表现
1. drawable 可以是图片文件，也可以是xml，常用于背景ImageView等
2. 代码中修改背景支持直接使用id设置，也支持使Drawable对象设置
3. 通常使context.getResourse().getDrawable(int resid) 的方式去获取Drawable对象，断点时可以发现不同的资源产生的Drawable类型不同
疑问
1. 为何可以对一个view设置 background 属性，设置 drawable 文件就可以使其能够显示一个图像
2. 当drawable设置为图片，或xml文件时，结果会变得不一样，底层是如何处理的
3. 假如当前使用了 shape 标签，直接在标签内定义的属性，及内部定义的标签如何被解析
源码探索
从源getDrawable()开始
给一个View设置背景最常用的方view.setBackground(Drawable drawable)一定不陌生，当我们想使用存放在drawable目录下的资源时，通常使context.getResourse().getDrawable(int resid) 的方式去获取，深入以下具体实现
public Drawable getDrawable(@DrawableRes int id, @Nullable Theme theme)
throws NotFoundException {
return getDrawableForDensity(id, 0, theme);
}
public Drawable getDrawableForDensity(@DrawableRes int id, int density, @Nullable Theme theme) {
final TypedValue value = obtainTempTypedValue();
try {
// 读取configuration里的属性
final ResourcesImpl impl = mResourcesImpl;
impl.getValueForDensity(id, density, value, true);
// 获取drawable
return loadDrawable(value, id, density, theme);
} finally {
releaseTempTypedValue(value);
}
}
Drawable loadDrawable(@NonNull TypedValue value, int id, int density, @Nullable Theme theme)
throws NotFoundException {
// 最终走到实现类的加载drawable方法
return mResourcesImpl.loadDrawable(this, value, id, density, theme);
}@Nullable
Drawable loadDrawable(@NonNull Resources wrapper, @NonNull TypedValue value, int id,
int density, @Nullable Resources.Theme theme)
throws NotFoundException {
...
try {
...
// 纯色背景，看起来设置了color即可，实际上仍然还是一个ColorDrawable对象
final boolean isColorDrawable;
final DrawableCache caches;
final long key;
...
// 判断是否有缓存可以用，对于同配置的同一id的资源，会优先取上次解析缓存返回，省去解析流程
// 题外话，这就是为什么drawable会有mutate()方法隔离属性，因为不同view持有的drawable是同一个对象
if (!mPreloading && useCache) {
final Drawable cachedDrawable = caches.getInstance(key, wrapper, theme);
if (cachedDrawable != null) {
cachedDrawable.setChangingConfigurations(value.changingConfigurations);
return cachedDrawable;
}
}
Drawable dr;
boolean needsNewDrawableAfterCache = false;
if (cs != null) {
dr = cs.newDrawable(wrapper);
} else if (isColorDrawable) {
dr = new ColorDrawable(value.data);
} else {
// 创建drawable对象并加入缓存
dr = loadDrawableForCookie(wrapper, value, id, density);
}
...
return dr;
} catch (Exception e) {
...
}
}loadDrawable
private Drawable loadDrawableForCookie(@NonNull Resources wrapper, @NonNull TypedValue value,
int id, int density) {
...
try {
...
try {
// 解析文件，区分文件类型1.xml 2.常规图片类型
if (file.endsWith(".xml")) {
final String typeName = getResourceTypeName(id);
if (typeName != null && typeName.equals("color")) {
dr = loadColorOrXmlDrawable(wrapper, value, id, density, file);
} else {
dr = loadXmlDrawable(wrapper, value, id, density, file);
}
} else {
final InputStream is = mAssets.openNonAsset(
value.assetCookie, file, AssetManager.ACCESS_STREAMING);
final AssetInputStream ais = (AssetInputStream) is;
dr = decodeImageDrawable(ais, wrapper, value);
}
} finally {
stack.pop();
}
} catch (Exception | StackOverflowError e) {
Trace.traceEnd(Trace.TRACE_TAG_RESOURCES);
final NotFoundException rnf = new NotFoundException(
"File " + file + " from drawable resource ID #0x" + Integer.toHexString(id));
rnf.initCause(e);
throw rnf;
}
...
return dr;
}
private Drawable loadXmlDrawable(@NonNull Resources wrapper, @NonNull TypedValue value,
int id, int density, String file)
throws IOException, XmlPullParserException {
try (
XmlResourceParser rp =
loadXmlResourceParser(file, id, value.assetCookie, "drawable")
) {
// 最终走到了Drawable的解析xml的方法内
return Drawable.createFromXmlForDensity(wrapper, rp, density, null);
}
}createFromXmlForDensity
public static Drawable createFromXmlForDensity(@NonNull Resources r,
@NonNull XmlPullParser parser, int density, @Nullable Theme theme)
throws XmlPullParserException, IOException {
...
