FrameLayout绘制 源码详解

一直以来都想搞清楚Android中View控件的绘制过程，怎奈自己以前能力不济再加上懒惰，自制力欠缺导致这件事情一直处于半将半就的状态。模模糊糊的了解到绘制分为measure测量、layout布局、draw绘制三步，但是针对其中涉及到的一些点一直有些恍惚，像方法measure()和onMeasure()、layout()和onLayout()、draw()和onDraw()相互之间的关系，它们各自的职责以及相互是如何配合的。其中尤以onMeasure在父子之间的调用顺序最让自己疑惑，今天通过分析FragmentLayout的绘制过程，让我们将这些疑问统统解决掉。也许你会问，为什么是FrameLayout?因为我觉得他最简单，是个软柿子，要怎滴！哈哈，言归正传！

这里有一点需要强调一下：分析的源码是Android 6.0的（API 23），如果跟您手边的源码有出入，大概是版本不同导致的！毕竟，Google在一直重构、优化Android的SDK嘛！

首先看一下FrameLayout的继承关系

FrameLayout –> ViewGroup –> View

onMeasure过程分析

在分析FrameLayout的onMeasure之前，我们先陈述一些基础知识点：我们知道跟一个控件尺寸相关的属性有(这里一height举例)layout_height、maxHeight、minHeight等，在这里我们都知道layout_height有三种可能的取值：MATCH_PARENT=-1、WRAP_CONTENT=-2和具体的数值，相信具体的含义大家都知道；还有onMeasure的int型参数高2位代表measureMode,低30位代表measureSize，而measureMode有三种可能的取值：MeasureSpec.EXACTLY、MeasureSpec.AT_MOST、MeasureSpec.UNSPECIFIED，MeasureSpec.EXACTLY代表子控件应该严格按照measureSize大小赋值，MeasureSpec.AT_MOST代表子控件最大为measureSize的大小而MeasureSpec.UNSPECIFIED代表不做任何限制。现在看来还一目了然，等到父控件套子控件，子控件再套孙子控件，父、子、孙子的layout_height再赋予不同的值得时候，是父控件先确定measure结果，还是子控件先确定measure结果，父子之间如何协商、onMeasure调用几次、layout_margin、padding、minHeight和maxHeight如何起作用？等等问题袭来，真是一团乱麻，下面从对FrameLayout的解剖中探寻究竟吧！

onMeasure(int widthMeasureSpec, int heightMeasureSpec)源码(==代码的说明在代码的注释中==)：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        int count = getChildCount();        final boolean measureMatchParentChildren =                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;//是否需要重新测绘属性为MATCH_PARENT的子控件，在下面会用到        mMatchParentChildren.clear();        int maxHeight = 0;        int maxWidth = 0;        int childState = 0;        //首先，将父控件的padding和子控件的margin考虑在内，先来测绘子控件的大小，measureChildWithMargins函数会在下边展开        for (int i = 0; i < count; i++) {            final View child = getChildAt(i);            if (mMeasureAllChildren || child.getVisibility() != GONE) {                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);                final LayoutParams lp = (LayoutParams) child.getLayoutParams();                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());                if (measureMatchParentChildren) {                    if (lp.width == LayoutParams.MATCH_PARENT ||                            lp.height == LayoutParams.MATCH_PARENT) {                        mMatchParentChildren.add(child);                    }                }            }        }        // Account for padding too        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();        // Check against our minimum height and width        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());        // Check against our foreground's minimum height and width        final Drawable drawable = getForeground();        if (drawable != null) {            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());        }        //从子控件的最大值（子控件的大小加上padding和foregroundPadding综合考虑的大小），背景的最小值、前景的最小值和控件的最小值属性综合考虑，取得其中的最大值，再考虑到父控件的measureSpec得到父控件也就是FrameLayout的大小。resolveSizeAndState在下边展开        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),                resolveSizeAndState(maxHeight, heightMeasureSpec,                        childState << MEASURED_HEIGHT_STATE_SHIFT));        //如果需要重新测量属性为match_parent的子控件则重新测量        count = mMatchParentChildren.size();        if (count > 1) {            for (int i = 0; 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(0, 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(0, 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);            }        }    }

