要理解View的测量，首先要了解MeasureSpec，系统在测量view的宽高时，要先确定MeasureSpec。

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。

 

接下来我们看看View的onMeasure方法：

/**     * 
     * Measure the view and its content to determine the measured width and the     * measured height. This method is invoked by {
    @link #measure(int, int)} and     * should be overridden by subclasses to provide accurate and efficient     * measurement of their contents.     * 
     *protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));    }

该方法是由measure来调用的，view宽高的测量值最终是通过setMeasuredDimension来设置，具体实现不去深究。

我们看看getDefaultSize方法：

/**     * Utility to return a default size. Uses the supplied size if the     * MeasureSpec imposed no constraints. Will get larger if allowed     * by the MeasureSpec.     *     * @param size Default size for this view     * @param measureSpec Constraints imposed by the parent     * @return The size this view should be.     */    public static int getDefaultSize(int size, int measureSpec) {        int result = size;        int specMode = MeasureSpec.getMode(measureSpec);        int specSize = MeasureSpec.getSize(measureSpec);        switch (specMode) {        case MeasureSpec.UNSPECIFIED:            result = size;            break;        case MeasureSpec.AT_MOST:        case MeasureSpec.EXACTLY:            result = specSize;            break;        }        return result;    }

 

先从MeasureSpec中获取的测量模式specMode和该模式下的大小specSize，然后对测量模式进行判断，如果是UNSPECIFIED，那么结果就是参数size，对于这个size是什么等会讨论。如果是AT_MOST和EXACTLY，那么结果就是view测量后的大小specSize。

那么这个参数size到底是什么呢？它是由实参getSuggestedMinimumWidth()和getSuggestedMinimumHeight()传下来的，我们进去getSuggestedMinimumWidth()看看。

/**     * Returns the suggested minimum width that the view should use. This     * returns the maximum of the view's minimum width)     * and the background's minimum width     *  ({
    @link android.graphics.drawable.Drawable#getMinimumWidth()}).     * 
     * When being used in {
    @link #onMeasure(int, int)}, the caller should still     * ensure the returned width is within the requirements of the parent.     *     * @return The suggested minimum width of the view.     */    protected int getSuggestedMinimumWidth() {        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());    }

 

从代码我们可以看出如果该view设置了背景，那么就返回max(mMinWidth, mBackground.getMinimumWidth())，否则返回mMinWidth。

mMinWidth是什么呢？它是对应android：minWidth这个属性的值，若是不指定，那么默认为0.

max(mMinWidth, mBackground.getMinimumWidth())：返回mMinWidth和mBackground.getMinimumWidth()两者中的最大值。

mBackground.getMinimumWidth()的代码如下：

/**     * Returns the minimum width suggested by this Drawable. If a View uses this     * Drawable as a background, it is suggested that the View use at least this     * value for its width. (There will be some scenarios where this will not be     * possible.) This value should INCLUDE any padding.     *     * @return The minimum width suggested by this Drawable. If this Drawable     *         doesn't have a suggested minimum width, 0 is returned.     */    public int getMinimumWidth() {        final int intrinsicWidth = getIntrinsicWidth();        return intrinsicWidth > 0 ? intrinsicWidth : 0;    }

它返回的是Drawable的原始宽度，如果Drawable有原始宽度的话。否则返回0.

 

因此若view有设置背景，那么size就是android：minWidth和背景最小宽度这两者的最大值，若没有设置背景，那么size的值就是android：minWidth的值。