Borderless button

读Google的文档的时候发现的这个，想起来，我之前在即时通讯项目是这样干的，直接在button的xml属性中指定下面的属性：

android:background=null;

One design that can be useful is a "borderless" button. Borderless buttons resemble basic buttons except that they have no borders or background but still change appearance during different states, such as when clicked.

To create a borderless button, apply the

borderlessButtonStyle

style
to the button. For example:
当使用无缝隙的button的时候，当他的state改变的时候，其appearance 回随着其一起改变。

<Button
    android:id="@+id/button_send"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage"
    style="?android:attr/borderlessButtonStyle" />

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