SRevealSideController and company class.

SDirectionPanGestureRecognizer.h

#import <Foundation/Foundation.h>
#import <UIKit/UIGestureRecognizerSubclass.h>

typedef NS_ENUM(NSUInteger, SDirectionPanGestureRecognizerDirection) {
    SDirectionPanGestureRecognizerDirectionHorizontal,
    SDirectionPanGestureRecognizerDirectionVertical
};

//  Subclass of UIPanGestureRecognizer that only enable one direction pan gesture.
//  The intended direction should be set by the property direction.
@interface SDirectionPanGestureRecognizer : UIPanGestureRecognize

//  Gesture's movement along the x axis, only valid when gesture's direction is horizontal.
@property (nonatomic, assign) NSUInteger moveX;

//  Gesture's movement along the y axis, only valid when gesture's direction is vertical.
@property (nonatomic, assign) NSUInteger moveY;

//  The intended direction should be set by this property. 
@property (nonatomic, assign) SDirectionPanGestureRecognizerDirection direction;

//  Not a designated initalizer, but a convenient one.
- (id)initWithTarget:(id)target action:(SEL)action direction:(SDirectionPanGestureRecognizerDirection)direction;
 
@end

SDirectionPanGestureRecognizer.m

import "SDirectionPanGestureRecognizer.h"

const static NSUInteger kDirectionPanDistanceThreshold = 5;

@interface SDirectionPanGestureRecognizer()

@property (nonatomic, assign) BOOL directionPan;

@end

@implementation SDirectionPanGestureRecognizer

//  Designated initializer.
- (id)initWithTarget:(id)target action:(SEL)action
{
    self = [super initWithTarget:target action:action];
    if (self) {
        [self reset];
    }
    return self;
}

- (id)initWithTarget:(id)target action:(SEL)action direction:(SDirectionPanGestureRecognizerDirection)direction
{
    self = [self initWithTarget:target action:action];
    if (self) {
        self.direction = direction;
    }
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesMoved:touches withEvent:event];
    if (self.state == UIGestureRecognizerStateFailed) return;
    CGPoint nowPoint = [[touches anyObject] locationInView:self.view];
    CGPoint prePoint = [[touches anyObject] locationInView:self.view];
    self.moveX += nowPoint.x - prePoint.x;
    self.moveY += nowPoint.y - prePoint.y;
    if (!directionPan) {
        if (self.direction == SDirectionPanGestureRecognizerDirectionHorizontal) {
            if (abs(self.moveX) > kDirectionPanDistanceThreshold) {
                self.directionPan = YES;
            }   else {
                self.state = UIGestureRecognizerStateFailed;
            }
        }   else if (self.direction == SDirectionPanGestureRecognizerDirectionVertical) {
            if (abs(self.moveY) > kDirectionPanDistanceThreshold) {
                self.directionPan = YES;
            }   else {
                self.state = UIGestureRecognizerStateFailed;
            }
        }
    }
}

- (void)reset
{
    [super reset];
    self.directionPan = NO;
    self.moveX = 0;
    self.moveY = 0;
}

@end

SRevealSideController.h

#import <UIKit/UIKit.h>

UIKIT_EXTERN NSString* SRevealSideControllerWillRevealSideControllerNotification;
UIKIT_EXTERN NSString* SRevealSideControllerDidRevealSideControllerNotification;
UIKIT_EXTERN NSString* SRevealSideControllerWillHideSideControllerNotification;
UIKIT_EXTERN NSString* SRevealSideControllerDidHideSideControllerNotification;

//  Custom container controller class contain two children view controllers:
//  Side controller, central controller.
//  Central controller will be fixed to fill the entire application frame.(SRevealSideController will normally be a root view controller)
//  Side controller will be hidden on the left side defaulty. And can be revealed and hided by a horizontal pan gesture or programatically.
//  That is to say: side controller will follow a horizontal pan's movement to traverse between its hidden position to fully revealed position.
@interface SRevealSideController : UIViewController<UIGestureRecognizerDelegate>

//  Side Controller.
@property (nonatomic, strong) UIViewController *sideController;

