Android的xml资料中的@和?的区别

Android的xml文件中的@和?的区别

@大家已经司空见惯。比如@+id/...  @string/...

看下面代码：

    <declare-styleable name="ImageView">        <!-- Sets a drawable as the content of this ImageView. -->        <attr name="src" format="reference|color" />        <!-- Controls how the image should be resized or moved to match the size             of this ImageView. -->        <attr name="scaleType">            <enum name="matrix" value="0" />            <enum name="fitXY" value="1" />            <enum name="fitStart" value="2" />            <enum name="fitCenter" value="3" />            <enum name="fitEnd" value="4" />            <enum name="center" value="5" />            <enum name="centerCrop" value="6" />            <enum name="centerInside" value="7" />        </attr>        <!-- Set this to true if you want the ImageView to adjust its bounds             to preserve the aspect ratio of its drawable. -->        <attr name="adjustViewBounds" format="boolean" />        <!-- An optional argument to supply a maximum width for this view.             See {see android.widget.ImageView#setMaxWidth} for details. -->        <attr name="maxWidth" format="dimension" />        <!-- An optional argument to supply a maximum height for this view.             See {see android.widget.ImageView#setMaxHeight} for details. -->        <attr name="maxHeight" format="dimension" />        <!-- Set a tinting color for the image -->        <attr name="tint" format="color" />        <!-- If true, the image view will be baseline aligned with based on its             bottom edge -->        <attr name="baselineAlignBottom" format="boolean" />         <!-- If true, the image will be cropped to fit within its padding -->        <attr name="cropToPadding" format="boolean" />    </declare-styleable>

同样，Android系统也为Theme定义了很多属性（可以看看attrs.xml文件），其中每个主题属性的名称都一一对应D:\android-sdk\platforms\android-16\data\res\values目录下的themes.xml文件。当要用到主题属性的时候，就不需要特别指定android:attr/，可以直接在？后面加上属性名。