今天是Fragment专辑的最后一篇,说明一下最常用的写法:
一般通过add、show、hide相结合的方法来控制Fragment的显示和隐藏,上一篇已经说过,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。
贴出代码:
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{ 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: if (oneFragment == null){ oneFragment = new OneFragment(); } if (oneFragment.isAdded()){ transaction.show(oneFragment); }else{ transaction.add(R.id.fragment_container,oneFragment); } if (twoFragment != null && twoFragment.isAdded()){ transaction.hide(twoFragment); } if (threeFragment != null && threeFragment.isAdded()){ transaction.hide(threeFragment); } break; case R.id.two: if (twoFragment == null){ twoFragment = new TwoFragment(); } if (twoFragment.isAdded()){ transaction.show(twoFragment); }else{ transaction.add(R.id.fragment_container,twoFragment); } if (oneFragment != null && oneFragment.isAdded()){ transaction.hide(oneFragment); } if (threeFragment != null && threeFragment.isAdded()){ transaction.hide(threeFragment); } break; case R.id.three: if (threeFragment == null){ threeFragment = new ThreeFragment(); } if (threeFragment.isAdded()){ transaction.show(threeFragment); }else{ transaction.add(R.id.fragment_container,threeFragment); } 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); }}
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代码中已经写好了注释,就不再详细解释,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
@Override public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) { super.onSaveInstanceState(outState, outPersistentState); }
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)。
outState.putSerializable(key, obj);outState.putParcelable(key, obj);
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,还不了解如何使用这两个接口的少年可以去百度资料。
@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); } ...... }
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(one != null){ oneFragment = new OneFragment();}...
我是使用的第二个方法,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{ 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 (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: oneFragment = fm.findFragmentByTag("OneFragment"); if (oneFragment == null){ oneFragment = new OneFragment(); } if (oneFragment.isAdded()){ transaction.show(oneFragment); }else{ transaction.add(R.id.fragment_container,oneFragment, "OneFragment"); } if (twoFragment != null && twoFragment.isAdded()){ transaction.hide(twoFragment); } if (threeFragment != null && threeFragment.isAdded()){ transaction.hide(threeFragment); } break; case R.id.two: twoFragment = fm.findFragmentByTag("TwoFragment"); if (twoFragment == null){ twoFragment = new TwoFragment(); } if (twoFragment.isAdded()){ transaction.show(twoFragment); }else{ transaction.add(R.id.fragment_container,twoFragment, "TwoFragment"); } if (oneFragment != null && oneFragment.isAdded()){ transaction.hide(oneFragment); } if (threeFragment != null && threeFragment.isAdded()){ transaction.hide(threeFragment); } break; case R.id.three: threeFragment = fm.findFragmentByTag("ThreeFragment"); if (threeFragment == null){ threeFragment = new ThreeFragment(); } if (threeFragment.isAdded()){ transaction.show(threeFragment); }else{ transaction.add(R.id.fragment_container,threeFragment); } if (twoFragment != null && twoFragment.isAdded()){ transaction.hide(twoFragment); } if (oneFragment != null && oneFragment.isAdded()){ transaction.hide(oneFragment); } break; default: break; } transaction.commitAllowingStateLoss(); }}
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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" tools:context="com.example.lzp.MainActivity" android:orientation="vertical"> <FrameLayout android:id="@+id/fragment_container" android:layout_width="match_parent" android:layout_height="0dp" android:layout_weight="1"/> <RadioGroup android:id="@+id/tab_radio" android:layout_width="match_parent" android:layout_height="50dp" android:orientation="horizontal"> <RadioButton android:id="@+id/one" android:layout_width="0dp" android:layout_weight="1" android:layout_height="match_parent" android:checked="true" android:text="1"/> <RadioButton android:id="@+id/two" android:layout_width="0dp" android:layout_weight="1" android:layout_height="match_parent" android:text="2"/> <RadioButton android:id="@+id/three" android:layout_width="0dp" android:layout_weight="1" android:layout_height="match_parent" android:text="3"/> </RadioGroup></LinearLayout>
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如果还有其他更加方便简单的方法,请大牛留言告知,让大家瞻仰学习一下。
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