ColorDrawable

ColorDrawable 是最简单的 Drawable，也是平时用的最多的，比如：

    android:background="@color/colorAccent"

对于 Drawable 而言，看 draw 函数基本就够了：

    @Override    public void draw(Canvas canvas) {        final ColorFilter colorFilter = mPaint.getColorFilter();        if ((mColorState.mUseColor >>> 24) != 0 || colorFilter != null || mTintFilter != null) {            if (colorFilter == null) {                mPaint.setColorFilter(mTintFilter);            }            mPaint.setColor(mColorState.mUseColor);            canvas.drawRect(getBounds(), mPaint);            // Restore original color filter.            mPaint.setColorFilter(colorFilter);        }    }