搞明白这个花的时间有点多，最后将代码copy出来运行断点后才知道原来是这样子的

方法一：

img.setAdjustViewBounds(true);

解析：

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
       ····
位置一
            if (mAdjustViewBounds) {
                resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;
                resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;

                desiredAspect = (float) w / (float) h;
            }
        }

       ·····
位置二
        if (resizeWidth || resizeHeight) {
            /* If we get here, it means we want to resize to match the
                drawables aspect ratio, and we have the freedom to change at
                least one dimension.
            */

            // Get the max possible width given our constraints
            widthSize = resolveAdjustedSize(w + pleft + pright, mMaxWidth, widthMeasureSpec);

            // Get the max possible height given our constraints
            heightSize = resolveAdjustedSize(h + ptop + pbottom, mMaxHeight, heightMeasureSpec);

            if (desiredAspect != 0.0f) {
                // See what our actual aspect ratio is
                final float actualAspect = (float)(widthSize - pleft - pright) /
                                        (heightSize - ptop - pbottom);

                if (Math.abs(actualAspect - desiredAspect) > 0.0000001) {

                    boolean done = false;

                    // Try adjusting width to be proportional to height
                    if (resizeWidth) {
                        int newWidth = (int)(desiredAspect * (heightSize - ptop - pbottom)) +
                                pleft + pright;

                        // Allow the width to outgrow its original estimate if height is fixed.
                        if (!resizeHeight && !sCompatAdjustViewBounds) {
                            widthSize = resolveAdjustedSize(newWidth, mMaxWidth, widthMeasureSpec);
                        }

                        if (newWidth <= widthSize) {
                            widthSize = newWidth;
                            done = true;
                        }
                    }

                    // Try adjusting height to be proportional to width
                    if (!done && resizeHeight) {
                        int newHeight = (int)((widthSize - pleft - pright) / desiredAspect) +
                                ptop + pbottom;

                        // Allow the height to outgrow its original estimate if width is fixed.
                        if (!resizeWidth && !sCompatAdjustViewBounds) {
                            heightSize = resolveAdjustedSize(newHeight, mMaxHeight,
                                    heightMeasureSpec);
                        }

                        if (newHeight <= heightSize) {
                            heightSize = newHeight;
                        }
                    }
                }
            }
        } else {
            /* We are either don't want to preserve the drawables aspect ratio,
               or we are not allowed to change view dimensions. Just measure in
               the normal way.
            */
            w += pleft + pright;
            h += ptop + pbottom;

            w = Math.max(w, getSuggestedMinimumWidth());
            h = Math.max(h, getSuggestedMinimumHeight());

            widthSize = resolveSizeAndState(w, widthMeasureSpec, 0);
            heightSize = resolveSizeAndState(h, heightMeasureSpec, 0);
        }

        setMeasuredDimension(widthSize, heightSize);
    }

1.上面那一行解决问题的代码的含义是改变mAdjustViewBounds的值
2.根据代码二的位置可以看出，imageview的测量其实分了两种情况，默认情况是在一定空间中按比例显示最小的尺寸（else中的代码），其次是按比例放大或缩小（为wrap的情况赋值），mAdjustViewBounds则控制这两种情况。
mAdjustViewBounds默认为false，这里我们需要改为true，来满足宽度充满全屏，高度按比例确定

方法二：

自定义Image
参考：
ImageView宽度填满屏幕，高度自适应

后记

推荐：Android获取View的宽度和高度
如有问题，欢迎批评指正

喵印～～～