers];
            [self _layoutSideContainers:NO duration:0.0f];
        }
    }
}

- (void)styleContainer:(UIView *)container animate:(BOOL)animate duration:(NSTimeInterval)duration {
    UIBezierPath *shadowPath = [UIBezierPath bezierPathWithRoundedRect:container.bounds cornerRadius:0.0f];
    if (animate) {
        CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"shadowPath"];
        animation.fromValue = (id)container.layer.shadowPath;
        animation.toValue = (id)shadowPath.CGPath;
        animation.duration = duration;
        [container.layer addAnimation:animation forKey:@"shadowPath"];
    }
    container.layer.shadowPath = shadowPath.CGPath;	
    container.layer.shadowColor = [UIColor blackColor].CGColor;
    container.layer.shadowRadius = 10.0f;
    container.layer.shadowOpacity = 0.75f;
    container.clipsToBounds = NO;
}

- (void)stylePanel:(UIView *)panel {
    panel.layer.cornerRadius = 6.0f;
    panel.clipsToBounds = YES;
}

- (void)_configureContainers {
    self.leftPanelContainer.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleRightMargin;
    self.rightPanelContainer.autoresizingMask = UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleHeight;
    self.centerPanelContainer.frame =  self.view.bounds;
    self.centerPanelContainer.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
}

- (void)_layoutSideContainers:(BOOL)animate duration:(NSTimeInterval)duration {
    CGRect leftFrame = self.view.bounds;
    CGRect rightFrame = self.view.bounds;
    if (self.style == JASidePanelMultipleActive) {
        // left panel container
        leftFrame.size.width = self.leftVisibleWidth;
        leftFrame.origin.x = self.centerPanelContainer.frame.origin.x - leftFrame.size.width;
        
        // right panel container
        rightFrame.size.width = self.rightVisibleWidth;
        rightFrame.origin.x = self.centerPanelContainer.frame.origin.x + self.centerPanelContainer.frame.size.width;
    }
    self.leftPanelContainer.frame = leftFrame;
    self.rightPanelContainer.frame = rightFrame;
    [self styleContainer:self.leftPanelContainer animate:animate duration:duration];	
    [self styleContainer:self.rightPanelContainer animate:animate duration:duration];	
}

- (void)_layoutSidePanels {
    if (self.rightPanel.isViewLoaded) {
        CGRect frame = self.rightPanelContainer.bounds;
        if (self.shouldResizeRightPanel) {
            frame.origin.x = self.rightPanelContainer.bounds.size.width - self.rightVisibleWidth;
            frame.size.width = self.rightVisibleWidth;
        }
        self.rightPanel.view.frame = frame;
    }
    if (self.leftPanel.isViewLoaded) {
        CGRect frame = self.leftPanelContainer.bounds;
        if (self.shouldResizeLeftPanel) {
            frame.size.width = self.leftVisibleWidth;
        }
        self.leftPanel.view.frame = frame;
    }
}

#pragma mark - Panels

- (void)setCenterPanel:(UIViewController *)centerPanel {
    UIViewController *previous = _centerPanel;
    if (centerPanel != _centerPanel) {
        [_centerPanel removeObserver:self forKeyPath:@"view"];
        [_centerPanel removeObserver:self forKeyPath:@"viewControllers"];
        _centerPanel = centerPanel;
        [_centerPanel addObserver:self forKeyPath:@"viewControllers" options:0 context:&ja_kvoContext];
        [_centerPanel addObserver:self forKeyPath:@"view" options:NSKeyValueObservingOptionInitial context:&ja_kvoContext];
        if (self.state == JASidePanelCenterVisible) {
            self.visiblePanel = _centerPanel;
        }
    }
    if (self.isViewLoaded && self.state == JASidePanelCenterVisible) {
        [self _swapCenter:previous with:_centerPanel];
    } else if (self.isViewLoaded) {
        // update the state immediately to prevent user interaction on the side panels while animating
        JASidePanelState previousState = self.state;
        self.state = JASidePanelCenterVisible;
        [UIView animateWithDuration:0.2f animations:^{
            // first move the centerPanel offscreen
            CGFloat x = (previousState == JASidePanelLeftVisible) ? self.view.bounds.size.width : -self.view.bounds.size.width;
            _centerPanelRestingFrame.origin.x = x;
            self.centerPanelContainer.frame = _centerPanelRestingFrame;
        } completion:^(__unused BOOL finished) {
            [self _swapCenter:previous with:_centerPanel];
            [self _showCenterPanel:YES bounce:NO];
        }];
    }
}

