当xml是这样时：
效果是：
Screenshot_1525569407.png
将ImageView的marginTop改成150dp后，效果是：
Screenshot_1525569332.png
图片的宽明显发生了改变。不知道这是不是bug。
通过看源码，我觉得是RelativeLayout测量方法的问题。RelativeLayout测量child的宽高时，先用去掉margins的宽高测量出child的宽，然后用测出的child的宽再去测量child的高。在测量宽时，用的height是RelativeLayout的高减去margin，ImageView的adjustViewBounds会根据这个height去计算出width，所以会导致marginTop越大，height越小，width越小。
下面是自己看源码的分析：
首先是RelativeLayout的onMeasure方法：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
。。。
View[] views = mSortedHorizontalChildren;
int count = views.length;
for (int i = 0; i < count; i++) {
View child = views[i];
if (child.getVisibility() != GONE) {
LayoutParams params = (LayoutParams) child.getLayoutParams();
int[] rules = params.getRules(layoutDirection);
applyHorizontalSizeRules(params, myWidth, rules);
measureChildHorizontal(child, params, myWidth, myHeight);
if (positionChildHorizontal(child, params, myWidth, isWrapContentWidth)) {
offsetHorizontalAxis = true;
}
}
}
。。。
}
在onMeasure中通过measureChildHorizontal计算出child的宽，顺便说下myWidth和myHeight是屏幕宽高。measureChildHorizontal的代码如下：
private void measureChildHorizontal(
View child, LayoutParams params, int myWidth, int myHeight) {
final int childWidthMeasureSpec = getChildMeasureSpec(params.mLeft, params.mRight,
params.width, params.leftMargin, params.rightMargin, mPaddingLeft, mPaddingRight,
myWidth);
final int childHeightMeasureSpec;
if (myHeight < 0 && !mAllowBrokenMeasureSpecs) {
if (params.height >= 0) {
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
params.height, MeasureSpec.EXACTLY);
} else {
// Negative values in a mySize/myWidth/myWidth value in
// RelativeLayout measurement is code for, "we got an
// unspecified mode in the RelativeLayout's measure spec."
// Carry it forward.
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
}
} else {
final int maxHeight;
if (mMeasureVerticalWithPaddingMargin) {
maxHeight = Math.max(0, myHeight - mPaddingTop - mPaddingBottom
- params.topMargin - params.bottomMargin);
} else {
maxHeight = Math.max(0, myHeight);
}
final int heightMode;
if (params.height == LayoutParams.MATCH_PARENT) {
heightMode = MeasureSpec.EXACTLY;
} else {
heightMode = MeasureSpec.AT_MOST;
}
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(maxHeight, heightMode);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
measureChildHorizontal中最后通过child.measure()测量了child的宽和高。测量高用的MeasureSpec的size是RelativeLayout去除margin后的高度。所以这里使用不同的marginTop会得到不同的高。测量宽用的MeasureSpec通过getChildMeasureSpec获得。getChildMeasureSpec的代码如下：
private int getChildMeasureSpec(int childStart, int childEnd,
int childSize, int startMargin, int endMargin, int startPadding,
int endPadding, int mySize) {
int childSpecMode = 0;
int childSpecSize = 0;
// Negative values in a mySize value in RelativeLayout
// measurement is code for, "we got an unspecified mode in the
// RelativeLayout's measure spec."
final boolean isUnspecified = mySize < 0;
if (isUnspecified && !mAllowBrokenMeasureSpecs) {
if (childStart != VALUE_NOT_SET && childEnd != VALUE_NOT_SET) {
// Constraints fixed both edges, so child has an exact size.
childSpecSize = Math.max(0, childEnd - childStart);
childSpecMode = MeasureSpec.EXACTLY;
} else if (childSize >= 0) {
// The child specified an exact size.
childSpecSize = childSize;
childSpecMode = MeasureSpec.EXACTLY;
} else {
// Allow the child to be whatever size it wants.
childSpecSize = 0;
childSpecMode = MeasureSpec.UNSPECIFIED;
}
return MeasureSpec.makeMeasureSpec(childSpecSize, childSpecMode);
}
// Figure out start and end bounds.
int tempStart = childStart;
int tempEnd = childEnd;
// If the view did not express a layout constraint for an edge, use
// view's margins and our padding
if (tempStart == VALUE_NOT_SET) {
tempStart = startPadding + startMargin;
}
if (tempEnd == VALUE_NOT_SET) {
tempEnd = mySize - endPadding - endMargin;
}
// Figure out maximum size available to this view
final int maxAvailable = tempEnd - tempStart;
if (childStart != VALUE_NOT_SET && childEnd != VALUE_NOT_SET) {
// Constraints fixed both edges, so child must be an exact size.
