當xml是這樣時：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="300dp"
        android:layout_marginTop="0dp"
        android:adjustViewBounds="true"
        android:background="#ff0000"
        android:src="@drawable/ic_720_1280"/>

</RelativeLayout>

效果是：

Screenshot_1525569407.png

將ImageView的marginTop改成150dp后，效果是：

Screenshot_1525569332.png

下面是自己看源碼的分析：
首先是RelativeLayout的onMeasure方法：

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        
        棉安。。铸抑。
        
        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;
                }
            }
        }

        贡耽。。鹊汛。

    }

在onMeasure中通過measureChildHorizontal計算出child的寬蒲赂，順便說下myWidth和myHeight是屏幕寬高。measureChildHorizontal的代碼如下：

    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中最后通過child.measure()測量了child的寬和高刁憋。測量高用的MeasureSpec的size是RelativeLayout去除margin后的高度滥嘴。所以這里使用不同的marginTop會得到不同的高。測量寬用的MeasureSpec通過getChildMeasureSpec獲得至耻。getChildMeasureSpec的代碼如下：

    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若皱、childEnd == VALUE_NOT_SET成立镊叁，這段代碼最后生成的MeasureSpce的size是去除margin的mySize，而mySize是傳入的屏幕寬走触。
這里的childStart == VALUE_NOT_SET是因為measureChildHorizontal之前執(zhí)行了applyHorizontalSizeRules晦譬，applyHorizontalSizeRules將layoutParams的mLeft和mRight都改成了VALUE_NOT_SET，applyHorizontalSizeRules代碼如下：

    private void applyHorizontalSizeRules(LayoutParams childParams, int myWidth, int[] rules) {
        ...

        childParams.mLeft = VALUE_NOT_SET;
        childParams.mRight = VALUE_NOT_SET;

        ...
    }

measureChildHorizontal的分析就結(jié)束了饺汹』滋恚總結(jié)下，measureChildHorizontal最終會調(diào)用child.measure方法兜辞，傳遞的寬MeasureSpce是size=RelativeLayout寬減去margins迎瞧、mode=AT_MOST，高MeasureSpce是size=RelativeLayout高減去margins逸吵、mode=AT_MOST凶硅。
接下來就是ImageView的onMeasure方法了。代碼如下：

    @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不成立扫皱。
然后mAdjustViewBounds==true成立足绅，mAdjustViewBounds==true是因為我們在xml中設(shè)置了android:adjustViewBounds="true"，這個會調(diào)用setAdjustViewBounds方法韩脑，該方法將mAdjustViewBounds標記為true氢妈。
然后我們傳入的MeasureSpce的mode都不是Exactly，得到resizeWidth和resizeHeight都是true段多。
在resizeWidth或resizeHeight為true的情況下首量，會根據(jù)View的比例和圖片比例重新計算width或高。這里我們720*1280比例是會導(dǎo)致width改變进苍。當傳入的heightMeasureSpce的size變小時width也就跟著變小了加缘。根據(jù)前面的分析，當marginTop變大時觉啊，heightMeasureSpce的size會變小拣宏，width也變小。

代碼分析到這里就結(jié)束了杠人。如果覺得有寫的不對的地方勋乾，希望能指出。