Android Floating Context Menu的使用方法

1. 概述

Android Context Menu有两种，即floating context menu和contextual action mode下的context menu。本文讨论前者，后者是在Android 3.0开始引入的。

2. 主要思路

要使用floating context menu，包括如下几个主要步骤（适用于Activity和Fragment)：

对于要关联context menu的view，调用registerForContextMenu(View)进行注册；
重载onCreateContextMenu()，其目的在于创建具体的context menu；
重载onContextItemSelected()，其目的在于，选择了某一个context menu item的时候，进行适当的处理。

3. 示例

这里是以为ListView创建一个context menu为例，其中ListView部分的介绍，请参考“ListView的例子”。

为了保持文章或示例的独立性，我们把相关的代码全部贴出来。

3.1 应用的布局文件

<?xml version="1.0" encoding="utf-8"?>

<!-- hello_context_menu.xml -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<ListView
android:id="@+id/list_view_id"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
</ListView>

</LinearLayout>

3.2 ListView中每一个元素的 布局文件

<!-- student_item.xml -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal" >

<TextView
android:id="@+id/student_name"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="2" />

<TextView
android:id="@+id/student_score"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1" />

</LinearLayout>

3.3 上下文菜单的资源文件

<?xml version="1.0" encoding="utf-8"?>

<!-- context_menu.xml -->
<menu xmlns:android="http://schemas.android.com/apk/res/android" >
<item
android:id="@+id/first_context_menu_item"
android:title="@string/first_context_menu_item"/>

<item
android:id="@+id/second_context_menu_item"
android:title="@string/second_context_menu_item"/>
</menu>

3.4 字符串资源

<string name="first_context_menu_item">first item</string>
<string name="second_context_menu_item">second item</string>

3.5 Java代码

package com.example.hellocontextmenu;

import java.util.ArrayList;
import java.util.HashMap;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.ContextMenu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ContextMenu.ContextMenuInfo;
import android.widget.ListView;
import android.widget.SimpleAdapter;

public class MainActivity extends Activity {

private static final String TAG = "MainActivity";

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

ListView listView = (ListView) this.findViewById(R.id.list_view_id);
setData(listView);

this.registerForContextMenu(listView);

}

private void setData(ListView listView) {
// each item's layout resource id
int student_item_id = R.layout.student_item;

// columns names
String[] columnNames = new String[] { "name", "score" };

// resource ids
int[] ids = new int[] { R.id.student_name, R.id.student_score };

// the data to be displayed
ArrayList<HashMap<String, Object>> students = new ArrayList<HashMap<String, Object>>();
HashMap<String, Object> map = null;
for (int i = 1; i <= 10; i++) {
map = new HashMap<String, Object>();
map.put(columnNames[0], "student-" + i);
map.put(columnNames[1], String.valueOf(i * 10));
students.add(map);
}

listView.setAdapter(new SimpleAdapter(this, students, student_item_id,
columnNames, ids));

}

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {
super.onCreateContextMenu(menu, v, menuInfo);

MenuInflater inflater = this.getMenuInflater();
inflater.inflate(R.menu.context_menu, menu);
}

@Override
public boolean onContextItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.first_context_menu_item:
Log.d(TAG, "first context menu item selected");
return true;

case R.id.second_context_menu_item:
Log.d(TAG, "second context menu item selected");
return true;

default:
return super.onContextItemSelected(item);
}

}
}

3.6 运行效果

4. 关于ContextMenuInfo

在Android官网的Menu Guide中，提到ContextMenuInfo可用于标识当前所选择的view，以及view中的item，从而做一些特殊的处理。原文如下：

The callback method parameters include the View that the user selected and a ContextMenu.ContextMenuInfo
object that provides additional information about the item selected. If your activity has several views
that each provide a different context menu, you might use these parameters to determine which context
menu to inflate.

本节就来分析这个ContextMenuInfo，并通过一些例子进行说明。

4.1 ContextMenuInfo接口

先大概了解这个对象，查看ContextMenuInfo这个class，发现有个toString()方法。增加调试打印：

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {
super.onCreateContextMenu(menu, v, menuInfo);

MenuInflater inflater = this.getMenuInflater();
inflater.inflate(R.menu.context_menu, menu);

Log.d(TAG, menuInfo.toString());
}

运行结果如下：

android.widget.AdapterView$AdapterContextMenuInfo@41d217b8

实际上，这个toString()并不是ContextMenuInfo重载的，而是缺省的继承Object的该方法。ContextMenuInfo本身是一个接口：

/**
* Additional information regarding the creation of the context menu.  For example,
* {@link AdapterView}s use this to pass the exact item position within the adapter
* that initiated the context menu.
*/
public interface ContextMenuInfo {
}

4.2 AdapterView

本例使用的是ListView，而它是（间接）继承自AdapterView。在AdapterView中，定义了ContextMenuInfo的一个实现，即AdapterContextMenuInfo：

/**
* Extra menu information provided to the
* {@link android.view.View.OnCreateContextMenuListener#onCreateContextMenu(ContextMenu, View, ContextMenuInfo) }
* callback when a context menu is brought up for this AdapterView.
*
*/
public static class AdapterContextMenuInfo implements ContextMenu.ContextMenuInfo {

public AdapterContextMenuInfo(View targetView, int position, long id) {
this.targetView = targetView;
this.position = position;
this.id = id;
}

/**
* The child view for which the context menu is being displayed. This
* will be one of the children of this AdapterView.
*/
public View targetView;

/**
* The position in the adapter for which the context menu is being
* displayed.
*/
public int position;

/**
* The row id of the item for which the context menu is being displayed.
*/
public long id;
}

这里的targetView、position、id就是典型的各种适配器中的概念。由此，我们就可以确定前面提到的onCreateContextMenu()及onContextItemSelected()中menuInfo对象的详细数据了。

4.3 分析ContextMenuInfo对象

根据前面的分析，我们可以根据所选择的ListView中的具体数据，来加载不同的context menu，——对应onCreateContextMenu()。还可以在onContextItemSelected()中对具体的选中对象做特殊的处理。下面的就是Android官网Menu Guide中的示例代码：

@Override
public boolean onContextItemSelected(MenuItem item) {
AdapterContextMenuInfo info = (AdapterContextMenuInfo) item.getMenuInfo();
switch (item.getItemId()) {
case R.id.edit:
editNote(info.id);
return true;
case R.id.delete:
deleteNote(info.id);
return true;
default:
return super.onContextItemSelected(item);
}
}

5. ContextMenuInfo的使用示例

我们接下来在之前的代码增加一些处理：根据选择的不同的对象，inflater不同的context menu：共3种，除了之前的一种之外，另增加了更新学生分数的两个上下文菜单。

5.1 增加学生分数的menu资源

<?xml version="1.0" encoding="utf-8"?>

<!-- add_context_menu.xml -->

<menu xmlns:android="http://schemas.android.com/apk/res/android" >

<item
android:id="@+id/add_score"
android:title="@string/add_score"/>

</menu>

5.2 减少学生分数的menu资源

<?xml version="1.0" encoding="utf-8"?>

<!-- minus_context_menu.xml -->

<menu xmlns:android="http://schemas.android.com/apk/res/android" >
<item
android:id="@+id/minus_score"
android:title="@string/minus_score"/>

</menu>

5.3 新增的字符串资源

<string name="add_score">Add Score</string>
<string name="minus_score">Minus Score</string>

5.4 Java代码

package com.example.hellocontextmenu;

import java.util.ArrayList;
import java.util.HashMap;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.ContextMenu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ContextMenu.ContextMenuInfo;
import android.widget.AdapterView.AdapterContextMenuInfo;
import android.widget.BaseAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;

public class MainActivity extends Activity {

private static final String TAG = "MainActivity";
private ListView listView = null;
private BaseAdapter adapter = null;

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

listView = (ListView) this.findViewById(R.id.list_view_id);
setData();

this.registerForContextMenu(listView);

}

private void setData() {
// each item's layout resource id
int student_item_id = R.layout.student_item;

// columns names
String[] columnNames = new String[] { "name", "score" };

// resource ids
int[] ids = new int[] { R.id.student_name, R.id.student_score };

// the data to be displayed
ArrayList<HashMap<String, Object>> students = new ArrayList<HashMap<String, Object>>();
HashMap<String, Object> map = null;
for (int i = 1; i <= 10; i++) {
map = new HashMap<String, Object>();
map.put(columnNames[0], "student-" + i);
map.put(columnNames[1], String.valueOf(i * 10));
students.add(map);
}

adapter = new SimpleAdapter(this, students, student_item_id,
columnNames, ids);
listView.setAdapter(adapter);
}

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {
super.onCreateContextMenu(menu, v, menuInfo);

AdapterContextMenuInfo info = (AdapterContextMenuInfo)menuInfo;
dumpInfo(info);

MenuInflater inflater = this.getMenuInflater();

if (info.position > 8) {
inflater.inflate(R.menu.context_menu, menu);
} else {
if (info.position % 2 == 0) {
inflater.inflate(R.menu.add_context_menu, menu);
} else {
inflater.inflate(R.menu.minus_context_menu, menu);
}
}
}

private void dumpInfo(AdapterContextMenuInfo info) {
HashMap<String, Object> student = (HashMap<String, Object>) adapter.getItem(info.position);

Toast.makeText(this, student.toString(),Toast.LENGTH_LONG).show();
}

private void updateScore(ContextMenuInfo menuInfo, int incremental) {
AdapterContextMenuInfo info = (AdapterContextMenuInfo)menuInfo;
HashMap<String, Object> student = (HashMap<String, Object>) adapter.getItem(info.position);
Log.d(TAG, "old: " + student.toString());

String studentName = (String) student.get("name");
String studentScore = (String) student.get("score");
int score = Integer.parseInt(studentScore);

student.put("score", String.valueOf(score + incremental));

adapter.notifyDataSetChanged();

Log.d(TAG, "new: " + student.toString());
}

@Override
public boolean onContextItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.first_context_menu_item:
Log.d(TAG, "first context menu item selected");
return true;

case R.id.second_context_menu_item:
Log.d(TAG, "second context menu item selected");
return true;

case R.id.add_score:
updateScore(item.getMenuInfo(), 1);
return true;

case R.id.minus_score:
updateScore(item.getMenuInfo(), -1);
return true;

default:
return super.onContextItemSelected(item);
}

}
}

5.5 运行效果

logcat日志：

09-07 20:17:29.835: D/MainActivity(13101): old: {score=20, name=student-2}
09-07 20:17:29.835: D/MainActivity(13101): new: {score=19, name=student-2}
09-07 20:17:40.745: D/MainActivity(13101): old: {score=40, name=student-4}
09-07 20:17:40.745: D/MainActivity(13101): new: {score=39, name=student-4}
09-07 20:18:02.755: D/MainActivity(13101): second context menu item selected
09-07 20:18:11.265: D/MainActivity(13101): old: {score=10, name=student-1}
09-07 20:18:11.265: D/MainActivity(13101): new: {score=11, name=student-1}

上下文菜单的截图：

6. 多个View的情况

现在前面代码的基础上，增加一个View。此时在onCreateContextMenu()中，通过传入的View参数来区分不同的View对象，从而加载不同的context menu。

6.1 布局文件

增加一个Button：

<?xml version="1.0" encoding="utf-8"?>

<!-- hello_context_menu.xml -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

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

<ListView
android:id="@+id/list_view_id"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
</ListView>

