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FAQ.md

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FAQ

ChangesDetector detects changes based on methods matches and same that you need to implement. If you implement them wrongly this can lead to omitting your changes in recycler view or flooding changed to recycler view that was not necessary.

Bugs (aka: your bugs)

Bug #1

public class Data implements SimpleDetector.Detectable<Data> {

    private final long id;
    @Nonnull
    private final String name;

    Data(long id, @Nonnull String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public boolean matches(@Nonnull Data item) {
        return Objects.equals(item.id, id);
    }

    @Override
    public boolean same(@Nonnull Data item) {
        return equals(item);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id);
    }
}

Because your equals method doesn't take care of name, changing name will not be informed to recycler view.

You need to change your equals method.

public class Data implements SimpleDetector.Detectable<Data> {
    // ...
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id) &&
          Objects.equals(name, data.name); // This was missing
    }
    // ...
}

Bug #2

public class Data implements SimpleDetector.Detectable<Data> {

    private final long id;
    @Nonnull
    private final Object o;

    Data(long id, @Nonnull Object o) {
        this.id = id;
        this.o = o;
    }

    @Override
    public boolean matches(@Nonnull Data item) {
        return Objects.equals(item.id, id);
    }

    @Override
    public boolean same(@Nonnull Data item) {
        return equals(item);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id) &&
           Objects.equals(o, data.o);
    }
}

This code is not buggy but probably you will invoke bug invoking changes detector.

If you execute:

adapter.call(Arrays.toList(Data(1, new Object())), new Data(2, new Object()));
adapter.call(Arrays.toList(Data(1, new Object())), new Data(2, new Object()));

You expect that changes detector will detect that nothing was changed, but actually change was made. Changes detector will detect that Object o was changed according to method equals where Objects.equal(new Object(), new Object()) == false.

There are two solutions:

  • use objects that implement equals method
  • use the same instance of objects
final Object instance = new Object()
adapter.call(Arrays.toList(Data(1, instance)), new Data(2, instance));
adapter.call(Arrays.toList(Data(1, instance)), new Data(2, instance));

Bug #2.1

Similar bug can be done with RxJava

public class Data implements SimpleDetector.Detectable<Data> {

    private final long id;
    @Nonnull
    private final Observable<String> observable;

    Data(long id, @Nonnull Observable<String> observable) {
        this.id = id;
        this.observable = observable;
    }

    @Override
    public boolean matches(@Nonnull Data item) {
        return Objects.equals(item.id, id);
    }

    @Override
    public boolean same(@Nonnull Data item) {
        return equals(item);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id) &&
           Objects.equals(observable, data.observable);
    }
}

So wrong is:

adapter.call(Arrays.toList(Data(1, Observable.just(""))), new Data(2, Observable.just("")));
adapter.call(Arrays.toList(Data(1, Observable.just(""))), new Data(2, Observable.just("")));

should be:

final Observable<String> instance = Observable.just("")
adapter.call(Arrays.toList(Data(1, instance)), new Data(2, instance));
adapter.call(Arrays.toList(Data(1, instance)), new Data(2, instance));

So wrong is:

final Observable<String> instance = Observable.just("")
adapter.call(Arrays.toList(Data(1, instance.observeOn(uiScheduler))), new Data(2, instance.observeOn(uiScheduler)));
adapter.call(Arrays.toList(Data(1, instance.observeOn(uiScheduler))), new Data(2, instance.observeOn(uiScheduler)));

should be:

final Observable<String> instance = Observable.just("").observeOn(uiScheduler)
adapter.call(Arrays.toList(Data(1, instance)), new Data(2, instance));
adapter.call(Arrays.toList(Data(1, instance)), new Data(2, instance));

Bug #3

This usually can happen when using RxJava

public class Data implements SimpleDetector.Detectable<Data> {

    private final long id;
    @Nonnull
    private final Observable<String> observable;

    Data(long id, @Nonnull Observable<String> observable, Scheduler uiScheduler) {
        this.id = id;
        this.observable = observable.observeOn(uiScheduler);
    }

    @Override
    public boolean matches(@Nonnull Data item) {
        return Objects.equals(item.id, id);
    }

    public Observable<String> observable() {
        return observable;
    }

    @Override
    public boolean same(@Nonnull Data item) {
        return equals(item);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id) &&
           Objects.equals(observable, data.observable);
    }
}

In this case, calling:

final Observable<String> instance = Observable.just("")
adapter.call(Arrays.toList(Data(1, instance)), new Data(2, instance));
adapter.call(Arrays.toList(Data(1, instance)), new Data(2, instance));

new Data() will always generate new objects so equals will never be true.