// 调用静态方法创建drawable对象
Drawable drawable = createFromXmlInnerForDensity(r, parser, attrs, density, theme);
if (drawable == null) {
throw new RuntimeException("Unknown initial tag: " + parser.getName());
}
return drawable;
}
static Drawable createFromXmlInnerForDensity(@NonNull Resources r,
@NonNull XmlPullParser parser, @NonNull AttributeSet attrs, int density,
@Nullable Theme theme) throws XmlPullParserException, IOException {
return r.getDrawableInflater().inflateFromXmlForDensity(parser.getName(), parser, attrs,
density, theme);
}inflate
到这一步代码已经没法在sdk中查看了，剩余的代码在sdk内，继续分析
可以看到实际原生也是使用的xml标签解析，不同的xml对应不同类型的drawable实现，这里基本已经包含了所有的自带drawable了
系统还有部分其他Drawable，不支持标签使用，常用于代码使用，比如PaintDrawable,ShapeDrawable等等
<shape>标签及其实现
<shape android:shape="rectangle">
<solid android:color="@android:color/holo_blue_dark"/>
</shape>通过这种写法，可以生成一个颜色为暗蓝色的背景，那么它是如何知道我们设置的颜色，并绘制出来呢
Android中的自定义标签一般会预先定义
attr文件内，根据其描述及其解析位置，可以找寻到其实现类
通过选android:color属性，找到以下源码
<!-- Used to fill the shape of GradientDrawable with a solid color. -->
<declare-styleable name="GradientDrawableSolid">
<!-- Solid color for the gradient shape. -->
<attr name="color" format="color" />
</declare-styleable>
源码干脆直接说明该属性被GradientDrawable使用
基于第四部中的标签对照，确实该标签对应的实现类为GradientDrawable
GradientDrawable
当我们使用shape标签时，将会创建一个GradientDrawable对象用于绘制，那么除了attr之外，还有一些内部标签是怎么解析的呢
public void inflate(@NonNull Resources r, @NonNull XmlPullParser parser,
@NonNull AttributeSet attrs, @Nullable Theme theme)
throws XmlPullParserException, IOException {
super.inflate(r, parser, attrs, theme);
mGradientState.setDensity(Drawable.resolveDensity(r, 0));
// 自定义View使用的同款attr解析
final TypedArray a = obtainAttributes(r, theme, attrs, R.styleable.GradientDrawable);
updateStateFromTypedArray(a);
a.recycle();
// 解析子标签
inflateChildElements(r, parser, attrs, theme);
updateLocalState(r);
}
private void inflateChildElements(Resources r, XmlPullParser parser, AttributeSet attrs,
Theme theme) throws XmlPullParserException, IOException {
TypedArray a;
int type;
final int innerDepth = parser.getDepth() + 1;
int depth;
while ((type=parser.next()) != XmlPullParser.END_DOCUMENT
&& ((depth=parser.getDepth()) >= innerDepth
|| type != XmlPullParser.END_TAG)) {
if (type != XmlPullParser.START_TAG) {
continue;
}
if (depth > innerDepth) {
continue;
}
String name = parser.getName();
// 常见的size，solid，stroke等在此解析
if (name.equals("size")) {
a = obtainAttributes(r, theme, attrs, R.styleable.GradientDrawableSize);
updateGradientDrawableSize(a);
a.recycle();
} else if (name.equals("gradient")) {
a = obtainAttributes(r, theme, attrs, R.styleable.GradientDrawableGradient);
updateGradientDrawableGradient(r, a);
a.recycle();
} else if (name.equals("solid")) {
a = obtainAttributes(r, theme, attrs, R.styleable.GradientDrawableSolid);
updateGradientDrawableSolid(a);
a.recycle();
} else if (name.equals("stroke")) {
a = obtainAttributes(r, theme, attrs, R.styleable.GradientDrawableStroke);
updateGradientDrawableStroke(a);
a.recycle();
} else if (name.equals("corners")) {
a = obtainAttributes(r, theme, attrs, R.styleable.DrawableCorners);
updateDrawableCorners(a);
a.recycle();
} else if (name.equals("padding")) {
a = obtainAttributes(r, theme, attrs, R.styleable.GradientDrawablePadding);
updateGradientDrawablePadding(a);
a.recycle();
} else {
Log.w("drawable", "Bad element under <shape>: " + name);
}
}
}其它细节
1. inflate方法是在创建后最先调用的方法，部分初始化逻辑可以放在这里
2. 在通过xml创建时，存在创建失败的情况，如不存在的标签，此时会走另一个方法进行创建，提供了可扩展性
当标签为drawable且存在class属性时，尝试使用class内容初始化，否则使用标签名初始化
流程总结
自定义阴影drawable
原生elevation不支持所有版本，颜色修改也有版本限制
原生elevation光源固定，导致不同屏幕位置的控件，阴影效果不完全相同
定义基类
public class BaseShadowDrawable extends Drawable {
protected Paint paint;
// shadow x轴偏移
protected float shadowDx;
// shdaowY轴偏移
protected float shadowDy;
// shadow模糊半径
protected float shadowRadius;
// shadow颜色
protected int shadowColor;
public BaseShadowDrawable() {
paint = new Paint(Paint.ANTI_ALIAS_FLAG);
