LinearGradient

线性渲染：

public class LinearGradient extends Shader {

    private static final int TYPE_COLORS_AND_POSITIONS = 1;
    private static final int TYPE_COLOR_START_AND_COLOR_END = 2;

    /**
     * 线性渲染的类型：可以是TYPE_COLORS_AND_POSITIONS类型，
     * 也可以是TYPE_COLOR_START_AND_COLOR_END类型。
     */
    private int mType;

    private float mX0;
    private float mY0;
    private float mX1;
    private float mY1;
    private int[] mColors;
    private float[] mPositions;
    private int mColor0;
    private int mColor1;

    private TileMode mTileMode;

    /**
     * 创建一个沿着直线绘制线性渐变的渲染器。
     *
     * @param x0           渐变线起点的x坐标
     * @param y0           渐变线起点的y坐标
     * @param x1           渐变线终点的x坐标
     * @param y1           渐变线终点的y坐标
     * @param colors       沿渐变线分布的颜色
     * @param positions    可能为null。颜色数组中每个对应颜色的相对位置[0..1]。
     * 					   如果为null，则颜色沿渐变线均匀分布。
     * @param tile         渲染器拼接模式
    */
    public LinearGradient(float x0, float y0, float x1, float y1, @NonNull @ColorInt int colors[],
            @Nullable float positions[], @NonNull TileMode tile) {
        if (colors.length < 2) {
            throw new IllegalArgumentException("needs >= 2 number of colors");
        }
        if (positions != null && colors.length != positions.length) {
            throw new IllegalArgumentException("color and position arrays must be of equal length");
        }
        mType = TYPE_COLORS_AND_POSITIONS;
        mX0 = x0;
        mY0 = y0;
        mX1 = x1;
        mY1 = y1;
        mColors = colors.clone();
        mPositions = positions != null ? positions.clone() : null;
        mTileMode = tile;
    }

    /**
     * 创建一个沿着直线绘制线性渐变的渲染器。
     *
     * @param x0       渐变线起点的x坐标
     * @param y0       渐变线起点的y坐标
     * @param x1       渐变线终点的x坐标
     * @param y1       渐变线终点的y坐标
     * @param color0   渐变线起点的颜色
     * @param color1   渐变线终点的颜色
     * @param tile     渲染器拼接模式
    */
    public LinearGradient(float x0, float y0, float x1, float y1,
            @ColorInt int color0, @ColorInt int color1,
            @NonNull TileMode tile) {
        mType = TYPE_COLOR_START_AND_COLOR_END;
        mX0 = x0;
        mY0 = y0;
        mX1 = x1;
        mY1 = y1;
        mColor0 = color0;
        mColor1 = color1;
        mColors = null;
        mPositions = null;
        mTileMode = tile;
    }

    @Override
    long createNativeInstance(long nativeMatrix) {
        if (mType == TYPE_COLORS_AND_POSITIONS) {
            return nativeCreate1(nativeMatrix, mX0, mY0, mX1, mY1,
                    mColors, mPositions, mTileMode.nativeInt);
        } else { // TYPE_COLOR_START_AND_COLOR_END
            return nativeCreate2(nativeMatrix, mX0, mY0, mX1, mY1,
                    mColor0, mColor1, mTileMode.nativeInt);
        }
    }

    /** @hide */
    @Override
    protected Shader copy() {
        final LinearGradient copy;
        if (mType == TYPE_COLORS_AND_POSITIONS) {
            copy = new LinearGradient(mX0, mY0, mX1, mY1, mColors.clone(),
                    mPositions != null ? mPositions.clone() : null, mTileMode);
        } else { // TYPE_COLOR_START_AND_COLOR_END
            copy = new LinearGradient(mX0, mY0, mX1, mY1, mColor0, mColor1, mTileMode);
        }
        copyLocalMatrix(copy);
        return copy;
    }

    private native long nativeCreate1(long matrix, float x0, float y0, float x1, float y1,
            int colors[], float positions[], int tileMode);
    private native long nativeCreate2(long matrix, float x0, float y0, float x1, float y1,
            int color0, int color1, int tileMode);
}