/*

     File: Quiz.m

 Abstract: Model class for a Quiz.  Manages loading the quiz 

 data (questions, answers) from a plist file, vending questions, and recording 

 responses.

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 */

#import "Quiz.h"

NSString * const QuestionsListKey = @"QuestionList";

NSString * const QuestionTextKey = @"QuestionText";

NSString * const AnswerTextKey = @"AnswerText";

NSString * const AnswerKey = @"Answer";

@interface Quiz ()

/// Array of Question objects.

@property (strong) NSArray *questions;

@property (readwrite) NSUInteger correctlyAnsweredQuestions;

@property (readwrite) NSUInteger answeredQuestions;

@end

@implementation Quiz

//| ----------------------------------------------------------------------------

//! Initializes and retuns a newly created Quiz with the questions from a plist

//! file at questionsURL.

//

//! @param  questionsURL

//!         A url for a plist file containing a list of questions.

///

- (id)initWithQuestionsPlistAtURL:(NSURL*)questionsURL

{

    self = [super init];

    if (self)

    {

        NSData *questionsPlistData;

        NSDictionary *questionsPlistDict;

        NSArray *questionsList;

        // Load and deserialize the plist

        questionsPlistData = [NSData dataWithContentsOfURL:questionsURL];

        questionsPlistDict = [NSPropertyListSerialization propertyListWithData:questionsPlistData

                                                                       options:NSPropertyListImmutable

                                                                        format:nil

                                                                         error:NULL];

        questionsList = questionsPlistDict[QuestionsListKey];

        // Create a temporary NSMutableArray to hold the Question objects 

        // that will be created from the data in questionsList.

        NSMutableArray *questions = [NSMutableArray arrayWithCapacity:questionsList.count];

        for (NSDictionary *questionDict in questionsList) {

            Question *question = [[Question alloc] initWithQuestionDict:questionDict];

            // Observe changes to the selectedResponse property of the newly 

            // created Question object.  When a change is observed, the Quiz may 

            // need to update its answeredQuestions and

            // correctlyAnsweredQuestions properties.

            [question addObserver:self

                       forKeyPath:@"selectedResponse"

                          options:(NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld)

                          context:NULL];

            [questions addObject:question];

        }

        // Create an immutable array from the mutable questions array and  

        // assign it to the ivar backing the questions property.

        _questions = [[NSArray alloc] initWithArray:questions];

    }

    return self;

}

//| ----------------------------------------------------------------------------

- (void)dealloc

{

    // Unregister Key-Value observing notifications for all Questions.

    for (Question *question in self.questions)

        [question removeObserver:self forKeyPath:@"selectedResponse" context:NULL];

}

//| ----------------------------------------------------------------------------

//! Manual implementation of the getter for the percentageScore property.

//

//  Because percentageScore is a function of the correctlyAnsweredQuestions

//  and answeredQuestions, it is computed as needed instead of stored.

//

- (float)percentageScore

{

    return (float)self.correctlyAnsweredQuestions/(float)self.answeredQuestions;

}

//| ----------------------------------------------------------------------------

//! Manual implementation of the getter for the totalQuestions property.

//

- (NSUInteger)totalQuestions

{

    return self.questions.count;

}

//| ----------------------------------------------------------------------------

//! Returns the Question at the specified index.

//

//  Implementing this method allows other objects to use the subscript

//  operator to access Questions by their index.

//     Ex: Question *q = myQuiz[0];

//

- (Question*)objectAtIndexedSubscript:(NSInteger)idx

{

    // Call through to -questionAtIndex:

    return [self questionAtIndex:idx];

}

//| ----------------------------------------------------------------------------

//! Returns the Question at the specified index.

//

- (Question*)questionAtIndex:(NSUInteger)idx

{

    return [self.questions objectAtIndex:idx];

}

//| ----------------------------------------------------------------------------

//! Resets the Quiz, clearing the current score.

//

- (void)resetQuiz

{

    // NSArray forwards calls to -setValue:forKey: to all the objects 

    // within the array.

    [self.questions setValue:@(NSNotFound) forKey:@"selectedResponse"];

}

#pragma mark - 

#pragma mark Key Value Observing

//| ----------------------------------------------------------------------------

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key

{

    if ([key isEqualToString:@"percentageScore"])

        // percentageScore is derived from answeredQuestions and

        // correctlyAnsweredQuestions.

        return [NSSet setWithObjects:@"answeredQuestions", @"correctlyAnsweredQuestions", nil];

    else

        return [super keyPathsForValuesAffectingValueForKey:key];

}

//| ----------------------------------------------------------------------------

//  Callback for key-value observing (KVO) change notifications.

//

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context

{

    if ([keyPath isEqualToString:@"selectedResponse"])

    // The selectedResponse of a Question changed.

    {

        NSInteger previouslySelectedResponse = [change[NSKeyValueChangeOldKey] integerValue];

        NSInteger selectedResponse = [change[NSKeyValueChangeNewKey] integerValue];

        if (previouslySelectedResponse == NSNotFound && selectedResponse != NSNotFound)

            // If the question had not been answered but now is, increment

            // answeredQuestions.

            self.answeredQuestions++;

        else if (previouslySelectedResponse != NSNotFound && selectedResponse == NSNotFound)

            // If the question had been answered but no longer is, decrement

            // answeredQuestions. This will occur when the quiz is being reset.

            self.answeredQuestions--;

        if (selectedResponse == [(Question*)object correctResponse] &&

            previouslySelectedResponse != [(Question*)object correctResponse])

            // If the current response is correct and the previous response was

            // incoorect, increment correctlyAnsweredQuestions.

            self.correctlyAnsweredQuestions++;

        else if (previouslySelectedResponse == [(Question*)object correctResponse] &&

                 selectedResponse != [(Question*)object correctResponse])

            // If the current response is incorrect and the previous response was

            // coorect, decrement correctlyAnsweredQuestions.

            self.correctlyAnsweredQuestions--;

    }

    else

        // Always invoke the super's implementation for unhandled keyPath's.

        return [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];

}

@end