关于ImageView的源码简单解析

今天在设置ImageView的图片时无意遇到一些问题，区分了一下设置background和src的区别，顺便看了一下View的setBackgroundDrawable源码(ImageView继承View调用的是View的方法)。将理解的简单总结一下，以便下一次接着看~
源码：

    /**     * @deprecated use {@link #setBackground(Drawable)} instead     */    @Deprecated    public void setBackgroundDrawable(Drawable background) {        computeOpaqueFlags();/*background为null，直接return*/        if (background == mBackground) {            return;        }/*requestLayout初始化为false，用于最后requestLayout()方法对requestLayout变量进行判断，如果为true，就调用requestLayout()方法。requestLayout()：当view确定自身已经不再适合现有的区域时，该view本身调用这个方法要求parent view重新调用他的onMeasure onLayout来对重新设置自己位置。*/        boolean requestLayout = false;        mBackgroundResource = 0;/*下面这一小段代码是用来判断，当view中已有内容时，清除已有内容的相关属性。其中调用unscheduleDrawable()方法在后面有介绍，主要是用来解除原有内容的相关事件，这块我也没有理解太清楚，只能理解为这样。。。*/        /*         * Regardless of whether we're setting a new background or not, we want         * to clear the previous drawable.         */        if (mBackground != null) {            mBackground.setCallback(null);            unscheduleDrawable(mBackground);        }/*sThreadLocal系统临时创建的矩形，用于未来扩展*/        if (background != null) {            Rect padding = sThreadLocal.get();            if (padding == null) {                padding = new Rect();                sThreadLocal.set(padding);            }            resetResolvedDrawables();           /*设置图片是从左到右还是从右到左对齐，依据父组件*/ background.setLayoutDirection(getLayoutDirection());            if (background.getPadding(padding)) {                resetResolvedPadding();                switch (background.getLayoutDirection()) {                    case LAYOUT_DIRECTION_RTL:                        mUserPaddingLeftInitial = padding.right;                        mUserPaddingRightInitial = padding.left;                        internalSetPadding(padding.right, padding.top, padding.left, padding.bottom);                        break;                    case LAYOUT_DIRECTION_LTR:                    default:                        mUserPaddingLeftInitial = padding.left;                        mUserPaddingRightInitial = padding.right;                        internalSetPadding(padding.left, padding.top, padding.right, padding.bottom);                }                mLeftPaddingDefined = false;                mRightPaddingDefined = false;            }/*当原本没用内容，或者原本的内容的最小大小和新的内容的最小大小不相同时，设置 requestLayout为true，调用 requestLayout()方法*/            // Compare the minimum sizes of the old Drawable and the new.  If there isn't an old or            // if it has a different minimum size, we should layout again            if (mBackground == null || mBackground.getMinimumHeight() != background.getMinimumHeight() ||                    mBackground.getMinimumWidth() != background.getMinimumWidth()) {                requestLayout = true;            }            background.setCallback(this);            if (background.isStateful()) {                background.setState(getDrawableState());            }            background.setVisible(getVisibility() == VISIBLE, false);            mBackground = background;            if ((mPrivateFlags & PFLAG_SKIP_DRAW) != 0) {                mPrivateFlags &= ~PFLAG_SKIP_DRAW;                mPrivateFlags |= PFLAG_ONLY_DRAWS_BACKGROUND;                requestLayout = true;            }        } else {//新的内容为空时            /* Remove the background */            mBackground = null;            if ((mPrivateFlags & PFLAG_ONLY_DRAWS_BACKGROUND) != 0) {                /*                 * This view ONLY drew the background before and we're removing                 * the background, so now it won't draw anything                 * (hence we SKIP_DRAW)                 */                mPrivateFlags &= ~PFLAG_ONLY_DRAWS_BACKGROUND;                mPrivateFlags |= PFLAG_SKIP_DRAW;            }            /*             * When the background is set, we try to apply its padding to this             * View. When the background is removed, we don't touch this View's             * padding. This is noted in the Javadocs. Hence, we don't need to             * requestLayout(), the invalidate() below is sufficient.             */            // The old background's minimum size could have affected this            // View's layout, so let's requestLayout            requestLayout = true;        }        computeOpaqueFlags();// requestLayout()方法        if (requestLayout) {            requestLayout();        }        mBackgroundSizeChanged = true;        invalidate(true);    }

以上只是为了以后学习记录，如有错误希望指出，感谢~