android中activity与fragment之间通信原理以及例题演示
效果图
首先,如果你想在android3.0及以下版本使用fragment,你必须引用android-support-v4.jar这个包
然后你写的activity不能再继承自Activity类了,而是要继承android.support.v4.app.FragmentActivity,一些其他的父类
也有相应的变化.
由于在android的实现机制中fragment和activity会被分别实例化为两个不相干的对象,他们之间的联系由activity的
一个成员对象SupportFragmentManager来维护.fragment实例化后会到activity中的SupportFragmentManager
去注册一下,这个动作封装在fragment对象的onAttach中,所以你可以在fragment中声明一些回调接口,当fragment
调用onAttach时,将这些回调接口实例化,这样fragment就能调用各个activity的成员函数了,当然activity必须implements
这些接口,否则会报ClassCastError
异常!
fragment和activity的回调机制又是OOP的一次完美演绎!
下面通过一个例子来说明:
我把Activity的UI分为两个部分,左边和右边,左边用来放置点击的按钮(LeftFragment)
右边用来放置对应点击后显示的信息(RightFragment).
Activity的布局layout文件:main.xml
<?xml version="1.0" encoding="utf-8"?>
<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="horizontal"
tools:context=".MainActivity">
<LinearLayout
android:id="@+id/left_layout"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="2"
android:orientation="vertical"
/>
<LinearLayout
android:id="@+id/right_layout"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="8"
android:orientation="vertical"/>
</LinearLayout>
LeftFragment的布局layout:fragment_left.xml
<?xml version="1.0" encoding="utf-8"?>
<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="horizontal"
tools:context=".MainActivity">
<LinearLayout
android:id="@+id/left_layout"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="2"
android:orientation="vertical"
/>
<LinearLayout
android:id="@+id/right_layout"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="8"
android:orientation="vertical"/>
</LinearLayout>
RightFragment的布局layout:fragment_right.xml
- 这里需要说明一下 用约束布局
ConstraintLayout
是为了屏幕适配 如果用其他的布局 会影响TextView的文字显示 - 简单点说就是每行文字的宽度对不齐!
<?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"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/right_show_message"
android:layout_width="0dp"
android:layout_height="0dp"
android:background="@android:color/holo_blue_dark"
android:textColor="@android:color/white"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>
以上是两个fragment和一个Activity的布局文件,下面来看他们的java文件
MainActivity:
public class MainActivity extends AppCompatActivity implements LeftFragment.MyListener {
/**
* 得到RightFragment中显示信息的控件
*/
private TextView showMessageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.e("TAG", "Activity--->onCreate");
FragmentManager manager = getSupportFragmentManager();
FragmentTransaction ft = manager.beginTransaction();
// 动态增加Fragment
RightFragment rightFragment = new RightFragment();
LeftFragment leftFragment = new LeftFragment();
ft.add(R.id.left_layout, leftFragment, "leftFragment");
ft.add(R.id.right_layout, rightFragment, "rightFragment");
ft.commit();
}
/**
* 实现MyListener,当LeftFragment中点击第一页的时候,让RightFragment显示第一页信息
* 同理当点击第二页的时候,RightFragment显示第二页信息
*
* @param index 显示的页数
*/
public void showMessage(int index) {
if (index == 1) {
showMessageView.setText(R.string.first_page);
}
if (index == 2) {
showMessageView.setText(R.string.second_page);
}
if (index == 3) {
showMessageView.setText(R.string.third_page);
}
}
@Override
protected void onResume() {
super.onResume();
Log.e("TAG", "Activity--->onResume");
showMessageView = findViewById(R.id.right_show_message);
}
}
LeftFragment:
public class LeftFragment extends Fragment {
private MyListener myListener;
private Button firstButton;
private Button secondButton;
private Button thirdButton;
/**
* Fragment第一次属于Activity时调用,在onCreate之前调用
*
* @param context
*/
@Override
public void onAttach(Context context) {
super.onAttach(context);
Log.e("TAG", "LeftFragment--->onAttach");
myListener = (MyListener) context;
}
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
Log.e("TAG", "LeftFragment--->onCreate");
super.onCreate(savedInstanceState);
}
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
Log.e("TAG", "LeftFragment--->onCreateView");
return inflater.inflate(R.layout.fragment_left, container, false);
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
Log.e("TAG", "LeftFragment--->onActivityCreated");
}
@Override
public void onResume() {
super.onResume();
Log.e("TAG", "LeftFragment--->onResume");
firstButton = getActivity().findViewById(R.id.first_button);
secondButton = getActivity().findViewById(R.id.second_button);
thirdButton = getActivity().findViewById(R.id.third_button);
MyButtonClickListener clickListener = new MyButtonClickListener();
firstButton.setOnClickListener(clickListener);
secondButton.setOnClickListener(clickListener);
thirdButton.setOnClickListener(clickListener);
}
/**
* Activity要实现这个接口,这样Fragment和Activity就可以共享事件触发的资源了
*/
public interface MyListener {
void showMessage(int index);
}
/**
* 按钮键的监听
*/
class MyButtonClickListener implements View.OnClickListener {
@Override
public void onClick(View view) {
Button button = (Button) view;
if (button == firstButton) {
myListener.showMessage(1);
}
if (button == secondButton) {
myListener.showMessage(2);
}
if (button == thirdButton) {
myListener.showMessage(3);
}
}
}
}
RightFragment:
public class RightFragment extends Fragment {
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
Log.e("TAG", "RightFragment--->onCreate");
super.onCreate(savedInstanceState);
}
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
Log.e("TAG", "RightFragment--->onCreateView");
return inflater.inflate(R.layout.fragment_right, container, false);
}
}
注意:
Fragment的生命周期和Activity生命周期之间的关系。在Activity里动态生成Fragment,首先是Activity调用onCreate()
方法但是这时候还没有加载到Fragment里的组件,当Fragment调用其onCreateView()方法后,Activity才能得到
Fragment中的组件
这里最关键的就是Fragment要有一个接口和这个接口的引用,而这个接口需要Activity去实现它。当Fragment调用onAttach(Context context)方法的时候,将这个activity传递给这个接口引用,这样,就可以和Activity进行交互了.