- (void)_swapCenter:(UIViewController *)previous with:(UIViewController *)next {
    if (previous != next) {
        [previous willMoveToParentViewController:nil];
        [previous.view removeFromSuperview];
        [previous removeFromParentViewController];
        
        if (next) {
            [self _loadCenterPanel];
            [self addChildViewController:next];
            [self.centerPanelContainer addSubview:next.view];
            [next didMoveToParentViewController:self];
        }
    }
}

- (void)setLeftPanel:(UIViewController *)leftPanel {
    if (leftPanel != _leftPanel) {
        [_leftPanel willMoveToParentViewController:nil];
        [_leftPanel.view removeFromSuperview];
        [_leftPanel removeFromParentViewController];
        _leftPanel = leftPanel;
        if (_leftPanel) {
            [self addChildViewController:_leftPanel];
            [_leftPanel didMoveToParentViewController:self];
            [self _placeButtonForLeftPanel];
        }
        if (self.state == JASidePanelLeftVisible) {
            self.visiblePanel = _leftPanel;
        }
    }
}

- (void)setRightPanel:(UIViewController *)rightPanel {
    if (rightPanel != _rightPanel) {
        [_rightPanel willMoveToParentViewController:nil];
        [_rightPanel.view removeFromSuperview];
        [_rightPanel removeFromParentViewController];
        _rightPanel = rightPanel;
        if (_rightPanel) {
            [self addChildViewController:_rightPanel];
            [_rightPanel didMoveToParentViewController:self];
        }
        if (self.state == JASidePanelRightVisible) {
            self.visiblePanel = _rightPanel;
        }
    }
}

#pragma mark - Panel Buttons

- (void)_placeButtonForLeftPanel {
    if (self.leftPanel) {
        UIViewController *buttonController = self.centerPanel;
        if ([buttonController isKindOfClass:[UINavigationController class]]) {
            UINavigationController *nav = (UINavigationController *)buttonController;
            if ([nav.viewControllers count] > 0) {
                buttonController = [nav.viewControllers objectAtIndex:0];
            }
        }
        if (!buttonController.navigationItem.leftBarButtonItem) {   
            buttonController.navigationItem.leftBarButtonItem = [self leftButtonForCenterPanel];
        }
    }	
}

#pragma mark - Gesture Recognizer Delegate

- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer {
    if (gestureRecognizer.view == self.tapView) {
        return YES;
    } else if (self.panningLimitedToTopViewController && ![self _isOnTopLevelViewController:self.centerPanel]) {
        return NO;
    } else if ([gestureRecognizer isKindOfClass:[UIPanGestureRecognizer class]]) {
        UIPanGestureRecognizer *pan = (UIPanGestureRecognizer *)gestureRecognizer;
        CGPoint translate = [pan translationInView:self.centerPanelContainer];
        // determine if right swipe is allowed
        if (translate.x < 0 && ! self.allowRightSwipe) {
            return NO;
        }
        // determine if left swipe is allowed
        if (translate.x > 0 && ! self.allowLeftSwipe) {
            return NO;
        }
        BOOL possible = translate.x != 0 && ((fabsf(translate.y) / fabsf(translate.x)) < 1.0f);
        if (possible && ((translate.x > 0 && self.leftPanel) || (translate.x < 0 && self.rightPanel))) {
            return YES;
        }
    }
    return NO;
}

