android - 使用 View.BaseSavedState 覆盖 View.onSaveInstanceState() 和 View.onRestoreInstanceState()？

Question

假设您想从现有的 View 实现派生自己的 View 类，添加一些值，从而维护一些变量，这些变量以有意义的方式表示您的 View 的状态。

如果您的 View 能像其他 View 一样自动保存其状态(如果分配了 ID)，那就太好了，所以您会想要覆盖 onRestoreInstanceState() 和 onSaveInstanceState().

当然，你需要调用你的基类各自的方法，你需要结合你的状态信息和你的基类。

显然，唯一安全的方法是将父类(super class)的 Parcelable 包装在自己的 Parcelable 中，这样 key 就不会混淆。

现在有 View.BaseSavedState及其有趣的 getSuperState() 方法，但我不知何故无法理解这如何真正增加了将基类的 Parcelable 存储在 Bundle 中的值(value)连同派生的 View 的状态值并返回它。另一方面，也许其他一些系统组件会期望所有 InstanceState 信息都是 View.AbsSavedState 类型(例如，这样可以调用 getSuperState())？

有什么经验可以分享吗？

Answer 1

为了补充 James Chen 的回答，here是如何使用此方法的完整示例，基于博客 article查尔斯·哈利 (Charles Harley) 着。

链接中的代码:

public class LockCombinationPicker extends LinearLayout {
    private NumberPicker numberPicker1;
    private NumberPicker numberPicker2;
    private NumberPicker numberPicker3;

    public LockCombinationPicker(Context context) {
        this(context, null);
    }

    public LockCombinationPicker(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public LockCombinationPicker(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        loadViews();
    }

    private void loadViews() {
        LayoutInflater.from(getContext()).inflate(R.layout.lock_combination_picker, this, true);
        numberPicker1 = (NumberPicker) findViewById(R.id.number1);
        numberPicker1.setMinValue(0);
        numberPicker1.setMaxValue(10);
        numberPicker2 = (NumberPicker) findViewById(R.id.number2);
        numberPicker2.setMinValue(0);
        numberPicker2.setMaxValue(10);
        numberPicker3 = (NumberPicker) findViewById(R.id.number3);
        numberPicker3.setMinValue(0);
        numberPicker3.setMaxValue(10);
    }

    @Override
    protected Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();
        return new SavedState(superState, numberPicker1.getValue(), numberPicker2.getValue(), numberPicker3.getValue());
    }

    @Override
    protected void onRestoreInstanceState(Parcelable state) {
        SavedState savedState = (SavedState) state;
        super.onRestoreInstanceState(savedState.getSuperState());
        numberPicker1.setValue(savedState.getNumber1());
        numberPicker2.setValue(savedState.getNumber2());
        numberPicker3.setValue(savedState.getNumber3());
    }

    @Override
    protected void dispatchSaveInstanceState(SparseArray<Parcelable> container) {
        // As we save our own instance state, ensure our children don't save and restore their state as well.
        super.dispatchFreezeSelfOnly(container);
    }

    @Override
    protected void dispatchRestoreInstanceState(SparseArray<Parcelable> container) {
        /** See comment in {@link #dispatchSaveInstanceState(android.util.SparseArray)} */
        super.dispatchThawSelfOnly(container);
    }

    /**
     * Convenience class to save / restore the lock combination picker state. Looks clumsy but once created is easy to maintain and use.
     */
    protected static class SavedState extends BaseSavedState {
        private final int number1;
        private final int number2;
        private final int number3;

        private SavedState(Parcelable superState, int number1, int number2, int number3) {
            super(superState);
            this.number1 = number1;
            this.number2 = number2;
            this.number3 = number3;
        }

        private SavedState(Parcel in) {
            super(in);
            number1 = in.readInt();
            number2 = in.readInt();
            number3 = in.readInt();
        }

        public int getNumber1() {
            return number1;
        }

        public int getNumber2() {
            return number2;
        }

        public int getNumber3() {
            return number3;
        }

        @Override
        public void writeToParcel(Parcel destination, int flags) {
            super.writeToParcel(destination, flags);
            destination.writeInt(number1);
            destination.writeInt(number2);
            destination.writeInt(number3);
        }

        public static final Parcelable.Creator<SavedState> CREATOR = new Creator<SavedState>() {
            public SavedState createFromParcel(Parcel in) {
                return new SavedState(in);
            }

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