一、Drawable

在Android系统张，图形图像的绘制需要在画布上进行操作和处理，但是绘制需要了解很多细节以及可能要进行一些复杂的处理，因此系统提供了一个被称之为Drawable的类来进行绘制处理。通过这个类可以减少我们的绘制工作和使用成本，同时系统也提供了众多的Drawable的派生类比如单色、图形、位图、裁剪、动画等等来完成一些常见的绘制需求。Drawable是一个抽象的可绘制类。他主要是提供了一个可绘制的区域bound属性以及一个draw成员函数，不同的派生类通过重载draw函数的实现而产生不同的绘制结果。如下是Drawable的加载流程。

从Resource.getDrawable会判断是否.xml结尾，不是的话走6，7步，如果从xml中读取，需要

getResource.getDrawable -> ResourceImpl.loadDrawableForCookie -> Drawable.createFromXml -> drawableInflater.inflateFromXmlForDensity -> drawable.inflate(Resources r, XmlPullParser parser, AttributeSet attrs, Resources.Theme theme)

Resources的作用是将整个过程进行了封装、同时实现了资源的缓存。因此，为了更加直白的了解加载过程，以上步骤我们可以精简如下：

Drawable.createFromXml -> drawableInflater.inflateFromXmlForDensity -> drawable.inflate(Resources r, XmlPullParser parser, AttributeSet attrs, Resources.Theme theme)

注意:Drawable和drawable，前者是类，后者是类的实例，同样drawableInflater也是类的实例。

二、流程分析和方法解析

Drawable.createFromXml是静态调用，实际上整个过程是XmlPull的解析。最终，会调用到createFromXmlInnerForDensity

 @NonNull
    public static Drawable createFromXmlForDensity(@NonNull Resources r,
            @NonNull XmlPullParser parser, int density, @Nullable Theme theme)
            throws XmlPullParserException, IOException {
        AttributeSet attrs = Xml.asAttributeSet(parser);

        int type;
        //noinspection StatementWithEmptyBody
        while ((type=parser.next()) != XmlPullParser.START_TAG
                && type != XmlPullParser.END_DOCUMENT) {
            // Empty loop.
        }

        if (type != XmlPullParser.START_TAG) {
            throw new XmlPullParserException("No start tag found");
        }

        Drawable drawable = createFromXmlInnerForDensity(r, parser, attrs, density, theme);

        if (drawable == null) {
            throw new RuntimeException("Unknown initial tag: " + parser.getName());
        }

        return drawable;
    }

  @NonNull
    static Drawable createFromXmlInnerForDensity(@NonNull Resources r,
            @NonNull XmlPullParser parser, @NonNull AttributeSet attrs, int density,
            @Nullable Theme theme) throws XmlPullParserException, IOException {

         //通过Resources里面的getDrawableInflater得到DrawableInflater的实例
        return r.getDrawableInflater().inflateFromXmlForDensity(parser.getName(), parser, attrs,
                density, theme);
    }

drawableInflater.inflateFromXmlForDensity 方法用来加载Drawable资源，如果不是我们自定义的Drawable类，逻辑流程通常如下解析：

 @NonNull
    public Drawable inflateFromXml(@NonNull String name, @NonNull XmlPullParser parser,
            @NonNull AttributeSet attrs, @Nullable Theme theme)
            throws XmlPullParserException, IOException {
       
        if (name.equals("drawable")) { //无意义的drawable
            name = attrs.getAttributeValue(null, "class");
            if (name == null) {
                throw new InflateException("<drawable> tag must specify class attribute");
            }
        }
        Drawable drawable = inflateFromTag(name); //解析处Drawable的实例
        if (drawable == null) {
            drawable = inflateFromClass(name);
        }
        drawable.inflate(mRes, parser, attrs, theme); 
       //得到drawable实例，通过drawable.inflate去实现属性的解析
        return drawable;  //返回实例
    }

inflateFromTag源码如下：

@NonNull
    @SuppressWarnings("deprecation")
    private Drawable inflateFromTag(@NonNull String name) {
        switch (name) {
            case "selector":
                return new StateListDrawable();
            case "animated-selector":
                return new AnimatedStateListDrawable();
            case "level-list":
                return new LevelListDrawable();
            case "layer-list":
                return new LayerDrawable();
            case "transition":
                return new TransitionDrawable();
            case "ripple":
                return new RippleDrawable();
            case "color":
                return new ColorDrawable();
            case "shape":
                return new GradientDrawable();
            case "vector":
                return new VectorDrawable();
            case "animated-vector":
                return new AnimatedVectorDrawable();
            case "scale":
                return new ScaleDrawable();
            case "clip":
                return new ClipDrawable();
            case "rotate":
                return new RotateDrawable();
            case "animated-rotate":
                return new AnimatedRotateDrawable();
            case "animation-list":
                return new AnimationDrawable();
            case "inset":
                return new InsetDrawable();
            case "bitmap":
                return new BitmapDrawable();
            case "nine-patch":
                return new NinePatchDrawable();
            default:
                return null;
        }
    }

