Android 之 Fragment 生命周期、常用 API、回退栈

前言

• 本篇介绍 Fragment 的生命周期、常用 API 、简单使用、回退栈
• 参考博客：http://blog.csdn.net/lmj623565791/article/details/37970961
• 参考博客：http://blog.csdn.net/lmj623565791/article/details/37992017

正文

一、Fragment 的由来

Fragment 是在 Android 3.0 之后引入的，主要是为了解决手机和屏幕的适配问题，同时，也能相应解决 activity 中代码量过大的问题，不过，从 Fragment 问世以来，相关的争论就不断，有同意使用的，也有反对使用的，反对的大部分原因就是坑太多，不好用，但是就我来看，用的人还是很多，而且本人也在经常使用，所以就来篇 Fragment 的大总结。

二、Fragment 的生命周期

Fragment 是嵌套在 Activity 中使用的，生命周期也与 Activity 极其相似，除了与 Activity 相同的几个生命周期方法之外，另加了几个与 Activity 相交互的几个方法，如下图所示：

几个额外的方法的介绍如下：

• onAttach(Context context) ：当 Fragment 与 Activity 发生关联的时候调用，（ Activity 传递到此方法内）

  此方法内部还是调用的 onAttach(Activity activity) 方法

• onCreateView ( LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState )

  创建 Fragment 视图的时候调用，

• onViewCreated(View view, @Nullable Bundle savedInstanceState)

  在 Activity 的 onCreate() 方法已返回时调用。

• onDestroyView()

  与 onCreateView 对应，当该 Fragment 的视图被移除时调用

• onDetach()

  与 onAttach 相对应，当 Fragment 与 Activity 关联被取消时调用

三、不同包下的 Fragment 的区别

Fragment 分两个不同包下：

android.support.v4.app.Fragment；

android.app.Fragment；

区别如下：

1. 兼容版本不同
   v4 包下的是为了兼容之 1.6 系统。
   另一个包下的是兼容 3.0 以上系统。不过这个现在没啥用了，因为现在的手机基本都在 4.0 以上了

1. 大家都知道 Fragment 是嵌套在 Activity 中使用的，对 Activity 的要求不同
   v4 包下的就要求这个 Activity 必须继承自FragmentActivity
   另一个包下的不要求，其中 FragmentActivity 集成自 Activity

2. 获取 FragmentManager 的方式不同

   v4 包下的是通过：getSupportFragmentManager();
   另一个包下的是：getFragmentManager();

注意：
关于 Fragment 的一切都要配套使用，注意导包问题否则容易出现类似这种错误： wrong 2nd argument type found ‘android.app.Fragment.’ required ‘Android.support.v4.app.Fragment’

四、Fragment 常用 API

1. FragmentManager

   这是 Fragment 管理者，要想操作 Fragment 必须先获取管理者，根据不同的包，有两种获取方式：getFragmentManager() // v4 中，getSupportFragmentManager

2. FragmentTransaction

   所有的操作都是基于此类的，调用此类的一系列方法来管理 Fragment ，
   获取方式：fragmentManager.beginTransaction()；方法大致如下：

• add()

  往Activity中添加一个Fragment

• 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。

• replace()

  使用另一个 Fragment 替换当前的，实际上就是 remove() 和 add() 的合体

• hide()

  隐藏当前的 Fragment，仅仅是设为不可见，并不会销毁

• show()

  显示之前隐藏的 Fragment

• detach()

  会将 view 从 UI 中移除,和 remove() 不同,此时 fragment 的状态依然由 FragmentManager 维护。

• attach()

  重建 view 视图，附加到 UI 上并显示。

• commit()

  提交一个事务，此方法必须要在 Activity 的 onSaveInstanceState 方法之前调用

五、使用 Fragment 的最简单的操作

1. 两个 Fragment

public class FirstFragment extends Fragment {

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

public class SecondFragment extends Fragment {
    
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View inflate = inflater.inflate(R.layout.secondfragment_layout, container, false);
        return inflate;
    }

2. Activity 中

public class MainActivity extends AppCompatActivity {


    @BindView(R.id.btn)
    Button btn;
    @BindView(R.id.fragment)
    FrameLayout fragment;
    private FragmentManager fragmentManager;
    private FirstFragment firstFragment;
    private FragmentTransaction fragmentTransaction;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
        
        setDefaultFragment();
    }

    /**
     * 设置默认的 fragment
     */
    private void setDefaultFragment() {

        fragmentManager = getSupportFragmentManager();
        fragmentTransaction = fragmentManager.beginTransaction();
        firstFragment = new FirstFragment();
        fragmentTransaction.add(R.id.fragment, firstFragment, "first").commit();
    }


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

        FragmentManager supportFragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction1 = supportFragmentManager.beginTransaction();
        SecondFragment secondFragment = new SecondFragment();
        fragmentTransaction1.replace(R.id.fragment, secondFragment, "second");
        fragmentTransaction1.commit();
    }
}

