截取MeasureSpec类的源代码出来看看

 /**
     * A MeasureSpec encapsulates the layout requirements passed from parent to child.
     * Each MeasureSpec represents a requirement for either the width or the height.
     * A MeasureSpec is comprised of a size and a mode. There are three possible
     * modes:
     * <dl>
     * <dt>UNSPECIFIED</dt>
     * <dd>
     * The parent has not imposed any constraint on the child. It can be whatever size
     * it wants.
     * </dd>
     *
     * <dt>EXACTLY</dt>
     * <dd>
     * 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.
     * </dd>
     *
     * <dt>AT_MOST</dt>
     * <dd>
     * The child can be as large as it wants up to the specified size.
     * </dd>
     * </dl>
     *
     * MeasureSpecs are implemented as ints to reduce object allocation. This class
     * is provided to pack and unpack the &lt;size, mode&gt; tuple into the int.
     */
    public static class MeasureSpec {
        private static final int MODE_SHIFT = 30;
        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

        /**
         * 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;

        /**
         * Creates a measure specification based on the supplied size and mode.
         *
         * The mode must always be one of the following:
         * <ul>
         *  <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li>
         *  <li>{@link android.view.View.MeasureSpec#EXACTLY}</li>
         *  <li>{@link android.view.View.MeasureSpec#AT_MOST}</li>
         * </ul>
         *
         * <p><strong>Note:</strong> On API level 17 and lower, makeMeasureSpec's
         * implementation was such that the order of arguments did not matter
         * and overflow in either value could impact the resulting MeasureSpec.
         * {@link android.widget.RelativeLayout} was affected by this bug.
         * Apps targeting API levels greater than 17 will get the fixed, more strict
         * behavior.</p>
         *
         * @param size the size of the measure specification
         * @param mode the mode of the measure specification
         * @return the measure specification based on size and mode
         */
        public static int makeMeasureSpec(int size, int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }

        /**
         * Like {@link #makeMeasureSpec(int, int)}, but any spec with a mode of UNSPECIFIED
         * will automatically get a size of 0. Older apps expect this.
         *
         * @hide internal use only for compatibility with system widgets and older apps
         */
        public static int makeSafeMeasureSpec(int size, int mode) {
            if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
                return 0;
            }
            return makeMeasureSpec(size, mode);
        }

        /**
         * Extracts the mode from the supplied measure specification.
         *
         * @param measureSpec the measure specification to extract the mode from
         * @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
         *         {@link android.view.View.MeasureSpec#AT_MOST} or
         *         {@link android.view.View.MeasureSpec#EXACTLY}
         */
        public static int getMode(int measureSpec) {
            return (measureSpec & MODE_MASK);
        }

        /**
         * Extracts the size from the supplied measure specification.
         *
         * @param measureSpec the measure specification to extract the size from
         * @return the size in pixels defined in the supplied measure specification
         */
        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);
        }

        /**
         * Returns a String representation of the specified measure
         * specification.
         *
         * @param measureSpec the measure specification to convert to a String
         * @return a String with the following format: "MeasureSpec: MODE SIZE"
         */
        public static String toString(int measureSpec) {
            int mode = getMode(measureSpec);
            int size = getSize(measureSpec);

            StringBuilder sb = new StringBuilder("MeasureSpec: ");

            if (mode == UNSPECIFIED)
                sb.append("UNSPECIFIED ");
            else if (mode == EXACTLY)
                sb.append("EXACTLY ");
            else if (mode == AT_MOST)
                sb.append("AT_MOST ");
            else
                sb.append(mode).append(" ");

            sb.append(size);
            return sb.toString();
        }
    }

  

       这里我介绍一点点MeasureSpec类的知识，源码是这样注释的：

A MeasureSpec encapsulates the layout requirements passed from parent to child.

       意思是说，一个MeasureSpec封装了从parent view传递到 child view 的布局要求。

       在自定义view 的时候，有时候需要重写onMeasure(int widthMeasureSpec, int heightMeasureSpec)方法。事实上参数widthMeasureSpec就是父布局传递过来的一个MeasureSpec

       比如onMeasure的参数widthMeasureSpec这个值由高2位和低30位组成，高2位保存的值叫specMode，可以通过MeasureSpec.getMode()获取；低30位为specSize，同样可以由MeasureSpec.getSize()获取。

我是如何知道高2位和低30位这些信息的呢？请看如下代码：

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

获取Mode的时候，直接用一个与的位运算来得到Mode，再看MODE_MASK的定义：

private static final int MODE_SHIFT = 30;
private static final int MODE_MASK  = 0x3 << MODE_SHIFT;