Android Paint 透明度

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paint.setAlpha() 实际修改了当前 paint 颜色的透明度值，所以 paint.setColor() 操作也会覆盖事先设置的透明度。

Usage

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// alpha will be reset.
paint.setAlpha(alpha);
...
paint.setColor(color);

// keep alpha
paint.setColor(color);
...
paint.setAlpha(alpha);

or 

paint.setColor((paint.getAlpha() << 24) | (color & 0x00ffffff));

Android Source

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/**
* Helper to setColor(), that only assigns the color's alpha value,
* leaving its r,g,b values unchanged. Results are undefined if the alpha
* value is outside of the range [0..255]
*
* @param a set the alpha component [0..255] of the paint's color.
*/
public void setAlpha(int a) {
    ColorSpace cs = Color.colorSpace(mColor);
    float r = Color.red(mColor);
    float g = Color.green(mColor);
    float b = Color.blue(mColor);
    mColor = Color.pack(r, g, b, a * (1.0f / 255), cs);
    nSetAlpha(mNativePaint, a);
}

/**
* Set the paint's color. Note that the color is an int containing alpha
* as well as r,g,b. This 32bit value is not premultiplied, meaning that
* its alpha can be any value, regardless of the values of r,g,b.
* See the Color class for more details.
*
* @param color The new color (including alpha) to set in the paint.
*/
public void setColor(@ColorInt int color) {
    nSetColor(mNativePaint, color);
    mColor = Color.pack(color);
}