從源碼上分析Android View保存數據狀態

構造函數調用派生類創建對象時傳入他們的savedstate，其實就是一個序列化數據的寫入，恢復數據無非就是從這個序列裡面讀取出剛剛寫入的數據。好了，我們再來分析數據恢復的過程。
從上面那張圖中，我們不難看出數據的恢復首先會調用restoreHierarchyState(SparseArray container)這個方法，然後再調dispatchRestoreInstanceState(SparseArray container)，最後調onRestoreInstanceState(Parcelable state)。所以我們接下一步步往下看。

4、restoreHierarchyState(SparseArray container)

    /**
     * Restore this view hierarchy's frozen state from the given container.
     *
     * @param container The SparseArray which holds previously frozen states.
     *
     * @see #saveHierarchyState(android.util.SparseArray)
     * @see #dispatchRestoreInstanceState(android.util.SparseArray)
     * @see #onRestoreInstanceState(android.os.Parcelable)
     */
    public void restoreHierarchyState(SparseArray container) {
        dispatchRestoreInstanceState(container);
    }

即從給定容器中恢復此視圖層次結構的凍結狀態。跟剛剛保存數據是一個相反的過程。

5、dispatchRestoreInstanceState(SparseArray container)

    /**
     * Called by {@link #restoreHierarchyState(android.util.SparseArray)} to retrieve the
     * state for this view and its children. May be overridden to modify how restoring
     * happens to a view's children; for example, some views may want to not store state
     * for their children.
     *
     * @param container The SparseArray which holds previously saved state.
     *
     * @see #dispatchSaveInstanceState(android.util.SparseArray)
     * @see #restoreHierarchyState(android.util.SparseArray)
     * @see #onRestoreInstanceState(android.os.Parcelable)
     */
    protected void dispatchRestoreInstanceState(SparseArray container) {
        if (mID != NO_ID) {
            Parcelable state = container.get(mID);
            if (state != null) {
                // Log.i("View", "Restoreing #" + Integer.toHexString(mID)
                // + ": " + state);
                mPrivateFlags &= ~PFLAG_SAVE_STATE_CALLED;
                onRestoreInstanceState(state);
                if ((mPrivateFlags & PFLAG_SAVE_STATE_CALLED) == 0) {
                    throw new IllegalStateException(
                            "Derived class did not call super.onRestoreInstanceState()");
                }
            }
        }
    }

還記得保存數據的時候安卓是怎麼干的嗎？嘿嘿，把當前view的ID當作鍵，把Parcelable當作值，保存到給定的container Map容器裡面。那麼現在是恢復我們之前保存的數據，那當然是要從Map容器裡面把數據讀取出來。即根據當前view的ID找出相應的Parcelable值，然後一次同時，把這個Parcelable值傳給onRestoreInstanceState()。那麼我們順著往下看onRestoreInstanceState()到底干了啥。
6、onRestoreInstanceState(Parcelable state)

 /**
     * Hook allowing a view to re-apply a representation of its internal state that had previously
     * been generated by {@link #onSaveInstanceState}. This function will never be called with a
     * null state.
     *
     * @param state The frozen state that had previously been returned by
     *        {@link #onSaveInstanceState}.
     *
     * @see #onSaveInstanceState()
     * @see #restoreHierarchyState(android.util.SparseArray)
     * @see #dispatchRestoreInstanceState(android.util.SparseArray)
     */
    @CallSuper
    protected void onRestoreInstanceState(Parcelable state) {
        mPrivateFlags |= PFLAG_SAVE_STATE_CALLED;
        if (state != null && !(state instanceof AbsSavedState)) {
            throw new IllegalArgumentException("Wrong state class, expecting View State but "
                    + "received " + state.getClass().toString() + " instead. This usually happens "
                    + "when two views of different type have the same id in the same hierarchy. "
                    + "This view's id is " + ViewDebug.resolveId(mContext, getId()) + ". Make sure "
                    + "other views do not use the same id.");
        }
        if (state != null && state instanceof BaseSavedState) {
            mStartActivityRequestWho = ((BaseSavedState) state).mStartActivityRequestWhoSaved;
        }
    }

看到沒，這個過程就是從BaseSavedState裡把之前寫進去的帶有數據屬性的變量給讀取出來。好了，Android系統系統控件的狀態整個保存以及恢復的過程到此分析完成。接下來我們來看看如果是我們自定義的控件，我們應該如何來保存我們的狀態數據。既然安卓系統控件的狀態保存我們都掌握了，那麼毫無懸念我們就按照安卓系統的方案走呗。這裡我舉例看看我的自定義控件的數據保存是怎麼干的，我這裡需要自定義一個輪播效果的引導頁，那肯定得把當前頁保存起來，不然不小心旋轉屏幕或者按下back鍵，再進入就不是離開時候的那個頁面了。來看看我是怎麼保存的。

  @Override
    public void onRestoreInstanceState(Parcelable state) {
        SavedState savedState = (SavedState) state;
        super.onRestoreInstanceState(savedState.getSuperState());
        mCurrentPage = savedState.currentPage;
        mSnapPage = savedState.currentPage;
        requestLayout();
    }

    @Override
    public Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();
        SavedState savedState = new SavedState(superState);
        savedState.currentPage = mCurrentPage;
        return savedState;
    }

    static class SavedState extends BaseSavedState {
        int currentPage;

        public SavedState(Parcelable superState) {
            super(superState);
        }

        private SavedState(Parcel in) {
            super(in);
            currentPage = in.readInt();
        }

        @Override
        public void writeToParcel(Parcel dest, int flags) {
            super.writeToParcel(dest, flags);
            dest.writeInt(currentPage);
        }

        @SuppressWarnings("UnusedDeclaration")
        public static final Creator CREATOR = new Creator() {
            @Override
            public SavedState createFromParcel(Parcel in) {
                return new SavedState(in);
            }

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

看到了吧，無非是按照Android系統控件保存的那幾個步驟，先把數據保存起來，需要的時候再把它取出來。
再來看看最常見的CheckBox是怎麼保存狀態。

static class SavedState extends BaseSavedState {
        boolean checked;

        /**
         * Constructor called from {@link CompoundButton#onSaveInstanceState()}
         */
        SavedState(Parcelable superState) {
            super(superState);
        }

        /**
         * Constructor called from {@link #CREATOR}
         */
        private SavedState(Parcel in) {
            super(in);
            checked = (Boolean)in.readValue(null);
        }

        @Override
        public void writeToParcel(Parcel out, int flags) {
            super.writeToParcel(out, flags);
            out.writeValue(checked);
        }

        @Override
        public String toString() {
            return "CompoundButton.SavedState{"
                    + Integer.toHexString(System.identityHashCode(this))
                    + " checked=" + checked + "}";
        }

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

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

    @Override
    public Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();

        SavedState ss = new SavedState(superState);

        ss.checked = isChecked();
        return ss;
    }

    @Override
    public void onRestoreInstanceState(Parcelable state) {
        SavedState ss = (SavedState) state;

        super.onRestoreInstanceState(ss.getSuperState());
        setChecked(ss.checked);
        requestLayout();
    }

總結：在安卓中有一個類(View.BaseSavedState)專門做數據保存這件事情。
(1)通過繼承它來實現保存上一級的狀態同時允許你保存自定義的屬性。在onRestoreInstanceState()期間我們則需要做相反的事情
(2)從指定的Parcelable中獲取上一級的狀態，同時讓你的父類通過調用super.onRestoreInstanceState(ss.getSuperState())來恢復它的狀態。之後我們才能恢復我們自己的狀態