Fragment的常用写法

一般通过add、show、hide相结合的方法来控制Fragment的显示和隐藏，这样不会再重写一遍Fragment的生命周期，节省了时间和内存，当然特殊需求除外。

package com.example.lzp;

import android.os.PersistableBundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.RadioGroup;

public class MainActivity extends FragmentActivity implements RadioGroup.OnCheckedChangeListener{

    // 负责全局的FragmentManager, 建议使用support包中的FramengManager类
    private FragmentManager fm;
    private Fragment oneFragment, twoFragment, threeFragment;

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

        fm = getSupportFragmentManager();
        RadioGroup tabRadio = (RadioGroup) findViewById(R.id.tab_radio);
        tabRadio.setOnCheckedChangeListener(this);
        if (oneFragment == null){
            oneFragment = new OneFragment();
        }
        fm.beginTransaction().add(R.id.fragment_container, oneFragment).commit();
    }

    @Override
    public void onCheckedChanged(RadioGroup group, int checkedId) {
        FragmentTransaction transaction = fm.beginTransaction();
        switch (checkedId){
            case R.id.one:
                // 判断是否为空
                if (oneFragment == null){
                    oneFragment = new OneFragment();
                }
                // 判断是否添加这个fragment对象是否已经添加到容器中
                // 如果已经添加过了，就show，如果没有添加就add
                if (oneFragment.isAdded()){
                    transaction.show(oneFragment);
                }else{
                    transaction.add(R.id.fragment_container,oneFragment);
                }
                // 隐藏掉其他的两个fragment
                if (twoFragment != null && twoFragment.isAdded()){
                    transaction.hide(twoFragment);
                }
                if (threeFragment != null && threeFragment.isAdded()){
                    transaction.hide(threeFragment);
                }
                break;

            case R.id.two:
                // 判断是否为空
                if (twoFragment == null){
                    twoFragment = new TwoFragment();
                }
                // 判断是否添加这个fragment对象是否已经添加到容器中
                // 如果已经添加过了，就show，如果没有添加就add
                if (twoFragment.isAdded()){
                    transaction.show(twoFragment);
                }else{
                    transaction.add(R.id.fragment_container,twoFragment);
                }
                // 隐藏掉其他的两个fragment
                if (oneFragment != null && oneFragment.isAdded()){
                    transaction.hide(oneFragment);
                }
                if (threeFragment != null && threeFragment.isAdded()){
                    transaction.hide(threeFragment);
                }
                break;
            case R.id.three:
                // 判断是否为空
                if (threeFragment == null){
                    threeFragment = new ThreeFragment();
                }
                // 判断是否添加这个fragment对象是否已经添加到容器中
                // 如果已经添加过了，就show，如果没有添加就add
                if (threeFragment.isAdded()){
                    transaction.show(threeFragment);
                }else{
                    transaction.add(R.id.fragment_container,threeFragment);
                }
                // 隐藏掉其他的两个fragment
                if (twoFragment != null && twoFragment.isAdded()){
                    transaction.hide(twoFragment);
                }
                if (oneFragment != null && oneFragment.isAdded()){
                    transaction.hide(oneFragment);
                }
                break;
            default:
                break;
        }
        transaction.commitAllowingStateLoss();
    }

    @Override
    public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
        super.onSaveInstanceState(outState, outPersistentState);
    }
}

很多初学的开发者认为写出上面的代码就可以了。实际上在功能是没有问题的，但是却有一个隐藏的巨大bug，假如MainActivity被回收，那也就说明MainActivity中所有的对象都被回收了，包括之前创建过的三个fragment，下次回到MainActivity的时候还会再创建一遍，判断是否已经添加过还是返回false，这样的情况出现多次，容器中就会添加了很多的同一类型的fragnment，最终出现内存溢出。
解决办法1： 
借助Activity的onSaveInstanceState()

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

        if (savedInstanceState != null){
            //obj = savedInstanceState.getSerializable(key);
            obj = savedInstanceState.getParcelableArray(key);
        }

        ......
}

@Override
public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
        super.onSaveInstanceState(outState, outPersistentState);
        //outState.putSerializable(key, obj);
        outState.putParcelable(key, obj);
}