#pragma mark - Pan Gestures

- (void)_addPanGestureToView:(UIView *)view {
    UIPanGestureRecognizer *panGesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(_handlePan:)];
    panGesture.delegate = self;
    panGesture.maximumNumberOfTouches = 1;
    panGesture.minimumNumberOfTouches = 1;
    [view addGestureRecognizer:panGesture];	
}

- (void)_handlePan:(UIGestureRecognizer *)sender {
    if ([sender isKindOfClass:[UIPanGestureRecognizer class]]) {
        UIPanGestureRecognizer *pan = (UIPanGestureRecognizer *)sender;
        
        if (pan.state == UIGestureRecognizerStateBegan) {
            _locationBeforePan = self.centerPanelContainer.frame.origin;
        }
        
        CGPoint translate = [pan translationInView:self.centerPanelContainer];
        CGRect frame = _centerPanelRestingFrame;
        frame.origin.x += [self _correctMovement:translate.x];
        self.centerPanelContainer.frame = frame;
        
        // if center panel has focus, make sure correct side panel is revealed
        if (self.state == JASidePanelCenterVisible) {
            if (frame.origin.x > 0.0f) {
                [self _loadLeftPanel];
            } else if(frame.origin.x < 0.0f) {
                [self _loadRightPanel];
            }
        }
        
        if (sender.state == UIGestureRecognizerStateEnded) {
            CGFloat deltaX =  frame.origin.x - _locationBeforePan.x;			
            if ([self _validateThreshold:deltaX]) {
                [self _completePan:deltaX];
            } else {
                [self _undoPan];
            }
        } else if (sender.state == UIGestureRecognizerStateCancelled) {
            [self _undoPan];
        }
    }
}

- (void)_completePan:(CGFloat)deltaX {
    switch (self.state) {
        case JASidePanelCenterVisible: {
            if (deltaX > 0.0f) {
                [self _showLeftPanel:YES bounce:self.bounceOnSidePanelOpen];
            } else {
                [self _showRightPanel:YES bounce:self.bounceOnSidePanelOpen];
            }
            break;
		}
        case JASidePanelLeftVisible: {
            [self _showCenterPanel:YES bounce:self.bounceOnSidePanelClose];
            break;
		}
        case JASidePanelRightVisible: {
            [self _showCenterPanel:YES bounce:self.bounceOnSidePanelClose];
            break;
		}
    }
}

- (void)_undoPan {
    switch (self.state) {
        case JASidePanelCenterVisible: {
            [self _showCenterPanel:YES bounce:NO];
            break;
		}
        case JASidePanelLeftVisible: {
            [self _showLeftPanel:YES bounce:NO];
            break;
		}
        case JASidePanelRightVisible: {
            [self _showRightPanel:YES bounce:NO];
		}
    }
}

#pragma mark - Tap Gesture

- (void)setTapView:(UIView *)tapView {
    if (tapView != _tapView) {
        [_tapView removeFromSuperview];
        _tapView = tapView;
        if (_tapView) {
            _tapView.frame = self.centerPanelContainer.bounds;
            _tapView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
            [self _addTapGestureToView:_tapView];
            if (self.recognizesPanGesture) {
                [self _addPanGestureToView:_tapView];
            }
            [self.centerPanelContainer addSubview:_tapView];
        }
    }
}

- (void)_addTapGestureToView:(UIView *)view {
    UITapGestureRecognizer *tapGesture = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(_centerPanelTapped:)];
    [view addGestureRecognizer:tapGesture];	
}

- (void)_centerPanelTapped:(__unused UIGestureRecognizer *)gesture {
    [self _showCenterPanel:YES bounce:NO];
}

