Android中ViewPager+Fragment禁止预加载问题

使用ViewPager+Fragment组合加载数据，由于ViewPager的内部机制所限制，会默认至少预加载一个。但有时我们必须禁止预加载，比如第1个和第2个页面可以任意浏览，第3个页面需要登录才能浏览，此时会导致在切换到第2个页面时，便跳转到登录页面。

ViewPager.setOffscreenPageLimit(0)

可不可以设置ViewPager的预加载为0，不就解决问题了吗？经过测试发现，即使设置vp.setOffscreenPageLimit(0)，也会被强制拽回1！

查看Viewpager的setOffscreenPageLimit()源码：

private static final int DEFAULT_OFFSCREEN_PAGES = 1;

/**
 * Set the number of pages that should be retained to either side of the
 * current page in the view hierarchy in an idle state. Pages beyond this
 * limit will be recreated from the adapter when needed.
 *
 * <p>This is offered as an optimization. If you know in advance the number
 * of pages you will need to support or have lazy-loading mechanisms in place
 * on your pages, tweaking this setting can have benefits in perceived smoothness
 * of paging animations and interaction. If you have a small number of pages (3-4)
 * that you can keep active all at once, less time will be spent in layout for
 * newly created view subtrees as the user pages back and forth.</p>
 *
 * <p>You should keep this limit low, especially if your pages have complex layouts.
 * This setting defaults to 1.</p>
 *
 * @param limit How many pages will be kept offscreen in an idle state.
 */
public void setOffscreenPageLimit(int limit) {
    if (limit < DEFAULT_OFFSCREEN_PAGES) {
        Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to "
                + DEFAULT_OFFSCREEN_PAGES);
        limit = DEFAULT_OFFSCREEN_PAGES;
    }
    if (limit != mOffscreenPageLimit) {
        mOffscreenPageLimit = limit;
        populate();
    }
}

我们发现，即使设置为0，还是会在判断后设为默认值1。

Fragment.setUserVisibleHint(boolean isVisibleToUser)

Fragment中有一个setUserVisibleHint(boolean isVisibleToUser)方法，这个方法就是告诉用户，UI对用户是否可见，那么我们在这里去加载数据会怎么样呢？

因为ViewPager会加载好多Fragment，为了节省内容等会在Fragment不可见的某个时候调用onDestroyView()将用户界面销毁掉，但是Fragment的实例还在，所以可能第一次加载没有问题，但是再次回到第一个Fragment去加载时就会出现UI对用户可见但是视图还没有初始化的现象。

最终解决方案LazyLoadFragment

LazyLoadFragment.java

/**
 * Fragment预加载问题的解决方案：
 * 1.可以懒加载的Fragment
 * 2.切换到其他页面时停止加载数据（可选）
 */
public abstract class LazyLoadFragment extends Fragment {
    /**
     * 视图是否已经初初始化
     */
    protected boolean isInit = false;
    protected boolean isLoad = false;
    protected final String TAG = "LazyLoadFragment";
    private View view;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        view = inflater.inflate(setContentView(), container, false);
        isInit = true;
        /**初始化的时候去加载数据**/
        isCanLoadData();
        return view;
    }

    /**
     * 视图是否已经对用户可见，系统的方法
     */
    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        isCanLoadData();
    }

    /**
     * 是否可以加载数据
     * 可以加载数据的条件：
     * 1.视图已经初始化
     * 2.视图对用户可见
     */
    private void isCanLoadData() {
        if (!isInit) {
            return;
        }

        if (getUserVisibleHint()) {
            lazyLoad();
            isLoad = true;
        } else {
            if (isLoad) {
                stopLoad();
            }
        }
    }

    /**
     * 视图销毁的时候讲Fragment是否初始化的状态变为false
     */
    @Override
    public void onDestroyView() {
        super.onDestroyView();
        isInit = false;
        isLoad = false;

    }

    /**
     * 设置Fragment要显示的布局
     *
     * @return 布局的layoutId
     */
    protected abstract int setContentView();

    /**
     * 获取设置的布局
     *
     * @return
     */
    protected View getContentView() {
        return view;
    }

    /**
     * 找出对应的控件
     *
     * @param id
     * @param <T>
     * @return
     */
    protected <T extends View> T findViewById(int id) {
        return (T) getContentView().findViewById(id);
    }

    /**
     * 当视图初始化并且对用户可见的时候去真正的加载数据
     */
    protected abstract void lazyLoad();

    /**
     * 当视图已经对用户不可见并且加载过数据，如果需要在切换到其他页面时停止加载数据，可以调用此方法
     */
    protected void stopLoad() {

    }
}

Fragment1.java

public class Fragment1 extends LazyLoadFragment {

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        Log.i(TAG, "Fragment1: onCreateView");
        return super.onCreateView(inflater, container, savedInstanceState);
    }

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        Log.i(TAG, "Fragment1: setUserVisibleHint(" + isVisibleToUser + ")");
        super.setUserVisibleHint(isVisibleToUser);
    }

    @Override
    public int setContentView() {
        return R.layout.fragment_1;
    }

    @Override
    protected void lazyLoad() {
        Log.d(TAG, "Fragment1: lazyLoad");
    }

    @Override
    protected void stopLoad() {
        Log.d(TAG, "Fragment1: stopLoad");
    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity {
    private ViewPager viewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        viewPager = (ViewPager) findViewById(R.id.vp);
        initData();
    }

    private void initData() {
        final List<Fragment> fragments = new ArrayList<>();
        fragments.add(new Fragment1());
        fragments.add(new Fragment2());
        fragments.add(new Fragment3());
        fragments.add(new Fragment4());
        fragments.add(new Fragment5());
        viewPager.setOffscreenPageLimit(0);
        viewPager.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) {
            @Override
            public Fragment getItem(int position) {
                return fragments.get(position);
            }

            @Override
            public int getCount() {
                return fragments.size();
            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        Log.d("yezhou", "MainActivity: onResume");
    }
}

日志记录

App Strat -> Fragment1

Fragment1: setUserVisibleHint(false)
Fragment2: setUserVisibleHint(false)
Fragment1: setUserVisibleHint(true)
Fragment1: onCreateView
Fragment1: lazyLoad
Fragment2: onCreateView

Fragment1 -> Fragment2

Fragment3: setUserVisibleHint(false)
Fragment1: setUserVisibleHint(false)
Fragment1: stopLoad
Fragment2: setUserVisibleHint(true)
Fragment2: lazyLoad
Fragment3: onCreateView

Fragment2 -> Fragment3

Fragment4: setUserVisibleHint(false)
Fragment2: setUserVisibleHint(false)
Fragment2: stopLoad
setUserVisibleHint(true)
Fragment3: lazyLoad
Fragment4: onCreateView

Fragment3 -> Fragment2

Fragment1: setUserVisibleHint(false)
Fragment3: setUserVisibleHint(false)
Fragment3: stopLoad
Fragment2: setUserVisibleHint(true)
Fragment2: lazyLoad
Fragment1: onCreateView

Activity onResume -> Fragment1

Fragment1: setUserVisibleHint(false)
Fragment2: setUserVisibleHint(false)
Fragment1: setUserVisibleHint(true)
Fragment1: onCreateView
Fragment1: lazyLoad
Fragment2: onCreateView