Question

新手问题，

我有3个Class，其中3个是来自NSOBject的子类。

集合类，有2个属性， masterSong为NSMutableSet （强，非原子）和 listOfPlaylists为NSMutableArray （强，非原子））
播放列表

有2个属性， playListName为NSString （copy，nonatomic）， songList为NSMutableArray （强，非原子）

3。 歌曲课程

拥有4个属性：标题，艺术家，专辑，游戏时间为NSString（复制，非原子）。

masterSong将包含Song对象，listOfPlaylist将包含播放列表对象。

而songList仅存储对Song对象的引用。

我想通过在masterSong中查找标题，艺术家或专辑的歌曲来为Collection Class创建removeSong方法。

如果查找找到1，它将返回NSSet，返回的NSSet将作为参数在minusSet：方法中从masterSong和所有Playlist.songList中删除。

但是，我无法弄清楚如何写下NSPredicate语法来过滤掉包含Song Object的masterSong中的.title或.album或.artist。

这是我到目前为止所得到的 Collection.m

- (void) removeSong: (NSString *)zSong{

    //remove from reference playlist


    NSSet *targets = [self lookUpTitle:zSong];

    if ([targets count]>0) {
        [self.masterSongs minusSet:targets];

        for (Playlist *playlist in listOfPlaylists) {
            // -all objects converts the set into array.

            [playlist.songList removeObjectsInArray:[targets allObjects]];
        }
    }
    else
        ;
}

执行lookUpTitle方法时Xcode抛出异常，因为由于未捕获的异常'NSInvalidArgumentException'而终止应用程序，原因：'无法查找值（标题：不返回的整点，艺术家：Cafe Bleu，专辑：CBSB，游戏时间：3：56）字符串（你知道什么）;值不是字符串

- (NSSet *) lookUpTitle: (NSString *)aName {

    NSString *filters = @"%K CONTAINS [cd] %@";
    NSPredicate *filter = [NSPredicate predicateWithFormat:filters, @"title", aName];

    NSSet *result = [masterSongs filteredSetUsingPredicate:filter];

    if ([result count] == 0) {
        NSLog(@"not found");

        return nil;
    }
    else{
        return result;
    }
}

我知道主要问题是在lookUpTitle方法

 NSPredicate *filter = [NSPredicate predicateWithFormat:filters, @"title", aName];

 NSSet *result = [masterSongs filteredSetUsingPredicate:filter];

过滤器以错误的方式过滤，而它应该过滤NSString对象（标题，艺术家或专辑），但是masterSong包含Song对象，如何使用NSPredicate访问放置在masterSong内的Song对象中的标题，艺术家，专辑？

这种情况的其他任何有效方法吗？

Predicate Programming Guide仅显示包含字符串

我找到了其他帖子which has similar topic，但是块读取起来很困惑，但在我的情况下仍然无效。

编辑按响应者＃1的要求添加部分代码（Class.m方法）。这里是与我的问题相关的.m文件中的一些方法

的main.m

    #import <Foundation/Foundation.h>
#import "MusicCollection.h"
#import "Playlist.h"
#import "Song.h"
int main(int argc, const char * argv[])
{

    @autoreleasepool {

        // insert code here...
       // NSLog(@"Hello, World!");



        //create Songs

        Song *aSong = [[Song alloc]initWithTitle:@"Whole point of no return" withArtist:@"Cafe Bleu" withAlbum:@"CBSB" withPlaytime:@"3:56"];
        Song *bSong  = [[Song alloc]initWithTitle:@"Council Meetin'" withArtist:@"Cafe Bleu" withAlbum:@"CBSB" withPlaytime:@"4:00"];

        Song *cSong = [[Song alloc]initWithTitle:@"Hayate" withArtist:@"Spitz" withAlbum:@"Indigo Chiheisen" withPlaytime:@"4:21"];

        Song *dSong = [[Song alloc]initWithTitle:@"I have a Dreams" withArtist:@"WestLife" withAlbum:@"Season" withPlaytime:@"4:11"];

        Song *eSong = [[Song alloc]initWithTitle:@"What Do You Know" withArtist:@"David Choi" withAlbum:@"Tomorrow" withPlaytime:@"3:46"];


        //create playList

        Playlist *playlistA = [[Playlist alloc]initWithName:@"Playlist A"];

        Playlist *playListB = [[Playlist alloc]initWithName:@"Playlist B"];

    //store Song A & B to Playlist A and Song A,B,C to playlist B

        [playlistA addSong:aSong];

        [playlistA addSong:bSong];

        [playListB addSong:aSong];

        [playListB addSong:bSong];

        [playListB addSong:cSong];

//        [playListB removeSong:eSong];


//Creating Master Collection

        MusicCollection *myCollection = [[MusicCollection alloc]initWithName:@"Library"];

        [myCollection addPlaylist:playlistA];
        [myCollection addPlaylist:playListB];
        [myCollection addSong:eSong];
        [myCollection addSong:dSong];

        [myCollection removePlaylist:playListB];
        [myCollection removeSong:aSong];

        NSSet *container2 = [myCollection lookUpTitle:@"What"];

    NSLog(@"%@",container2);


