Arms-merchants

这是在基于总的Fragment数量为5，并且第一次数据加载已经完成，从0的tab切换到2的tab的过程，limit为1，分析2的创建以及3的预加载，还有0的destory：

    void populate(int newCurrentItem) {
        //newCurrentItem =2
        ViewPager.ItemInfo oldCurInfo = null;
        if (mCurItem != newCurrentItem) {
            //oldCurInfo = 0
            oldCurInfo = infoForPosition(mCurItem);
            //mCurItem = 2
            mCurItem = newCurrentItem;
        }

        mAdapter.startUpdate(this);
        //mOffscreenPageLimit = 1
        final int pageLimit = mOffscreenPageLimit;
        //startPos = 1
        final int startPos = Math.max(0, mCurItem - pageLimit);
        //n= 5
        final int N = mAdapter.getCount();
        //endPos = 3
        final int endPos = Math.min(N - 1, mCurItem + pageLimit);

        // Locate the currently focused item or add it if needed.
        int curIndex = -1;
        ViewPager.ItemInfo curItem = null;
        // mItems.size() = 2 (0,1)

        for (curIndex = 0; curIndex < mItems.size(); curIndex++) {
            final ViewPager.ItemInfo ii = mItems.get(curIndex);
            if (ii.position >= mCurItem) {
                if (ii.position == mCurItem) curItem = ii;
                break;
            }
        }
        //curIndex =2
        if (curItem == null && N > 0) {
            curItem = addNewItem(mCurItem, curIndex);
        }
        //把index =2的fragment加入了

        // Fill 3x the available width or up to the number of offscreen
        // pages requested to either side, whichever is larger.
        // If we have no current item we have no work to do.
        if (curItem != null) {
            float extraWidthLeft = 0.f;
            //itemIndex = 1
            int itemIndex = curIndex - 1;
            //拿到1的ItemInfo
            ViewPager.ItemInfo ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
            final int clientWidth = getClientWidth();
            final float leftWidthNeeded = clientWidth <= 0 ? 0 :
                    2.f - curItem.widthFactor + (float) getPaddingLeft() / (float) clientWidth;
            //pos = 1
            for (int pos = mCurItem - 1; pos >= 0; pos--) {
                if (extraWidthLeft >= leftWidthNeeded && pos < startPos) {
                    if (ii == null) {
                        break;
                    }
                    if (pos == ii.position && !ii.scrolling) {
                        mItems.remove(itemIndex);
                        mAdapter.destroyItem(this, pos, ii.object);
                        if (DEBUG) {
                            Log.i(TAG, "populate() - destroyItem() with pos: " + pos
                                    + " view: " + ((View) ii.object));
                        }
                        itemIndex--;
                        curIndex--;
                        ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                    }
                } else if (ii != null && pos == ii.position) {
                    extraWidthLeft += ii.widthFactor;
                    itemIndex--;
                    //在1的时候拿到0的ItemInfo
                    ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                } else {
                    ii = addNewItem(pos, itemIndex + 1);
                    extraWidthLeft += ii.widthFactor;
                    curIndex++;
                    ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                }
            }

            float extraWidthRight = curItem.widthFactor;
            itemIndex = curIndex + 1;
            if (extraWidthRight < 2.f) {
                ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                final float rightWidthNeeded = clientWidth <= 0 ? 0 :
                        (float) getPaddingRight() / (float) clientWidth + 2.f;
                for (int pos = mCurItem + 1; pos < N; pos++) {
                    if (extraWidthRight >= rightWidthNeeded && pos > endPos) {
                        if (ii == null) {
                            break;
                        }
                        if (pos == ii.position && !ii.scrolling) {
                            mItems.remove(itemIndex);
                            mAdapter.destroyItem(this, pos, ii.object);
                            if (DEBUG) {
                                Log.i(TAG, "populate() - destroyItem() with pos: " + pos
                                        + " view: " + ((View) ii.object));
                            }
                            ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                        }
                    } else if (ii != null && pos == ii.position) {
                        extraWidthRight += ii.widthFactor;
                        itemIndex++;
                        ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                    } else {
                        ii = addNewItem(pos, itemIndex);
                        itemIndex++;
                        extraWidthRight += ii.widthFactor;
                        ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                    }
                }
            }

            calculatePageOffsets(curItem, curIndex, oldCurInfo);

            mAdapter.setPrimaryItem(this, mCurItem, curItem.object);
        }

        mAdapter.finishUpdate(this);

        // Check width measurement of current pages and drawing sort order.
        // Update LayoutParams as needed.
        //3
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            final ViewPager.LayoutParams lp = (ViewPager.LayoutParams) child.getLayoutParams();
            lp.childIndex = i;
            if (!lp.isDecor && lp.widthFactor == 0.f) {
                // 0 means requery the adapter for this, it doesn't have a valid width.
                final ViewPager.ItemInfo ii = infoForChild(child);
                if (ii != null) {
                    lp.widthFactor = ii.widthFactor;
                    lp.position = ii.position;
                }
            }
        }
        sortChildDrawingOrder();

        if (hasFocus()) {
            View currentFocused = findFocus();
            ViewPager.ItemInfo ii = currentFocused != null ? infoForAnyChild(currentFocused) : null;
            if (ii == null || ii.position != mCurItem) {
                for (int i = 0; i < getChildCount(); i++) {
                    View child = getChildAt(i);
                    ii = infoForChild(child);
                    if (ii != null && ii.position == mCurItem) {
                        if (child.requestFocus(View.FOCUS_FORWARD)) {
                            break;
                        }
                    }
                }
            }
        }
    }