fragment – 蓝天白云

　　我们知道，当app的屏幕进行旋转或者其他的配置变化的时候，Fragment会执行它的周期变化，重新创建Fragment并加载（除非设置了setRetainInstance(true))，那么在重建的时候Fragment中的变量就会被重新初始化，运行中的状态将无法保留。

　　setArgument中的Bundle可以用来存放各种需要保持的数据，即使Fragment由于配置改变而重建，这些保存的数据也不会被清除。

　　下面我们将通过一个实例来测试一下，在实例的Fragment中共有3个TextView和一个Button，为了使测试的内容更清晰，下面把三个TextView对应的字符串列表如下：

textview1->banana, 　　textview2->apple, 　　textview3->pear

　　对应的三个字符串：

mParam1: 在Fragment.newInstance中传入，并保存在setArgument中，但是在点击按钮时未更新到Argument中
mParam2: Fragment中的私有变量，和Argument无任何交互
mParam3: 在Fragment.newInstance中传入，并保存在setArgument中，并且在点击按钮时更新到了Argument中

　　测试程序的布局代码和java代码如下：

　　Fragment部局文件的代码

<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.bcoder.myapplication.TestFragment"
              android:orientation="vertical">

    <!-- TODO: Update blank fragment layout -->


    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="textview1"
        android:id="@+id/textview1"/>

    <TextView
        android:text="TextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/textview2"/>

    <TextView
        android:text="TextView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/textview3"/>

    <Button
        android:text="Button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/button1"/>

</LinearLayout>

<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.bcoder.myapplication.TestFragment"

android:orientation="vertical">

<TextView

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:text="textview1"

android:id="@+id/textview1"/>

<TextView

android:text="TextView"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:id="@+id/textview2"/>

<TextView

android:text="TextView"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:id="@+id/textview3"/>

<Button

android:text="Button"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:id="@+id/button1"/>

</LinearLayout>

　　Fragment的Java代码

package com.bcoder.myapplication;

import android.content.Context;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;


public class TestFragment extends Fragment {
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM3 = "param3";

    private String mParam1;
    private String mParam2;
    private String mParam3;

    TextView mTextView1;
    TextView mTextView2;
    TextView mTextView3;
    Button button1;

    View fragmentView;

    public TestFragment() {
        // Required empty public constructor
    }


    public static TestFragment newInstance(String param1, String param3) {
        TestFragment fragment = new TestFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM3, param3);
        fragment.setArguments(args);

        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam3 = getArguments().getString(ARG_PARAM3);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        fragmentView = inflater.inflate(R.layout.fragment_test, container, false);

        initViews();

        mTextView1.setText(mParam1);
        mTextView2.setText(mParam2);
        mTextView3.setText(mParam3);

        return fragmentView;
    }

    private void initViews() {
        mTextView1 = (TextView) fragmentView.findViewById(R.id.textview1);
        mTextView2 = (TextView) fragmentView.findViewById(R.id.textview2);
        mTextView3 = (TextView) fragmentView.findViewById(R.id.textview3);

        button1 = (Button) fragmentView.findViewById(R.id.button1);
        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                mParam1 = "banana2";
                mTextView1.setText(mParam1);

                mParam2 = "apple";
                mTextView2.setText(mParam2);

                mParam3 = "pear click";
                getArguments().putString(ARG_PARAM3, mParam3);
                mTextView3.setText(mParam3);
            }
        });
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
    }

    @Override
    public void onDetach() {
        super.onDetach();
    }

}

package com.bcoder.myapplication;

import android.content.Context;

import android.net.Uri;

import android.os.Bundle;

import android.support.v4.app.Fragment;

import android.view.LayoutInflater;

import android.view.View;

import android.view.ViewGroup;

import android.widget.Button;

import android.widget.TextView;

public class TestFragment extends Fragment {

private static final String ARG_PARAM1 = "param1";

private static final String ARG_PARAM3 = "param3";

private String mParam1;

private String mParam2;

private String mParam3;

TextView mTextView1;

TextView mTextView2;

TextView mTextView3;

Button button1;

View fragmentView;

public TestFragment() {

// Required empty public constructor

}

public static TestFragment newInstance(String param1, String param3) {

TestFragment fragment = new TestFragment();

Bundle args = new Bundle();

args.putString(ARG_PARAM1, param1);

args.putString(ARG_PARAM3, param3);

fragment.setArguments(args);

return fragment;

}

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

if (getArguments() != null) {

mParam1 = getArguments().getString(ARG_PARAM1);

mParam3 = getArguments().getString(ARG_PARAM3);

}

@Override

public View onCreateView(LayoutInflater inflater, ViewGroup container,

Bundle savedInstanceState) {

fragmentView = inflater.inflate(R.layout.fragment_test, container, false);

initViews();

mTextView1.setText(mParam1);

mTextView2.setText(mParam2);

mTextView3.setText(mParam3);

return fragmentView;

}

private void initViews() {

mTextView1 = (TextView) fragmentView.findViewById(R.id.textview1);

mTextView2 = (TextView) fragmentView.findViewById(R.id.textview2);

mTextView3 = (TextView) fragmentView.findViewById(R.id.textview3);

button1 = (Button) fragmentView.findViewById(R.id.button1);

button1.setOnClickListener(new View.OnClickListener() {

@Override

public void onClick(View view) {

mParam1 = "banana2";

mTextView1.setText(mParam1);

mParam2 = "apple";

mTextView2.setText(mParam2);

mParam3 = "pear click";

getArguments().putString(ARG_PARAM3, mParam3);

mTextView3.setText(mParam3);

}

});

}

@Override

public void onAttach(Context context) {

super.onAttach(context);

}

@Override

public void onDetach() {

super.onDetach();

}

　　主Activity的代码

package com.bcoder.myapplication;

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.LinearLayout;

public class MainActivity extends AppCompatActivity {
    LinearLayout layoutContainer;

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

        initViews();

        TestFragment fragment = TestFragment.newInstance("banana", "pear");

        FragmentManager manager = getSupportFragmentManager();
        FragmentTransaction transaction = manager.beginTransaction();
        transaction.add(R.id.layoutFragmentContainer, fragment);
        transaction.commit();
    }

    private void initViews() {
        layoutContainer = (LinearLayout) findViewById(R.id.layoutFragmentContainer);
    }
}

package com.bcoder.myapplication;

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.LinearLayout;

public class MainActivity extends AppCompatActivity {

LinearLayout layoutContainer;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

initViews();

TestFragment fragment = TestFragment.newInstance("banana", "pear");

FragmentManager manager = getSupportFragmentManager();

FragmentTransaction transaction = manager.beginTransaction();

transaction.add(R.id.layoutFragmentContainer, fragment);

transaction.commit();

}

private void initViews() {

layoutContainer = (LinearLayout) findViewById(R.id.layoutFragmentContainer);

}

　　主Activity的布局文件代码

<?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:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.bcoder.myapplication.MainActivity"
    android:orientation="vertical">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/layoutFragmentContainer">
    </LinearLayout>
</LinearLayout>

<?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:id="@+id/activity_main"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:paddingBottom="@dimen/activity_vertical_margin"

android:paddingLeft="@dimen/activity_horizontal_margin"

android:paddingRight="@dimen/activity_horizontal_margin"

android:paddingTop="@dimen/activity_vertical_margin"

tools:context="com.bcoder.myapplication.MainActivity"

android:orientation="vertical">

<LinearLayout

android:orientation="vertical"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:id="@+id/layoutFragmentContainer">

</LinearLayout>

　　程序启动时显示默认如下内容：

　　textview1和textview3的值取自getArgument并显示在了屏幕上，当点击按钮时会重新给三个String变量赋值，并显示到textview中，如下图：