    //    NSLog(@"%@",myCollection);
    }
    return 0;
}

Collection.m

-(instancetype) initWithName: (NSString *)aName{
    self = [super init];
    if (self) {

        self.name           = [NSMutableString stringWithString:aName];
        self.listOfPlaylists = [NSMutableArray array];
        self.masterSongs = [NSMutableSet set];


    }
    return self;
}

-(instancetype) init{

    return  [self initWithName:@""];

}

-(void) addPlaylist: (Playlist *)aPlayList{

    if ([listOfPlaylists containsObject:aPlayList]==YES) {

    }
    else
        [listOfPlaylists addObject:aPlayList];


}

-(void) removePlaylist: (Playlist *)aPlayList{
    if ([listOfPlaylists containsObject:aPlayList]) {

        [listOfPlaylists removeObjectIdenticalTo:aPlayList];
    }

    else{
        ;
    }

}

- (void) displaySong{

    NSLog(@"displaying all song in Collection");

    NSLog(@" %@",self.masterSongs);

}

- (void) addSong :(Song *)aSong{
    if (![masterSongs containsObject:aSong]) {

        [masterSongs addObject:aSong];

    }
}

- (void) removeSong: (NSString *)zSong{

    //remove from reference playlist


    NSSet *targets = [self lookUpTitle:zSong];

    if ([targets count]>0) {
        [self.masterSongs minusSet:targets];

        for (Playlist *playlist in listOfPlaylists) {
            // -all objects converts the set into array.

            [playlist.songList removeObjectsInArray:[targets allObjects]];
        }
    }
    else
        ;
}



 - (NSSet *) lookUpTitle: (NSString *)aName {

    NSString *filters = @"%K CONTAINS [cd] %@";
    NSPredicate *filter = [NSPredicate predicateWithFormat:filters, @"title", aName];

    NSSet *result = [masterSongs filteredSetUsingPredicate:filter];

    if ([result count] == 0) {
        NSLog(@"not found");

        return nil;
    }
    else{
        return result;
    }
}

Playlist.m

- (void) addSong :(Song *)aSong{

    if (![songList containsObject:aSong]) {
        [songList addObject:aSong];
    }

}

- (void) removeSong: (Song *)aSong{
    if ([songList containsObject:aSong]){
    [self.songList removeObjectIdenticalTo:aSong];
    }
}
- (instancetype) initWithName: (NSString *)aPLName{
    self = [super init];

    if (self) {
        self.songList = [NSMutableArray array];
        self.playlistName = aPLName;
    }
    return self;
}

- (instancetype)init{
    return  [self initWithName:@""];
}

Song.m

 - (instancetype) initWithTitle: (NSString *)aTitle withArtist: (NSString *)anArtist withAlbum: (NSString *)aAlbum withPlaytime: (NSString *)playingTime{

    self = [super init];

    if (self) {
        self.artist = [NSMutableString stringWithString:anArtist];
        self.album  = [NSMutableString stringWithString:aAlbum];
        self.title =   [NSMutableString stringWithString:aTitle];
        self.playtime = [NSMutableString stringWithString:playingTime];

    }
       return self;
}

-(instancetype) init{
    return  [self initWithTitle:@"null" withArtist:@"null" withAlbum:@"null" withPlaytime:@"null"];
}

Answer 1

您不需要使用-predicateWithBlock，在您的情况下使用以下谓词就足够了

[NSPredicate predicateWithFormat:@"SELF.title CONTAINS %@", @"Layla"];

修改

Jeffery Thomas是对的，问题出在这一行 - 实际上编译器会发出警告

[myCollection removeSong:aSong];

你要传递给-removeSong:一个歌曲对象，而不是歌曲的标题。如果您要传递给-removeSong:，Song对象会将方法签名从- (void)removeSong:(NSString *)songTitle更改为-(void)removeSong:(Song*)song，并相应地更改方法实现，以将该歌曲的标题传递给谓词一个NSString

- (void) removeSong: (Song *)zSong { //remove from reference playlist NSSet *targets = [self lookUpTitle:zSong.title]; if ([targets count]>0) { [self.masterSongs minusSet:targets]; for (Playlist *playlist in self.listOfPlaylists) { // -all objects converts the set into array. [playlist.songList removeObjectsInArray:[targets allObjects]]; } } }

从现在开始，请不要忽略编译器警告，以免过去几天调试这种微妙的错误。

Answer 2

你重载了-removeSong:的含义，这让你感到困惑。在Playlist中，-removeSong:获取了Song个实例，但在Collection中，-removeSong:为歌曲标题提供了NSString个实例。

问题是您将Song的实例传递给期望NSString的版本。

- (void)removeSong:(NSString *)zSong
{
    NSSet *targets = [self lookUpTitle:zSong];
    // …
}

应该是

- (void)removeSong:(Song *)aSong
{
    NSSet *targets = [self lookUpTitle:aSong.title];
    // …
}

或

- (void)removeSongWithTitle:(NSString *)aTitle
{
    NSSet *targets = [self lookUpTitle:aTitle];
    // …
}

我更改了方法的第二个版本的名称，以清楚地显示预期参数是什么。如果要使用第二个版本，可以传递消息[myCollection removeSongWithTitle:aSong.title]

如何使用NSPredicate过滤包含来自其他类的复合对象的NSMutableSet？

2 个答案: