Viewpager pageradapter notifydatasetchanged invalid problem

As we all know , Whether we are using RecyclerView still ListView when , After updating the data source, it will call adapter Of notifyDataSetChanged() Method to update the view . and PagerAdapter Namely ViewPager The use of adapter, But in practice , It is found that updating the data source does not necessarily update the view .

Let's take a look at Google's right PagerAdapter This method of notifyDataSetChanged() The definition of

 

 

 

  Personally, I don't think the definition of Google's method is a good explanation for the problems I encounter in my work , So try the source code ...

/**
 * This method should be called by the application if the data backing this adapter has changed
 * and associated views should update.
 */
public void notifyDataSetChanged() {
    synchronized (this) {
        if (mViewPagerObserver != null) {
            mViewPagerObserver.onChanged();
        }
    }
    mObservable.notifyChanged();
}

 At first glance, it is the observer mode , Here we only focus on mViewPagerObserver, See where it is assigned .

void setViewPagerObserver(DataSetObserver observer) {
    synchronized (this) {
        mViewPagerObserver = observer;
    }
}

Let's see where this method is called .

/**
 * Set a PagerAdapter that will supply views for this pager as needed.
 *
 * @param adapter Adapter to use
 */
public void setAdapter(PagerAdapter adapter) {
    if (mAdapter != null) {
        mAdapter.setViewPagerObserver(null);
        mAdapter.startUpdate(this);
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
            mAdapter.destroyItem(this, ii.position, ii.object);
        }
        mAdapter.finishUpdate(this);
        mItems.clear();
        removeNonDecorViews();
        mCurItem = 0;
        scrollTo(0, 0);
    }

    final PagerAdapter oldAdapter = mAdapter;
    mAdapter = adapter;
    mExpectedAdapterCount = 0;

    if (mAdapter != null) {
        if (mObserver == null) {
            mObserver = new PagerObserver();
        }
        mAdapter.setViewPagerObserver(mObserver);
        mPopulatePending = false;
        final boolean wasFirstLayout = mFirstLayout;
        mFirstLayout = true;
        mExpectedAdapterCount = mAdapter.getCount();
        if (mRestoredCurItem >= 0) {
            mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);
            setCurrentItemInternal(mRestoredCurItem, false, true);
            mRestoredCurItem = -1;
            mRestoredAdapterState = null;
            mRestoredClassLoader = null;
        } else if (!wasFirstLayout) {
            populate();
        } else {
            requestLayout();
        }
    }

    // Dispatch the change to any listeners
    if (mAdapterChangeListeners != null && !mAdapterChangeListeners.isEmpty()) {
        for (int i = 0, count = mAdapterChangeListeners.size(); i < count; i++) {
            mAdapterChangeListeners.get(i).onAdapterChanged(this, oldAdapter, adapter);
        }
    }
}

In the above code, it is seen in setAdapter Assign values in this method , And see mObserver = new PagerObserver();mObserver This object is also created in this method , I want to see others PagerObserver This class , Is a private inner class .

private class PagerObserver extends DataSetObserver {
    PagerObserver() {
    }

    @Override
    public void onChanged() {
        dataSetChanged();
    }
    @Override
    public void onInvalidated() {
        dataSetChanged();
    }
}

Now it's clear ,notifyDataSetChanged() In the method mViewPagerObserver.onChanged() Execution is dataSetChanged() Method , Let's see dataSetChanged() This method .

void dataSetChanged() {
    // This method only gets called if our observer is attached, so mAdapter is non-null.

    final int adapterCount = mAdapter.getCount();
    mExpectedAdapterCount = adapterCount;
    boolean needPopulate = mItems.size() < mOffscreenPageLimit * 2 + 1
            && mItems.size() < adapterCount;
    int newCurrItem = mCurItem;

    boolean isUpdating = false;
    for (int i = 0; i < mItems.size(); i++) {
        final ItemInfo ii = mItems.get(i);
        final int newPos = mAdapter.getItemPosition(ii.object);

        if (newPos == PagerAdapter.POSITION_UNCHANGED) {
            continue;
        }

        if (newPos == PagerAdapter.POSITION_NONE) {
            mItems.remove(i);
            i--;

            if (!isUpdating) {
                mAdapter.startUpdate(this);
                isUpdating = true;
            }

            mAdapter.destroyItem(this, ii.position, ii.object);
            needPopulate = true;

            if (mCurItem == ii.position) {
                // Keep the current item in the valid range
                newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                needPopulate = true;
            }
            continue;
        }

        if (ii.position != newPos) {
            if (ii.position == mCurItem) {
                // Our current item changed position. Follow it.
                newCurrItem = newPos;
            }

            ii.position = newPos;
            needPopulate = true;
        }
    }

    if (isUpdating) {
        mAdapter.finishUpdate(this);
    }

    Collections.sort(mItems, COMPARATOR);

    if (needPopulate) {
        // Reset our known page widths; populate will recompute them.
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            if (!lp.isDecor) {
                lp.widthFactor = 0.f;
            }
        }

        setCurrentItemInternal(newCurrItem, false, true);
        requestLayout();
    }
}

See the red mark , I want to see others getItemPosition() This method ,

/**
 * Called when the host view is attempting to determine if an item's position
 * has changed. Returns {@link #POSITION_UNCHANGED} if the position of the given
 * item has not changed or {@link #POSITION_NONE} if the item is no longer present
 * in the adapter.
 *
 * <p>The default implementation assumes that items will never
 * change position and always returns {@link #POSITION_UNCHANGED}.
 *
 * @param object Object representing an item, previously returned by a call to
 *               {@link #instantiateItem(View, int)}.
 * @return object's new position index from [0, {@link #getCount()}),
 *         {@link #POSITION_UNCHANGED} if the object's position has not changed,
 *         or {@link #POSITION_NONE} if the item is no longer present.
 */
public int getItemPosition(Object object) {
    return POSITION_UNCHANGED;
}

Default return POSITION_UNCHANGED, This leads to the update of the data source , But the view doesn't change .

So my solution is to rewrite getItemPosition() This method , To return to POSITION_NONE. Of course, this method costs a lot of system , Do it every time destroyItem() Method , If the amount of data is small , It will cause a lot of unnecessary expenses .