MeasureSpec字面意思可以理解为“测量规格”，在测量过程中，系统会将View的LayoutParams根据父容器的规格转换生成相应的MeasureSpec，然后根据这个MeasureSpec测量出view的尺寸。也就是说，view的MeasureSpec是由自己的LayoutParams和父容器所给的MeasureSpec共同决定的。

MeasureSpec

• MeasureSpec封装了从父级传递给子级的布局要求。 每个MeasureSpec代表宽度或高度的要求。
• MeasureSpecs实现为32位的int，由size和mode组成，高2位代表mode，低30位代表size，它通过将mode和size打包成一个int值来减少对象内存分配，并提供打包和解包的方法。
• mode分类：
  • UNSPECIFIED：父容器不对View有任何限制，给它想要的任何尺寸。一般用于系统内部，表示一种测量状态。
  • EXACTLY：父容器已经检测出view的精确大小，这时候view的大小就是size所指定的值。它对应于LayoutParams中的match_parent和具体数值两种模式。
  • AT_MOST：父容器指定了一个可用大小，即size，子view的大小不能大于这个值，具体值要看vew的实现。它对应于LayoutParams中的wrap_content。

    public static class MeasureSpec {
        private static final int MODE_SHIFT = 30;
        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

        /** @hide */
        @IntDef({UNSPECIFIED, EXACTLY, AT_MOST})
        @Retention(RetentionPolicy.SOURCE)
        public @interface MeasureSpecMode {}

        public static final int UNSPECIFIED = 0 << MODE_SHIFT;

        public static final int EXACTLY     = 1 << MODE_SHIFT;

        public static final int AT_MOST     = 2 << MODE_SHIFT;

        public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
                                          @MeasureSpecMode int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }

        public static int makeSafeMeasureSpec(int size, int mode) {
            if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
                return 0;
            }
            return makeMeasureSpec(size, mode);
        }

        @MeasureSpecMode
        public static int getMode(int measureSpec) {
            //noinspection ResourceType
            return (measureSpec & MODE_MASK);
        }

        public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }

        static int adjust(int measureSpec, int delta) {
            final int mode = getMode(measureSpec);
            int size = getSize(measureSpec);
            if (mode == UNSPECIFIED) {
                // No need to adjust size for UNSPECIFIED mode.
                return makeMeasureSpec(size, UNSPECIFIED);
            }
            size += delta;
            if (size < 0) {
                Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size +
                        ") spec: " + toString(measureSpec) + " delta: " + delta);
                size = 0;
            }
            return makeMeasureSpec(size, mode);
        }
    }

MeasureSpec与LayoutParams的关系

普通view的measure参数是由父容器传递而来，这里我们可以看下ViewGroup的measureChildWithMargins方法：

    /**
    * @param child 要测量的子view
    * @param parentWidthMeasureSpec 该容器对子view的宽度要求
    * @param widthUsed 父级水平使用的额外空间（可能是父级的其他子级），可以理解为已经被占用的空间
    * @param parentHeightMeasureSpec 该容器对子view的高度要求
    * @param heightUsed 父级垂直使用的额外空间（可能是父级的其他子级）
    */
    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);
    }

可以看到子view的测量参数是由getChildMeasureSpec方法得到的，它和父容器（即该viewgroup）对子view的测量要求、父容器的pandding、子view的margin、父容器的已占空间都由关系：

    /**
    * @param spec 该容器的测量要求
    * @param padding 可以简单理解为子view和该容器之间的间隔，由多个值决定
    * @param childDimension 子view的期望值，即子view的LayoutParams的宽高
    */
    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);
    }

该方法主要就是前面提到的，根据父容器的测量规格，将View的LayoutParams转换生成一个MeasureSpec。上述代码可以简单翻译成下面这个图表：