前面在CardView简析中简要讲了CardView的基本属性和其实现逻辑，本文基于CardView的实现原理，并基于CardView的源码进行修改，实现对CardView阴影效果的修改。
首先需要明确的一点，CardView的阴影效果是没提供api进行修改的，只能通过cardElevation改变投影深度。
分析原因
CardView是通过CardViewImpl来静态代理圆角和阴影绘制以及受他们影响的padding处理逻辑的，先来看看这段代码：
public class CardView extends FrameLayout {
...
private static final CardViewImpl IMPL;
static {
if (Build.VERSION.SDK_INT >= 21) {
IMPL = new CardViewApi21Impl();
} else if (Build.VERSION.SDK_INT >= 17) {
IMPL = new CardViewApi17Impl();
} else {
IMPL = new CardViewBaseImpl();
}
IMPL.initStatic();
}
...
}从代码中我们了解到这代理接口是在CardView的类加载的时候初始化的，并且是根据安卓版本不同创建了其不同的实现类。
那接下来看看这个接口到底提供了哪些方法：
interface CardViewImpl {
//初始化方法，里面逻辑主要是创建Drawable并设置为CardView的背景
void initialize(CardViewDelegate cardView, Context context, ColorStateList backgroundColor,
float radius, float elevation, float maxElevation);
//设置圆角半径
void setRadius(CardViewDelegate cardView, float radius);
float getRadius(CardViewDelegate cardView);
//设置阴影高度
void setElevation(CardViewDelegate cardView, float elevation);
float getElevation(CardViewDelegate cardView);
//全局内容的初始化，主要是针对不同安卓版本下的RoundRectDrawableWithShadow.sRoundRectHelper进行初始化
void initStatic();
void setMaxElevation(CardViewDelegate cardView, float maxElevation);
float getMaxElevation(CardViewDelegate cardView);
float getMinWidth(CardViewDelegate cardView);
float getMinHeight(CardViewDelegate cardView);
void updatePadding(CardViewDelegate cardView);
void onCompatPaddingChanged(CardViewDelegate cardView);
void onPreventCornerOverlapChanged(CardViewDelegate cardView);
void setBackgroundColor(CardViewDelegate cardView, @Nullable ColorStateList color);
ColorStateList getBackgroundColor(CardViewDelegate cardView);
}这里主要关注以下两个方法：
- initStatic 这个方法是在CardView创建CardViewImpl对象的静态代码块中调用的。
- initialize 这个方法实在CardView的构造方法中调用，主要逻辑是CardView将他被代理过程中需要用到的对象传入代理对象中，并初始化，即创建RoundRectDrawableWithShadow/RoundRectDrawable，设置阴影相关参数。
下面分别对比下CardViewBaseImpl、CardViewApi17Impl和CardViewApi21Impl之间这两个方法的实现到底有何不同。
class CardViewBaseImpl implements CardViewImpl {
...
@Override
public void initStatic() {
// Draws a round rect using 7 draw operations. This is faster than using
// canvas.drawRoundRect before JBMR1 because API 11-16 used alpha mask textures to draw
// shapes.
RoundRectDrawableWithShadow.sRoundRectHelper =
new RoundRectDrawableWithShadow.RoundRectHelper() {
@Override
public void drawRoundRect(Canvas canvas, RectF bounds, float cornerRadius,
Paint paint) {
final float twoRadius = cornerRadius * 2;
final float innerWidth = bounds.width() - twoRadius - 1;
final float innerHeight = bounds.height() - twoRadius - 1;
if (cornerRadius >= 1f) {
// increment corner radius to account for half pixels.
float roundedCornerRadius = cornerRadius + .5f;
mCornerRect.set(-roundedCornerRadius, -roundedCornerRadius, roundedCornerRadius,
roundedCornerRadius);
int saved = canvas.save();
canvas.translate(bounds.left + roundedCornerRadius,
bounds.top + roundedCornerRadius);
canvas.drawArc(mCornerRect, 180, 90, true, paint);
canvas.translate(innerWidth, 0);
canvas.rotate(90);
canvas.drawArc(mCornerRect, 180, 90, true, paint);
canvas.translate(innerHeight, 0);
canvas.rotate(90);
canvas.drawArc(mCornerRect, 180, 90, true, paint);
canvas.translate(innerWidth, 0);
canvas.rotate(90);
canvas.drawArc(mCornerRect, 180, 90, true, paint);
canvas.restoreToCount(saved);
//draw top and bottom pieces
canvas.drawRect(bounds.left + roundedCornerRadius - 1f, bounds.top,
bounds.right - roundedCornerRadius + 1f,
bounds.top + roundedCornerRadius, paint);
canvas.drawRect(bounds.left + roundedCornerRadius - 1f,
bounds.bottom - roundedCornerRadius,
bounds.right - roundedCornerRadius + 1f, bounds.bottom, paint);
}
// center
canvas.drawRect(bounds.left, bounds.top + cornerRadius,
bounds.right, bounds.bottom - cornerRadius , paint);
}
};
}
@Override
public void initialize(CardViewDelegate cardView, Context context,
ColorStateList backgroundColor, float radius, float elevation, float maxElevation) {
RoundRectDrawableWithShadow background = createBackground(context, backgroundColor, radius,
elevation, maxElevation);
background.setAddPaddingForCorners(cardView.getPreventCornerOverlap());
cardView.setCardBackground(background);
updatePadding(cardView);
}
...
