Fragment事务及Fragment实现选项卡功能

Fragment事务

Fragment类引入到Honeycomb时，为协同工作，Activity类被更改为含有FragmentManager类，FragmentManager类负责管理fragment并将它们的视图添加到activity的视图层级结构中。

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FragmentManager类具体管理的是：
1. fragment队列
2. fragment事务的回退栈

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fragment事务被用来 添加、移除、附加、分离或替换 fragment队列中的fragment。这是使用fragment在运行时组装和重新组装用户界面的核心方式。FragmentManager管理者fragment事务的回退栈。

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FragmentManager.beginTransaction()方法创建并返回FragmentTransaction实例。FragmentTransaction类使用了一个fluent interface接口方法，通过该方法配置FragmentTransaction返回FragmentTransaction类对象，而不是void，由此可得到一个FragmentTransaction队列。

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activity因设备旋转或回收内存被销毁后重建时，Activity.onCreate()方法会响应activity重建而被调用。activity被销毁时，它的FragmentManager会将fragment队列保存下来。这样，activity重建时，新的FragmentManager会首先获取保存的队列，然后重建fragment队列，从而恢复到原来的状态。

Fragment实现选项卡功能

package com.ashzheng.studydemo.demo4;

import android.app.Activity;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.widget.RadioGroup;

import com.ashzheng.studydemo.R;

public class Demo4Activity extends Activity {
    private Demo4Fragment1 fragment1;
    private Demo4Fragment2 fragment2;
    private Demo4Fragment3 fragment3;
    private Demo4Fragment4 fragment4;

    private FragmentManager fragmentManager;

    private RadioGroup radioGroup;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_demo4);

        radioGroup = (RadioGroup) findViewById(R.id.demo4_rg);
        radioGroup.setOnCheckedChangeListener(new RadioGroup.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(RadioGroup radioGroup, int i) {
                selectFragment(i);
            }
        });

        fragmentManager = getFragmentManager();
        selectFragment(R.id.demo4_rbt3);
    }


    private void selectFragment(int checkedId) {

        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        hideFragment(fragmentTransaction);

        switch (checkedId) {
            case R.id.demo4_rbt1:
                if (null == fragment1) {
                    fragment1 = new Demo4Fragment1();
                    fragmentTransaction.add(R.id.demo4_layout, fragment1);
                } else {
                    fragmentTransaction.show(fragment1);
                }
                break;
            case R.id.demo4_rbt2:
                if (null == fragment2) {
                    fragment2 = new Demo4Fragment2();
                    fragmentTransaction.add(R.id.demo4_layout, fragment2);
                } else {
                    fragmentTransaction.show(fragment2);
                }
                break;
            case R.id.demo4_rbt3:
                if (null == fragment3) {
                    fragment3 = new Demo4Fragment3();
                    fragmentTransaction.add(R.id.demo4_layout, fragment3);
                } else {
                    fragmentTransaction.show(fragment3);
                }
                break;
            case R.id.demo4_rbt4:
                if (null == fragment4) {
                    fragment4 = new Demo4Fragment4();
                    fragmentTransaction.add(R.id.demo4_layout, fragment4);
                } else {
                    fragmentTransaction.show(fragment4);
                }
                break;
        }
        fragmentTransaction.commit();
    }

    private void hideFragment(FragmentTransaction fragmentTransaction) {
        if(null != fragment1){
            fragmentTransaction.hide(fragment1);
        }
        if(null != fragment2){
            fragmentTransaction.hide(fragment2);
        }
        if(null != fragment3){
            fragmentTransaction.hide(fragment3);
        }
        if(null != fragment4){
            fragmentTransaction.hide(fragment4);
        }

    }

}