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View在测量时的MeasureSpec由什么决定?

时间:2017-6-18 8:24:20  作者:06评比  来源:06评比  查看:9906  评论:0
内容摘要: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。 ...
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。

复制代码
/**
     * 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);
    }
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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。

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/**
     * 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);
    }
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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。

 

以上便是对MeasureSpec由何决定的分析。

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