}@RequiresApi(17)
class CardViewApi17Impl extends CardViewBaseImpl {
@Override
public void initStatic() {
RoundRectDrawableWithShadow.sRoundRectHelper =
new RoundRectDrawableWithShadow.RoundRectHelper() {
@Override
public void drawRoundRect(Canvas canvas, RectF bounds, float cornerRadius,
Paint paint) {
canvas.drawRoundRect(bounds, cornerRadius, cornerRadius, paint);
}
};
}
}@RequiresApi(21)
class CardViewApi21Impl implements CardViewImpl {
...
@Override
public void initialize(CardViewDelegate cardView, Context context,
ColorStateList backgroundColor, float radius, float elevation, float maxElevation) {
final RoundRectDrawable background = new RoundRectDrawable(backgroundColor, radius);
cardView.setCardBackground(background);
View view = cardView.getCardView();
view.setClipToOutline(true);
view.setElevation(elevation);
setMaxElevation(cardView, maxElevation);
}
@Override
public void initStatic() {
}
...
}由上面的代码得知，安卓L以上，是以RoundRectDrawable来实现圆角，View.setElevation来提供阴影，Elevation在安卓5.0以上的所有View中都支持，直接由RenderNode渲染阴影。而在L以下，就只能通过Canvas绘制出阴影，所以RoundRectDrawableWithShadow就是为了满足圆角和阴影而出现的。initStatic中对RoundRectDrawableWithShadow.sRoundRectHelper的实现不同，drawRoundRect是在RoundRectDrawableWithShadow的draw方法里面调用的
class RoundRectDrawableWithShadow extends Drawable {
@Override
public void draw(Canvas canvas) {
if (mDirty) {
buildComponents(getBounds());
mDirty = false;
}
canvas.translate(0, mRawShadowSize / 2);
drawShadow(canvas);
canvas.translate(0, -mRawShadowSize / 2);
sRoundRectHelper.drawRoundRect(canvas, mCardBounds, mCornerRadius, mPaint);
}
}由此得知，CardView的阴影效果是应用层的统一效果，并未开放扩展和修改。那么既然分析清楚原因了，且清楚了CardView是在什么地方实现阴影的，那我们就自己可以进行修改。有两种方式可以修改这个效果，一种是通过反射替换CardView.IMPL，另一种方法，就是直接参照CardView的源码修改。当然，两种方法都需要自己实现CardViewImpl。
实现自定义阴影
先来看看对比图：
具体实现思路就是通过自定义CardViewImpl从而改变CardView阴影实现逻辑。下面我就阐述下我自己的实现简要过程。
我同样是通过代理模式，来实现对CardView的绘制管理，ShadowLayoutImpl对应CardViewImpl
public interface ShadowLayoutImpl {
void initialize(ShadowLayoutDelegate cardView, Context context, ColorStateList backgroundColor,
float radius,int shadowStartColor,int shadowEndColor, float elevation, float maxElevation);
void setRadius(ShadowLayoutDelegate cardView, float radius);
float getRadius(ShadowLayoutDelegate cardView);
void setElevation(ShadowLayoutDelegate cardView, float elevation);
float getElevation(ShadowLayoutDelegate cardView);
void initStatic();
void setMaxElevation(ShadowLayoutDelegate cardView, float maxElevation);
float getMaxElevation(ShadowLayoutDelegate cardView);
float getMinWidth(ShadowLayoutDelegate cardView);
float getMinHeight(ShadowLayoutDelegate cardView);
void updatePadding(ShadowLayoutDelegate cardView);
void onCompatPaddingChanged(ShadowLayoutDelegate cardView);
void onPreventCornerOverlapChanged(ShadowLayoutDelegate cardView);
void setBackgroundColor(ShadowLayoutDelegate cardView, @Nullable ColorStateList color);
ColorStateList getBackgroundColor(ShadowLayoutDelegate cardView);
}
由于我们不需要安卓L或CardView提供的阴影效果。所以ShadowLayoutImpl只实现了ShadowLayoutBaseImpl和ShadowLayoutApi17Impl。它们的区别只是在于对于圆角的实现，因为Canvas.drawRoundRect(bounds, cornerRadius, cornerRadius, paint) 只在SDK>=17才有。它们都同样使用了RoundRectDrawableWithShadow，下面贴出实现阴影的绘制逻辑：
public class RoundRectDrawableWithShadow extends Drawable {
...