You should write your Data:

public class Data implements SimpleDetector.Detectable<Data> {

    private final long id;
    @Nonnull
    private final Observable<String> observable;
    @Nonnull
    private final Scheduler uiScheduler;

    Data(long id, @Nonnull Observable<String> observable, Scheduler uiScheduler) {
        this.id = id;
        this.observable = observable;
        this.uiScheduler = uiScheduler;
    }

    @Override
    public boolean matches(@Nonnull Data item) {
        return Objects.equals(item.id, id);
    }

    public Observable<String> observable() {
        return observable.observeOn(uiScheduler);
    }

    @Override
    public boolean same(@Nonnull Data item) {
        return equals(item);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id) &&
           Objects.equals(observable, data.observable) &&
           Objects.equals(uiScheduler, data.uiScheduler);
    }
}

Bug #4

The same issue can happen without using RxJava:

public class Data implements SimpleDetector.Detectable<Data> {

    private final long id;
    @Nonnull
    private final int value;

    Data(long id, @Nonnull java.util.Random random) {
        this.id = id;
        this.value = random.nextInt();
    }

    @Override
    public boolean matches(@Nonnull Data item) {
        return Objects.equals(item.id, id);
    }

    @Override
    public boolean same(@Nonnull Data item) {
        return equals(item);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id) &&
           Objects.equals(value, data.value);
    }
}

This is definitely what you shouldn't do.

Bug #5

When using RxJava with adapter (this is not strictly related to ChangesDetector) there is one more thing to remember.

public class Data implements SimpleDetector.Detectable<Data> {

    private final long id;
    @Nonnull
    private final Observable<String> observable;
    @Nonnull
    private final Scheduler uiScheduler;

    Data(long id, @Nonnull Observable<String> observable, Scheduler uiScheduler) {
        this.id = id;
        this.observable = observable;
        this.uiScheduler = uiScheduler;
    }

    @Override
    public boolean matches(@Nonnull Data item) {
        return Objects.equals(item.id, id);
    }

    public Observable<String> observable() {
        return observable.observeOn(uiScheduler);
    }

    @Override
    public boolean same(@Nonnull Data item) {
        return equals(item);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id) &&
           Objects.equals(observable, data.observable) &&
           Objects.equals(uiScheduler, data.uiScheduler);
    }
}

This code actually have some bug. Because RecyclerView recycle it's old items. So in case when we have some items:

adapter.call(Arrays.toList(
    new Data(1, Observable.just("1").delay(1, TimeUnite.SECONDS)),
    new Data(2, Observable.just("2").delay(1, TimeUnite.SECONDS)),
    new Data(3, Observable.just("3").delay(1, TimeUnite.SECONDS)),
    new Data(4, Observable.just("4").delay(1, TimeUnite.SECONDS)),
    new Data(5, Observable.just("5").delay(1, TimeUnite.SECONDS))
    // ...
))

List will be populated correctly and values ["1", "2", "3", "4"] will be populated after second. But after user will scroll something weird happens. We will see ["2", "3", "4", "1"] and then, after a second, it will change to ["2", "3", "4", "5"], because Observable<String> observable() does not have start value and first item was recycled to populate fifth element.

So you need to change your code to:

public class Data {
    // ...
    public Observable<String> observable() {
        return observable.observeOn(uiScheduler).startWith("");
    }
    // ...
}

Bug #6

While using RxJava trying skipping some fields in equals can lead to bugs

public class Data implements SimpleDetector.Detectable<Data> {

    private final long id;
    @Nonnull
    private final Observable<String> observable;

    Data(long id, @Nonnull Observable<String> observable) {
        this.id = id;
        this.observable = observable;
    }

    @Override
    public boolean matches(@Nonnull Data item) {
        return Objects.equals(item.id, id);
    }

    public Observable<String> observable() {
        return observable;
    }

    @Override
    public boolean same(@Nonnull Data item) {
        return equals(item);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Data)) return false;
        final Data data = (Data) o;
        return Objects.equals(id, data.id);
    }
}

So when you type:

final Observable<String> instance = Observable.just("")
adapter.call(Arrays.toList(Data(1, instance.observeOn(uiScheduler))), new Data(2, instance.observeOn(uiScheduler)));
adapter.call(Arrays.toList(Data(1, instance.observeOn(uiScheduler))), new Data(2, instance.observeOn(uiScheduler)));

everything is ok, but when you are going to change value of observable, nothing will be populated to RecyclerView:

adapter.call(Arrays.toList(Data(1, instance.observeOn(uiScheduler))), new Data(2, Observable.just("new value").observeOn(uiScheduler)));

So if you would like to be on safe site - include all fields in matches.

TIPS

Using Mateusz's AutoValue plugin usually your code will be less potential to bugs.