ViewGroup.measureChildWithMargins源码

protected void measureChildWithMargins(View child,            int parentWidthMeasureSpec, int widthUsed,            int parentHeightMeasureSpec, int heightUsed) {        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();        //根据父控件的padding和子控件margin以及父控件的measureSpec和子控件的lp.width决定子控件的measureSpec，getChildMeasureSpec会在下边展开        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);        //进行子控件的测量工作        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);    }

ViewGroup.getChildMeasureSpec的源码

/***这个函数有几个遵从的原则，或许能帮助更好的理解代码：1、如果子控件指定了具体的大小则MeasureSpec的模式为EXACTLY值为指定大小；2、如果子控件的需求跟父控件的需求有冲突则以父控件为准；*/public static int getChildMeasureSpec(int spec, int padding, int childDimension) {        int specMode = MeasureSpec.getMode(spec);        int specSize = MeasureSpec.getSize(spec);        int size = Math.max(0, specSize - padding);        int resultSize = 0;        int resultMode = 0;        switch (specMode) {        // Parent has imposed an exact size on us        case MeasureSpec.EXACTLY:            if (childDimension >= 0) {                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 >= 0) {                // 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 >= 0) {                // 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 ? 0 : 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 ? 0 : size;                resultMode = MeasureSpec.UNSPECIFIED;            }            break;        }        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);    }

View.resolveSizeAndState源码

/***还是需求跟供给冲突了，以供给为准（FrameLayout想要的大小跟measureSpec冲突，以measureSpec为准）*/public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {        final int specMode = MeasureSpec.getMode(measureSpec);        final int specSize = MeasureSpec.getSize(measureSpec);        final int result;        switch (specMode) {            case MeasureSpec.AT_MOST:                if (specSize < size) {                    result = specSize | MEASURED_STATE_TOO_SMALL;                } else {                    result = size;                }                break;            case MeasureSpec.EXACTLY:                result = specSize;                break;            case MeasureSpec.UNSPECIFIED:            default:                result = size;        }        return result | (childMeasuredState & MEASURED_STATE_MASK);    }

onLayout过程分析

onMeasure测量出控件自身的大小，而onLayout通过调用子控件的layout决定子控件的位置和大小(这里的大小是控件最终呈现的状态，一般就是控件测量的大小)，onLayout的过程跟FrameLayout的gravity和子控件的layout_gravity属性有关，下边看一下FrameLayout的onLayout过程：

onLayout(boolean changed, int left, int top, int right, int bottom源码

/***这里需要明确一下坐标系，子控件的所在的坐标系是由父控件确定的。拿这里举例，FrameLayout的onLayout的参数值，是以FrameLayout的父控件左上角为坐标原点的坐标系中的值。而接下调用child.layout确定FrameLayout的子控件的位置和大小的函数中的参数值是以FrameLayout左上角为原点的坐标系中的值*/protected void onLayout(boolean changed, int left, int top, int right, int bottom) {        layoutChildren(left, top, right, bottom, false /* no force left gravity */);    }

layoutChildren源码

void layoutChildren(int left, int top, int right, int bottom,                                  boolean forceLeftGravity) {        final int count = getChildCount();        final int parentLeft = getPaddingLeftWithForeground();        final int parentRight = right - left - getPaddingRightWithForeground();        final int parentTop = getPaddingTopWithForeground();        final int parentBottom = bottom - top - getPaddingBottomWithForeground();        for (int i = 0; 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 == -1) {                    gravity = DEFAULT_CHILD_GRAVITY;                }                //布局是从右到左还是从左到右                final int layoutDirection = getLayoutDirection();                //将布局中的start、end转化为left、right                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);                final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {                    case Gravity.CENTER_HORIZONTAL:                        childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +                        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) / 2 +                        lp.topMargin - lp.bottomMargin;                        break;                    case Gravity.BOTTOM:                        childTop = parentBottom - height - lp.bottomMargin;                        break;                    default:                        childTop = parentTop + lp.topMargin;                }                child.layout(childLeft, childTop, childLeft + width, childTop + height);            }        }    }

总结

FrameLayout是几大布局中最简单的一个，比较适合入门。通过分析FrameLayout对控件的绘制过程有个大概的了解。其他Layout的measure过程可能更复杂，需要更多的父子控件的协商。