#pragma mark - Internal Methods

- (CGFloat)_correctMovement:(CGFloat)movement {
    CGFloat position = _centerPanelRestingFrame.origin.x + movement;
    if (self.state == JASidePanelCenterVisible) {
        if ((position > 0.0f && !self.leftPanel) || (position < 0.0f && !self.rightPanel)) {
            return 0.0f;
        } else if (!self.allowLeftOverpan && position > self.leftVisibleWidth) {
            return self.leftVisibleWidth;
        } else if (!self.allowRightOverpan && position < -self.rightVisibleWidth) {
            return -self.rightVisibleWidth;
        }
    } else if (self.state == JASidePanelRightVisible && !self.allowRightOverpan) {
        if (position < -self.rightVisibleWidth) {
            return 0.0f;
        } else if (position > self.rightPanelContainer.frame.origin.x) {
            return self.rightPanelContainer.frame.origin.x - _centerPanelRestingFrame.origin.x;
        }
    } else if (self.state == JASidePanelLeftVisible  && !self.allowLeftOverpan) {
        if (position > self.leftVisibleWidth) {
            return 0.0f;
        } else if (position < self.leftPanelContainer.frame.origin.x) {
            return self.leftPanelContainer.frame.origin.x - _centerPanelRestingFrame.origin.x;
        }
    }
    return movement;
}

- (BOOL)_validateThreshold:(CGFloat)movement {
    CGFloat minimum = floorf(self.view.bounds.size.width * self.minimumMovePercentage);
    switch (self.state) {
        case JASidePanelLeftVisible: {
            return movement <= -minimum;
		}
        case JASidePanelCenterVisible: {
            return fabsf(movement) >= minimum;
		}
        case JASidePanelRightVisible: {
            return movement >= minimum;
		}
    }
    return NO;
}

- (BOOL)_isOnTopLevelViewController:(UIViewController *)root {
    if ([root isKindOfClass:[UINavigationController class]]) {
        UINavigationController *nav = (UINavigationController *)root;
        return [nav.viewControllers count] == 1;
    } else if ([root isKindOfClass:[UITabBarController class]]) {
        UITabBarController *tab = (UITabBarController *)root;
        return [self _isOnTopLevelViewController:tab.selectedViewController];
    }
    return root != nil;
}

#pragma mark - Loading Panels

- (void)_loadCenterPanel {
    [self _placeButtonForLeftPanel];
    
    _centerPanel.view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
    _centerPanel.view.frame = self.centerPanelContainer.bounds;
    [self stylePanel:_centerPanel.view];
}

- (void)_loadLeftPanel {
    self.rightPanelContainer.hidden = YES;
    if (self.leftPanelContainer.hidden && self.leftPanel) {
        
        if (!_leftPanel.view.superview) {
            [self _layoutSidePanels];
            _leftPanel.view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
            [self stylePanel:_leftPanel.view];
            [self.leftPanelContainer addSubview:_leftPanel.view];
        }
        
        self.leftPanelContainer.hidden = NO;
    }
}

- (void)_loadRightPanel {
    self.leftPanelContainer.hidden = YES;
    if (self.rightPanelContainer.hidden && self.rightPanel) {
        
        if (!_rightPanel.view.superview) {
            [self _layoutSidePanels];
            _rightPanel.view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
            [self stylePanel:_rightPanel.view];
            [self.rightPanelContainer addSubview:_rightPanel.view];
        }
        
        self.rightPanelContainer.hidden = NO;
    }
}

- (void)_unloadPanels {
    if (self.canUnloadLeftPanel && self.leftPanel.isViewLoaded) {
        [self.leftPanel.view removeFromSuperview];
    }
    if (self.canUnloadRightPanel && self.rightPanel.isViewLoaded) {
        [self.rightPanel.view removeFromSuperview];
    }
}

#pragma mark - Animation

- (CGFloat)_calculatedDuration {
    CGFloat remaining = fabsf(self.centerPanelContainer.frame.origin.x - _centerPanelRestingFrame.origin.x);	
    CGFloat max = _locationBeforePan.x == _centerPanelRestingFrame.origin.x ? remaining : fabsf(_locationBeforePan.x - _centerPanelRestingFrame.origin.x);
    return max > 0.0f ? self.maximumAnimationDuration * (remaining / max) : self.maximumAnimationDuration;
}