那么drawable.inflate方法是如何实现的？

Drawable本身是抽象类，根据不同实现去解析属性，我们以ShapeDrawable为例，一般的通过TypeArray解析当前节点的属性，如果存在子元素继续遍历。

  @Override
    public void inflate(Resources r, XmlPullParser parser, AttributeSet attrs, Theme theme)
            throws XmlPullParserException, IOException {
        super.inflate(r, parser, attrs, theme);

        final TypedArray a = obtainAttributes(r, theme, attrs, R.styleable.ShapeDrawable);
        updateStateFromTypedArray(a);
        a.recycle();

        int type;
        final int outerDepth = parser.getDepth();
        while ((type = parser.next()) != XmlPullParser.END_DOCUMENT
                && (type != XmlPullParser.END_TAG || parser.getDepth() > outerDepth)) {
            if (type != XmlPullParser.START_TAG) {
                continue;
            }

            final String name = parser.getName();
            // 解析子节点
            if (!inflateTag(name, r, parser, attrs)) {
                android.util.Log.w("drawable", "Unknown element: " + name +
                        " for ShapeDrawable " + this);
            }
        }

        // Update local properties.
        updateLocalState();
    }

三、实现自定义Drawable类的加载

通常我们说的自定义drawable是自定义xml文件，如果实现一种可以复用并且Android系统中没有内置的Drawable，此外实现多个布局文件的引用，当然你可以说完全可以将代码自定义到静态方法中，实现多次引用也是可以，不过我们按照Android的建议，图形化的对象尽量以xml形式呈现。

下面，我们定义一个形状如下的Drawable：

3.1、原理分析

那么，要实现“自定义Drawable类的加载”需求，比如要进行技术可行性分析，那我们的依据是什么呢？

在DrawableInflater中，除了通过inflateFromTag优先解析Drawable之外，我们发现同样提供了inflateFromClass，通过这种方式我们同样可以得到Drawable子类的实例。

   Drawable drawable = inflateFromTag(name); //解析处Drawable的实例
        if (drawable == null) {
            drawable = inflateFromClass(name);
        }

inflateFromClass的实现如下：

@NonNull
private Drawable inflateFromClass(@NonNull String className) {
        try {
            Constructor<? extends Drawable> constructor;
            synchronized (CONSTRUCTOR_MAP) {
                constructor = CONSTRUCTOR_MAP.get(className);
                if (constructor == null) {
                    //通过ClassLoader加载Drawable类，然后转为Drawable类
                    final Class<? extends Drawable> clazz =
                            mClassLoader.loadClass(className).asSubclass(Drawable.class);
                    constructor = clazz.getConstructor();
                    CONSTRUCTOR_MAP.put(className, constructor);
                }
            }
            return constructor.newInstance();  //创建Drawable实例
        } catch (Exception e) {
           //省略
       }
     return null;
 }

注意：我们通过ClassLoader去加载类，那么还要注意一个事情就是混淆，混淆时我们必须注意我们自定义的Drawable类不能被混淆，否则无法加载。

-keepclassmembers class * extends android.graphics.drawable.Drawable{
    public void *(android.view.View);
}

3.2、代码实例

[1]定义图形

首先，我们需要定义一个Shape图形，在Android系统中，实现圆角圆弧最好的方式是通过Path实现。

public  class RadiusBorderShape extends Shape {


        private Path mPath;
        @ColorInt
        private int color;  //边框颜色
        private  float strokeWidth; //线宽
        private float[] radius;  //各个角的radius
        @ColorInt
        private int backgroundColor; //背景填充颜色

    public void setColor(@ColorInt  int color) {
            this.color = color;
        }

  public void setRadius(float[] radius) {

        if(radius==null || radius.length<4){
            this.radius = new float[4];
        }else{
            this.radius = radius;
        }
        for (int i=0;i<this.radius.length;i++){
            float v = this.radius[i];
            if(v<0) {
                this.radius[i] = 0f;
            }
        }
    }

    public void setStrokeWidth(float strokeWidth) {
            if(strokeWidth<0) {
                strokeWidth = 0;
            }
            this.strokeWidth = strokeWidth;
        }

        public RadiusBorderShape(){
            mPath = new Path();
            this.strokeWidth = 5f;
            this.color = Color.RED;
            this.backgroundColor = Color.GREEN;
            this.radius = new float[]{5f,0f,20f,30f};
        }

