Android Room:处理与LiveData的关系
所以我有两个模型(表),Android Room:处理与LiveData的关系,android,android-room,android-livedata,Android,Android Room,Android Livedata,所以我有两个模型(表),词组和词,它们之间的关系是一对多的。 因此,我的模型和Dao看起来像: @Entity public class WordGroup { @PrimaryKey private int id; @Ignore private List<Word> words; public int getId() { return id; } public void setId(int id) {
词组
和词
,它们之间的关系是一对多的。
因此,我的模型和Dao
看起来像:
@Entity
public class WordGroup {
@PrimaryKey
private int id;
@Ignore
private List<Word> words;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public List<Word> getWords() {
return words;
}
public void setWords(List<Word> words) {
this.words = words;
}
}
@Entity
public class Word {
@PrimaryKey
private int id;
@ForeignKey(entity = WordGroup.class, parentColumns = "id", childColumns = "wordGroupId")
private int wordGroupId;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getWordGroupId() {
return wordGroupId;
}
public void setWordGroupId(int wordGroupId) {
this.wordGroupId = wordGroupId;
}
}
现在我想写一个方法,选择所有WordGroup
s,并为每个WordGroup
设置它们的Word
s。方法的签名应类似于:
public LiveData<List<WordGroup>> getAllWordGroupsWithWords();
public LiveData getAllWordGroupsWithWords();
我尝试了更多,在MediatorLiveData
的帮助下,我想出了这个解决方案:
public LiveData<List<WordGroup>> getAllWordGroupsWithWords() {
LiveData<List<WordGroup>> wordGroupsLiveData = dao.selectAllWordGroups();
LiveData<List<WordGroup>> outputWordGroups = Transformations.switchMap(wordGroupsLiveData, inputWordGroups -> {
MediatorLiveData<List<WordGroup>> wordGroupsMediatorLiveData = new MediatorLiveData<>();
for (WordGroup wordGroup : inputWordGroups) {
wordGroupsMediatorLiveData.addSource(dao.selectWordsByWordGroupId(wordGroup.getId()), words -> {
wordGroup.setWords(words);
wordGroupsMediatorLiveData.postValue(inputWordGroups);
});
}
return wordGroupsMediatorLiveData;
});
return outputWordGroups;
}
public LiveData getAllWordGroupsWithWords(){
LiveData wordGroupsLiveData=dao。选择AllWordGroups();
LiveData outputWordGroups=Transformations.switchMap(wordGroupsLiveData,inputWordGroups->{
MediatorLiveData wordGroupsMediatorLiveData=新的MediatorLiveData();
for(字组字组:inputWordGroups){
wordgroupsmeditorlivedata.addSource(dao.selectWordsByWordGroupId(wordGroup.getId()),words->{
wordGroup.setWords(words);
wordGroupsMediatorLiveData.postValue(inputWordGroups);
});
}
返回字GroupsMediatorLiveData;
});
返回outputWordGroups;
}
现在这正是我想要的方式,但是有一些大问题
- 首先,如果您在
上使用方法MediatorLiveData
,它将多次调用观察者,我可以通过使用setValue
减少它调用观察者的次数postValue
- 第二,即使您只更改一个单词,它也会再次调用每个
单词组的观察者
public LiveData<List<WordGroup>> getAllWordGroupsWithWords();
public LiveData<List<WordGroup>> getAllWordGroupsWithWords() {
LiveData<List<WordGroup>> wordGroupsLiveData = dao.selectAllWordGroups();
LiveData<List<WordGroup>> outputWordGroups = Transformations.switchMap(wordGroupsLiveData, inputWordGroups -> {
MediatorLiveData<List<WordGroup>> wordGroupsMediatorLiveData = new MediatorLiveData<>();
for (WordGroup wordGroup : inputWordGroups) {
wordGroupsMediatorLiveData.addSource(dao.selectWordsByWordGroupId(wordGroup.getId()), words -> {
wordGroup.setWords(words);
wordGroupsMediatorLiveData.postValue(inputWordGroups);
});
}
return wordGroupsMediatorLiveData;
});
return outputWordGroups;
}