- (void)_animateCenterPanel:(BOOL)shouldBounce completion:(void (^)(BOOL finished))completion {
    CGFloat bounceDistance = (_centerPanelRestingFrame.origin.x - self.centerPanelContainer.frame.origin.x) * self.bouncePercentage;
    
    // looks bad if we bounce when the center panel grows
    if (_centerPanelRestingFrame.size.width > self.centerPanelContainer.frame.size.width) {
        shouldBounce = NO;
    }
    
    CGFloat duration = [self _calculatedDuration];
    [UIView animateWithDuration:duration delay:0.0f options:UIViewAnimationOptionCurveLinear|UIViewAnimationOptionLayoutSubviews animations:^{
        self.centerPanelContainer.frame = _centerPanelRestingFrame;
        [self styleContainer:self.centerPanelContainer animate:YES duration:duration];
        if (self.style == JASidePanelMultipleActive) {
            [self _layoutSideContainers:NO duration:0.0f];
        }
    } completion:^(BOOL finished) {
        if (shouldBounce) {
            // make sure correct panel is displayed under the bounce
            if (self.state == JASidePanelCenterVisible) {
                if (bounceDistance > 0.0f) {
                    [self _loadLeftPanel];
                } else {
                    [self _loadRightPanel];
                }
            }
            // animate the bounce
            [UIView animateWithDuration:self.bounceDuration delay:0.0f options:UIViewAnimationOptionCurveEaseOut animations:^{
                CGRect bounceFrame = _centerPanelRestingFrame;
                bounceFrame.origin.x += bounceDistance;
                self.centerPanelContainer.frame = bounceFrame;
            } completion:^(__unused BOOL finished2) {
                [UIView animateWithDuration:self.bounceDuration delay:0.0f options:UIViewAnimationOptionCurveEaseIn animations:^{
                    self.centerPanelContainer.frame = _centerPanelRestingFrame;				
                } completion:completion];
            }];
        } else if (completion) {
            completion(finished);
        }
    }];
}

#pragma mark - Panel Sizing

- (CGRect)_adjustCenterFrame {
    CGRect frame = self.view.bounds;
    switch (self.state) {
        case JASidePanelCenterVisible: {
            frame.origin.x = 0.0f;
            if (self.style == JASidePanelMultipleActive) {
                frame.size.width = self.view.bounds.size.width;	
            }
            break;
		}
        case JASidePanelLeftVisible: {
            frame.origin.x = self.leftVisibleWidth;
            if (self.style == JASidePanelMultipleActive) {
                frame.size.width = self.view.bounds.size.width - self.leftVisibleWidth;
            }
            break;
		}
        case JASidePanelRightVisible: {
            frame.origin.x = -self.rightVisibleWidth;
            if (self.style == JASidePanelMultipleActive) {
                frame.origin.x = 0.0f;
                frame.size.width = self.view.bounds.size.width - self.rightVisibleWidth;
            }
            break;
		}
    }
    _centerPanelRestingFrame = frame;
    return _centerPanelRestingFrame;
}

- (CGFloat)leftVisibleWidth {
    if (self.centerPanelHidden && self.shouldResizeLeftPanel) {
        return self.view.bounds.size.width;
    } else {
        return self.leftFixedWidth ? self.leftFixedWidth : floorf(self.view.bounds.size.width * self.leftGapPercentage);
    }
}

- (CGFloat)rightVisibleWidth {
    if (self.centerPanelHidden && self.shouldResizeRightPanel) {
        return self.view.bounds.size.width;
    } else {
        return self.rightFixedWidth ? self.rightFixedWidth : floorf(self.view.bounds.size.width * self.rightGapPercentage);
    }    
}

#pragma mark - Showing Panels

- (void)_showLeftPanel:(BOOL)animated bounce:(BOOL)shouldBounce {
    self.state = JASidePanelLeftVisible;
    [self _loadLeftPanel];
    