@Override
public void draw(Canvas canvas) {
if (mDirty) {
buildComponents(getBounds());
mDirty = false;
}
// canvas.translate(0, mRawShadowSize / 2);
drawShadow(canvas);
// canvas.translate(0, -mRawShadowSize / 2);
sRoundRectHelper.drawRoundRect(canvas, mCardBounds, mCornerRadius, mPaint);
}
private void drawShadow(Canvas canvas) {
final float edgeShadowTop = -mCornerRadius - mShadowSize;
// final float inset = 0;
final float inset = mCornerRadius + mInsetShadow ;
final boolean drawHorizontalEdges = mCardBounds.width() - 2 * inset > 0;
final boolean drawVerticalEdges = mCardBounds.height() - 2 * inset > 0;
int saved=0;
// LT
saved = canvas.save();
canvas.translate(mCardBounds.left + inset, mCardBounds.top + inset);
canvas.drawPath(mCornerShadowPath, mCornerShadowPaint);
if (drawHorizontalEdges) {
canvas.drawRect(0, edgeShadowTop,
mCardBounds.width() - 2 * inset, -mCornerRadius,
mEdgeShadowPaint);
}
canvas.restoreToCount(saved);
// RB
saved = canvas.save();
canvas.translate(mCardBounds.right - inset, mCardBounds.bottom - inset);
canvas.rotate(180f);
canvas.drawPath(mCornerShadowPath, mCornerShadowPaint);
if (drawHorizontalEdges) {
canvas.drawRect(0, edgeShadowTop,
mCardBounds.width() - 2 * inset, -mCornerRadius ,
mEdgeShadowPaint);
}
canvas.restoreToCount(saved);
// LB
saved = canvas.save();
canvas.translate(mCardBounds.left + inset, mCardBounds.bottom - inset);
canvas.rotate(270f);
canvas.drawPath(mCornerShadowPath, mCornerShadowPaint);
if (drawVerticalEdges) {
canvas.drawRect(0, edgeShadowTop,
mCardBounds.height() - 2 * inset, -mCornerRadius, mEdgeShadowPaint);
}
canvas.restoreToCount(saved);
// RT
saved = canvas.save();
canvas.translate(mCardBounds.right - inset, mCardBounds.top + inset);
canvas.rotate(90f);
canvas.drawPath(mCornerShadowPath, mCornerShadowPaint);
if (drawVerticalEdges) {
canvas.drawRect(0, edgeShadowTop,
mCardBounds.height() - 2 * inset, -mCornerRadius, mEdgeShadowPaint);
}
canvas.restoreToCount(saved);
}
private void buildShadowCorners() {
RectF innerBounds = new RectF(-mCornerRadius, -mCornerRadius, mCornerRadius, mCornerRadius);
RectF outerBounds = new RectF(innerBounds);
outerBounds.inset(-mShadowSize, -mShadowSize);
if (mCornerShadowPath == null) {
mCornerShadowPath = new Path();
} else {
mCornerShadowPath.reset();
}
mCornerShadowPath.setFillType(Path.FillType.EVEN_ODD);
mCornerShadowPath.moveTo(-mCornerRadius, 0);
mCornerShadowPath.rLineTo(-mShadowSize, 0);
// outer arc
mCornerShadowPath.arcTo(outerBounds, 180f, 90f, false);
// inner arc
mCornerShadowPath.arcTo(innerBounds, 270f, -90f, false);
mCornerShadowPath.close();
float startRatio = mCornerRadius / (mCornerRadius + mShadowSize)*.2f;
mCornerShadowPaint.setShader(new RadialGradient(0, 0, mCornerRadius + mShadowSize,
new int[]{mShadowStartColor, mShadowStartColor, mShadowEndColor},
new float[]{0f, startRatio, 1f},
Shader.TileMode.CLAMP));
// we offset the content shadowSize/2 pixels up to make it more realistic.
// this is why edge shadow shader has some extra space
// When drawing bottom edge shadow, we use that extra space.
mEdgeShadowPaint.setShader(new LinearGradient(0, -mCornerRadius + mShadowSize, 0,
-mCornerRadius - mShadowSize,
new int[]{mShadowStartColor, mShadowStartColor, mShadowEndColor},
new float[]{0f, .1f, 1f}, Shader.TileMode.CLAMP));
mEdgeShadowPaint.setAntiAlias(false);
}
private void buildComponents(Rect bounds) {
// Card is offset SHADOW_MULTIPLIER * maxShadowSize to account for the shadow shift.
// We could have different top-bottom offsets to avoid extra gap above but in that case
// center aligning Views inside the CardView would be problematic.
final float verticalOffset = mRawMaxShadowSize * SHADOW_MULTIPLIER;
mCardBounds.set(bounds.left + mRawMaxShadowSize, bounds.top + verticalOffset,
bounds.right - mRawMaxShadowSize, bounds.bottom - verticalOffset);
buildShadowCorners();
}
...
}自定义参数和CardView差不多，所以使用方法也差不多。
attrs.xml:
到这里，本文差不多结束了，由于代码全贴出来没必要，所以下面提供源码，供大家参考。