今天在看android froyo的launcher2 源码的时候，在launcher.xml中看到有这么一段代码：

<com.android.launcher2.DragLayer
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:launcher="http://schemas.android.com/apk/res/com.android.launcher"

    android:id="@+id/drag_layer"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <include layout="@layout/all_apps" />

    <!-- The workspace contains 3 screens of cells -->
    <com.android.launcher2.Workspace
        android:id="@+id/workspace"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scrollbars="horizontal"
        android:fadeScrollbars="true"
        launcher:defaultScreen="2">

注意到其中的两处：

xmlns:launcher=”http://schemas.android.com/apk/res/com.android.launcher”

和

launcher:defaultScreen="2"

可以看出在这个布局文件中，使用了自定义属性。

以前没遇到过，既然这里碰到了，就顺便学习下，下面就写个简单的示例，权当学习笔记，便于以后查阅。

1. 定义一些自定义属性

建立一个属性xml文件： values/attrs.xml, 内容如下：

<?xml version="1.0" encoding="utf-8" ?>
<resources>
    <!-- the relation between the icon and text. -->
    <attr name="relation">
        <enum name="icon_left"     value="0" />
        <enum name="icon_right"    value="1" />
        <enum name="icon_above"    value="2" />
        <enum name="icon_below"    value="3" />
    </attr>
    
    <skip />
    
    <declare-styleable name="IconText">
         <attr name="relation" />
         <attr name="icon"         format="reference" />  
         <attr name="text"         format="string" />
       <attr name="text_size"     format="dimension" />
       <attr name="text_color"    format="integer" />
       <attr name="space"         format="dimension" />
    </declare-styleable>
    
</resources>

解释如下：

属性relation有4种可选值：icon_left, icon_right, icon_above,icon_below.

属性icon的可选值为引用： 例如："@/drawbable/icon".

属性text的可选值为string， 例如： "Hello world", 也可是string的引用"@string/hello".

属性text_size的可选值为尺寸大小，例如：20sp、18dip、20px等.

属性text_color的可选值为整数，例如："0xfffffff"， 也可以是color的引用"@color/white".

2. 定义一个能够处理这些属性值的view或者layout类

package com.braincol.viewattrs;


import android.content.Context;
import android.content.res.TypedArray;
import android.util.AttributeSet;
import android.util.Log;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;

public class IconTextView  extends LinearLayout {
    private final static String TAG = "IconTextView";
    private final int ICON_LEFT = 0;
    private final int ICON_RIGHT = 1;
    private final int ICON_ABOVE = 2;
    private final int ICON_BELOW = 3;

    private TextView mTextView;
    private ImageView mImageView;

    private int mRelation = ICON_LEFT;
    private String mText = "";
    private int mIconId;
    private float mTextSize;
    private int mSpace;

    public IconTextView(Context context, AttributeSet attrs){
        super(context, attrs);
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.IconText);

        mRelation = a.getInt(R.styleable.IconText_relation, ICON_LEFT);
        Log.d(TAG,"mRelation: "+mRelation);

        mText = a.getString(R.styleable.IconText_text);
        Log.d(TAG,"mText: "+mText);

        mTextSize = a.getDimensionPixelSize(R.styleable.IconText_text_size, 12);
        Log.d(TAG,"mTextSize: "+mTextSize);

        mSpace = a.getDimensionPixelSize(R.styleable.IconText_space, 5);
        Log.d(TAG,"mSpace: "+mSpace);

        mIconId = a.getResourceId(R.styleable.IconText_icon, R.drawable.icon);
        Log.d(TAG,"mIconId: "+mIconId);

        a.recycle();

        mTextView = new TextView(context);
        mTextView.setText(mText);
        mTextView.setTextSize(mTextSize);

        mImageView = new ImageView(context);
        mImageView.setImageResource(mIconId);    

        int left    = 0;
        int top     = 0;
        int right    = 0;
        int bottom    = 0;
        int orientation = HORIZONTAL;
        int textViewIndex = 0;
        switch(mRelation){
        case ICON_ABOVE:
            orientation = VERTICAL;
            bottom = mSpace;
            textViewIndex = 1;
            break;
        case ICON_BELOW:
            orientation = VERTICAL;
            top = mSpace;
            break;
        case ICON_LEFT:
            right = mSpace;
            textViewIndex = 1;
            break;
        case ICON_RIGHT:
            left = mSpace;
            break;
        }
        this.setOrientation(orientation);
        this.addView(mImageView);
        mImageView.setPadding(left, top, right, bottom);
        this.addView(mTextView, textViewIndex);
    }
}

可以看出这个LinearLayout 子类IconTextView中只有两个元素，ImageView 和mTextView，通过从xml配置文件中读取属性值来决定icon和text的内容、相对位置及其它属性。

3. 在layout布局文件中使用这个自定布局及其属性

layout/main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<com.braincol.viewattrs.IconTextView
    android:id="@+id/icontext_01"
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:icontext="http://schemas.android.com/apk/res/com.braincol.viewattrs"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    icontext:relation="icon_left"
    icontext:icon="@drawable/hi"
    icontext:text="hi,how are you!"
    icontext:text_size="12sp"
  />
  
</LinearLayout>

可以看到我们在这个布局文件中加入了一个新的命名空间：

xmlns:icontext="http://schemas.android.com/apk/res/com.braincol.viewattrs"   

并使用我们自定义的那些属性：

icontext:relation="icon_left"
icontext:icon="@drawable/hi"
icontext:text="hi, how are you !"
icontext:text_size="30sp"

4. 在Activity中使用该布局

package com.braincol.viewattrs;

import android.app.Activity;
import android.os.Bundle;

public class ViewAttrs extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

运行效果：