        @Override
        public void draw(Canvas canvas, Paint paint) {


            Paint.Style  old_style = paint.getStyle();
            int old_color = paint.getColor();
            float old_strokeWidth = paint.getStrokeWidth();


            paint.setStrokeWidth(this.strokeWidth);


            int backgroundId = canvas.save();
            canvas.translate(strokeWidth,strokeWidth);

            drawBackground(canvas, paint);
            drawBorder(canvas, paint);

            canvas.restoreToCount(backgroundId);

            paint.setStyle(old_style);
            paint.setColor(old_color);
            paint.setStrokeWidth(old_strokeWidth);


        }

    private void drawBorder(Canvas canvas, Paint paint) {
        paint.setStyle(Paint.Style.STROKE);
        paint.setColor(this.color);
        canvas.scale(1, 1);
        canvas.drawPath(mPath, paint);

    }

    private void drawBackground(Canvas canvas, Paint paint) {
       final Path.FillType fillType = mPath.getFillType();

        int borderId = canvas.save();
        paint.setStyle(Paint.Style.FILL);
        paint.setColor(this.backgroundColor);

        if(this.backgroundColor!=Color.TRANSPARENT){
            mPath.setFillType(Path.FillType.WINDING);  //填充，兼容低版本无法填充的问题
        }
        canvas.drawPath(mPath, paint);
        canvas.restoreToCount(borderId);
        mPath.setFillType(fillType);//还原
    }

    @Override
        protected void onResize(float width, float height) {
            super.onResize(width, height);

            float w =  width - strokeWidth*2;  //减去左右侧的线宽
            float h =  height - strokeWidth*2; //减去上下侧的线宽
            mPath.reset();

            if(w<=0 && h<=0){
                return;
            }
            float leftTopThresold = radius[0];
            mPath.moveTo(0,leftTopThresold);
            //从180度处顺时针旋转，增量90度
            mPath.arcTo(new RectF(0,0,leftTopThresold,leftTopThresold),  180f, 90f);

            float rightTopThresold = radius[1];
            mPath.lineTo(w-rightTopThresold,0);
            mPath.arcTo(new RectF(w-rightTopThresold,0,w,rightTopThresold),  270f, 90f);

            float rightBottomThresold = radius[2];
            mPath.lineTo(w,h-rightBottomThresold);
            mPath.arcTo(new RectF(w-rightBottomThresold,h-rightBottomThresold,w,h),  0f, 90f);

            float leftBottomThresold = radius[3];
            mPath.lineTo(leftBottomThresold,h);
            mPath.arcTo(new RectF(0,h-leftBottomThresold,leftBottomThresold,h),  90f, 90f);


            mPath.lineTo(0,leftTopThresold);
            mPath.close();

        }

        @Override
        public Shape clone() throws CloneNotSupportedException {
            final RadiusBorderShape shape = (RadiusBorderShape) super.clone();
            shape.mPath = new Path(mPath);
            shape.radius = radius;
            shape.strokeWidth = strokeWidth;
            shape.color = color;
            return shape;
        }


    public void setBackgroundColor(int backgroundColor) {
            this.backgroundColor = backgroundColor;
    }
}

在这个类中，最终要的2个方法是onResize和draw方法，shape.onResize在Drawable中会被drawable.onBoundsChanged调用，从而实现Drawable大小的监听。

[2]定义Drawable

public class RadiusRectDrawable extends ShapeDrawable {


    private int backgroundColor;
    private RadiusBorderShape shape;

    @Override
    public void inflate(Resources r, XmlPullParser parser, AttributeSet attrs, Resources.Theme theme) throws XmlPullParserException, IOException {

        TypedArray array = RadiusRectDrawable.obtainAttributes(r, theme, attrs, R.styleable.RadiusRectDrawable);
        if(array==null) return;
        backgroundColor = array.getColor(R.styleable.RadiusRectDrawable_backgroundColor, Color.TRANSPARENT);
        array.recycle();

        super.inflate(r, parser, attrs, theme);
    }


//低版本api兼容
@Override
    public void inflate(@NonNull Resources r, @NonNull XmlPullParser parser, @NonNull AttributeSet attrs) throws XmlPullParserException, IOException {
        TypedArray array = RadiusRectDrawable.obtainAttributes(r, null, attrs, R.styleable.RadiusRectDrawable);
        if(array==null) return;
        backgroundColor = array.getColor(R.styleable.RadiusRectDrawable_backgroundColor, Color.TRANSPARENT);
        array.recycle();
        super.inflate(r, parser, attrs);
    }