//  Central Controller.
@property (nonatomic, strong) UIViewController *centralController;

//  Designated Initializer.
- (id)initWithSideController:(UIViewController *)sideController centralController:(UIViewController *)centralController;

//  Reveal side controller with an animation.
//  Only valid when side controller being hidden.
- (void)revealSideController;

//  Hide side controller with an animation.
//  Only valid when side controller being fully revealed.
- (void)hideSideController;

@end

@interface UIViewController(SRevealSideController)

//  If a view controller has an ancestor as a SRevealSideController, return one, otherwise return nil.
- (id)revealSideController;

@end

SRevealSideController.m

#import "SRevealSideController.h"
#import "SDirectionPanGestureRecognizer.h"

const static NSUInteger kSRevealSideControllerSideControllerWidth = 320;
const static NSUInteger kSRevealSideControllerSideControllerShadowWidth = 10;
const static NSUInteger kSRevealSideControllerSideControllerDefiniteHideXPosition = -280;
const static CGFloat kSRevealSideControllerRevealHideAnimationDuration = 0.5;
const static CGFloat kSRevealSideControllerDimViewMaximumAlpha = 0.5;
const static NSString *kSRevealSideControllerSideControllerShadowImageName = @"SideController-Shadow.png";
const static NSString *kSRevealSideControllerSideControllerShadowImageEdgeInsets = CGEdgeInsetsZero;  //  Placeholder value;

const NSString *SRevealSideControllerWillRevealSideControllerNotification = @"SRevealSideControllerWillRevealSideControllerNotification";
const NSString *SRevealSideControllerDidRevealSideControllerNotification = @"SRevealSideControllerDidRevealSideControllerNotification";
const NSString *SRevealSideControllerWillHideSideControllerNotification = @"SRevealSideControllerWillHideSideControllerNotification";
const NSString *SRevealSideControllerDidHideSideControllerNotification = @"SRevealSideControllerDidHideSideControllerNotification";

typedef NS_ENUM(NSUInteger, SRevealSideControllerSideControllerState) {
    SRevealSideControllerSideControllerStateHidden,
    SRevealSideControllerSideControllerStateRevealed,
    SRevealSideControllerSideControllerStateMovingFromRevealed,
    SRevealSideControllerSideControllerStateMovingFromHidden
};

@interface SRevealSideController()

//  Controller's root view.
@property (nonatomic, strong) UIView *containerView;

//  Wrapper superview for sideController's view and sideControllerShadowView;
@property (nonatomic, strong) UIView *sideControllerContainerView;

//  Custom view object to fake as a shadow of side controller.
@property (nonatomic, strong) UIView *sideControllerShadowView;

//  Custom view with black color background and alpha 0.0.
//  Positioned below sideControllerContainerView and above central controller's view.
//  Its size is application frame size, same as central controller.
//  And its alpha value will increase along with side controller revealing from left to right.
@property (nonatomic, strong) UIView *centralControllerDimView;

//  Horizontal pan gesture recognizer on containerView.
@property (nonatomic, strong) SDirectionPanGestureRecognizer *horizontalPan;

// Side controller state
@property (nonatomic, assign) SRevealSideControllerSideControllerState *sideControllerState;

@end

@implementation SRevealSideController

#pragma mark - Life Cycle

- (id)initWithSideController:(UIViewController *)sideController centralController:(UIViewController *)centralController
{
    self = [super init];
    if (self) {
        _sideController = sideController;
        _centralController = centralController;
    }
    return self;
}

- (void)loadView
{
    CGSize applicationFrame = [UIApplication mainScreen] frame];
    
    //  Alloc and create all views.
    _containerView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, applicationFrame.width, applicationFrame.height);
    _sideControllerContainerView = [[UIView alloc] initWithFrame:CGRectMake(-kSRevealSideControllerSideControllerWidth, 0, kSRevealSideControllerSideControllerWidth, applicationFrame.height)];
    //  Shadow view'frame is outside of side controller contrainer view's.
    _sideControllerShadowView = [[UIView alloc] initWithFrame:CGRectMake(kSRevealSideControllerSideControllerWidth, 0, kSRevealSideControllerSideControllerShadowWidth, applicationFrame.height)]; 
    _centralControllerDimView = [[UIView alloc] initWithFrame:_containerView.frame];

