tablayout+viewpager+fragment组合使用以及懒加载机制

在开发中经常用到TabLayout+ViewPager+Fragment的组合。如下图所示：

首先介绍一下使用方法：

xml文件

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 <android.support.design.widget.TabLayout                android:id="@+id/tab_layout"                android:layout_width="match_parent"                android:layout_height="wrap_content"                android:background="#EF8D11"                app:tabIndicatorColor="#EF4A11"                app:tabMode="scrollable"                app:tabSelectedTextColor="#FFFFFF"                app:tabTextAppearance="@style/MyTabLayoutTextAppearance"                app:tabTextColor="#FFFFFF" /><android.support.v4.view.ViewPager            android:id="@+id/view_pager"            android:layout_width="match_parent"            android:layout_height="match_parent" />

属性说明：
android:background="#EF8D11" 背景色
app:tabIndicatorColor="#EF4A11" tab文字下方的那条线的颜色
app:tabMode="scrollable" 如果tab过多超出屏幕宽度可以水平滚动
app:tabSelectedTextColor="#FFFFFF" tab被选中的时候文字的颜色
app:tabTextColor="#FFFFFF" tab未被选中时文字的颜色
app:tabTextAppearance="@style/MyTabLayoutTextAppearance" 自定义字体大小（一般使用默认即可）

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<style name="MyTabLayoutTextAppearance" parent="TextAppearance.AppCompat.Widget.ActionBar.Title.Inverse">    <item name="android:textSize">12sp</item></style>

java 代码

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private void setTabs() {        HomePagerAdapter homePagerAdapter = new HomePagerAdapter(getSupportFragmentManager());        homePagerAdapter.addTab(new FragmentOne(), "tab1");        homePagerAdapter.addTab(new FragmentTwo(), "tab2");        homePagerAdapter.addTab(new FragmentThree(), "tab3");        viewPager.setAdapter(homePagerAdapter);        //把tabLayout和Viewpager关联起来        tabLayout.setupWithViewPager(viewPager);    }

HomePagerAdapter.java

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package com.jetsun.demo;import android.support.v4.app.Fragment;import android.support.v4.app.FragmentManager;import android.support.v4.app.FragmentStatePagerAdapter;import java.util.ArrayList;import java.util.List;/** * Overview: */public class HomePagerAdapter extends FragmentStatePagerAdapter {    private List<Fragment> fragments = new ArrayList<>();    private List<String> titles = new ArrayList<>();    public HomePagerAdapter(FragmentManager fm) {        super(fm);    }    public void addTab(Fragment fragment, String title) {        fragments.add(fragment);        titles.add(title);    }    @Override    public CharSequence getPageTitle(int position) {        return titles.get(position);    }    @Override    public Fragment getItem(int position) {        return fragments.get(position);    }    @Override    public int getCount() {        return fragments.size();    }}

懒加载机制

默认情况下，当显示tab1时，tab2已经加载了（已经调用了生命周期的oncreate,oncreateview,onactivitycreate,onresume方法），也就是默认加载当前fragment两边的一个fragment。加载的个数可以通过viewpager.setoffscreenpagelimit方法控制，模式是viewpager.setoffscreenpagelimit(1).最少也就是1，即使设置成0也默认预加载一个。假如现在的需求是要在fragment显示出来的时候才加载数据，那么通过viewpager.setoffscreenpagelimit方法是无法做到控制的。
下面就介绍另外一种方法来实现这种需求：

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package com.jetsun.demo;import android.os.Bundle;import android.support.annotation.Nullable;import android.support.v4.app.Fragment;import android.support.v4.widget.SwipeRefreshLayout;import android.support.v7.widget.RecyclerView;import android.util.Log;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import butterknife.Bind;import butterknife.ButterKnife;/** * Overview: */public class FragmentDemo extends Fragment {    @Bind(R.id.recycler_view)    RecyclerView recyclerView;    @Bind(R.id.swipe_refresh_layout)    SwipeRefreshLayout swipeRefreshLayout;    //控件是否已经初始化    private boolean isCreateView = false;    @Nullable    @Override    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {        View view = inflater.inflate(R.layout.fragment, null, false);        ButterKnife.bind(this, view);        initViews();        isCreateView = true;        return view;    }    private void initViews() {       //初始化控件    }//此方法在控件初始化前调用，所以不能在此方法中直接操作控件会出现空指针    @Override    public void setUserVisibleHint(boolean isVisibleToUser) {        super.setUserVisibleHint(isVisibleToUser);        if (isVisibleToUser && isCreateView) {            lazyLoad();        }    }    private void lazyLoad() {        //加载数据操作    }    @Override    public void onActivityCreated(@Nullable Bundle savedInstanceState) {        super.onActivityCreated(savedInstanceState);        //第一个fragment会调用        if (getUserVisibleHint())            lazyLoad();    }    @Override    public void onDestroyView() {        super.onDestroyView();        ButterKnife.unbind(this);    }}

在实际开发中，可以把上面的代码抽取到BaseFragment中使用。上面这个方法实现了每次fragment显示出来的时候加载数据，但是这样切换回来的时候还会重复加载，我们只需加载一次就好了。所以改进一下：

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package com.jetsun.demo;import android.os.Bundle;import android.support.annotation.Nullable;import android.support.v4.app.Fragment;import android.support.v4.widget.SwipeRefreshLayout;import android.support.v7.widget.RecyclerView;import android.util.Log;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import butterknife.Bind;import butterknife.ButterKnife;/** * Overview: */public class FragmentDemo extends Fragment {    @Bind(R.id.recycler_view)    RecyclerView recyclerView;    @Bind(R.id.swipe_refresh_layout)    SwipeRefreshLayout swipeRefreshLayout;    //控件是否已经初始化    private boolean isCreateView = false;    //是否已经加载过数据    private boolean isLoadData = false;    @Nullable    @Override    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {        View view = inflater.inflate(R.layout.fragment, null, false);        ButterKnife.bind(this, view);        initViews();        isCreateView = true;        return view;    }    private void initViews() {       //初始化控件    }//此方法在控件初始化前调用，所以不能在此方法中直接操作控件会出现空指针    @Override    public void setUserVisibleHint(boolean isVisibleToUser) {        super.setUserVisibleHint(isVisibleToUser);        if (isVisibleToUser && isCreateView) {            lazyLoad();        }    }    private void lazyLoad() {    //如果没有加载过就加载，否则就不再加载了        if(!isLoadData){                //加载数据操作                isLoadData=true;        }    }    @Override    public void onActivityCreated(@Nullable Bundle savedInstanceState) {        super.onActivityCreated(savedInstanceState);        //第一个fragment会调用        if (getUserVisibleHint())            lazyLoad();    }    @Override    public void onDestroyView() {        super.onDestroyView();        ButterKnife.unbind(this);    }}

#tablayout