　　这时三个字符串变量都有了新的值，那么当我们把屏幕进行一下旋转操作后，看结果如何呢？

　　由图可知，字符串1回到了最初的状态（创建Fragment时传入的值），而字符串2被清除了。

　　字符串3因为在点击按钮的时候重新被保存在getArgument中，而没有任何的变化的被保留下来了。

　　因此大家在给Fragment传入参数的时候最好通过setArgument进行保存，而不是简单的通过一个变量进行保存。

注意：

setArgument只能在Fragment中创建后(MyFragment fragment = new MyFragment())立即调用，如果在Fragment被附加到Activity中后再调用就会报错，可以在按钮的点击事件中添加一下setArgument的调用进行一下试验，应用将被异常终止。

　　做了一个小的app，其中用到了tab形式的actionBar和多个fragment，后来因为需求需要在某两个fragment中加入context menu，加入后添加onContextItemSelected事件响应菜单按下的效果，一开始运行正常，在两个fragment中打开context menu正常，两个fragment中的菜单响应事件也正常，后来改动了一些代码后突然发现菜单响应的不正确了，只有一个fragment中的菜单有响应，另外一个没响应。经过一番修改后，发现是onContextItemSelected的返回值影响到了菜单点击事件的执行。

　　我们先来看一下onContextItemSelected的官方描述：

This hook is called whenever an item in a context menu is selected. The default implementation simply returns false to have the normal processing happen (calling the item's Runnable or sending a message to its Handler as appropriate). You can use this method for any items for which you would like to do processing without those other facilities.

Use getMenuInfo() to get extra information set by the View that added this menu item.

Derived classes should call through to the base class for it to perform the default menu handling.

Parameters
item	The context menu item that was selected.

Returns
boolean Return false to allow normal context menu processing to proceed, true to consume it here.

This hook is called whenever an item in a context menu is selected. The default implementation simply returns false to have the normal processing happen (calling the item's Runnable or sending a message to its Handler as appropriate). You can use this method for any items for which you would like to do processing without those other facilities.

Use getMenuInfo() to get extra information set by the View that added this menu item.

Derived classes should call through to the base class for it to perform the default menu handling.

Parameters

item The context menu item that was selected.

Returns

boolean Return false to allow normal context menu processing to proceed, true to consume it here.

　　我们主要看这个返回值部分，返回为了一布尔型变量，当返回false值时由系统去决定此菜单事件的响应流程，返回true则表示菜单响应到此为止，停止后面的流程。

　　本人没做深入研究，猜想大概流程是这样的，因为在fragment中调用context menu中通过如下方式

	public void onCreateContextMenu(ContextMenu menu, View v,
			ContextMenuInfo menuInfo) {
		getActivity().getMenuInflater().inflate(R.menu.menu_goodlist_item, menu);
	}

public void onCreateContextMenu(ContextMenu menu, View v,

ContextMenuInfo menuInfo) {

getActivity().getMenuInflater().inflate(R.menu.menu_goodlist_item, menu);

}

　　是通过父activity来填充这个弹出菜单，所以首先是父activity先接收到onContextItemSelected事件，如果父activity中不做处理，则activity会把这个事件分发到各个fragment中去，本人在此次犯的错误就是在子fragment中的onContextItemSelected事件中返回了true值，所以只在一个fragment中响应了事件，另外一个没有响应。

　　既然在所有的fragment中都会响应onContextItemSelected这个事件，那么我们在每个fragment中处理该事件的时候就需要做一些判断，以避免多个fragment同时执行这个事件，这个是非常重要的，因为不做判断很可能出现一些莫名奇妙的而且不好测试的问题。如下例子：

	@Override
	public boolean onContextItemSelected(MenuItem item) {
		super.onContextItemSelected(item);
		if(item.getItemId() == R.id.menu_edit_good || item.getItemId() == R.id.menu_delete_good){
		...
		}

		return false;
	}

@Override

public boolean onContextItemSelected(MenuItem item) {

super.onContextItemSelected(item);

if(item.getItemId() == R.id.menu_edit_good || item.getItemId() == R.id.menu_delete_good){

...

}

return false;

}

为什么要在Fragment中使用setArgument？

在多fragment应用中遇到的onContextItemSelected一个小问题