MeasureSpec和LayoutParams

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LayoutParams和MeasureSpec

说到View的Measure就离不开MeasureSpec。那么MeasureSpec到底是什么呢？它与我们熟悉的LayoutParams有什么联系呢？

简单的说：

MeasureSpec是一个int值;

MeasureSpec决定了View的measuredWidth/Height；

父容器的MeasureSpec与子View的LayoutParams一同决定了子View的MeasureSpec。

下面我们详细看一下它们是什么。

LayoutParams

LayoutParams是View布局信息的载体，它包含了View应有的width、height等信息。一般来说，不同的ViewGroup都会实现自己的LayoutParams子类，用于支持额外的属性，比如margin、weight、gravity等。

当我们在xml文件里定义一个View时，我们为它指定的属性会被装载到AttributeSet传递给View，并在inflate的时候通过generateLayoutParams(AttributeSet)方法生成相应的LayoutParams。

我们也可以在代码中动态定义一个View，或者动态更新View的LayoutParams，像下面这样：

    TextView textView = new TextView(this);    textView.setText("TextView");    textView.setLayoutParams(      new LinearLayout.LayoutParams(            LinearLayout.LayoutParams.MATCH_PARENT,            LinearLayout.LayoutParams.MATCH_PARENT ) );

MeasureSpec

MeasureSpec直译为“测量规格“，它作为View的测量的约束条件，直接影响View的测量结果。为了减少对象分配，MeasureSpec被实现为一个32位int值，该int值由两部分组成：

0～29：SpecSize，规格尺寸；
30～31：SpecMode，测量模式。

    private static final int MODE_SHIFT = 30;    /**     * Measure specification mode: The parent has not imposed any constraint     * on the child. It can be whatever size it wants.     */    public static final int UNSPECIFIED = 0 << MODE_SHIFT;    /**     * Measure specification mode: The parent has determined an exact size     * for the child. The child is going to be given those bounds regardless     * of how big it wants to be.     */    public static final int EXACTLY     = 1 << MODE_SHIFT;    /**     * Measure specification mode: The child can be as large as it wants up     * to the specified size.     */    public static final int AT_MOST     = 2 << MODE_SHIFT;

如上所示，Measure中定义的测量模式共有UNSPECIFIED、EXACTLY以及AT_MOST三种。注释中对三种测量模式的描述很清楚，它们分别表示：

UNSPECIFIED：表示父容器不对子View做任何约束；
EXACTLY：表示父容器已经确定了子View的尺寸，子View的尺寸就是SpecSize；
AT_MOST：表示父容器为子View指定了一个最大尺寸SpecSize，子View最大不能超过该尺寸；

为了方便，Measure类中提供了数个静态方法用于从int值中分离SpecMode、SpecSize以及将SpecMode、SpecSize打包成一个int值。

LayoutParams与MeasureSpec的关系

现在，我们了解了LayoutParams和MeasureSpec，那么MeasureSpec是如何生成的呢？它与LayoutParams又有什么关系呢？

在View的测量过程中，父容器会结合自己的MeasureSpec以及子View的LayoutParams，生成子View的MeasureSpec，然后再在此MeasureSpec的约束下对子View进行测量。那么，父容器的MeasureSpec、子View的LayoutParams与生成的子View 的MeasureSpec是怎么的关系呢？

简单的说，父容器在自己的MeasureSpec和子View的LayoutParams的共同作用下生成子View的MeasureSpec，然后子View在MeasureSpec约束下进行自己的测量。该过程自然要发生在measure阶段，并且子View在父容器执行onMeasure时被测量，而ViewGroup没有提供onMeasure的实现（因为不同的ViewGroup的measure过程是不同的），但是ViewGroup提供了measureChildren方法，下面看一下measureChildren：

    protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {        final int size = mChildrenCount;        final View[] children = mChildren;        for (int i = 0; i < size; ++i) {            final View child = children[i];            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {                measureChild(child, widthMeasureSpec, heightMeasureSpec);            }        }    }

measureChildren方法简单的遍历子View，然后调用measureChild方法，下面看一下measureChild方法：

    protected void measureChild(View child, int parentWidthMeasureSpec,            int parentHeightMeasureSpec) {        final LayoutParams lp = child.getLayoutParams();        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,                mPaddingLeft + mPaddingRight, lp.width);        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,                mPaddingTop + mPaddingBottom, lp.height);        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);    }

measureChild方法里通过getChildMeasureSpec方法生成子View的MeasureSpec，然后调用子View的measure方法对子View进行测量，下面看一下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(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;        }        //noinspection ResourceType        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);    }

上面的方法的逻辑很清晰，就是根据specMode以及来自LayoutParams的childDimension生成相应的MeasureSpec。childDimension有三类值，分别是MATCH_PARENT、WRAP_CONTENT以及确定的尺寸值。总的来说，子View的specMode有下面的对应关系：

MATCH_PARENT WRAP_CONTENT 准确的尺寸值 EXACTLY EXACTLY AT_MOST AT_MOST AT_MOST AT_MOST UNSPECIFIED UNSPECIFIED UNSPECIFIED

在childDimension的值为MATCH_PARENT或者WRAP_CONTENT时，specSize的值为size（UNSPECIFIED模式为0或者size），也即父容器specSize的值减去padding。childDimension的值为准确的尺寸值时，specSize = childDimension。

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