无法将对象添加到可变数组 [英] Adding an object to Mutable array is not possible

问题描述

我试图将Song *对象添加到Mutable数组中，但我感到很困惑，因为尽管添加了对象，但数组的数量却没有增加.

I am trying add a Song* object to a Mutable array and I am stumped as the count of the array is not increasing in spite of adding the object.

Song.h

#import <Foundation/Foundation.h>

@interface Song : NSObject

@property(copy, nonatomic) NSString *title, *album, *artist;
@property(copy, nonatomic) NSNumber *playTime;

@end

Song.m

#import "Song.h"

@implementation Song

@end

播放列表.h

#import <Foundation/Foundation.h>
@class Song;

@interface Playlist : NSObject

@property(copy, nonatomic) NSMutableArray *playListArray;

-(void) addSong: (Song *) tempSongToBeAdded;
-(void) removeSong: (Song *) tempSongToBeremoved;
-(void) listOfSongs;
-(NSUInteger) entries;

@end

播放列表.m

#import "Playlist.h"
#import "Song.h"

@implementation Playlist

-(void) addSong: (Song *) tempSongToBeAdded{
    NSLog(@"%s song is being added.", [tempSongToBeAdded.title UTF8String]);
    [self.playListArray addObject:tempSongToBeAdded];
}
-(void) removeSong: (Song *) tempSongToBeremoved{
    [self.playListArray removeObject:tempSongToBeremoved];
}

-(NSUInteger) entries{
    return [self.playListArray count];
}


    -(void) listOfSongs{
        NSLog(@"Hi");
        for (Song *tempSong in self.playListArray) {
            NSLog(@"title: %s", [tempSong.title UTF8String]);
        }
    }

@end

Main.m

#import <Foundation/Foundation.h>
#import "Song.h"
#import "Playlist.h"

int main(int argc, const char * argv[])
{

@autoreleasepool {

    Song *song1 = [[Song alloc] init];
    song1.title = @"Manasa";
    song1.album = @"Ye Maya Chesava";
    song1.artist = @"A. R. Rahman";
    song1.playTime = [NSNumber numberWithDouble:4.56];

    Playlist *playlist1 = [[Playlist alloc] init];

    [playlist1 addSong:song1];
    NSLog(@"The total number of songs are %lu",[playlist1 entries]);
    [playlist1 listOfSongs];




    }
return 0;
}

我在播放列表中获得的条目为0，在歌曲列表中获得的条目为空.我没有收到任何编译错误，也不知道为什么没有将对象添加到数组中.

I am getting the entries in the playlist as 0 and and the list of songs as empty. I am not getting any compile errors and I have no idea why the objects are not getting added to the array.

推荐答案

您的变量playListArray从未初始化，并且始终为nil.您需要使用以下命令对其进行初始化:

Your variable playListArray is never initialized and is always nil. You need to initialize it using:

playListArray = [[NSMutableArray] alloc] init];

您可以在初始化该对象的Playlist类中添加init方法.

You can add an init method in your Playlist class where you initialize this object.

- (id)init
{
    self = [super init];
    if (self)
    {
        playListArray = [[NSMutableArray] alloc] init];
    }
    return self;
}

问题似乎在于您如何声明该属性

The problem seems to be how you declared the property

@property(copy, nonatomic) NSMutableArray *playListArray;

它声明为copy，这意味着即使执行playListArray = [[NSMutableArray] alloc] init]，变量playListArray也会获取初始化数组的副本，但是copy协议是从NSArray继承的，而不是从NSMutableArray，因此您获得了一个不可变的数组.您可以在NSMutableArray文档中进行检查.您需要将copy更改为retain(您不使用ARC，对吗?).

It is declared as copy, that means that even when you do playListArray = [[NSMutableArray] alloc] init], your variable playListArray gets a copy of the initialized array, but the copy protocol is inherited from NSArray, not from NSMutableArray, so you get an immutable array. You can check this in the NSMutableArray documentation. You need to change copy for retain (you're not using ARC, right?).

事实上，我发现您对大多数属性都使用copy，如果没有特殊原因，我会将其更改为retain.

In fact I see that you're using copy for most of your properties, if there's no particular reason for this, I would change them to retain.

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