</LinearLayout>

6.2 字符串资源

<string name="button_text">Try to Click Me</string>

6.3 Java代码

主要修改两个地方：

为Button注册上下文菜单；
创建上下文菜单的时候，要区分不同的view，做差异化处理

package com.example.hellocontextmenu;

import java.util.ArrayList;
import java.util.HashMap;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.ContextMenu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ContextMenu.ContextMenuInfo;
import android.widget.AdapterView.AdapterContextMenuInfo;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;

public class MainActivity extends Activity {

private static final String TAG = "MainActivity";
private ListView listView = null;
private BaseAdapter adapter = null;
private Button button = null;

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

listView = (ListView) this.findViewById(R.id.list_view_id);
setData();

button = (Button) this.findViewById(R.id.button_id);
this.registerForContextMenu(button);

this.registerForContextMenu(listView);

Log.d(TAG, "onCreate()");
}

private void setData() {
// each item's layout resource id
int student_item_id = R.layout.student_item;

// columns names
String[] columnNames = new String[] { "name", "score" };

// resource ids
int[] ids = new int[] { R.id.student_name, R.id.student_score };

// the data to be displayed
ArrayList<HashMap<String, Object>> students = new ArrayList<HashMap<String, Object>>();
HashMap<String, Object> map = null;
for (int i = 1; i <= 10; i++) {
map = new HashMap<String, Object>();
map.put(columnNames[0], "student-" + i);
map.put(columnNames[1], String.valueOf(i * 10));
students.add(map);
}

adapter = new SimpleAdapter(this, students, student_item_id,
columnNames, ids);
listView.setAdapter(adapter);
}

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {
super.onCreateContextMenu(menu, v, menuInfo);

MenuInflater inflater = this.getMenuInflater();

if (v == button) {
inflater.inflate(R.menu.context_menu, menu);
return;
}

// Now the view is the ListView object
AdapterContextMenuInfo info = (AdapterContextMenuInfo)menuInfo;
dumpInfo(info);

if (info.position % 2 == 0) {
inflater.inflate(R.menu.add_context_menu, menu);
} else {
inflater.inflate(R.menu.minus_context_menu, menu);
}
}

private void dumpInfo(AdapterContextMenuInfo info) {
HashMap<String, Object> student = (HashMap<String, Object>) adapter.getItem(info.position);

Toast.makeText(this, student.toString(),Toast.LENGTH_LONG).show();
}

private void updateScore(ContextMenuInfo menuInfo, int incremental) {
AdapterContextMenuInfo info = (AdapterContextMenuInfo)menuInfo;
HashMap<String, Object> student = (HashMap<String, Object>) adapter.getItem(info.position);
Log.d(TAG, "old: " + student.toString());

String studentName = (String) student.get("name");
String studentScore = (String) student.get("score");
int score = Integer.parseInt(studentScore);

student.put("score", String.valueOf(score + incremental));

adapter.notifyDataSetChanged();

Log.d(TAG, "new: " + student.toString());
}

@Override
public boolean onContextItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.first_context_menu_item:
Log.d(TAG, "first context menu item selected");
return true;

case R.id.second_context_menu_item:
Log.d(TAG, "second context menu item selected");
return true;

case R.id.add_score:
updateScore(item.getMenuInfo(), 1);
return true;

case R.id.minus_score:
updateScore(item.getMenuInfo(), -1);
return true;

default:
return super.onContextItemSelected(item);
}

}
}

6.4 使用习惯

以上我们为Button注册上下文菜单，仅仅是一种示例，用于说明如何区分不同的View对象。

虽然可以给各种View注册上下文菜单，但从用户使用习惯上来讲，我们通常只会为特定的一些类别注册这种行为。比如给编辑框注册，长按之后可以Select/SelectAll/Copy/Paste；另外更常见的是ListView， GridView等。

7. 小结

从本文示例来看，floating context menu使用起来还是很方便的。当然，其功能还是有一定的局限性，为此Android3.0引入了contextual action mode的概念。