FrameLayout的onMearsure()与onLayout()源码浅析

前言

之前一直对View绘制过程只知道个大概,最近看了扔物线的视频,突然来了兴趣,于是找了一个比较简单的ViewGroup来分析下源码,巩固下理解。

onMeasure

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();//子View的数量

        //①判断是否需要再次测量子View
        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear();

        int maxHeight = ;//子View占的最大高度
        int maxWidth = ;//子View占的最大宽度
        int childState = ;

        for (int i = ; i < count; i++) {
            final View child = getChildAt(i);
            //如果子View的Visibility属性不为GONE,或者mMeasureAllChildren为True则测量子View的宽高
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                //②开始测量子View的宽高与MeasureMode
                measureChildWithMargins(child, widthMeasureSpec, , heightMeasureSpec, );
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                //带上Margin计算出子View占的最大宽度与最大高度
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);

                childState = combineMeasuredStates(childState, child.getMeasuredState());
                //如果测量被测量的子ViewLayoutParams为Match_Parent,添加这些子View到需要重新测量的子View集合
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

        // Account for padding too FrameLayout最后的宽高还需要加上间距值
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width ,比较当前FrameLayout的宽高是否小于用户设置的最小值
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        ...

        //③根据最后计算出的最大宽高,设置FrameLayout的宽高
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));


        count = mMatchParentChildren.size();
        if (count > ) {
            //根据LayoutParams的宽高是否为Match_Parent与重新计算过后确定的FrameLayout的宽高,重新计算子View宽高
            for (int i = ; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }

                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

分析

①判断是否需要再次测量Match_parent的子View

final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;

这里先判断下FrameLayout本身的MeasureSpec是否都是EXACTLY,精确模式那么FrameLayout的宽高都是确定了的,子View在测量时是Match_Parent,子View的的MeasureSpec也会变成EXACTLY,这样就没有再重新测量子View的必要了。可以看下ViewGroup中的getChildMeasureSpec()方法。

case MeasureSpec.EXACTLY:
            if (childDimension >= ) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

②开始测量子View的宽高与MeasureMode

measureChildWithMargins(child, widthMeasureSpec, , heightMeasureSpec, );

我们现在具体来看

protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

可以看到这里就是把Margin和Padding加起来再调用getChildMeasureSpec()方法

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(, specSize - padding);

        int resultSize = ;
        int resultMode = ;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= ) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= ) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= ) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ?  : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ?  : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

getChildMeasureSpec()这个方法就是根据父View的MesasureSpec来确定FrameLayout期望子View的MearsureSpec,这部分相关很多博客都说的很好,我就不重复了。

疑惑思考

onMeasure()代码里面看到Match_Parent的子View再FrameLayout不是EXACTIVITY的测量模式时计算了两次,看代码的时候一直没有想通,为什么需要计算两次呢?

看着代码想了很久,分别看了FramLayout的几种情况,最后发现,如果Framelayout的MeasureMode是AT_MOST的话,第一轮测量子View时,全部子View的Size的最大值还不能确定是多少,所以需要等到一轮测量完成后,FrameLayout的宽高确定了,最后才能给LayoutParams为Match_Parent的子View重新算出宽高。

onLayout

FrameLayout中就一个方法layoutChildren()

@Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        layoutChildren(left, top, right, bottom, false /* no force left gravity */);
    }

具体来分析下layoutChildren:

void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
        final int count = getChildCount();

        //算出各未知的的padding
        final int parentLeft = getPaddingLeftWithForeground();
        final int parentRight = right - left - getPaddingRightWithForeground();

        final int parentTop = getPaddingTopWithForeground();
        final int parentBottom = bottom - top - getPaddingBottomWithForeground();

    //和onMeasure类似,循环确定各子View的位置并调用子View的Layout方法
        for (int i = ; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                final int width = child.getMeasuredWidth();
                final int height = child.getMeasuredHeight();

                int childLeft;
                int childTop;

                int gravity = lp.gravity;
                if (gravity == -) {
                    gravity = DEFAULT_CHILD_GRAVITY;
                }

                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

                //确定子View的Left与Top的坐标
                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                //①这里可能稍微有点抽象需要多想想
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) /  +
                        lp.leftMargin - lp.rightMargin;
                        break;
                    case Gravity.RIGHT:
                        if (!forceLeftGravity) {
                            childLeft = parentRight - width - lp.rightMargin;
                            break;
                        }
                    case Gravity.LEFT:
                    default:
                        childLeft = parentLeft + lp.leftMargin;
                }

                switch (verticalGravity) {
                    case Gravity.TOP:
                        childTop = parentTop + lp.topMargin;
                        break;
                    case Gravity.CENTER_VERTICAL:
                        childTop = parentTop + (parentBottom - parentTop - height) /  +
                        lp.topMargin - lp.bottomMargin;
                        break;
                    case Gravity.BOTTOM:
                        childTop = parentBottom - height - lp.bottomMargin;
                        break;
                    default:
                        childTop = parentTop + lp.topMargin;
                }

                //知道Left和Top加上宽高就能推出四个点的坐标了
                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }

分析

①这里可能稍微有点抽象需要多想想,特别是要求子View处于父View中心位置时。

case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) /  +
                        lp.leftMargin - lp.rightMargin;
                        break;

直接看这个可能想象起来可能有点复杂,所以我们要先拆一下

先假定父View的左边位置parentLeft是0,

(parentRight-parentLeft-width)/2=(parentRight-parentLeft)-width/2

(parentRight-parentLeft):求出ParentView的中点

(parentRight-parentLeft)-width/2:减去子View宽度的一半就是子View左边的位置

(parentRight-parentLeft)-width/2+lp.leftMargin - lp.rightMargin:

求出子View左边的位置加上Margin后的位置

parentLeft + (parentRight-parentLeft)-width/2+lp.leftMargin - lp.rightMargin,加上父View的左侧的位置就是子View最后的位置

后记

由于个人水平有限,可能某些地方分析的不太好或者有错误,特别是有很多地方用到了位操作的地方还是没看的太明白,希望大家多多理解,给予指正。

参考资料

HenCode

FrameLayout的onMearsure()与onLayout()源码浅析

前言

onMeasure

分析

①判断是否需要再次测量Match_parent的子View

②开始测量子View的宽高与MeasureMode

疑惑思考

onLayout

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