    [self _adjustCenterFrame];
    
    if (animated) {
        [self _animateCenterPanel:shouldBounce completion:nil];
    } else {
        self.centerPanelContainer.frame = _centerPanelRestingFrame;	
        [self styleContainer:self.centerPanelContainer animate:NO duration:0.0f];
        if (self.style == JASidePanelMultipleActive) {
            [self _layoutSideContainers:NO duration:0.0f];
        }
    }
    
    if (self.style == JASidePanelSingleActive) {
        self.tapView = [[UIView alloc] init];
    }
    [self _toggleScrollsToTopForCenter:NO left:YES right:NO];
}

- (void)_showRightPanel:(BOOL)animated bounce:(BOOL)shouldBounce {
    self.state = JASidePanelRightVisible;
    [self _loadRightPanel];
    
    [self _adjustCenterFrame];
    
    if (animated) {
        [self _animateCenterPanel:shouldBounce completion:nil];
    } else {
        self.centerPanelContainer.frame = _centerPanelRestingFrame;	
        [self styleContainer:self.centerPanelContainer animate:NO duration:0.0f];
        if (self.style == JASidePanelMultipleActive) {
            [self _layoutSideContainers:NO duration:0.0f];
        }
    }
    
    if (self.style == JASidePanelSingleActive) {
        self.tapView = [[UIView alloc] init];
    }
    [self _toggleScrollsToTopForCenter:NO left:NO right:YES];
}

- (void)_showCenterPanel:(BOOL)animated bounce:(BOOL)shouldBounce {
    self.state = JASidePanelCenterVisible;
    
    [self _adjustCenterFrame];
    
    if (animated) {
        [self _animateCenterPanel:shouldBounce completion:^(__unused BOOL finished) {
            self.leftPanelContainer.hidden = YES;
            self.rightPanelContainer.hidden = YES;
            [self _unloadPanels];
        }];
    } else {
        self.centerPanelContainer.frame = _centerPanelRestingFrame;	
        [self styleContainer:self.centerPanelContainer animate:NO duration:0.0f];
        if (self.style == JASidePanelMultipleActive) {
            [self _layoutSideContainers:NO duration:0.0f];
        }
        self.leftPanelContainer.hidden = YES;
        self.rightPanelContainer.hidden = YES;
        [self _unloadPanels];
    }
    
    self.tapView = nil;
    [self _toggleScrollsToTopForCenter:YES left:NO right:NO];
}

- (void)_hideCenterPanel {
    self.centerPanelContainer.hidden = YES;
    if (self.centerPanel.isViewLoaded) {
        [self.centerPanel.view removeFromSuperview];
    }
}

- (void)_unhideCenterPanel {
    self.centerPanelContainer.hidden = NO;
    if (!self.centerPanel.view.superview) {
        self.centerPanel.view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
        self.centerPanel.view.frame = self.centerPanelContainer.bounds;
        [self stylePanel:self.centerPanel.view];
        [self.centerPanelContainer addSubview:self.centerPanel.view];
    }
}

- (void)_toggleScrollsToTopForCenter:(BOOL)center left:(BOOL)left right:(BOOL)right {
    // iPhone only supports 1 active UIScrollViewController at a time
    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone) {
        [self _toggleScrollsToTop:center forView:self.centerPanelContainer];
        [self _toggleScrollsToTop:left forView:self.leftPanelContainer];
        [self _toggleScrollsToTop:right forView:self.rightPanelContainer];
    }
}

- (BOOL)_toggleScrollsToTop:(BOOL)enabled forView:(UIView *)view {
    if ([view isKindOfClass:[UIScrollView class]]) {
        UIScrollView *scrollView = (UIScrollView *)view;
        scrollView.scrollsToTop = enabled;
        return YES;
    } else {
        for (UIView *subview in view.subviews) {
            if([self _toggleScrollsToTop:enabled forView:subview]) {
                return YES;
            }
        }
    }
    return NO;
}