    @Override
    protected boolean inflateTag(String name, Resources r, XmlPullParser parser, AttributeSet attrs) {
        if("RadiusBorderShape".equals(name)){

            TypedArray array = r.obtainAttributes(attrs, R.styleable.RadiusRectDrawable);

            int lineColor = array.getColor(R.styleable.RadiusRectDrawable_lineColor, Color.TRANSPARENT);
            float lineWidth = array.getFloat(R.styleable.RadiusRectDrawable_lineWidth, 0f);

            float leftTopRadius = array.getDimensionPixelSize(R.styleable.RadiusRectDrawable_leftTop_radius, 0);
            float leftBottomRadius = array.getDimensionPixelSize(R.styleable.RadiusRectDrawable_leftBottom_radius, 0);
            float rightTopRadius = array.getDimensionPixelSize(R.styleable.RadiusRectDrawable_rightTop_radius, 0);
            float rightBottomRadius = array.getDimensionPixelSize(R.styleable.RadiusRectDrawable_rightBottom_radius, 0);

            if(shape==null){
                shape = new RadiusBorderShape();
            }
            shape.setColor(lineColor);
            shape.setStrokeWidth(lineWidth);
            shape.setRadius(new float[]{leftTopRadius,rightTopRadius,rightBottomRadius,leftBottomRadius});
            shape.setBackgroundColor(backgroundColor);
            if(shape!=getShape()){
                setShape(shape);
            }

            array.recycle();

            return true;
        }
        else{
            return super.inflateTag(name, r, parser, attrs);
        }
    }

    protected static @NonNull TypedArray obtainAttributes(@NonNull Resources res,
                                                          @Nullable Resources.Theme theme, @NonNull AttributeSet set, @NonNull int[] attrs) {
        if (theme == null) {
            return res.obtainAttributes(set, attrs);
        }
        return theme.obtainStyledAttributes(set, attrs, 0, 0);
    }
}

这个就是我们自己定义的Drawable类，当然，自定义往往需要自定义属性。

   <declare-styleable name="RadiusRectDrawable">
        <attr name="lineColor" format="color|reference"/>
        <attr name="backgroundColor" format="color|reference"/>
        <attr name="lineWidth" format="float|reference"/>
        <attr name="leftTop_radius" format="dimension|reference" />
        <attr name="leftBottom_radius" format="dimension|reference" />
        <attr name="rightBottom_radius" format="dimension|reference" />
        <attr name="rightTop_radius" format="dimension|reference" />
    </declare-styleable>

[3]定义drwable文件

自定义drawble的xml文件，安装惯例应该在drawable资源文件夹下，但是我们的编译器表现的有些不友好，要求sdk版本大于24（android 7.0)才行。

从ResourcesImpl.loadDrawableForCookie加载逻辑来看，文件加载主要通过2种方式，文件读取的核心代码如下：

 if (file.endsWith(".xml")) {
                    final XmlResourceParser rp = loadXmlResourceParser(
                            file, id, value.assetCookie, "drawable");
                    dr = Drawable.createFromXmlForDensity(wrapper, rp, density, null);
                    rp.close();
                } else {
                    final InputStream is = mAssets.openNonAsset(
                            value.assetCookie, file, AssetManager.ACCESS_STREAMING);
                    AssetInputStream ais = (AssetInputStream) is;
                    dr = decodeImageDrawable(ais, wrapper, value);
                }

一般代码实际上可以通过loadXmlResourceParser或者mAssets.openNonAsset加载，前者加载xml文件内置资源，后者加载图片文件内置资源。通过loadXmlResourceParser加载文件，最后一个参数制定的是drawable，但是从loadXmlResourceParser源码中并未使用第四个参数（篇幅有限，ResourcesImpl源码自行查看），也就是说，加载资源时并没有对资源文件所在目录进行校验。

因此说，编译器会校验类型，但运行时不会校验。这样我们可以将xml文件放置到非drawable目录，可以是Assets文件夹中，同样也可以是xml资源文件夹下。我们这里将定义文件放置到xml资源目录即可。

