Android的xml檔案中的@和?的區別

@大家已經司空見慣。比如@+id/...  @string/...

看下面代碼：

[html]
<ProgressBar 
    android:id="@+id/firstProgressBar" 
    style="?android:progressBarStyleHorizontal" 
    android:layout_width="200dp" 
    android:layout_height="wrap_content" 
    android:visibility="gone" /> 

    <ProgressBar
        android:id="@+id/firstProgressBar"
        style="?android:progressBarStyleHorizontal"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:visibility="gone" />
上面代碼中用到了“？”，那麼“？”是什麼意思呢？

“？”引用主題屬性，當您使用這個標記，你所提供的資源名必須能夠在主題屬性中找到，因為資源工具認為這個資源屬性是被期望得到的，您不需要明確的指出它的類型（也就是不需要寫全在哪個檔案中?android:attr/android:textDisabledColor）

那麼什麼又叫主題屬性呢？

你可以在Android的SDK的以下目錄中找到attrs.xml檔案

D:\android-sdk\platforms\android-16\data\res\values

開啟這個檔案可以看到，Android系統的屬性都定義在這個檔案中。

舉個例子（ImageView）：

[html]
<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> 

    <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>
以上是從attrs.xml檔案中提取的，可以看到ImageView的一些特有屬性（由於ImageView繼承自View，所以View的屬性自然就繼承了）。
同樣，Android系統也為Theme定義了很多屬性（可以看看attrs.xml檔案），其中每個主題屬性的名稱都一一對應D:\android-sdk\platforms\android-16\data\res\values目錄下的themes.xml檔案。當要用到主題屬性的時候，就不需要特別指定android:attr/，可以直接在？後面加上屬性名稱。