    //  Setup view's attributes and contents.(Gesture recognizer, autoresizingMask... )
    _containerView.multipleTouchEnabled = YES;
    UIImage *shadowImage = [[UIImage imageNamed:kSRevealSideControllerSideControllerShadowImageName] resizableImageWithEdgeInsets:kSRevealSideControllerSideControllerShadowImageEdgeInsets];
    _sideControllerShadowView.layer.contents = [shadowImage CGIImage];
    _centralControllerDimView.backgroundColor = [UIColor blackColor];
    _centralControllerDimView.alpha = 0.0;
    //  AutoresizingMask.
    _containerView.autoresizingMask = UIViewAutoresizingMaskFlexibleWidth | UIViewAutoresizingMaskFlexibleHeight | UIViewAutoresizingMaskFlexibleRightMargin | UIViewAutoresizingMaskFlexibleBottomMargin;
    _sideControllerContainerView = UIViewAutoresizingMaskFlexibleRightMargin | UIViewAutoresizingMaskFlexibleBottomMargin;
    _sideController.view = UIViewAutoresizingMaskFlexibleRightMargin | UIViewAutoresizingMaskFlexibleBottomMargin;
    _sideControllerShadowView = UIViewAutoresizingMaskFlexibleRightMargin | UIViewAutoresizingMaskFlexibleBottomMargin;
    _centralControllerDimView = UIViewAutoresizingMaskFlexibleWidth | UIViewAutoresizingMaskFlexibleHeight | UIViewAutoresizingMaskFlexibleRightMargin | UIViewAutoresizingMaskFlexibleBottomMargin;
    _centralController.view = UIViewAutoresizingMaskFlexibleWidth | UIViewAutoresizingMaskFlexibleHeight | UIViewAutoresizingMaskFlexibleRightMargin | UIViewAutoresizingMaskFlexibleBottomMargin;

    //  View Hierachy organization. 
    self.view = _containerView;
    [sideControllerContainerView addSubview:_sideControllerShadowView];
    [sideControllerContainerView addSubview:_sideController.view];
    [_containerView addSubview:_centralController.view];
    [_containerView addSubview:_centralControllerDimView];
    [_containerView addSubview:_sideControllerContainerView];
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    //  Horizontal pan gesture attachment.
    self.horizontalPan = [SDirectionPanGestureRecognizer alloc] initWithTarget:self action:@selector(handleHorizontalPan:) direction:SDirectionPanGestureRecognizerDirectionHorizontal];
    [self.containerView addGestureRecognizer:self.horizontalPan];

    //  Default side controller state.
    self.sideControllerState = SRevealSideControllerSideControllerStateHidden;
}

#pragma mark - Side Controller, Central Controller Management and Reveal/ Hide Side Controller.

- (void)handleHorizontalPan:(SDirectionPanGestureRecognizer *)horizontalPan
{   
    if (horizontalPan.state == UIGestureRecognizerStateChanged) {
        if (self.sideControllerState == SRevealSideControllerSideControllerStateHidden || self.sideControllerState == SRevealSideControllerSideControllerStateRevealed) {
            if (self.sideControllerState == SRevealSideControllerSideControllerStateHidden && horizontalPan.moveX > 0) {
                self.sideControllerState = SRevealSideControllerSideControllerStateMovingFromHidden;
                [[NSNotificationCenter defaultCenter] postNotificationName:SRevealSideControllerWillRevealSideControllerNotification object:self];
            }   else if (self.sideControllerState == SRevealSideControllerSideControllerStateRevealed && horizontalPan.moveX < 0) {
                self.sideControllerState = SRevealSideControllerSideControllerStateMovingFromRevealed;
                [[NSNotificationCenter defaultCenter] postNotificationName:SRevealSideControllerWillHideSideControllerNotification object:self];
            }   else {
                return;
            }
        }
        //  FIXME: math calculation may be buggy.
        NSUInteger moveX = horizontalPan.moveX;
        NSUInteger originX = self.sideControllerState == SRevealSideControllerSideControllerStateMovingFromHidden ? -kSRevealSideControllerSideControllerWidth : 0;
        CGFloat movePercentage = 0.0;
        if (moveX > 0) moveX = 0;
        if (moveX < -kSRevealSideControllerSideControllerWidth) moveX = -kSRevealSideControllerSideControllerWidth;
        self.sideControllerContainerView.origin.x = originX + moveX;
        movePercentage = fabs((kSRevealSideControllerSideControllerWidth + originX + moveX) / kSRevealSideControllerSideControllerWidth);
        self.sideControllerShadowView.alpha = round(movePercentage * 100.0) / 100.0;
        self.centralControllerDimView.alpha = round(movePercentage * 100.0) / 100.0 * kSRevealSideControllerDimViewMaximumAlpha;
    }   else if (horizontalPan.state == UIGestureRecognizerStateEnded) {
        NSUInteger originX = self.sideControllerContainerView.origin.x;
        if (originX == -kSRevealSideControllerSideControllerWidth) {
            self.sideControllerState = SRevealSideControllerSideControllerStateHidden;
            [[NSNotificationCenter defaultCenter] postNotificationName:SRevealSideControllerDidHideSideControllerNotification object:self];
            return;
        }   else if (originX == 0) {
            self.sideControllerState = SRevealSideControllerSideControllerStateRevealed;
            [[NSNotificationCenter defaultCenter] postNotificationName:SRevealSideControllerDidRevealSideControllerNotification object:self];
            return;
        }
        if (horizontalPan.moveX < 0 || originX < kSRevealSideControllerSideControllerDefiniteHideXPosition) {
            [self hideSideController];
        }   else {
            [self revealSideController];
        }
    }
}

- (void)revealSideController
{
    if (self.sideControllerState == SRevealSideControllerSideControllerStateHidden) {
        self.sideControllerState == SRevealSideControllerSideControllerStateMovingFromHidden;
        [[NSNotificationCenter defaultCenter] postNotificationName:SRevealSideControllerWillRevealSideControllerNotification object:self];
    }   else if (self.sideControllerState == SRevealSideControllerSideControllerStateRevealed) {
        return;
    }
    [UIView animateWithDuration:kSRevealSideControllerRevealHideAnimationDuration 
                          delay:0.0 
                        options:UIViewAnimationOptionCurveEaseOut 
                     animations:^{
                         self.sideControllerContainerView.origin.x = 0;
                         self.sideControllerShadowView.alpha = 1.0;
                         self.centralControllerDimView.alpha = kSRevealSideControllerDimViewMaximumAlpha;
                     } 
                     completion:^(BOOL finished){
                         self.sideControllerState == SRevealSideControllerSideControllerStateRevealed;
                         [[NSNotificationCenter defaultCenter] postNotificationName:SRevealSideControllerDidRevealSideControllerNotification object:self];
    }];
}

- (void)hideSideController
{
if (self.sideControllerState == SRevealSideControllerSideControllerStateRevealed) {
        self.sideControllerState == SRevealSideControllerSideControllerStateMovingFromRevealed;
        [[NSNotificationCenter defaultCenter] postNotificationName:SRevealSideControllerWillHideSideControllerNotification object:self];
    }   else if (self.sideControllerState == SRevealSideControllerSideControllerStateHidden) {
        return;
    }
    [UIView animateWithDuration:kSRevealSideControllerRevealHideAnimationDuration 
                          delay:0.0 
                        options:UIViewAnimationOptionCurveEaseOut 
                     animations:^{
                         self.sideControllerContainerView.origin.x = -kSRevealSideControllerSideControllerWidth;
                         self.sideControllerShadowView.alpha = 0.0;
                         self.centralControllerDimView.alpha = 0.0;
                     } completion:^(BOOL finished){
                         self.sideControllerState == SRevealSideControllerSideControllerStateHidden;
                         [[NSNotificationCenter defaultCenter] postNotificationName:SRevealSideControllerDidHideSideControllerNotification object:self];
    }];
}

@end