Android中添加Fragment的正确姿势

前言：

我们在平时开发中必定会运用到Fragment，经常会出现一些莫名其妙的问题，比如在旋转屏幕或被系统杀掉重启时，出现bug，这说明你添加Fragment的姿势不对！在这里我总结一下自己在项目中添加Fragment的方式，我且称之为正确姿势，如有不对的地方或者更好的方式，欢迎大家留言指出！

正文：

一、在一个Activity中添加一个Fragment的正确姿势

Activity中代码（直接拿去套用）：

//用户ID的KEY
public static final String KEY_UID = "KEY_UID";
private HomeFragment HomeFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_acts);

        addDetailFragment(savedInstanceState);
    }

    private void addDetailFragment(Bundle savedInstanceState) {
        //添加Fragment应以这种方式,当旋转屏幕和被系统杀掉重启时,才不会出问题
        if (savedInstanceState == null) {
            FragmentManager fm = getSupportFragmentManager(); //获取Fragment管理器对象
            FragmentTransaction ft = fm.beginTransaction(); //开启事务

            if (fm.findFragmentById(R.id.fragment_container) == null) {
                HomeFragment = new HomeFragment();
            } else {
                HomeFragment = (HomeFragment) fm.findFragmentById(R.id.videos_fragment_container);
                ft.remove(HomeFragment);
                fm.popBackStack();
                ft.commit();
                ft = fm.beginTransaction();
            }
            //传入一些参数
            Bundle bundle = new Bundle();
            bundle.putLong(KEY_UID, mUserId);
            ......
            HomeFragment.setArguments(bundle);

            ft.add(R.id.videos_fragment_container, HomeFragment); //将fragment添加到布局当中
            ft.commit(); //千万别忘记提交事务
        }
    }

Activity的layout中：

<FrameLayout
        android:id="@+id/fragment_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

二、在一个Activity中添加几个Fragment的正确姿势

现在的好多App进入主页后都会有首页、发现和我的三个页面，下面我就以此为例。还是看Activity中的代码：

    //定义代表各个界面的常量
    public static final int HOME_POS = 0; //首页
    public static final int DISCOVERY_POS = 1; //发现
    public static final int MINE_POS = 2; //我的

    private HomeFragment mHomeFragment; //首页Fragment
    private DiscoveryFragment mDiscoveryFragment; //发现Fragment
    private UserInfoFragment mMineFragment; //我的Fragment

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //状态变化时删除老的Fragment
        FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();
        Fragment fragment = fm.findFragmentById(R.id.frag_container);
        if (fragment != null) {
            ft.remove(fragment);
            fm.popBackStack();
            ft.commit();
        }
  }
  setSelection(HOME_POS); //默认选择首页

我们继续看setSelection()方法干了什么：

public void setSelection(int position) {

        FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction transaction = fm.beginTransaction();

        hideAllFragments(transaction); //先隐藏全部Fragment

        //然后是各个Fragment的创建和显示
        switch (position) {
            case HOME_POS:
                if (mHomeFragment == null) {
                    mHomeFragment = new HomeFragment();
                    transaction.add(R.id.frag_container, mHomeFragment);
                } else {
                    transaction.show(mHomeFragment);
                }
                mHomeTab.setSelected(true);

                break;
            case DISCOVERY_POS:
                if (mDiscoveryFragment == null) {
                    mDiscoveryFragment = new DiscoveryFragment();
                    transaction.add(R.id.frag_container, mDiscoveryFragment);
                } else {
                    transaction.show(mDiscoveryFragment);
                }
                mDiscoveryTab.setSelected(true);

                break;
            case MINE_POS:
                if (mMineFragment == null) {
                    mMineFragment = new UserInfoFragment();
                    transaction.add(R.id.frag_container, mMineFragment);
                } else {
                    transaction.show(mMineFragment);
                }
                mMineTab.setSelected(true);

                break;
        }

        transaction.commit(); //千万别忘记提交事务
    }

在显示某个Fragment之前，我将全部Fragment隐藏，这样可以确定不会显示出错，再看看hideAllFragments():

private void hideAllFragments(FragmentTransaction transaction) {
        if (mHomeFragment != null) transaction.hide(mHomeFragment);
        if (mDiscoveryFragment != null) transaction.hide(mDiscoveryFragment);
        if (mMineFragment != null) transaction.hide(mMineFragment);
    }

Activity的layout代码：

<FrameLayout
        android:id="@+id/frag_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

OK，之后再遇到添加Fragment的问题，直接copy就好啦！