Fragment不完全解析

Fragment是在Android 3.0 (API level 11)开始引入的,它能让你的app在现有基础上性能大幅度提高，并且占用内存降低，同样的界面Activity占用内存比Fragment要多，响应速度Fragment比Activty在中低端手机上快了很多，甚至能达到好几倍,同时给大屏幕（如平板电脑）上更加动态和灵活的 UI 设计提供支持.

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一、Fragment的生命周期

Fragment表示Activity中的行为或用户界面部分,其生命周期直接受宿主activity生命周期的影响。下图可以很直观反映出两者的关系：

Fragment比Activity多出的几个生命周期回调方法：

• onAttach(Activity)：

  called once the fragment is associated with its activity.（当Fragment和Activity相关联调用

• onCreateView(LayoutInflater, ViewGroup, Bundle)

  creates and returns the view hierarchy associated with the fragment.（创建Fragment的视图

• onActivityCreated(Bundle)

  tells the fragment that its activity has completed its own Activity.onCreate(). （当Activity.onCreate()完成时调用

• onDestroyView()

  allows the fragment to clean up resources associated with its View.

• onDetach()

  called immediately prior to the fragment no longer being associated with its activity

二、Fragment 的使用

静态使用

把Fragment当做普通的控件写入Activity的布局文件中，用布局文件调用Fragment，这里不再展开

动态使用

Fragment动态使用主要是动态添加，更新，删除.

在activity的布局文件中添加一个FrameLayout，作为fragment需要放入的ViewGroup

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="net.december1900.test.MainActivity">
    
    <FrameLayout
        android:id="@+id/content_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</RelativeLayout>

fragment和主activity

public class MainFragment extends Fragment{

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_main,container,false);
    }

}

public class MainActivity extends AppCompatActivity {

    private MainFragment mMainFragment;

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

    private void showFragment(){
        FragmentManager manager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = manager.beginTransaction();
        mMainFragment = new MainFragment();
        fragmentTransaction.replace(R.id.content_layout,mMainFragment);
        fragmentTransaction.commit();
    }
}

显示

Fragment常用的类

1. android.app.FragmentManager

   获取方式：getSupportFragmentManager();

2. android.app.FragmentTransaction

   FragmentTransaction transaction = fm.benginTransatcion();

   • transaction.add()

     往Activity中添加一个Fragment

   • transaction.remove()

     从Activity中移除一个Fragment，如果被移除的Fragment没有添加到回退栈，这个Fragment实例将会被销毁

   • transaction.replace()

     使用另一个Fragment替换当前的，可以理解为remove() + add()

   • transaction.hide()

     隐藏当前的Fragment，仅仅是设为不可见，并不会销毁

   • transaction.show()

     显示之前隐藏的Fragment

   • transatcion.commit()

总结起来：

如果你希望保留用户操作的面板，可以使用show(),hide()。其实是让Fragment的View setVisibility(true还是false)，不会调用生命周期

如果你不希望保留用户操作，可以使用replace()。其会销毁视图，即调用onDestoryView、onCreateView等一系列生命周期

三、管理Fragment的回退栈

我们可以通过Activity维护一个回退栈来保存每次Fragment事务发生的变化。如果将Fragment任务添加到回退栈，当用户点击后退按钮时，将看到上一次的保存的Fragment。一旦Fragment完全从后退栈中弹出，用户再次点击后退键，则会退出当前Activity。

添加一个Fragment事务到回退栈：
FragmentTransaction.addToBackStack(String)

主activity

public class MainActivity extends AppCompatActivity {

    private MainFragment mMainFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        showFragment("MainFragment");
    }

    private void showFragment(String tag) {
        FragmentManager manager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = manager.beginTransaction();
        mMainFragment = new MainFragment();
        fragmentTransaction.add(R.id.content_layout, mMainFragment, tag);
        fragmentTransaction.addToBackStack(tag);
        fragmentTransaction.commit();

    }

}

MainFragment

public class MainFragment extends Fragment {

    @BindView(R.id.btn_main)
    Button mBtnMain;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_main, container, false);
        ButterKnife.bind(this, view);
        return view;
    }


    @OnClick(R.id.btn_main)
    public void onViewClicked() {

        showFragment("SecondFragment");
    }

    private void showFragment(String tag) {
        SecondFragment secondFragment = new SecondFragment();
        FragmentManager manager = getFragmentManager();
        FragmentTransaction transaction = manager.beginTransaction();
        transaction.replace(R.id.content_layout, secondFragment, tag);
        transaction.addToBackStack(tag);
        transaction.commit();

    }
}

SecondFragment

public class SecondFragment extends Fragment {

    @BindView(R.id.btn_second)
    Button mBtnSecond;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_second, container, false);
        ButterKnife.bind(this, view);
        return view;
    }


    @OnClick(R.id.btn_second)
    public void onViewClicked() {
        showFragment("ThirdFragment");
    }

    private void showFragment(String tag) {
        ThirdFragment thirdFragment = new ThirdFragment();
        FragmentManager manager = getFragmentManager();
        FragmentTransaction transaction = manager.beginTransaction();
        transaction.add(R.id.content_layout, thirdFragment, tag);
        transaction.addToBackStack(tag);
        transaction.commit();
    }
}

显示

总体来说，我们调用addToBackStack(tag);将当前的事务添加到了回退栈，所以MainFragment实例不会被销毁，但是视图层次依然会被销毁，即会调用onDestoryView和onCreateView，但是使用add（）方法的SecondFragment视图层依然存在