Android学习笔记（Android Studio） 4-2-1~2 Fragment详解（一、二）（不可不会的Activity和Fragment）

Android学习笔记4-2-1~4-2-2

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4-2-1 Fragment详解(一)

• Fragment有自己的生命周期
• Fragment依赖于Activity
• Fragment通过getActivity()可以获取所在的Activity；Activity通过FragmentManager的findFragmentById()或findFragmentByTag()获取Fragment
• Fragment和Activity是多对多的关系

4-2-1 Fragment详解(二)

• Fragment中getActivity()为null的问题
• 向Fragment传递参数

（一）（二）代码示例

• fragment_a.xml

    <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical" 
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center">
        <TextView
            android:id="@+id/tv_title"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textColor="#000"
            android:textSize="20sp"
            android:text="我是AFragment"
            android:gravity="center"/>
        
    </LinearLayout>
  

• 效果
  

• AFragment.java

    package com.ylw.helloworld.fragment;
    
    import android.app.Activity;
    import android.content.Context;
    import android.os.Bundle;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.TextView;
    
    import androidx.annotation.NonNull;
    import androidx.annotation.Nullable;
    import androidx.fragment.app.Fragment;
    
    import com.ylw.helloworld.R;
    
    public class AFragment extends Fragment {
    
        private TextView mTvTitle;
    //    private Activity mActivity;
    
        //向Fragment传递参数
        public static AFragment newInstance(String title){
            AFragment fragment = new AFragment();
            Bundle bundle = new Bundle();
            bundle.putString("title",title);
            fragment.setArguments(bundle);
            return fragment;
        }
    
        //设置布局文件
        @Nullable
        @Override
        public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
            View view = inflater.inflate(R.layout.fragment_a,container,false);
            return view;
        }
    
        //当布局文件创建完成后进行查找、调用等
        @Override
        public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
            super.onViewCreated(view, savedInstanceState);
            //
            mTvTitle = view.findViewById(R.id.tv_title);
    //        if (getActivity() != null){   //如果getActivity()为null的问题
    //            //
    //        }else{
    //            //
    //        }
            if (getArguments()!=null){  //向Fragment传递参数
                mTvTitle.setText(getArguments().getString("title"));
            }
    
        }
    
        @Override
        public void onAttach(Context context) { //与Activity重新保持关系
            super.onAttach(context);
    //        mActivity = (Activity) context;   //如果getActivity()为null的问题
        }
    
        @Override
        public void onDetach() {    //运行该方法会与Activity脱离关系
            super.onDetach();
            //取消异步
        }
    }
  

• fragment_b.xml

    <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical" 
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center">
        <TextView
            android:id="@+id/tv_title"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textColor="#000"
            android:textSize="20sp"
            android:text="我是BFragment"
            android:gravity="center"/>
        
    </LinearLayout>
  

• 效果
  

• BFragment.java

    package com.ylw.helloworld.fragment;
    
    import android.os.Bundle;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.TextView;
    
    import androidx.annotation.NonNull;
    import androidx.annotation.Nullable;
    import androidx.fragment.app.Fragment;
    
    import com.ylw.helloworld.R;
    
    public class BFragment extends Fragment {
    
        private TextView mTvTitle;
    
        //设置布局文件
        @Nullable
        @Override
        public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
            View view = inflater.inflate(R.layout.fragment_b,container,false);
            return view;
        }
    
        //当布局文件创建完成后进行查找、调用等
        @Override
        public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
            super.onViewCreated(view, savedInstanceState);
            //
            mTvTitle = view.findViewById(R.id.tv_title);
        }
    }
  

• activity_container.xml

    <?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    
        <Button
            android:id="@+id/btn_change"
            android:layout_width="match_parent"
            android:layout_height="50dp"
            android:text="更换Fragment"/>
    
        <FrameLayout
            android:id="@+id/fl_container"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_below="@+id/btn_change"
            />
    
    </RelativeLayout>
  

• 效果
  

• ContainerActivity.java

    package com.ylw.helloworld.fragment;
    
    import androidx.appcompat.app.AppCompatActivity;
    
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    
    import com.ylw.helloworld.R;
    
    public class ContainerActivity extends AppCompatActivity {
    
        private AFragment aFragment;
        private BFragment bFragment;
        private Button mBtnChange;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_container);
            mBtnChange = findViewById(R.id.btn_change);
            mBtnChange.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    if (bFragment == null) {
                        bFragment = new BFragment();
                    }
                    getSupportFragmentManager().beginTransaction().replace(R.id.fl_container,bFragment).commitAllowingStateLoss();
                }
            });
    
            //实例化AFragment
            //aFragment = new AFragment();
            aFragment = AFragment.newInstance("我是参数");//向Fragment传递参数
            //把AFragment添加到Activity中，记得调用commit/commitAllowingStateLoss,后者可以少一些错误
            getSupportFragmentManager().beginTransaction().add(R.id.fl_container,aFragment).commitAllowingStateLoss();
        }
    }
  

• 效果