Chapter 11 - Chat App/ChatAppFinished/Pods/FirebaseDatabase/Firebase/Database/FEventGenerator.m
/*
 * Copyright 2017 Google
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#import "FEventGenerator.h"
#import "FNode.h"
#import "FIRDatabaseQuery_Private.h"
#import "FQueryParams.h"
#import "FQuerySpec.h"
#import "FChange.h"
#import "FNamedNode.h"
#import "FEventRegistration.h"
#import "FEvent.h"
#import "FDataEvent.h"

@interface FEventGenerator ()
@property (nonatomic, strong) FQuerySpec *query;
@end

/**
* An EventGenerator is used to convert "raw" changes (fb.core.view.Change) as computed by the
* CacheDiffer into actual events (fb.core.view.Event) that can be raised.  See generateEventsForChanges()
* for details.
*/
@implementation FEventGenerator

- (id)initWithQuery:(FQuerySpec *)query {
    self = [super init];
    if (self) {
        self.query = query;
    }
    return self;
}

/**
* Given a set of raw changes (no moved events, and prevName not specified yet), and a set of EventRegistrations that
* should be notified of these changes, generate the actual events to be raised.
*
* Notes:
* - child_moved events will be synthesized at this time for any child_changed events that affect our index
* - prevName will be calculated based on the index ordering
*
* @param changes NSArray of FChange, not necessarily in order.
* @param registrations is NSArray of FEventRegistration.
* @return NSArray of FEvent.
*/
- (NSArray *) generateEventsForChanges:(NSArray *)changes
                            eventCache:(FIndexedNode *)eventCache
                    eventRegistrations:(NSArray *)registrations
{
    NSMutableArray *events = [[NSMutableArray alloc] init];

    // child_moved is index-specific, so check all our child_changed events to see if we need to materialize
    // child_moved events with this view's index
    NSMutableArray *moves = [[NSMutableArray alloc] init];
    for (FChange *change in changes) {
        if (change.type == FIRDataEventTypeChildChanged && [self.query.index indexedValueChangedBetween:change.oldIndexedNode.node
                                                                                                    and:change.indexedNode.node]) {
            FChange *moveChange = [[FChange alloc] initWithType:FIRDataEventTypeChildMoved
                                                    indexedNode:change.indexedNode
                                                       childKey:change.childKey
                                                 oldIndexedNode:nil];
            [moves addObject:moveChange];
        }
    }

    [self generateEvents:events forType:FIRDataEventTypeChildRemoved changes:changes eventCache:eventCache eventRegistrations:registrations];
    [self generateEvents:events forType:FIRDataEventTypeChildAdded changes:changes eventCache:eventCache eventRegistrations:registrations];
    [self generateEvents:events forType:FIRDataEventTypeChildMoved changes:moves eventCache:eventCache eventRegistrations:registrations];
    [self generateEvents:events forType:FIRDataEventTypeChildChanged changes:changes eventCache:eventCache eventRegistrations:registrations];
    [self generateEvents:events forType:FIRDataEventTypeValue changes:changes eventCache:eventCache eventRegistrations:registrations];

    return events;
}

- (void) generateEvents:(NSMutableArray *)events
                forType:(FIRDataEventType)eventType
                changes:(NSArray *)changes
             eventCache:(FIndexedNode *)eventCache
     eventRegistrations:(NSArray *)registrations
{
    NSMutableArray *filteredChanges = [[NSMutableArray alloc] init];
    for (FChange *change in changes) {
        if (change.type == eventType) {
            [filteredChanges addObject:change];
        }
    }

    id<FIndex> index = self.query.index;

    [filteredChanges sortUsingComparator:^NSComparisonResult(FChange *one, FChange *two) {
        if (one.childKey == nil || two.childKey == nil) {
            @throw [[NSException alloc] initWithName:@"InternalInconsistencyError"
                                              reason:@"Should only compare child_ events"
                                            userInfo:nil];
        }
        return [index compareKey:one.childKey
                         andNode:one.indexedNode.node
                      toOtherKey:two.childKey
                         andNode:two.indexedNode.node];
    }];

    for (FChange *change in filteredChanges) {
        for (id<FEventRegistration> registration in registrations) {
            if ([registration responseTo:eventType]) {
                id<FEvent> event = [self generateEventForChange:change registration:registration eventCache:eventCache];
                [events addObject:event];
            }
        }
    }
}

- (id<FEvent>) generateEventForChange:(FChange *)change
                         registration:(id<FEventRegistration>)registration
                           eventCache:(FIndexedNode *)eventCache
{
    FChange *materializedChange;
    if (change.type == FIRDataEventTypeValue || change.type == FIRDataEventTypeChildRemoved) {
        materializedChange = change;
    } else {
        NSString *prevChildKey = [eventCache predecessorForChildKey:change.childKey
                                                          childNode:change.indexedNode.node
                                                              index:self.query.index];
        materializedChange = [change changeWithPrevKey:prevChildKey];
    }
    return [registration createEventFrom:materializedChange query:self.query];
}

@end
