通过源码分析View的测量

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：

/**
     * <p>
     * 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.
     * </p>
     *
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()}).
     * <p>
     * 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。