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我的目标：我想使用 Transformations.distinctUntilChanged() 方法创建一个应用程序，该应用程序在每次更新 LiveData 对象时显示一条 toast 消息。LiveData 对象是一个简单的自定义 Integer 对象 (MyInteger)，每次单击按钮时递增 1。当 myInteger 超过 5 时，应用程序应该停止显示 toast 消息。要实现这个 >5 条件，应用程序必须重写 MyInteger 类的 equals() 方法（在 distinctUntilChanged() 类中调用 equals() 方法）。

问题：在下面的代码中，currentValue 和 previousValue 字段始终相同（第一次单击按钮除外）。因此，在 MainActivity 中未调用 onChanged，并且从第二次单击开始不会显示 Toast 消息。有关当前结果与我期望应该发生的情况，请参见下文。

请帮忙。同样，应用程序必须同时使用 distinctUntilChanged() 和覆盖的 equals() 方法。

Current result:
First time button clicked: currentValue=1 and previousValue=null, Toast Message shows.
Second time button clicked: currentValue=2 and previousValue=2, Toast Message does not show.
Third time button clicked: currentValue=3 and previousValue=3, Toast Message does not show.
Fourth time button clicked: currentValue=4 and previousValue=4, Toast Message does not show.
and so on...

Desired result:
First time button clicked: currentValue=1 and previousValue=null, Toast Message shows.
Second time button clicked: currentValue=2 and previousValue=1, Toast Message shows.
Third time button clicked: currentValue=3 and previousValue=2, Toast Message shows.
Fourth time button clicked: currentValue=4 and previousValue=3, Toast Message shows.
and so on...

块引用

public class MainActivity extends AppCompatActivity {

    private MainActivityViewModel mMainActivityViewModel;
    Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button = findViewById(R.id.my_button);
        MyInteger myInteger = new MyInteger(0);
        mMainActivityViewModel = new ViewModelProvider(this).get(MainActivityViewModel.class);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                myInteger.incrementByOne();
                mMainActivityViewModel.getLiveData().setValue(myInteger);
            }
        });

        Transformations.distinctUntilChanged(mMainActivityViewModel.getLiveData()).observe(this, new Observer<MyInteger>() {
            @Override
            public void onChanged(MyInteger myInteger) {
                Log.wtf("Transformations onChanged:", String.valueOf(myInteger.integer));
                Toast.makeText(MainActivity.this, "Transformations onChanged:"+String.valueOf(myInteger.integer), Toast.LENGTH_SHORT).show();
            }
        });
    }
}

块引用

public class MainActivityViewModel extends AndroidViewModel {

    public MutableLiveData<MyInteger> myIntegerMutableLiveData;

    public MainActivityViewModel(@NonNull Application application) {
        super(application);
    }

    public MutableLiveData<MyInteger> getLiveData() {

        if (myIntegerMutableLiveData == null) {
            myIntegerMutableLiveData = new MutableLiveData<>();
        }

//        MediatorLiveData transformedLiveData = (MediatorLiveData) Transformations.distinctUntilChanged(myIntegerMutableLiveData);

        return myIntegerMutableLiveData;
    }

}

块引用

public class MyInteger {
    
    public int integer;
    
    public MyInteger(Integer integer) {
        this.integer = integer;
    }

    public void incrementByOne() {
        integer++;
    }

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    MyInteger myInteger = (MyInteger) o;
    if ((myInteger.integer <= 5) && (integer == myInteger.integer)) {return true;}
    if ((myInteger.integer <= 5) && (integer != myInteger.integer)) {return false;}
    return (myInteger.integer > 5);
}

    @Override
    public int hashCode() {
        return Objects.hash(integer);
    }
}
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