Fragment

Fragment是可以嵌入Activity的UI片段，能让程序更加合理和充分地利用空间。

Fragment的基本用法

静态加载
1.  直接在宿主Activity的Layout中引入<fragment>

注意：
    name指定绑定的Fragment
    tag与id属性是必须添加的，作为Fragment的标记。

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 
    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"
    tools:context=".ui.activity.MainActivity">

    <fragment
        android:id="@+id/fra_id"
        android:name="com.w.review.ui.fragment.MyFragment"
        android:layout_width="match_parent"
        android:layout_height="300dp"
        android:tag="fra_demo" />

</android.support.constraint.ConstraintLayout>


2.  为Fragment设置布局
为了方便演示，就设置个纯色背景
<android.support.constraint.ConstraintLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorAccent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="我是Fragment"
        android:textColor="#FFF"
        android:textSize="18sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>


3.  设置Fragment类
public class MyFragment extends Fragment {

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

        //绑定样式
        return inflater.inflate(R.layout.fragment_demo, container, false);
    }
}

效果

动态加载

1.  在宿主Activity中留出一块布局，用来装载Fragment的样式
<android.support.constraint.ConstraintLayout
    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"
    tools:context=".ui.activity.MainActivity">

    <FrameLayout
        android:id="@+id/fra_content"

        android:layout_width="match_parent"
        android:layout_height="300dp" />

</android.support.constraint.ConstraintLayout>


2.  设置Fragment的布局
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorAccent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="我是动态加载的Fragment"
        android:textColor="#FFF"
        android:textSize="18sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>


3.设置Fragment类，和上面静态加载代码相同


4.  Activity中用代码动态加载Fragment
public class MainActivity extends AppCompatActivity {

    private MyFragment mFragment;

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

        initFragment();
    }

    private void initFragment(){
        mFragment = new MyFragment();
        FragmentTransaction transaction = getSupportFragmentManager()
                .beginTransaction();

        transaction
                .replace(R.id.fra_content,mFragment,"fra_demo")
                .commit();
    }
}

效果

Fragment的生命周期

Fragment依赖于Activity存在，它的生命周期相较于宿主Activity触发晚。

public class MyFragment extends Fragment {

    //Fragment和宿主 Activity 绑定
    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
    }

    //Fragment 和宿主 Activity 解除绑定
    @Override
    public void onDetach() {
        super.onDetach();
    }

    //开始创建
    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }


    //UI创建时，可见时调用
    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater,
                             @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {

        //绑定样式
        return null;
    }


    //宿主Activity的onCreate执行完成后回掉
    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
    }

    //UI销毁，变得不可见
    @Override
    public void onDestroyView() {
        super.onDestroyView();
    }

    @Override
    public void onStart() {
        super.onStart();
    }

    @Override
    public void onStop() {
        super.onStop();
    }

    @Override
    public void onResume() {
        super.onResume();
    }

    @Override
    public void onPause() {
        super.onPause();
    }

    //销毁，开始回收内存
    @Override
    public void onDestroy() {
        super.onDestroy();
    }
}

FragmentTransaction常用API

1.  add(int containerViewId, Fragment fragment, String tag);
    添加Fragment，UI效果是层层叠加。


    id：添加目标空间的id
    fragment：要添加的 fragment 实例
    tag：与添加的 fragment 对应，用于findFragmentByTag时查找到对应的Fragment


2.  replace(int containerViewId, Fragment fragment, String tag)
    先将之前添加的所有Fragment清除，然后再添加。


3.  detach(Fragment fragment)
    解除Fragment与宿主Activity的UI绑定，但是不调用Fragment的onDestroy()。
    即Fragment依旧存在，只是不与宿主关联。

4.  attach(Fragment fragment)
    重新绑定Fragment，调用onCreateView()。


5.  remove(Fragment fragment)
    调用Fragment的onDestroy()，彻底删除Fragment与Activity的关联。


6.  hide(Fragment fragment)
    隐藏Fragment，不调用任何生命周期方法只是隐藏UI，避免反复调用创建，消耗内存。

7.  show(Fragment fragment)
    将隐藏UI显示出来。

commit() 与 commitAllowingStateLoss() 提交区别

Activity在以下情况容易触发onSaveInstanceState(Bundle outState)
   按下HOME键
   按下电源(关闭屏幕显示)键
   屏幕方向切换
   Activity跳转切换

