Drawable资源之LayerDrawable资源

A LayerDrawable is a drawable object that manages an array of other drawables. Each drawable in the list is drawn in the order of the list—the last drawable in the list is drawn on top.

Each drawable is represented by an <item> element inside a single <layer-list> element.

file location:

res/drawable/filename.xml
The filename is used as the resource ID.

compiled resource datatype:

Resource pointer to a   LayerDrawable.

resource reference:

In Java:   R.drawable.filename
In XML:   @[package:]drawable/filename

syntax:

<?xml version="1.0" encoding="utf-8"?>
<layer-list
    xmlns:android="http://schemas.android.com/apk/res/android" >
    <item
        android:drawable="@[package:]drawable/drawable_resource"
        android:id="@[+][package:]id/resource_name"
        android:top="dimension"
        android:right="dimension"
        android:bottom="dimension"
        android:left="dimension" />
</layer-list>

 

elements:

<layer-list>

Required.  This must be the root element. Contains one or more   <item>  elements.

attributes:

xmlns:android

String.   Required.  Defines the XML namespace, which must be "http://schemas.android.com/apk/res/android".

<item>

Defines a drawable to place in the layer drawable, in a position defined by its attributes. Must be a child of a   <selector>  element. Accepts child   <bitmap>  elements.

attributes:

android:drawable

Drawable resource.   Required. Reference to a drawable resource.

android:id

Resource ID. A unique resource ID for this drawable. To create a new resource ID for this item, use the form:   "@+id/name". The plus symbol indicates that this should be created as a new ID. You can use this identifier to retrieve and modify the drawable with   View.findViewById()  or Activity.findViewById().

android:top

Integer. The top offset in pixels.

android:right

Integer. The right offset in pixels.

android:bottom

Integer. The bottom offset in pixels.

android:left

Integer. The left offset in pixels.

All drawable items are scaled to fit the size of the containing View, by default. Thus, placing your images in a layer list at different positions might increase the size of the View and some images scale as appropriate. To avoid scaling items in the list, use a <bitmap> element inside the <item> element to specify the drawable and define the gravity to something that does not scale, such as "center". For example, the following <item> defines an item that scales to fit its container View:

<itemandroid:drawable="@drawable/image"/>

To avoid scaling, the following example uses a <bitmap> element with centered gravity:

<item><bitmapandroid:src="@drawable/image"android:gravity="center"/></item>

example:

XML file saved at   res/drawable/layers.xml:

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
      <bitmap android:src="@drawable/android_red"
        android:gravity="center" />
    </item>
    <item android:top="10dp" android:left="10dp">
      <bitmap android:src="@drawable/android_green"
        android:gravity="center" />
    </item>
    <item android:top="20dp" android:left="20dp">
      <bitmap android:src="@drawable/android_blue"
        android:gravity="center" />
    </item>
</layer-list>

 

Notice that this example uses a nested <bitmap> element to define the drawable resource for each item with a "center" gravity. This ensures that none of the images are scaled to fit the size of the container, due to resizing caused by the offset images.

This layout XML applies the drawable to a View:

<ImageView
    android:layout_height="wrap_content"
    android:layout_width="wrap_content"
    android:src="@drawable/layers" />

 

The result is a stack of increasingly offset images: