<?xml version="1.0" encoding="utf-8"?>
<shape
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape=["rectangle" | "oval" | "line" | "ring"] >
    <corners
        android:radius="integer"
        android:topLeftRadius="integer"
        android:topRightRadius="integer"
        android:bottomLeftRadius="integer"
        android:bottomRightRadius="integer" />
    <gradient
        android:angle="integer"
        android:centerX="integer"
        android:centerY="integer"
        android:centerColor="integer"
        android:endColor="color"
        android:gradientRadius="integer"
        android:startColor="color"
        android:type=["linear" | "radial" | "sweep"]
        android:useLevel=["true" | "false"] />
    <padding
        android:left="integer"
        android:top="integer"
        android:right="integer"
        android:bottom="integer" />
    <size
        android:width="integer"
        android:height="integer" />
    <solid
        android:color="color" />
    <stroke
        android:width="integer"
        android:color="color"
        android:dashWidth="integer"
        android:dashGap="integer" />
</shape>

public View(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        final TypedArray a = context.obtainStyledAttributes(
                attrs, com.android.internal.R.styleable.View, defStyleAttr, defStyleRes);
        ....
        background = a.getDrawable(attr);
        ....
}

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")
        ) {
            return Drawable.createFromXmlForDensity(wrapper, rp, density, null);
        }
    }

@NonNull
    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);
    }

@NonNull
    Drawable inflateFromXmlForDensity(@NonNull String name, @NonNull XmlPullParser parser,
            @NonNull AttributeSet attrs, int density, @Nullable Theme theme)
            throws XmlPullParserException, IOException {
        ....
        Drawable drawable = inflateFromTag(name);
        if (drawable == null) {
            drawable = inflateFromClass(name);
        }
        drawable.setSrcDensityOverride(density);
        drawable.inflate(mRes, parser, attrs, theme);
        return drawable;
    }

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 "adaptive-icon":
                return new AdaptiveIconDrawable();
            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();
            case "animated-image":
                return new AnimatedImageDrawable();
            default:
                return null;
        }
    }

final static class GradientState extends ConstantState {
        public @Shape int mShape = RECTANGLE;
        public ColorStateList mSolidColors;
        public ColorStateList mStrokeColors;
        public int mStrokeWidth = -1;
        public float mStrokeDashWidth = 0.0f;
        public float mRadius = 0.0f;
        public float[] mRadiusArray = null;
        ....
}

@IntDef({RECTANGLE, OVAL, LINE, RING})
    @Retention(RetentionPolicy.SOURCE)
    public @interface Shape {}

@Override
    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));
        final TypedArray a = obtainAttributes(r, theme, attrs, R.styleable.GradientDrawable);
        updateStateFromTypedArray(a);
        a.recycle();
        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;
        ....
            String name = parser.getName();
            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);
            }
        }
    }

private void updateDrawableCorners(TypedArray a) {
        final GradientState st = mGradientState;
        // Account for any configuration changes.
        st.mChangingConfigurations |= a.getChangingConfigurations();
        // Extract the theme attributes, if any.
        st.mAttrCorners = a.extractThemeAttrs();
        final int radius = a.getDimensionPixelSize(
                R.styleable.DrawableCorners_radius, (int) st.mRadius);
        setCornerRadius(radius);
        // TODO: Update these to be themeable.
        final int topLeftRadius = a.getDimensionPixelSize(
                R.styleable.DrawableCorners_topLeftRadius, radius);
        final int topRightRadius = a.getDimensionPixelSize(
                R.styleable.DrawableCorners_topRightRadius, radius);
        final int bottomLeftRadius = a.getDimensionPixelSize(
                R.styleable.DrawableCorners_bottomLeftRadius, radius);
        final int bottomRightRadius = a.getDimensionPixelSize(
                R.styleable.DrawableCorners_bottomRightRadius, radius);
        if (topLeftRadius != radius || topRightRadius != radius ||
                bottomLeftRadius != radius || bottomRightRadius != radius) {
            // The corner radii are specified in clockwise order (see Path.addRoundRect())
            setCornerRadii(new float[] {
                    topLeftRadius, topLeftRadius,
                    topRightRadius, topRightRadius,
                    bottomRightRadius, bottomRightRadius,
                    bottomLeftRadius, bottomLeftRadius
            });
        }
    }

@Override
    public void draw(Canvas canvas) {
        1、判斷是否需要繪制，如果不需要繪制，則直接return
        if (!ensureValidRect()) {
            // nothing to draw
            return;
        }
        2、獲取各類變量，并依據(jù)useLayer變量設(shè)置對(duì)應(yīng)的屬性
        // remember the alpha values, in case we temporarily overwrite them
        // when we modulate them with mAlpha
        final int prevFillAlpha = mFillPaint.getAlpha();
        final int prevStrokeAlpha = mStrokePaint != null ? mStrokePaint.getAlpha() : 0;
        // compute the modulate alpha values
        final int currFillAlpha = modulateAlpha(prevFillAlpha);
        final int currStrokeAlpha = modulateAlpha(prevStrokeAlpha);
        final boolean haveStroke = currStrokeAlpha > 0 && mStrokePaint != null &&
                mStrokePaint.getStrokeWidth() > 0;
        final boolean haveFill = currFillAlpha > 0;
        final GradientState st = mGradientState;
        final ColorFilter colorFilter = mColorFilter != null ? mColorFilter : mBlendModeColorFilter;
        /*  we need a layer iff we're drawing both a fill and stroke, and the
            stroke is non-opaque, and our shapetype actually supports
            fill+stroke. Otherwise we can just draw the stroke (if any) on top
            of the fill (if any) without worrying about blending artifacts.
         */
        final boolean useLayer = haveStroke && haveFill && st.mShape != LINE &&
                 currStrokeAlpha < 255 && (mAlpha < 255 || colorFilter != null);
        /*  Drawing with a layer is slower than direct drawing, but it
            allows us to apply paint effects like alpha and colorfilter to
            the result of multiple separate draws. In our case, if the user
            asks for a non-opaque alpha value (via setAlpha), and we're
            stroking, then we need to apply the alpha AFTER we've drawn
            both the fill and the stroke.
        */
        if (useLayer) {
            if (mLayerPaint == null) {
                mLayerPaint = new Paint();
            }
            mLayerPaint.setDither(st.mDither);
            mLayerPaint.setAlpha(mAlpha);
            mLayerPaint.setColorFilter(colorFilter);
            float rad = mStrokePaint.getStrokeWidth();
            canvas.saveLayer(mRect.left - rad, mRect.top - rad,
                             mRect.right + rad, mRect.bottom + rad,
                             mLayerPaint);
            // don't perform the filter in our individual paints
            // since the layer will do it for us
            mFillPaint.setColorFilter(null);
            mStrokePaint.setColorFilter(null);
        } else {
            /*  if we're not using a layer, apply the dither/filter to our
                individual paints
            */
            mFillPaint.setAlpha(currFillAlpha);
            mFillPaint.setDither(st.mDither);
            mFillPaint.setColorFilter(colorFilter);
            if (colorFilter != null && st.mSolidColors == null) {
                mFillPaint.setColor(mAlpha << 24);
            }
            if (haveStroke) {
                mStrokePaint.setAlpha(currStrokeAlpha);
                mStrokePaint.setDither(st.mDither);
                mStrokePaint.setColorFilter(colorFilter);
            }
        }
        3、根據(jù)shape四種屬性繪制對(duì)應(yīng)的圖形
        switch (st.mShape) {
            case RECTANGLE:
            根據(jù)是否有角度，以及角度是否相同，分別采用canvas.drawRect、canvas.drawRoundRect、canvas.drawPath進(jìn)行繪制
            case OVAL:
            使用canvas.drawOval進(jìn)行繪制
            case LINE: 
            使用canvas.drawLine進(jìn)行繪制
            case RING:
            使用canvas.drawPath進(jìn)行繪制
        }
        4、恢復(fù)現(xiàn)場(chǎng)
        if (useLayer) {
            canvas.restore();
        } else {
            mFillPaint.setAlpha(prevFillAlpha);
            if (haveStroke) {
                mStrokePaint.setAlpha(prevStrokeAlpha);
            }
        }
    }

/**
     * This checks mGradientIsDirty, and if it is true, recomputes both our drawing
     * rectangle (mRect) and the gradient itself, since it depends on our
     * rectangle too.
     * @return true if the resulting rectangle is not empty, false otherwise
     */