在以上情况提交Fragment会抛出异常，
也就说尽量不能在onPause()，onStop()，onDestroy()，做commit()提交操作。

如果需要，就要用commitAllowingStateLoss()代替commit()。

---

Fragment的返回栈

Fragment默认不会响应Back键，因此点击Back键时，Fragment无回退状态。
如果想按下Back键回退到上一个碎片，就需要设置Fragment的返回栈。

设置Fragment
public class RedFragment extends Fragment {

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

        View view = new View(getContext());

        FrameLayout.LayoutParams params = new FrameLayout.LayoutParams(
                FrameLayout.LayoutParams.MATCH_PARENT, 56);

        view.setLayoutParams(params);
        params.setMargins(0, 0, 0, 20);

        view.setBackgroundColor(getResources().getColor(R.color.colorAccent));
        //绑定样式
        return view;
    }
}


public class MainActivity extends AppCompatActivity {

    private Button mBtnAddFra, mBrnPopupFra;

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

        initView();
        initEvent();
    }


    private void initEvent() {

        mBtnAddFra.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                loadFra();
            }
        });

        mBrnPopupFra.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                getSupportFragmentManager().popBackStack();
            }
        });
    }

    private void initView() {
        mBtnAddFra = findViewById(R.id.btn_add_fra);
        mBrnPopupFra = findViewById(R.id.btn_popup_fra);
    }

    private void loadFra() {
        FragmentTransaction fraTran = getSupportFragmentManager().beginTransaction();
        
          addToBackStack(String name)
          name 代表这次Transaction的名称
          可以根据这个名称找到相应的添加Fragment操作，进行回退操作
          popBackStack("popup1", POP_BACK_STACK_INCLUSIVE);
          也可以传null代表不记录该次操作
        
        fraTran.add(R.id.fra_content, new RedFragment(), "Tag")
                .addToBackStack(null)
                .commit();
    }
}

---

Fragment动画

主要是使用FragmentTransaction中setCustomAnimations()设置动画
1.v4的Transaction支持XML动画，但不支持属性动画。

FragmentTransaction setCustomAnimations (int enter, int exit, 
int popEnter, int popExit)

参数
  enter：当一个Fragment被添加added 或者绑定 attached到视图上时
  exit：当一个Fragment从视图上被移除removed或者解除绑定detached时
  popEnter：当调用popBackStack()弹栈后，栈顶Fragment重新被添加时
  popExit：当调用popBackStack()弹栈后，弹出的Fragment

slide_left_in.xml：左视图进入时动画
<set xmlns:android="http://schemas.android.com/apk/res/android">
    <translate
        android:duration="700"
        android:fromXDelta="-100%p"
        android:toXDelta="0%p" />

    <alpha
        android:duration="700"
        android:fromAlpha="0.5"
        android:toAlpha="1.0" />
</set>


slide_left_out.xml：左视图离开时动画
<set xmlns:android="http://schemas.android.com/apk/res/android">
    <translate
        android:duration="700"
        android:fromXDelta="0%p"
        android:toXDelta="-100%p" />

    <alpha
        android:duration="700"
        android:fromAlpha="1.0"
        android:toAlpha="0.5" />
</set>


slide_right_in.xml
<set xmlns:android="http://schemas.android.com/apk/res/android">
    <translate
        android:duration="700"
        android:fromXDelta="100%p"
        android:toXDelta="0%p" />

    <alpha
        android:duration="700"
        android:fromAlpha="0.5"
        android:toAlpha="1.0" />
</set>


slide_right_out.xml
<set xmlns:android="http://schemas.android.com/apk/res/android">
    <translate
        android:duration="700"
        android:fromXDelta="0%p"
        android:toXDelta="100%p" />

    <alpha
        android:duration="700"
        android:fromAlpha="1.0"
        android:toAlpha="0.5" />
</set>



public class MainActivity extends AppCompatActivity {

    private Button mBtnPrevious, mBtnNext;
    private RedFragment mRedFra;
    private BlueFragment mBlueFra;

