与 TabHost 相比，ViewPager 和 Fragments 行为不一致

Question

我正在尝试实现一个四标签 ViewPager，因为 TabHost 不支持滑动。这里的问题是，我在每个片段的 OnCreateView 中都有一些异步调用，并且除了我在使用 ViewPager 时滑动到的那个之外，还进行了来自下一个片段的调用。

但是，对于 TabHost，情况并非如此。仅从选定选项卡进行调用。是我应该解决的 ViewPager 问题还是我为 Fragment 使用了错误的回调方法？

这是项目的部分代码：

public class MainActivity extends FragmentActivity implements TabHost.OnTabChangeListener, ViewPager.OnPageChangeListener {

    private TabHost mTabHost;
    private ViewPager mViewPager;
    private HashMap<String, TabInfo> mapTabInfo = new HashMap<String, MainActivity.TabInfo>();
    private PagerAdapter mPagerAdapter;

    private class TabInfo {
        private String tag;
        private Class<?> clss;
        private Bundle args;
        private Fragment fragment;
        TabInfo(String tag, Class<?> clazz, Bundle args) {
            this.tag = tag;
            this.clss = clazz;
            this.args = args;
        }

    }

    class TabFactory implements TabContentFactory {

        private final Context mContext;

        /**
         * @param context
         */
        public TabFactory(Context context) {
            mContext = context;
        }

        public View createTabContent(String tag) {
            View v = new View(mContext);
            v.setMinimumWidth(0);
            v.setMinimumHeight(0);
            return v;
        }

    }

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Inflate the layout
        setContentView(R.layout.activity_main);
        // Initialise the TabHost
        this.initialiseTabHost(savedInstanceState);
        if (savedInstanceState != null) {
            mTabHost.setCurrentTabByTag(savedInstanceState.getString("tab")); 
        }
        // Intialise ViewPager
        this.intialiseViewPager();
    }

    protected void onSaveInstanceState(Bundle outState) {
        outState.putString("tab", mTabHost.getCurrentTabTag()); //save the tab selected
        super.onSaveInstanceState(outState);
    }

    private void intialiseViewPager() {

        List<Fragment> fragments = new Vector<Fragment>();
        fragments.add(new ActivityFeedFragment()); 
        fragments.add(new InboxFragment());
        fragments.add(new TaskFragment());
        fragments.add(new CalendarEventFragment());
        this.mPagerAdapter  = new PagerAdapter(super.getSupportFragmentManager(), fragments);

        this.mViewPager = (ViewPager)super.findViewById(R.id.viewpager);
        this.mViewPager.setAdapter(this.mPagerAdapter);
        this.mViewPager.setOnPageChangeListener(this);
    }

    private void initialiseTabHost(Bundle args) {
        mTabHost = (TabHost)findViewById(android.R.id.tabhost);
        mTabHost.setup();
        TabInfo tabInfo = null;
        MainActivity.AddTab(this, this.mTabHost, this.mTabHost.newTabSpec("Tab1").setIndicator("Tab 1"), ( tabInfo = new TabInfo("Tab1", ActivityFeedFragment.class, args)));
        this.mapTabInfo.put(tabInfo.tag, tabInfo);
        MainActivity.AddTab(this, this.mTabHost, this.mTabHost.newTabSpec("Tab2").setIndicator("Tab 2"), ( tabInfo = new TabInfo("Tab2", InboxFragment.class, args)));
        this.mapTabInfo.put(tabInfo.tag, tabInfo);
        MainActivity.AddTab(this, this.mTabHost, this.mTabHost.newTabSpec("Tab3").setIndicator("Tab 3"), ( tabInfo = new TabInfo("Tab3", TaskFragment.class, args)));
        this.mapTabInfo.put(tabInfo.tag, tabInfo);
        MainActivity.AddTab(this, this.mTabHost, this.mTabHost.newTabSpec("Tab4").setIndicator("Tab 4"), ( tabInfo = new TabInfo("Tab4", CalendarEventFragment.class, args)));
        this.mapTabInfo.put(tabInfo.tag, tabInfo);

        mTabHost.setOnTabChangedListener(this);
    }

    private static void AddTab(MainActivity activity, TabHost tabHost, TabHost.TabSpec tabSpec, TabInfo tabInfo) {
        // Attach a Tab view factory to the spec
        tabSpec.setContent(activity.new TabFactory(activity));
        tabHost.addTab(tabSpec);
    }

    public void onTabChanged(String tag) {
        int pos = this.mTabHost.getCurrentTab();
        this.mViewPager.setCurrentItem(pos, true);
    }

    @Override
    public void onPageScrolled(int position, float positionOffset,
            int positionOffsetPixels) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onPageSelected(int position) {
        // TODO Auto-generated method stub
        this.mTabHost.setCurrentTab(position);
    }

    @Override
    public void onPageScrollStateChanged(int state) {
        // TODO Auto-generated method stub

    }
}

另外，这是我正在使用的适配器

public class PagerAdapter extends FragmentStatePagerAdapter {

    private List<Fragment> fragments;     

    public PagerAdapter(FragmentManager fm, List<Fragment> fragments) {
        super(fm);
        this.fragments = fragments;
    }

    @Override
    public Fragment getItem(int position) {
        return this.fragments.get(position);
    }

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

如果有任何关于在 TabHost 中启用滑动手势的建议，我也愿意尝试。

Answer 1

默认ViewPager在当前页面之外创建并保留一页，如果你不希望你可以将setOffscreenPageLimit的ViewPager改为0

viewPager.setOffscreenPageLimit(0);

--

另一种解决方案是 setOnPageChangeListener 到 viewpager 并在选择片段时触发 Async 调用

private static class CustomOnPageChangeListener extends SimpleOnPageChangeListener
{
    @Override
    public void onPageSelected(int position)
    {
        switch(position){
        case 1:
             FirstFragment fragment1 = (FirstFragment) = FirstFragment.getInstance(); 
             fragment1.startAsyncProcess();
        break;
        case 2:
             SecondFragment fragment2 = (SecondFragment) = SecondFragment.getInstance(); 
             fragment2.startAsyncProcess();
        break;
        }

        super.onPageSelected(position);
    }
}

...

viewPager.setOnPageChangeListener(new CustomOnPageChangeListener());

Answer 2

ViewPager 加载当前片段之外的上一个和下一个片段，以提供流畅的用户体验。因此，来自其他片段的异步任务被调用。

我建议你在每个片段中保留公共静态布尔变量，在你的 OnPageSelected 方法中你可以设置这些变量。

然后在你的每个方法的onResume方法中检查这个变量的值是否为真，然后只执行你的异步任务。

如果你不喜欢静态变量，那么使用片段管理器访问你的片段并设置布尔值