点击侧滑菜单切换Fragment

1.用add方法进行添加

  @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        //得到Fragment的管理类
        FragmentManager fragmentManager = getSupportFragmentManager();
        //开启事务
        transaction = fragmentManager.beginTransaction();
       /**
        * 这是一个方法，用于隐藏所有的Fragment。
        */
        hideFragment(aFragment);
        hideFragment(bFragment);
        hideFragment(cFragment);
        hideFragment(dFragment);
        switch (menuItem.getItemId()) {
            case R.id.item1:
                if (aFragment == null) {//如果没有fragment,开始创建并且添加
                    aFragment = new AFragment();
                    transaction.add(R.id.fl, aFragment);
                } else {//有Fragment就让它显示
                    transaction.show(aFragment);
                }
                dl.closeDrawer(Gravity.LEFT);//关闭侧滑菜单
                break;
            case R.id.item2:
                if (bFragment == null) {
                    bFragment = new BFragment();
                    transaction.add(R.id.fl, bFragment);
                } else {
                    transaction.show(bFragment);
                }
                dl.closeDrawer(Gravity.LEFT);
                break;
            case R.id.item3:
                if (cFragment == null) {
                    cFragment = new CFragment();
                    transaction.add(R.id.fl, cFragment);
                } else {
                    transaction.show(cFragment);
                }
                dl.closeDrawer(Gravity.LEFT);
                break;
            case R.id.item4:
                if (dFragment == null) {
                    dFragment = new DFragment();
                    transaction.add(R.id.fl, dFragment);
                } else {
                    transaction.show(dFragment);
                }
                dl.closeDrawer(Gravity.LEFT);
                break;
        }
        transaction.commit();
        return true;
    }

    //隐藏Fragment
    private void hideFragment(Fragment fragment) {
        if (fragment != null) {
            transaction.hide(fragment);
        }
    }

2.用replace方法进行替换

 @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        switch (menuItem.getItemId()) {
            case R.id.item1:
                aFragment = new AFragment();
                getFragment(aFragment);
                dl.closeDrawer(Gravity.LEFT);
                break;
            case R.id.item2:
                bFragment = new BFragment();
                getFragment(bFragment);
                dl.closeDrawer(Gravity.LEFT);
                break;
            case R.id.item3:
                cFragment = new CFragment();
                getFragment(cFragment);
                dl.closeDrawer(Gravity.LEFT);
                break;
            case R.id.item4:
                dFragment = new DFragment();
                getFragment(dFragment);
                dl.closeDrawer(Gravity.LEFT);
                break;
        }
        transaction.commit();
        return true;
    }

    private void getFragment(Fragment fragment){
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.replace(R.id.fl,fragment);
        transaction.commit();
    }

add其中主要用了FragmentTransaction的show和hide方法，当然，用replace方法也能实现，但是replace方法相比而言 比用show和hide方法要浪费资源，如果加载的是网络数据用replace就比较耗费流量了，因为replace方法其实就是remove方法和add方法的结合，当我们加载布局文件后不需要显示的时候就remove掉，当用的时候再去加载，这个中间要耗资源，如果用show和hide方法的话，如果用到某个布局，那我们就show，不显示布局就hide，这样避免了重复加载.