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

        initView();
        initEvent();
        initFra();
    }

    private void initFra() {
        mRedFra = new RedFragment();
        mBlueFra = new BlueFragment();

        getSupportFragmentManager()
                .beginTransaction()
                .add(R.id.fra_content, mRedFra)
                .commit();
    }


    private void initEvent() {

        mBtnPrevious.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                getSupportFragmentManager()
                        .popBackStack();
            }
        });

        mBtnNext.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                getSupportFragmentManager()
                        .beginTransaction()
                        .setCustomAnimations(
                                R.anim.slide_right_in,
                                R.anim.slide_left_out,
                                R.anim.slide_left_in,
                                R.anim.slide_right_out
                        ).replace(R.id.fra_content, mBlueFra)
                        .addToBackStack(null)
                        .commit();
            }
        });
    }

    private void initView() {
        mBtnPrevious = findViewById(R.id.btn_previous);
        mBtnNext = findViewById(R.id.btn_next);
    }
}

---

Fragment通讯

1.Activity和Fragment通讯，使用Bundle

public class MyFragment extends Fragment {

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

        //拿到传递的数据
        String res = getArguments().get("key").toString();

        TextView textView = new TextView(getContext());
        textView.setLayoutParams(
                new ViewGroup.LayoutParams(
                        ViewGroup.LayoutParams.WRAP_CONTENT,
                        ViewGroup.LayoutParams.WRAP_CONTENT));

        textView.setText(res);
        return textView;
    }
}


public class MainActivity extends AppCompatActivity {

    private Button mBtnAddFra;
    private MyFragment mFragment;

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

        initView();
        initEvent();
    }

    private void addFra() {
        mFragment = new MyFragment();
        Bundle bundle = new Bundle();
        bundle.putString("key", "传递的数据");
        mFragment.setArguments(bundle);

        getSupportFragmentManager()
                .beginTransaction()
                .add(R.id.fra_content, mFragment, "fra")
                .commit();
    }


    private void initEvent() {

        mBtnAddFra.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                addFra();
            }
        });
    }

    private void initView() {
        mBtnAddFra = findViewById(R.id.btn_add_fra);
    }
}

效果

2.多个Fragment间通讯

Fragment必定依存于Activity，因此将Activity作为中间层，通过接口进行交互。

举个例子
在FragmentTwo监听显示FragmentOne中Button的点击次数。


1.  为了方便，选择静态加载Fragment布局
<LinearLayout 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"
    android:orientation="vertical"
    tools:context=".ui.activity.MainActivity">

    <fragment
        android:id="@+id/fra_red"
        android:name="com.w.review.ui.fragment.ShowFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:tag="showFra" />

    <fragment
        android:id="@+id/fra_blue"
        android:name="com.w.review.ui.fragment.ClickFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:tag="clickFra" />

</LinearLayout>

2.定义接口，由Activity实现
  你可以理解为这个接口成了Activity的父类
  当Fragment调用getActivity()时，根据多态特性，调用的是Activity的实现

public interface IClickCountListener {
    void clickCount(int index);
}


public class MainActivity extends AppCompatActivity implements IClickCountListener {

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

    @Override
    public void clickCount(int index) {
        ShowFragment showFrat = (ShowFragment) getSupportFragmentManager()
                .findFragmentByTag("showFra");
        showFrat.setCount(index);
    }
}

3.ClickFragment调用点击
public class ClickFragment extends Fragment {

    private View mView;
    private Button mBtnAddCount;

    private int index = 0;
    private IClickCountListener mClickCountListener;

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

        mView = inflater.inflate(R.layout.fragment_blue, container, false);
        mBtnAddCount = mView.findViewById(R.id.btn_add_count);
        return mView;
    }

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

        //获取Activity的引用，子类指向父类
        mClickCountListener = (IClickCountListener) getActivity();
        initEvent();
    }

    private void initEvent() {
        mBtnAddCount.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                index++;
                //调用的还是子类的实现
                mClickCountListener.clickCount(index);
            }
        });
    }
}

4.showFragment定义方法用于显示计数，当clickFragment数值改变时，相应的也会调用。

public class ShowFragment extends Fragment {

    private View mView;
    private TextView mTvCount;

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

        mView = inflater.inflate(R.layout.fragment_red, container, false);
        mTvCount = mView.findViewById(R.id.tv_count);
        return mView;
    }

    public void setCount(int index) {
        mTvCount.setText(String.valueOf(index));
    }
}

---

Fragment补充

1.屏幕旋转时Fragment数据丢失问题

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt("index", index);
}

2.inflate

inflate(R.layout.fragment_one, container, false);

参数
绑定Layout布局
通知View，将定义的布局挂载上去。
false表示自己手动挂载，不需要自动添加。