childSpecMode = isUnspecified ? MeasureSpec.UNSPECIFIED : MeasureSpec.EXACTLY;
childSpecSize = Math.max(0, maxAvailable);
} else {
if (childSize >= 0) {
// Child wanted an exact size. Give as much as possible.
childSpecMode = MeasureSpec.EXACTLY;
if (maxAvailable >= 0) {
// We have a maximum size in this dimension.
childSpecSize = Math.min(maxAvailable, childSize);
} else {
// We can grow in this dimension.
childSpecSize = childSize;
}
} else if (childSize == LayoutParams.MATCH_PARENT) {
// Child wanted to be as big as possible. Give all available
// space.
childSpecMode = isUnspecified ? MeasureSpec.UNSPECIFIED : MeasureSpec.EXACTLY;
childSpecSize = Math.max(0, maxAvailable);
} else if (childSize == LayoutParams.WRAP_CONTENT) {
// Child wants to wrap content. Use AT_MOST to communicate
// available space if we know our max size.
if (maxAvailable >= 0) {
// We have a maximum size in this dimension.
childSpecMode = MeasureSpec.AT_MOST;
childSpecSize = maxAvailable;
} else {
// We can grow in this dimension. Child can be as big as it
// wants.
childSpecMode = MeasureSpec.UNSPECIFIED;
childSpecSize = 0;
}
}
}
return MeasureSpec.makeMeasureSpec(childSpecSize, childSpecMode);
}
由于childStart == VALUE_NOT_SET、childEnd == VALUE_NOT_SET成立，这段代码最后生成的MeasureSpce的size是去除margin的mySize，而mySize是传入的屏幕宽。
这里的childStart == VALUE_NOT_SET是因为measureChildHorizontal之前执行了applyHorizontalSizeRules，applyHorizontalSizeRules将layoutParams的mLeft和mRight都改成了VALUE_NOT_SET，applyHorizontalSizeRules代码如下：
private void applyHorizontalSizeRules(LayoutParams childParams, int myWidth, int[] rules) {
...
childParams.mLeft = VALUE_NOT_SET;
childParams.mRight = VALUE_NOT_SET;
...
}
measureChildHorizontal的分析就结束了。总结下，measureChildHorizontal最终会调用child.measure方法，传递的宽MeasureSpce是size=RelativeLayout宽减去margins、mode=AT_MOST，高MeasureSpce是size=RelativeLayout高减去margins、mode=AT_MOST。
接下来就是ImageView的onMeasure方法了。代码如下：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
resolveUri();
int w;
int h;
// Desired aspect ratio of the view's contents (not including padding)
float desiredAspect = 0.0f;
// We are allowed to change the view's width
boolean resizeWidth = false;
// We are allowed to change the view's height
boolean resizeHeight = false;
final int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
if (mDrawable == null) {
// If no drawable, its intrinsic size is 0.
mDrawableWidth = -1;
mDrawableHeight = -1;
w = h = 0;
} else {
w = mDrawableWidth;
h = mDrawableHeight;
if (w <= 0) w = 1;
if (h <= 0) h = 1;
// We are supposed to adjust view bounds to match the aspect
// ratio of our drawable. See if that is possible.
if (mAdjustViewBounds) {
resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;
resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;
desiredAspect = (float) w / (float) h;
}
}
final int pleft = mPaddingLeft;
final int pright = mPaddingRight;
final int ptop = mPaddingTop;
final int pbottom = mPaddingBottom;
int widthSize;
int heightSize;
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);
}
首先mDrawable == null不成立。
然后mAdjustViewBounds==true成立，mAdjustViewBounds==true是因为我们在xml中设置了android:adjustViewBounds="true"，这个会调用setAdjustViewBounds方法，该方法将mAdjustViewBounds标记为true。
然后我们传入的MeasureSpce的mode都不是Exactly，得到resizeWidth和resizeHeight都是true。
在resizeWidth或resizeHeight为true的情况下，会根据View的比例和图片比例重新计算width或高。这里我们720*1280比例是会导致width改变。当传入的heightMeasureSpce的size变小时width也就跟着变小了。根据前面的分析，当marginTop变大时，heightMeasureSpce的size会变小，width也变小。
代码分析到这里就结束了。如果觉得有写的不对的地方，希望能指出。