我发现创建顶部阴影的唯一方法是修改 Android 兼容性 v7 CardView 项目的一些源代码。该项目将 CardView 类引入到较早的 Android 版本，因此还包括高程阴影。生成的阴影非常接近“真实”的高程阴影。
CardView 项目中的重要类是RoundRectDrawableWithShadow。
我这样称呼它：
float density = getResources().getDisplayMetrics().density;
float elevation = 2;
xlayout.setBackgroundDrawable(new RoundRectDrawableWithShadow(
getResources(), Color.BLACK, 0,
elevation*density, ((elevation +1 )*density) + 1
));
res/values/values.xml 中需要以下内容
<color name="cardview_shadow_end_color">#03000000</color>
<color name="cardview_shadow_start_color">#47000000</color>
<dimen name="cardview_compat_inset_shadow">1dp</dimen>
这里修改类RoundRectDrawableWithShadow：
/*
* Copyright (C) 2014 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package xxx;
import android.content.res.Resources;
import android.graphics.Canvas;
import android.graphics.ColorFilter;
import android.graphics.LinearGradient;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PixelFormat;
import android.graphics.RadialGradient;
import android.graphics.Rect;
import android.graphics.RectF;
import android.graphics.Shader;
import android.graphics.drawable.Drawable;
import xxx.R;
/**
* A rounded rectangle drawable which also includes a shadow around.
*/
public class RoundRectDrawableWithShadow extends Drawable {
// used to calculate content padding
final static double COS_45 = Math.cos(Math.toRadians(45));
final static float SHADOW_MULTIPLIER = 1.5f;
final int mInsetShadow; // extra shadow to avoid gaps between card and shadow
/*
* This helper is set by CardView implementations.
* <p>
* Prior to API 17, canvas.drawRoundRect is expensive; which is why we need this interface
* to draw efficient rounded rectangles before 17.
* */
static RoundRectHelper sRoundRectHelper;
Paint mPaint;
Paint mCornerShadowPaint;
Paint mEdgeShadowPaint;
final RectF mCardBounds;
float mCornerRadius;
Path mCornerShadowPath;
// updated value with inset
float mMaxShadowSize;
// actual value set by developer
float mRawMaxShadowSize;
// multiplied value to account for shadow offset
float mShadowSize;
// actual value set by developer
float mRawShadowSize;
private boolean mDirty = true;
private final int mShadowStartColor;
private final int mShadowEndColor;
private boolean mAddPaddingForCorners = true;
/**
* If shadow size is set to a value above max shadow, we print a warning
*/
private boolean mPrintedShadowClipWarning = false;
public RoundRectDrawableWithShadow(
Resources resources, int backgroundColor, float radius,
float shadowSize, float maxShadowSize
) {
mShadowStartColor = resources.getColor(R.color.cardview_shadow_start_color);
mShadowEndColor = resources.getColor(R.color.cardview_shadow_end_color);
mInsetShadow = resources.getDimensionPixelSize(R.dimen.cardview_compat_inset_shadow);
mPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
mPaint.setColor(backgroundColor);
mCornerShadowPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
mCornerShadowPaint.setStyle(Paint.Style.FILL);
mCornerRadius = (int) (radius + .5f);
mCardBounds = new RectF();
mEdgeShadowPaint = new Paint(mCornerShadowPaint);
mEdgeShadowPaint.setAntiAlias(false);
setShadowSize(shadowSize, maxShadowSize);
RoundRectDrawableWithShadow.sRoundRectHelper
= new RoundRectDrawableWithShadow.RoundRectHelper() {
@Override
public void drawRoundRect(Canvas canvas, RectF bounds, float cornerRadius,
Paint paint) {
canvas.drawRoundRect(bounds, cornerRadius, cornerRadius, paint);
}
};
}
/**
* Casts the value to an even integer.