源码内容如下：

<?xml version="1.0" encoding="utf-8"?>
<com.example.cc.myapplication.shape.RadiusRectDrawable
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    app:backgroundColor="@color/white"
    >
    <RadiusBorderShape
        app:lineColor="@color/colorAccent"
        app:lineWidth="5.5"
        app:leftTop_radius="50dip"
        app:leftBottom_radius="0dip"
        app:rightTop_radius="0dip"
        app:rightBottom_radius="0dip"
        />
</com.example.cc.myapplication.shape.RadiusRectDrawable>

[4]加载并使用

事实上由于编译工具的要求sdk api大于24才可以使用，因此，我们android:background="@xml/radius_border"显然存在问题，除非我们自行实现LayoutInfater.Factory2，通过自定义的方式去拦截和解析，但是由于篇幅问题，这里我们通过一般代码加载。

public class ResourceUtils {

    private static final HashMap<String, Constructor<? extends Drawable>> CONSTRUCTOR_MAP =
            new HashMap<>();

    private Context context;

    private ResourceUtils(Context context){
        this.context = context;
    }

    public Context getContext() {
        return context;
    }
    //加载drawable
    public  static Drawable getDrawable(Context context, int xmlShapeId){
        try {
            ResourceUtils resourceUtils = new ResourceUtils(context);
            return resourceUtils.parseDrawable(xmlShapeId);
        }catch (Exception e){
            e.printStackTrace();
        }
        return null;
    }

    private Drawable  parseDrawable(int xmlId)  { //R.xml.radius_border)
        Drawable drawable = null;
        try{

            if(Build.VERSION.SDK_INT<24) {
                drawable = parseDrawableFromClass(xmlId);
            }
            if(drawable!=null){
                return drawable;
            }

            Context context = getContext();
            Resources resources = context.getResources();
            XmlResourceParser xmlParse = resources.getXml(xmlId);
            if(Build.VERSION.SDK_INT>=21) {
                drawable = Drawable.createFromXml(resources, xmlParse, context.getTheme());
            }else{
                drawable = Drawable.createFromXml(resources, xmlParse);
            }
            xmlParse.close();
        }catch (Exception e){
            e.printStackTrace();
        }
        return drawable;
    }

    private Drawable parseDrawableFromClass(int xmlId){
        Drawable drawable = null;
        try {
            Context context = getContext();

            Resources resources = context.getResources();
            XmlResourceParser xmlParse = resources.getXml(xmlId);

            AttributeSet attrs = Xml.asAttributeSet(xmlParse);
            int type;
            while ((type = xmlParse.next()) != XmlPullParser.START_TAG
                    && type != XmlPullParser.END_DOCUMENT) {
            }

            if (type != XmlPullParser.START_TAG) {
                throw new XmlPullParserException("No start tag found");
            }
            drawable = inflateFromClass(xmlParse.getName());
            if(drawable==null) return null;

            if (Build.VERSION.SDK_INT >= 21) {
                drawable.inflate(resources, xmlParse, attrs, context.getTheme());
            } else {
                drawable.inflate(resources, xmlParse, attrs);

            }
        }catch (Exception e){
            e.printStackTrace();
        }

        return  drawable;
    }


    @NonNull
    private Drawable inflateFromClass(@NonNull String className) {
        try {
            Constructor<? extends Drawable> constructor;
            synchronized (CONSTRUCTOR_MAP) {
                constructor = CONSTRUCTOR_MAP.get(className);
                if (constructor == null) {
                    //通过ClassLoader加载Drawable类，然后转为Drawable类
                    final Class<? extends Drawable> clazz =
                            getClass().getClassLoader().loadClass(className).asSubclass(Drawable.class);
                    constructor = clazz.getConstructor();
                    CONSTRUCTOR_MAP.put(className, constructor);
                }
            }
            return constructor.newInstance();  //创建Drawable实例
        } catch (Exception e) {
            //省略
        }
        return null;
    }
}

当然，用法我们以ImageView为例

Drawable drawable = ResourceUtils.getDrawable(mContext,R.xml.radius_border);

myImageView.setBackgroundDrawable(drawable);

四、总结

我们通过这种方式成功实现了自定义Drawable的加载，DrawableInflater作为加载引擎和路由，我们应该充分利用这种关系，作为Inflater，同样LayoutInflater.Factory值得我们去实践。

附录：

1）LayoutInflater.Factory2加载机制请参阅如下链接：

https://my.oschina.net/ososchina/blog/405904

2）DrawableInflater请参阅如下链接：

https://github.com/aosp-mirror/platform_frameworks_base/blob/master/graphics/java/android/graphics/drawable/DrawableInflater.java