android - Parcelable 接口(interface)保留对象引用？ &Parcelable.CREATOR 是不是叫了？

Question

我正在使用 Parcelable 接口(interface)将我的对象传递到 fragment 中。根据 Android 的文档，它是一个“类的接口(interface)，其实例可以写入 Parcel 并从中恢复”[1] .
我期待每次当 ParcelableObject 通过 Bundle 传递到另一个 fragment 时，对象被写入 Parcel 然后 Parcelable.CREATOR 应该重新创建对象并初始化对象的变量。
如果我将我的可打包对象 (X) 放入 Bundle 并将该包设置为我的新 fragment 的参数，这对我来说有点意外。然后我在新 fragment 的 onCreateView 方法中从 bundle 中读取我的 parcelable 对象 (Y)。我在 onCreateView (Y) 中获得的对象与我在前一个 fragment 中放入 Bundle 的对象 (X) 相同(意思是 X == Y)。更令人惊讶的是，Parcelable.CREATOR 甚至没有被调用。
他们在 Android 的文档中写到 Bundle:“从 String 值到各种 Parcelable 类型的映射。” [2] .好吧，这也许可以解释为什么写入的对象与读取的对象相同，但为什么他们需要 Parcelable.CREATOR 接口(interface)，即使没有它也能正常工作？

现在让我们更具体一点:

我有一个 ParentPagerFragment，它包含一个 ViewPager，它是 3 个 ChildTextViewFragment 的父级

这里是 ParentPagerFragment 实现的重要部分。

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    View v = inflater.inflate(R.layout.fragment_pager, container, false);

    List<ChildTextViewFragment> viewFragments = new ArrayList<>();

    MyEntity mEntity1 = new MyEntity();
    mEntity1.setId(1l);
    MyEntity mEntity2 = new MyEntity();
    mEntity2.setId(2l);
    MyEntity mEntity3 = new MyEntity();
    mEntity3.setId(3l);

    viewFragments.add(ChildTextViewFragment.newInstance(mEntity1));
    viewFragments.add(ChildTextViewFragment.newInstance(mEntity2));
    viewFragments.add(ChildTextViewFragment.newInstance(mEntity3));

    MyPagerAdapter mPagerAdapter = new MyPagerAdapter(getChildFragmentManager(), viewFragments);

    ViewPager mViewPager = (ViewPager) v.findViewById(R.id.pager);
    mViewPager.setAdapter(mPagerAdapter);

    return v;
}

ChildTextViewFragment 只显示一个简单的TextView

ChildTextViewFragment::newInstance 实现

public static ChildTextViewFragment newInstance(MyEntity mEntity) {
        ChildTextViewFragment fragment = new ChildTextViewFragment();
        Bundle bundle = new Bundle();

        Log.d(MyEntityParcelable.TAG, "ChildTextViewFragment::newInstance : creating Parcelable : mEntity has ID = "+ mEntity.getId() +" mEntity = " + mEntity);
        MyEntityParcelable entityPar = new MyEntityParcelable(mEntity);
        bundle.putParcelable(M_ENTITY_KEY, entityPar);
        fragment.setArguments(bundle);

        return fragment;
    }

ChildTextViewFragment::onCreateView 实现

@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.fragment_text, container, false);

        TextView mTextView = (TextView) view.findViewById(R.id.textView1);
        MyEntityParcelable mEntityPar = getArguments().getParcelable(M_ENTITY_KEY);
        MyEntity mEntity = mEntityPar.getMyEntity();
        Log.d(MyEntityParcelable.TAG, "ChildTextViewFragment::onCreateView : reading Parcelable : mEntity has ID = "+mEntity.getId()+" mEntity = " + mEntity);

        mTextView.setText("my id is: " + Long.toString(mEntity.getId()));

        return view;

    }

我的实体:

public class MyEntity {

    private long id;
    private String name;
    private String description;
    private String owner;