这个方法会在activi回收的时候回调，可以重写这个方法保存一些我们想保存的信息，view的状态信息一般已经被自动保存了，不需要我们特意去写（实际测试锤子和坚果手机除外）。上面两个方法帮助我们保存对象，这个类必须实现可序列化接口，android系统推荐的是使用第二个方法--Parcelable。下次回到Activity的时候去判断是否有保存的信息，取出之前保存的对象即可。

解决办法2：

通过给fragment设置标签来保证容器中只有同一个类型的对象

transaction.add(R.id.fragment_container,oneFragment, "oneFragment");

add方法中的第三个参数就是我们设置的标签，同一个类型我们设置一个唯一的标签，切换的时候通过标签获取对象：

FragmentManager fm = getSupportFragmentManager();
oneFragment = fm.findFragmentByTag("oneFragment");
if(oneFragment == null){
  oneFragment = new OneFragment(); 
}...

下面贴出第二种方法的完整代码：

package com.example.lzp;

import android.os.PersistableBundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.RadioGroup;

public class MainActivity extends FragmentActivity implements RadioGroup.OnCheckedChangeListener{

    // 负责全局的FragmentManager, 建议使用support包中的FramengManager类
    private FragmentManager fm;
    private Fragment oneFragment, twoFragment, threeFragment;

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

        fm = getSupportFragmentManager();
        RadioGroup tabRadio = (RadioGroup) findViewById(R.id.tab_radio);
        tabRadio.setOnCheckedChangeListener(this);
        // 如果不需要恢复之前的状态，默认显示的是第一个oneFragment
        if (savedInstanceState == null){
            if (oneFragment == null){
                oneFragment = new OneFragment();
            }
            fm.beginTransaction().add(R.id.fragment_container, oneFragment).commit();
        }

    }

    @Override
    public void onCheckedChanged(RadioGroup group, int checkedId) {
        FragmentTransaction transaction = fm.beginTransaction();
        switch (checkedId){
            case R.id.one:
                // 从容器通过标签获取相同类型的Fragment
                oneFragment = fm.findFragmentByTag("OneFragment");
                // 判断是否为空
                if (oneFragment == null){
                    oneFragment = new OneFragment();
                }
                // 判断是否添加这个fragment对象是否已经添加到容器中
                // 如果已经添加过了，就show，如果没有添加就add
                if (oneFragment.isAdded()){
                    transaction.show(oneFragment);
                }else{
                    transaction.add(R.id.fragment_container,oneFragment, "OneFragment");
                }
                // 隐藏掉其他的两个fragment
                if (twoFragment != null && twoFragment.isAdded()){
                    transaction.hide(twoFragment);
                }
                if (threeFragment != null && threeFragment.isAdded()){
                    transaction.hide(threeFragment);
                }
                break;

            case R.id.two:
                // 从容器通过标签获取相同类型的Fragment
                twoFragment = fm.findFragmentByTag("TwoFragment");
                // 判断是否为空
                if (twoFragment == null){
                    twoFragment = new TwoFragment();
                }
                // 判断是否添加这个fragment对象是否已经添加到容器中
                // 如果已经添加过了，就show，如果没有添加就add
                if (twoFragment.isAdded()){
                    transaction.show(twoFragment);
                }else{
                    transaction.add(R.id.fragment_container,twoFragment, "TwoFragment");
                }
                // 隐藏掉其他的两个fragment
                if (oneFragment != null && oneFragment.isAdded()){
                    transaction.hide(oneFragment);
                }
                if (threeFragment != null && threeFragment.isAdded()){
                    transaction.hide(threeFragment);
                }
                break;
            case R.id.three:
                // 从容器通过标签获取相同类型的Fragment
                threeFragment = fm.findFragmentByTag("ThreeFragment");
                // 判断是否为空
                if (threeFragment == null){
                    threeFragment = new ThreeFragment();
                }
                // 判断是否添加这个fragment对象是否已经添加到容器中
                // 如果已经添加过了，就show，如果没有添加就add
                if (threeFragment.isAdded()){
                    transaction.show(threeFragment);
                }else{
                    transaction.add(R.id.fragment_container,threeFragment);
                }
                // 隐藏掉其他的两个fragment
                if (twoFragment != null && twoFragment.isAdded()){
                    transaction.hide(twoFragment);
                }
                if (oneFragment != null && oneFragment.isAdded()){
                    transaction.hide(oneFragment);
                }
                break;
            default:
                break;
        }
        transaction.commitAllowingStateLoss();
    }

}

http://blog.csdn.net/u011315960/article/details/51192635