*/
private int toEven(float value) {
int i = (int) (value + .5f);
if (i % 2 == 1) {
return i - 1;
}
return i;
}
public void setAddPaddingForCorners(boolean addPaddingForCorners) {
mAddPaddingForCorners = addPaddingForCorners;
invalidateSelf();
}
@Override
public void setAlpha(int alpha) {
mPaint.setAlpha(alpha);
mCornerShadowPaint.setAlpha(alpha);
mEdgeShadowPaint.setAlpha(alpha);
}
@Override
protected void onBoundsChange(Rect bounds) {
super.onBoundsChange(bounds);
mDirty = true;
}
void setShadowSize(float shadowSize, float maxShadowSize) {
if (shadowSize < 0 || maxShadowSize < 0) {
throw new IllegalArgumentException("invalid shadow size");
}
shadowSize = toEven(shadowSize);
maxShadowSize = toEven(maxShadowSize);
if (shadowSize > maxShadowSize) {
shadowSize = maxShadowSize;
if (!mPrintedShadowClipWarning) {
mPrintedShadowClipWarning = true;
}
}
if (mRawShadowSize == shadowSize && mRawMaxShadowSize == maxShadowSize) {
return;
}
mRawShadowSize = shadowSize;
mRawMaxShadowSize = maxShadowSize;
mShadowSize = (int)(shadowSize * SHADOW_MULTIPLIER + mInsetShadow + .5f);
mMaxShadowSize = maxShadowSize + mInsetShadow;
mDirty = true;
invalidateSelf();
}
@Override
public boolean getPadding(Rect padding) {
int vOffset = (int) Math.ceil(calculateVerticalPadding(mRawMaxShadowSize, mCornerRadius,
mAddPaddingForCorners));
// int hOffset = (int) Math.ceil(calculateHorizontalPadding(mRawMaxShadowSize, mCornerRadius,
// mAddPaddingForCorners));
// padding.set(hOffset, vOffset, hOffset, vOffset);
padding.set(0, vOffset, 0, 0);
return true;
}
static float calculateVerticalPadding(float maxShadowSize, float cornerRadius,
boolean addPaddingForCorners) {
if (addPaddingForCorners) {
return (float) (maxShadowSize * SHADOW_MULTIPLIER + (1 - COS_45) * cornerRadius);
} else {
return maxShadowSize * SHADOW_MULTIPLIER;
}
}
static float calculateHorizontalPadding(float maxShadowSize, float cornerRadius,
boolean addPaddingForCorners) {
if (addPaddingForCorners) {
return (float) (maxShadowSize + (1 - COS_45) * cornerRadius);
} else {
return maxShadowSize;
}
}
@Override
public void setColorFilter(ColorFilter cf) {
mPaint.setColorFilter(cf);
mCornerShadowPaint.setColorFilter(cf);
mEdgeShadowPaint.setColorFilter(cf);
}
@Override
public int getOpacity() {
return PixelFormat.TRANSLUCENT;
}
void setCornerRadius(float radius) {
radius = (int) (radius + .5f);
if (mCornerRadius == radius) {
return;
}
mCornerRadius = radius;
mDirty = true;
invalidateSelf();
}
@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 insetVertical = mCornerRadius + mInsetShadow + mRawShadowSize / 2;
final float insetHorizontal = -mInsetShadow;
// LT top
int saved = canvas.save();
canvas.translate(mCardBounds.left + insetHorizontal, mCardBounds.top + insetVertical);
canvas.drawPath(mCornerShadowPath, mCornerShadowPaint);
canvas.drawRect(0, edgeShadowTop,
mCardBounds.width() - 2 * insetHorizontal, -mCornerRadius + mShadowSize,
mEdgeShadowPaint);
canvas.restoreToCount(saved);
// RT right
saved = canvas.save();
canvas.translate(mCardBounds.right - insetHorizontal, mCardBounds.top + insetVertical);
canvas.rotate(90f);
canvas.drawPath(mCornerShadowPath, mCornerShadowPaint);
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);
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, .5f, 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();
}
float getCornerRadius() {
return mCornerRadius;
}
void getMaxShadowAndCornerPadding(Rect into) {
getPadding(into);
}
void setShadowSize(float size) {
setShadowSize(size, mRawMaxShadowSize);
}
void setMaxShadowSize(float size) {
setShadowSize(mRawShadowSize, size);
}
float getShadowSize() {
return mRawShadowSize;
}
float getMaxShadowSize() {
return mRawMaxShadowSize;
}
float getMinWidth() {
final float content = 2 *
Math.max(mRawMaxShadowSize, mCornerRadius + mInsetShadow + mRawMaxShadowSize / 2);
return content + (mRawMaxShadowSize + mInsetShadow) * 2;
}
float getMinHeight() {
final float content = 2 * Math.max(mRawMaxShadowSize, mCornerRadius + mInsetShadow
+ mRawMaxShadowSize * SHADOW_MULTIPLIER / 2);
return content + (mRawMaxShadowSize * SHADOW_MULTIPLIER + mInsetShadow) * 2;
}
public void setColor(int color) {
mPaint.setColor(color);
invalidateSelf();
}
static interface RoundRectHelper {
void drawRoundRect(Canvas canvas, RectF bounds, float cornerRadius, Paint paint);
}
}
工作对我来说就像一个魅力。