    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    public String getOwner() {
        return owner;
    }
    public void setOwner(String owner) {
        this.owner = owner;
    }




}

MyEntityParcelable

public class MyEntityParcelable implements Parcelable {

    public static final String TAG = "PARCELABLE_TESTING";

    private MyEntity myEntity;

    public MyEntityParcelable(MyEntity mEntity) {
        this.myEntity = mEntity;
    }

    private MyEntityParcelable(Parcel in) {
        Log.d(TAG, "MyEntityParcelable::Parcel constructor : creating MyEntity from Parcel");
        myEntity = new MyEntity();
        myEntity.setId(in.readLong());
        myEntity.setName(in.readString());
        myEntity.setDescription(in.readString());
        myEntity.setOwner(in.readString());
    }

    @Override
    public int describeContents() {
        return this.hashCode();
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        Log.d(TAG, "MyEntityParcelable::writeToParcel : writing myEntity with ID =  " + myEntity.getId() + " to Parcel");
        dest.writeLong(myEntity.getId());
        dest.writeString(myEntity.getName());
        dest.writeString(myEntity.getDescription());
        dest.writeString(myEntity.getOwner());

    }

    /*
     * Parcelable interface must also have a static field called CREATOR, which
     * is an object implementing the Parcelable.Creator interface. Used to
     * un-marshal or de-serialize object from Parcel.
     */
    public static final Parcelable.Creator<MyEntityParcelable> CREATOR = new Parcelable.Creator<MyEntityParcelable>() {
        public MyEntityParcelable createFromParcel(Parcel in) {
            return new MyEntityParcelable(in);
        }

        public MyEntityParcelable[] newArray(int size) {
            return new MyEntityParcelable[size];
        }
    };

    public MyEntity getMyEntity() {
        return myEntity;
    }

}

正如我上面提到的，甚至没有调用 PARCELABLE.CREATOR，这里是演示控制流的日志。

13:55:14.946: ChildTextViewFragment::newInstance : creating Parcelable : mEntity has ID = 1 mEntity = com.example.nestedfragments.MyEntity@21208247
13:55:14.951: ChildTextViewFragment::newInstance : creating Parcelable : mEntity has ID = 2 mEntity = com.example.nestedfragments.MyEntity@1ecfc4f8
13:55:14.952: ChildTextViewFragment::newInstance : creating Parcelable : mEntity has ID = 3 mEntity = com.example.nestedfragments.MyEntity@3c4a9dd1
13:55:15.069: ChildTextViewFragment::onCreateView : reading Parcelable : mEntity has ID = 1 mEntity = com.example.nestedfragments.MyEntity@21208247
13:55:15.071: ChildTextViewFragment::onCreateView : reading Parcelable : mEntity has ID = 2 mEntity = com.example.nestedfragments.MyEntity@1ecfc4f8
13:55:33.614: ChildTextViewFragment::onCreateView : reading Parcelable : mEntity has ID = 3 mEntity = com.example.nestedfragments.MyEntity@3c4a9dd1

请注意对象标识在创建和读取时是相同的。

这是在 fragment 之间传递对象的正确方法吗？在哪些情况下调用 Parcelable.CREATOR 接口(interface)？

谢谢你的帮助!

Answer 1

So ok that's maybe what explains why is the written object identical with the read one, but why they need the Parcelable.CREATOR interface when it works as correct even without it ?

您的 Parcelable 未放入 Parcel 中，因此未使用 Parcelable.CREATOR。

Parcel 涉及IPC。因此，如果您尝试通过 startActivity() 额外传递相同的 Intent，那将涉及您的 CREATOR，如 startActivity()涉及IPC。

配置更改可能涉及包裹(他们可能会优化这种情况)。如果您的进程在后台终止，并且用户通过最近任务列表快速返回到您正在运行的任务，这将涉及一个 Parcel 和一个 CREATOR。