#pragma mark - Key Value Observing

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(__unused NSDictionary *)change context:(void *)context {
    if (context == &ja_kvoContext) {
        if ([keyPath isEqualToString:@"view"]) {
            if (self.centerPanel.isViewLoaded && self.recognizesPanGesture) {
                [self _addPanGestureToView:self.centerPanel.view];
            }
        } else if ([keyPath isEqualToString:@"viewControllers"] && object == self.centerPanel) {
            // view controllers have changed, need to replace the button
            [self _placeButtonForLeftPanel];
        }
    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

#pragma mark - Public Methods

- (UIBarButtonItem *)leftButtonForCenterPanel {
    return [[UIBarButtonItem alloc] initWithImage:[[self class] defaultImage] style:UIBarButtonItemStylePlain target:self action:@selector(toggleLeftPanel:)];
}

- (void)showLeftPanel:(BOOL)animated {
    [self showLeftPanelAnimated:animated];
}

- (void)showRightPanel:(BOOL)animated {
    [self showRightPanelAnimated:animated];
}

- (void)showCenterPanel:(BOOL)animated {
    [self showCenterPanelAnimated:animated];
}

- (void)showLeftPanelAnimated:(BOOL)animated {
    [self _showLeftPanel:animated bounce:NO];
}

- (void)showRightPanelAnimated:(BOOL)animated {
    [self _showRightPanel:animated bounce:NO];
}

- (void)showCenterPanelAnimated:(BOOL)animated {
    // make sure center panel isn't hidden
    if (_centerPanelHidden) {
        _centerPanelHidden = NO;
        [self _unhideCenterPanel];
    }
    [self _showCenterPanel:animated bounce:NO];
}

- (void)toggleLeftPanel:(__unused id)sender {
    if (self.state == JASidePanelLeftVisible) {
        [self _showCenterPanel:YES bounce:NO];
    } else if (self.state == JASidePanelCenterVisible) {
        [self _showLeftPanel:YES bounce:NO];
    }
}

- (void)toggleRightPanel:(__unused id)sender {
    if (self.state == JASidePanelRightVisible) {
        [self _showCenterPanel:YES bounce:NO];
    } else if (self.state == JASidePanelCenterVisible) {
        [self _showRightPanel:YES bounce:NO];
    }
}

- (void)setCenterPanelHidden:(BOOL)centerPanelHidden {
    [self setCenterPanelHidden:centerPanelHidden animated:NO duration:0.0];
}

- (void)setCenterPanelHidden:(BOOL)centerPanelHidden animated:(BOOL)animated duration:(NSTimeInterval) duration {
    if (centerPanelHidden != _centerPanelHidden && self.state != JASidePanelCenterVisible) {
        _centerPanelHidden = centerPanelHidden;
        duration = animated ? duration : 0.0f;
        if (centerPanelHidden) {
            [UIView animateWithDuration:duration animations:^{
                CGRect frame = self.centerPanelContainer.frame;
                frame.origin.x = self.state == JASidePanelLeftVisible ? self.centerPanelContainer.frame.size.width : -self.centerPanelContainer.frame.size.width;
                self.centerPanelContainer.frame = frame;
                if (self.shouldResizeLeftPanel || self.shouldResizeRightPanel) {
                    [self _layoutSidePanels];
                }
            } completion:^(__unused BOOL finished) {
                // need to double check in case the user tapped really fast
                if (_centerPanelHidden) {
                    [self _hideCenterPanel];
                }
            }];
        } else {
            [self _unhideCenterPanel];
            [UIView animateWithDuration:duration animations:^{
                if (self.state == JASidePanelLeftVisible) {
                    [self showLeftPanelAnimated:NO];
                } else {
                    [self showRightPanelAnimated:NO];
                }
                if (self.shouldResizeLeftPanel || self.shouldResizeRightPanel) {
                    [self _layoutSidePanels];
                }
            }];
        }
    }
}

@end