paint.setColor(Color.TRANSPARENT);
}
@Override
public void draw(@NonNull Canvas canvas) {
}
@Override
public void setAlpha(int alpha) {
}
@Override
public void setColorFilter(@Nullable ColorFilter colorFilter) {
}
@Override
public int getOpacity() {
return 0;
}
@Override
public void inflate(@NonNull Resources r, @NonNull XmlPullParser parser, @NonNull AttributeSet attrs, @Nullable Resources.Theme theme) throws IOException, XmlPullParserException {
super.inflate(r, parser, attrs, theme);
TypedArray typedArray = r.obtainAttributes(attrs, R.styleable.BaseShadowDrawable);
shadowColor = typedArray.getColor(R.styleable.BaseShadowDrawable_android_shadowColor, Color.GRAY);
shadowRadius = typedArray.getFloat(R.styleable.BaseShadowDrawable_android_shadowRadius, 5);
shadowDx = typedArray.getFloat(R.styleable.BaseShadowDrawable_android_shadowDx, 0);
shadowDy = typedArray.getFloat(R.styleable.BaseShadowDrawable_android_shadowDy, 0);
typedArray.recycle();
}
}矩形阴影实现类，可定义圆角
public class RectShadowDrawable extends BaseShadowDrawable {
protected int rectRadiusX;
protected int rectRadiusY;
@Override
public void draw(@NonNull Canvas canvas) {
paint.setShadowLayer(shadowRadius, shadowDx, shadowDy, shadowColor);
Rect rect = getBounds();
canvas.drawRoundRect(rect.left, rect.top, rect.right, rect.bottom, rectRadiusX, rectRadiusY, paint);
}
@Override
public void inflate(@NonNull Resources r, @NonNull XmlPullParser parser, @NonNull AttributeSet attrs, @Nullable Resources.Theme theme) throws IOException, XmlPullParserException {
super.inflate(r, parser, attrs, theme);
TypedArray typedArray = r.obtainAttributes(attrs, R.styleable.RectShadowDrawable);
int allRadius = typedArray.getDimensionPixelOffset(R.styleable.RectShadowDrawable_shadowRectRadius, 0);
rectRadiusX = typedArray.getDimensionPixelOffset(R.styleable.RectShadowDrawable_shadowRectRadiusX, allRadius);
rectRadiusY = typedArray.getDimensionPixelOffset(R.styleable.RectShadowDrawable_shadowRectRadiusY, allRadius);
typedArray.recycle();
}
}path阴影实现类,可使用vectordrawable的path
public class PathShadowDrawable extends BaseShadowDrawable {
protected Path path;
@Override
public void draw(@NonNull Canvas canvas) {
paint.setShadowLayer(shadowRadius, shadowDx, shadowDy, shadowColor);
canvas.drawPath(path, paint);
}
@Override
public void inflate(@NonNull Resources r, @NonNull XmlPullParser parser, @NonNull AttributeSet attrs, @Nullable Resources.Theme theme) throws IOException, XmlPullParserException {
super.inflate(r, parser, attrs, theme);
TypedArray typedArray = r.obtainAttributes(attrs, R.styleable.PathShadowDrawable);
String pathStr = typedArray.getString(R.styleable.PathShadowDrawable_shadowPath);
path = PathParser.createPathFromPathData(pathStr);
typedArray.recycle();
}
}使用方式
使用drawable标签+class属性自定义
<?xml version="1.0" encoding="utf-8"?>
<drawable class="com.bt.example.RectShadowDrawable"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:shadowColor="#2196F3"
android:shadowDx="10"
android:shadowDy="10"
android:shadowRadius="20"
app:shadowRectRadius="20px" />使用自定义标签
<?xml version="1.0" encoding="utf-8"?>
<com.bt.example.PathShadowDrawable
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:shadowColor="#2196F3"
android:shadowDx="0"
android:shadowDy="0"
android:shadowRadius="5"
app:shadowPath="M20.808,19.797C21.198,19.407 21.831,19.407 22.222,19.797L30,27.576L37.778,19.797C38.141,19.435 38.713,19.409 39.105,19.72L39.192,19.797L40.203,20.808C40.593,21.198 40.593,21.831 40.203,22.222L22.222,40.203C21.831,40.593 21.198,40.593 20.808,40.203L19.797,39.192C19.407,38.802 19.407,38.169 19.797,37.778L27.576,30L19.797,22.222C19.435,21.859 19.409,21.287 19.72,20.895L19.797,20.808ZM39.192,40.203L40.203,39.192C40.593,38.802 40.593,38.169 40.203,37.778L35.556,33.131C35.165,32.741 34.532,32.741 34.142,33.131L33.131,34.142C32.741,34.532 32.741,35.165 33.131,35.556L37.778,40.203C38.169,40.593 38.802,40.593 39.192,40.203Z" />效果
