@ And? In the xml file of Android? Difference

@ Everyone is familiar with it. For example, @ + id/... @ string /...

See the following code:

[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"/>
The above Code uses "?", Then "?" What does that mean?

"?" Reference topic attributes. When you use this tag, the resource name you provide must be located in the topic attributes, because the resource tool considers this resource attribute to be expected, you do not need to explicitly specify its type (that is, in which file you do not need to write all of it? Android: attr/android: textDisabledColor)

So what is theme attribute?

You can find the attrs. xml file in the following directory of the Android SDK

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

Open this file and you can see that the attributes of the Android system are defined in this file.

For example (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 shocould 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 shocould 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>
The above is extracted from the attrs. xml file. You can see some special attributes of ImageView (because ImageView inherits from View, the View attributes naturally inherit ).
Similarly, the Android system defines many attributes for Theme (you can see attrs. xml file). The names of each topic attribute correspond to themes in the Directory D: \ android-sdk \ platforms \ android-16 \ data \ res \ values. xml file. When topic attributes are used, you do not need to specify android: attr? Add the attribute name.