android - 如何将 alpha channel 添加到 xml 中的现有 Android 颜色

Question

我在 values/colors.xml 中有以下颜色:

<color name="grey_1">#0F0E10</color>

我想在渐变中引用这个颜色:

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:type="linear"
        android:angle="-90"
        android:startColor="#000F0E10"
        android:endColor="#990F0E10"/>
</shape>

但是，这复制了 RGB 颜色定义。理想情况下，我想写这样的东西:

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:type="linear"
        android:angle="-90"
        android:startColor="alpha(00, @color/grey_1)"
        android:endColor="alpha(99, @color/grey_1)"/>
</shape>

或者这个:

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:type="linear"
        android:angle="-90"
        android:startColor="@color/grey_1"
        android:startTransparency="#00"
        android:endColor="@color/grey_1"
        android:endTransparency="#99"/>
</shape>

这可能吗？

Answer 1

您可以在 API 23 或更高版本中使用 ColorStateList 执行此操作.

来自文档:

Starting with API 23, items may optionally define an android:alpha attribute to modify the base color's opacity. This attribute takes a either floating-point value between 0 and 1 or a theme attribute that resolves as such. The item's overall color is calculated by multiplying by the base color's alpha channel by the alpha value. For example, the following item represents the theme's accent color at 50% opacity:

<item android:state_enabled="false"
      android:color="?android:attr/colorAccent"
      android:alpha="0.5" />

因此，就我而言，我会这样做:

color/gradient_start_color.xml:

<item android:color="@color/grey_1"
      android:alpha="0" />

color/gradient_end_color.xml:

<item android:color="@color/grey_1"
      android:alpha="0.6" />

drawable/gradient.xml:

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:type="linear"
        android:angle="-90"
        android:startColor="@color/gradient_start_color"
        android:endColor="@color/gradient_end_color" />
</shape>