3. activity 中的 xml 文件如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context="com.example.administrator.fragment_text.MainActivity">

    <FrameLayout
        android:id="@+id/fragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
>

    </FrameLayout>

</LinearLayout>

点击按钮之后就能切换 Fragment

六、 Fragment 回退栈 ( 包含 Fragment 常用 API 调用生命周期方法详解 )

• 个人对回退栈的理解为： 当 Fragment 入栈之后，能够保存此 Fragment 的状态，通过返回键再次回到此 Fragment 的时候，数据依然存在！

• 回退栈就像 Activity 一样，先进后出，一层一层的

• 由于是真机测试，所以没法提供动图

• 测试逻辑大概是这样的：三个 Fragment 按顺序跳转，每个 Fragment 中都有一个输入框，测试是否能保存输入框中的内容，及调用的生命周期方法。

1. 三个 Fragment 分别为：

• FirstFragment

package com.example.administrator.fragment_text.huituizhan;

import android.content.Context;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;

import com.example.administrator.fragment_text.R;

import butterknife.BindView;
import butterknife.ButterKnife;
import butterknife.OnClick;
import butterknife.Unbinder;


/**
 * Created by Administrator on 2017/4/10.
 */

public class FirstFragment extends Fragment {

    @BindView(R.id.editText)
    EditText editText;
    @BindView(R.id.btn)
    Button btn;
    Unbinder unbinder;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

        Log.i("FirstFragment","onCreateView");
        View inflate = inflater.inflate(R.layout.huituizhan_first_fragment, container, false);
        unbinder = ButterKnife.bind(this, inflate);
        return inflate;
    }

    @Override
    public void onAttach(Context context) {
        Log.i("FirstFragment","onAttach");
        super.onAttach(context);
    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        Log.i("FirstFragment","onCreate");
        super.onCreate(savedInstanceState);
    }

    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        Log.i("FirstFragment","onViewCreated");
        super.onViewCreated(view, savedInstanceState);
    }

    @Override
    public void onStart() {
        Log.i("FirstFragment","onStart");
        super.onStart();
    }

    @Override
    public void onResume() {
        Log.i("FirstFragment","onResume");
        super.onResume();
    }

    @Override
    public void onPause() {
        Log.i("FirstFragment","onPause");
        super.onPause();
    }

    @Override
    public void onStop() {
        Log.i("FirstFragment","onStop");
        super.onStop();
    }

    @Override
    public void onDestroyView() {
        Log.i("FirstFragment","onDestroyView");
        super.onDestroyView();
        unbinder.unbind();
    }

    @Override
    public void onDestroy() {
        Log.i("FirstFragment","onDestroy");
        super.onDestroy();
    }

    @Override
    public void onDetach() {
        Log.i("FirstFragment","onDetach");
        super.onDetach();
    }
    /**
     * 点击按钮切换到 SecondFragment 
     */
    @OnClick(R.id.btn)
    public void onViewClicked() {

        FragmentManager fragmentManager = getFragmentManager();

        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

        SecondFragment secondFragment = new SecondFragment();

        fragmentTransaction.hide(this);
        fragmentTransaction.add(R.id.framelayout,secondFragment,"second");
        // 将 FirstFragment 添加至 回退栈
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();


    }
}

注意此处切换 Fragment 使用的是：

fragmentTransaction.hide(this);
fragmentTransaction.add(R.id.framelayout,secondFragment,"second");

• SecondFragment

package com.example.administrator.fragment_text.huituizhan;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;

import com.example.administrator.fragment_text.R;

import butterknife.BindView;
import butterknife.ButterKnife;
import butterknife.OnClick;
import butterknife.Unbinder;


/**
 * Created by Administrator on 2017/4/10.
 */

public class SecondFragment extends Fragment {

    @BindView(R.id.editText)
    EditText editText;
    @BindView(R.id.btn)
    Button btn;
    Unbinder unbinder;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View inflate = inflater.inflate(R.layout.huituizhan_second_fragment, container, false);
        unbinder = ButterKnife.bind(this, inflate);
        Log.i("SecondFragment","onCreateView");
        return inflate;
    }

    @Override
    public void onAttach(Context context) {
        Log.i("SecondFragment","onAttach");
        super.onAttach(context);
    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        Log.i("SecondFragment","onCreate");
        super.onCreate(savedInstanceState);
    }

    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        Log.i("SecondFragment","onViewCreated");
        super.onViewCreated(view, savedInstanceState);
    }

    @Override
    public void onStart() {
        Log.i("SecondFragment","onStart");
        super.onStart();
    }

    @Override
    public void onResume() {
        Log.i("SecondFragment","onResume");
        super.onResume();
    }

    @Override
    public void onPause() {
        Log.i("SecondFragment","onPause");
        super.onPause();
    }

    @Override
    public void onStop() {
        Log.i("SecondFragment","onStop");
        super.onStop();
    }

    @Override
    public void onDestroyView() {
        Log.i("SecondFragment","onDestroyView");
        super.onDestroyView();
        unbinder.unbind();
    }

    @Override
    public void onDestroy() {
        Log.i("SecondFragment","onDestroy");
        super.onDestroy();
    }

    @Override
    public void onDetach() {
        Log.i("SecondFragment","onDetach");
        super.onDetach();
    }

    @OnClick(R.id.btn)
    public void onViewClicked() {
        FragmentManager fragmentManager = getFragmentManager();

        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

        ThreeFragment threeFragment = new ThreeFragment();
        fragmentTransaction.replace(R.id.framelayout,threeFragment,"three");
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();
    }
}

注意此处切换 Fragment 使用的是：

fragmentTransaction.replace(R.id.framelayout,threeFragment,"three");

• ThreeFragment

package com.example.administrator.fragment_text.huituizhan;

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.example.administrator.fragment_text.R;

import butterknife.BindView;
import butterknife.ButterKnife;
import butterknife.OnClick;
import butterknife.Unbinder;


/**
 * Created by Administrator on 2017/4/10.
 */

public class ThreeFragment extends Fragment {

    @BindView(R.id.editText)
    EditText editText;
    @BindView(R.id.btn)
    Button btn;
    Unbinder unbinder;

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

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

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        unbinder.unbind();
    }

    @OnClick({R.id.editText, R.id.btn})
    public void onViewClicked(View view) {
        switch (view.getId()) {
            case R.id.editText:
                break;
            case R.id.btn:
                Toast.makeText(getActivity().getApplicationContext(),"最后一个了",Toast.LENGTH_SHORT)
                        .show();
                break;
        }
    }
}

2. Activity 中的代码为：

package com.example.administrator.fragment_text.huituizhan;

import android.os.Bundle;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.widget.FrameLayout;

import com.example.administrator.fragment_text.R;

import butterknife.BindView;
import butterknife.ButterKnife;

public class HuiTuiZhanActivity extends AppCompatActivity {

    @BindView(R.id.framelayout)
    FrameLayout framelayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_hui_tui_zhan);
        ButterKnife.bind(this);

        FragmentManager supportFragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = supportFragmentManager.beginTransaction();

        FirstFragment firstFragment = new FirstFragment();

        fragmentTransaction.add(R.id.framelayout,firstFragment,"first").commit();

    }
}

3. 运行之后，hide、add 与 replace 都能保存数据，但是生命周期不太一样，如下所示：

• FirstFragment 跳转到 SecondFragment 时：

04-22 13:43:10.160 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onAttach
04-22 13:43:10.160 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onCreate
04-22 13:43:10.160 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onCreateView
04-22 13:43:10.180 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onViewCreated
04-22 13:43:10.180 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onStart
04-22 13:43:10.180 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onResume
04-22 13:43:26.605 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onAttach
04-22 13:43:26.605 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onCreate
04-22 13:43:26.615 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onCreateView
04-22 13:43:26.620 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onViewCreated
04-22 13:43:26.620 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onStart
04-22 13:43:26.620 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onResume

注意： 此处切换的时候，没有立马调用 FirstFragment 的 onPause、onStop、onDestroyView

• SecondFragment 跳转到 ThreeFragment 时：

04-22 13:45:22.035 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onPause
04-22 13:45:22.035 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onStop
04-22 13:45:22.035 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onDestroyView
04-22 13:45:22.040 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onPause
04-22 13:45:22.040 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onStop
04-22 13:45:22.040 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onDestroyView

注意：此处是先调用 SecondFragment 的 onPause、onStop、onDestroyView，然后才调用 FirstFragment 的对应方法，

• 按返回键 ThreeFragment 回退到 SecondFragment 时：

04-22 13:51:26.075 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onCreateView
04-22 13:51:26.085 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onViewCreated
04-22 13:51:26.090 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onStart
04-22 13:51:26.090 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onResume
04-22 13:51:26.095 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onCreateView
04-22 13:51:26.100 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onViewCreated
04-22 13:51:26.100 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onStart
04-22 13:51:26.100 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onResume

注意： 此处竟然先调用的是 FirstFragment 的 onCreateView、onViewCreated、onStart、onResume

• 继续按返回键 SecondFragment 回退到 FirstFragment 时：

04-22 13:52:30.825 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onPause
04-22 13:52:30.825 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onStop
04-22 13:52:30.825 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onDestroyView
04-22 13:52:30.825 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onDestroy
04-22 13:52:30.825 15361-15361/com.example.administrator.fragment_text I/SecondFragment: onDetach

• 继续返回键 退出 Activity 时：

04-22 13:53:33.675 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onPause
04-22 13:53:34.045 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onStop
04-22 13:53:34.045 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onDestroyView
04-22 13:53:34.045 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onDestroy
04-22 13:53:34.045 15361-15361/com.example.administrator.fragment_text I/FirstFragment: onDetach

4. Fragment 退栈方式

这有个哥们写的非常详细：http://blog.csdn.net/u010049692/article/details/38944867