View在测量时的MeasureSpec由什么决定?

 

我们都知道系统要确定View的大小,首先得先获得MeasureSpec,再通过MeasureSpec来决定View的大小。

MeasureSpec(32为int值)由两部分组成:

SpecMode(高2位):测量模式。

SpecSize(低30位):某种测量模式下的规格大小。

 

SpecMode有3类:

UNSPECIFIED: 父容器不对view做大小限制,一般用于系统内部,表示一种测量状态。

EXACTLY:精确模式。对应于:LayoutPrams中的match_parent和具体数值。

AT_MOST:最大值模式。对应于LayoutParam中的wrap_content模式。

 

那么问题来了,这个MeasureSpec又是由什么决定的呢?我们从源代码里面入手。

ViewGroup里面有一个方法,叫做measureChildWithMargins,用来测量子view的大小的。

/**
     * Ask one of the children of this view to measure itself, taking into
     * account both the MeasureSpec requirements for this view and its padding
     * and margins. The child must have MarginLayoutParams The heavy lifting is
     * done in getChildMeasureSpec.
     *
     * @param child The child to measure
     * @param parentWidthMeasureSpec The width requirements for this view
     * @param widthUsed Extra space that has been used up by the parent
     *        horizontally (possibly by other children of the parent)
     * @param parentHeightMeasureSpec The height requirements for this view
     * @param heightUsed Extra space that has been used up by the parent
     *        vertically (possibly by other children of the parent)
     */
    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);
    }

可以看出,在代码里,会先获取childWidthMeasureSpec和childHeightMeasureSpec,然后再测量子元素 child.measure(childWidthMeasureSpec, childHeightMeasureSpec);

我们关键是要看这两个MeasureSpec是怎么获取到的,通过方法getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);我们可以看出MeasureSpec的获取不但与子元素本身的LayoutParam有关,还与父容器的MeasureSpec有关,当然,也与它的padding,margin有关。
我们进去看看,这部分代码有点长,不过里面的逻辑很简单,我们直接在源代码里面通过加注释来分析。

/**
     * Does the hard part of measureChildren: figuring out the MeasureSpec to
     * pass to a particular child. This method figures out the right MeasureSpec
     * for one dimension (height or width) of one child view.
     *
     * The goal is to combine information from our MeasureSpec with the
     * LayoutParams of the child to get the best possible results. For example,
     * if the this view knows its size (because its MeasureSpec has a mode of
     * EXACTLY), and the child has indicated in its LayoutParams that it wants
     * to be the same size as the parent, the parent should ask the child to
     * layout given an exact size.
     *
     * @param spec The requirements for this view
     * @param padding The padding of this view for the current dimension and
     *        margins, if applicable
     * @param childDimension How big the child wants to be in the current
     *        dimension
     * @return a MeasureSpec integer for the child
     */
    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec); //获取父容器的测量模式
        int specSize = MeasureSpec.getSize(spec); //获取父容器在该测量模式下的大小

        int size = Math.max(0, specSize - padding);

        int resultSize = 0//子元素的specSize
        int resultMode = 0//子元素的specMode

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY: //如果父容器的测量模式为EXACTLY
            if (childDimension >= 0) { //如果子元素的LayoutParam为具体数值>=0
                resultSize = childDimension; //那么子元素的specSize就是childDimension
                resultMode = MeasureSpec.EXACTLY; //那么子元素的specMode是EXACTLY
//以下的分析都是一样的,也就不写了
}
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
 //这里View.sUseZeroUnspecifiedMeasureSpec一直都是false,因此resultSize==0;
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); }


由以上的分析可以看出子元素的MeasureSpec的获取不但与子元素本身的LayoutParam有关,还与父容器的MeasureSpec有关,当然,也与它的padding,margin有关。

其实我们可以得出下面的结论:

 

 

            parentMeasureSpec

 

childLayoutParam

EXACTLY

AT_MOST

UNSPECIFIED

具体数值

EXACTLLY

childSize

EXACTLY

childSize

EXACTLY

childSize

Match_parent

EXACTLY

parentSize

AT_MOST

parentSize

UNSPECIFIED

0

Wrap_content

AT_MOST

parentSize

AT_MOST

parentSize

UNSPECIFIED

0

 parentSize是父容器的剩余空间。

从上面的表格也可以看出,当我们直接继承view来实现自定义控件的时候,需要重写onMeasure方法并设置wrap_content时候的自身大小,不然使用wrap_content得时候就相当于使用match_parent。我们可以用下面的模板:

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSpecMode=MeasureSpec.getMode(widthMeasureSpec);
        int heightSpecMode=MeasureSpec.getMode(heightMeasureSpec);

        int widthSpecSize=MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecSize=MeasureSpec.getSize(heightMeasureSpec);

        if (widthSpecMode==MeasureSpec.AT_MOST && heightSpecMode==MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(100,100);
        }
        else if (widthSpecMode==MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(100,heightSpecSize);
        }
        else if (heightSpecMode==MeasureSpec.AT_MOST)
        {
            setMeasuredDimension(widthSpecSize,100);
        }
    }

上面代码中的100是我们自己设置的数值,你也可以用其他数值,根据自己需要。

 

以上便是对MeasureSpec由何决定的分析。

posted @ 2017-06-17 17:17 妖久 阅读(...) 评论(...) 编辑 收藏