前言

ViewPager默认情况的加载，会默认预加载一个的布局到ViewPager中，这就时ViewPager的预加载。setOffscreenPageLimit可以通过这个方法去设置预加载布局的个数。

 public void setOffscreenPageLimit(int limit) {
        if (limit < 1) {
            Log.w("ViewPager", "Requested offscreen page limit " + limit + " too small; defaulting to " + 1);
            limit = 1;
        }

        if (limit != this.mOffscreenPageLimit) {
            this.mOffscreenPageLimit = limit;
            this.populate();
        }

    }

从上面可以看出，预加载的页面个数大于等于1。

缓存的是什么？

在缓存数量更新的时候，要执行缓存方法polulate()。去看看里面做了啥事

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

        if (this.mAdapter == null) {
            this.sortChildDrawingOrder();
        } else if (this.mPopulatePending) {
            this.sortChildDrawingOrder();
        } else if (this.getWindowToken() != null) {
            this.mAdapter.startUpdate(this);//被弃用了
            int pageLimit = this.mOffscreenPageLimit;//当前的缓存页面个数
            int startPos = Math.max(0, this.mCurItem - pageLimit);//计算缓存的开始位置
            int N = this.mAdapter.getCount();//adapter的子内容的数量
            int endPos = Math.min(N - 1, this.mCurItem + pageLimit);//计算缓存的结束位置
            if (N != this.mExpectedAdapterCount) {
                String resName;
                try {
                    resName = this.getResources().getResourceName(this.getId());
                } catch (NotFoundException var17) {
                    resName = Integer.toHexString(this.getId());
                }

                throw new IllegalStateException("The application's PagerAdapter changed the adapter's contents without calling PagerAdapter#notifyDataSetChanged! Expected adapter item count: " + this.mExpectedAdapterCount + ", found: " + N + " Pager id: " + resName + " Pager class: " + this.getClass() + " Problematic adapter: " + this.mAdapter.getClass());
            } else {
                int curIndex = true;
                ViewPager.ItemInfo curItem = null;
                //开始去找 ViewPager.ItemInfo
                int curIndex;
                for(curIndex = 0; curIndex < this.mItems.size(); ++curIndex) {
                    ViewPager.ItemInfo ii = (ViewPager.ItemInfo)this.mItems.get(curIndex);
                    if (ii.position >= this.mCurItem) {
                        if (ii.position == this.mCurItem) {
                            curItem = ii;
                        }
                        break;
                    }
                }
                //没找到就去创建 ViewPager.ItemInfo，并放入ArrayList<ViewPager.ItemInfo> mItems中
                if (curItem == null && N > 0) {
                    curItem = this.addNewItem(this.mCurItem, curIndex);
                }

                int itemIndex;
                ViewPager.ItemInfo ii;
                int i;
                 ·····一些计算操作，省略
                }
            
                this.mAdapter.finishUpdate(this);//完成条目的更新
                int childCount = this.getChildCount();

                for(itemIndex = 0; itemIndex < childCount; ++itemIndex) {
                    View child = this.getChildAt(itemIndex);
                    ViewPager.LayoutParams lp = (ViewPager.LayoutParams)child.getLayoutParams();
                    lp.childIndex = itemIndex;
                    if (!lp.isDecor && lp.widthFactor == 0.0F) {
                        ViewPager.ItemInfo ii = this.infoForChild(child);
                        if (ii != null) {
                            lp.widthFactor = ii.widthFactor;
                            lp.position = ii.position;
                        }
                    }
                }
                this.sortChildDrawingOrder()；
            }
        }
    }

前面提到了创建ViewPager.ItemInfo，这个是什么

 ViewPager.ItemInfo addNewItem(int position, int index) {
        ViewPager.ItemInfo ii = new ViewPager.ItemInfo();
        ii.position = position;
        注释①
        ii.object = this.mAdapter.instantiateItem(this, position);
        ii.widthFactor = this.mAdapter.getPageWidth(position);
        if (index >= 0 && index < this.mItems.size()) {
            this.mItems.add(index, ii);
        } else {
            this.mItems.add(ii);
        }

        return ii;
    }

注释①：调用instantiateItem来创建object，在FragmentPagerAdapter的instantiateItem这个方法中，创建的就是fragment。所以缓存的就是fragment，fragment在创建时，会有ui操作，网络操作，在还未可见的时候，就去初始化fragment非常消耗性能，所以懒加载的方式来加载，不去缓存fragment。