[AndroidTips]Declaring a custom android UI element using XML

定制Android UI

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本文详细介绍了如何在Android中创建自定义UI组件，包括在values/attrs.xml文件中声明属性、在自定义视图构造函数中初始化这些属性并读取其值的方法。此外，还提供了将自定义视图整合到布局文件中的实例。

http://stackoverflow.com/questions/2695646/declaring-a-custom-android-ui-element-using-xml

The Android Developer Guide has a section called Building Custom Components . Unfortunately, the discussion of XML attributes only covers declaring the control inside the layout file and not actually handling the values inside the class initialisation. The steps are as follows:

1. Declare attributes in `values\attrs.xml`

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="MyCustomView">
        <attr name="android:text"/>
        <attr name="android:textColor"/>            
        <attr name="extraInformation" format="string" />
    </declare-styleable>
</resources>

Notice the use of an unqualified name in the declare-styleable tag. Non-standard android attributes like extraInformation need to have their type declared. Tags declared in the superclass will be available in subclasses without having to be redeclared.

2. Create constructors

Since there are two constructors that use an AttributeSet for initialisation, it is convenient to create a separate initialisation method for the constructors to call.

private void init(AttributeSet attrs) { 
    TypedArray a=getContext().obtainStyledAttributes(
         attrs,
         R.styleable.MyCustomView);

    //Use a
    Log.i("test",a.getString(
         R.styleable.MyCustomView_android_text));
    Log.i("test",""+a.getColor(
         R.styleable.MyCustomView_android_textColor, Color.BLACK));
    Log.i("test",a.getString(
         R.styleable.MyCustomView_android_extraInformation));

    //Don't forget this
    a.recycle();
}

R.styleable.MyCustomView is an autogenerated int[] resource where each element is the ID of an attribute. Attributes are generated for each property in the XML by appending the attribute name to the element name. Attributes can then be retrieved from the TypedArray using various get functions. If the attribute is not defined in the defined in the XML, then null is returned. Except, of course, if the return type is a primitive, in which case the second argument is returned.

If you don't want to retrieve all of the attributes, it is possible to create this array manually.The ID for standard android attributes are included in android.R.attr , while attributes for this project are in R.attr .

int attrsWanted[]=new int[]{android.R.attr.text, R.attr.textColor};

Please note that you should not use anything in android.R.styleable , as per this thread it may change in the future. It is still in the documentation as being to view all these constants in the one place is useful.

3. Use it in a layout files such as `layout\main.xml`

Include the namespace declaration xmlns:app="http://schemas.android.com/apk/res/com.mycompany.projectname" in the top level xml element.

<com.mycompany.projectname.MyCustomView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@android:color/transparent"
    android:text="Test text"
    android:textColor="#FFFFFF"
app:extraInformation="My extra information";
/>

Reference the custom view using the fully qualified name.

Android LabelView Sample

If you want a complete example, look at the android label view sample.

LabelView.java

TypedArray a=context.obtainStyledAttributes(attrs, R.styleable.LabelView);
 CharSequences=a.getString(R.styleable.LabelView_text);

attrs.xml

<declare-styleable name="LabelView">
    <attr name="text"format="string"/>
    <attr name="textColor"format="color"/>
    <attr name="textSize"format="dimension"/>
</declare-styleable>

custom_view_1.xml

<com.example.android.apis.view.LabelView
    android:background="@drawable/blue"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    app:text="Blue"app:textSize="20dp"/>

This is contained in a LinearLayout with a namespace attribute:

xmlns:app="http://schemas.android.com/apk/res/com.example.android.apis"

[AndroidTips]Declaring a custom android UI element using XML

1. Declare attributes in values\attrs.xml

2. Create constructors

3. Use it in a layout files such as layout\main.xml

Android LabelView Sample

Links

1. Declare attributes in `values\attrs.xml`

3. Use it in a layout files such as `layout\main.xml`