Android中自定义View的onMeasure以及MeasureSpec使用

一般来说，自定义控件都会去重写View的onMeasure方法，因为该方法指定该控件在屏幕上的大小。

@Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {     ///your code    }

onMeasure传入的两个参数是由上一层控件传入的大小，有多种情况，重写该方法时需要对计算控件的实际大小，然后调用setMeasuredDimension(int, int)设置实际大小。

onMeasure传入的widthMeasureSpec和heightMeasureSpec不是一般的尺寸数值，而是将模式和尺寸组合在一起的数值。我们需要通过int mode = MeasureSpec.getMode(widthMeasureSpec)得到模式，用int size = MeasureSpec.getSize(widthMeasureSpec)得到尺寸。

mode共有三种情况，取值分别为MeasureSpec.UNSPECIFIED, MeasureSpec.EXACTLY, MeasureSpec.AT_MOST。

/**         * 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;

MeasureSpec.EXACTLY是精确尺寸，当我们将控件的layout_width或layout_height指定为具体数值时如andorid:layout_width="50dip"，或者为android:layout_width="fill_parent"，都是控件大小已经确定的情况，都是精确尺寸。

MeasureSpec.AT_MOST是最大尺寸，当控件的layout_width或layout_height指定为WRAP_CONTENT时，控件大小一般随着控件的子空间或内容进行变化，此时控件尺寸只要不超过父控件允许的最大尺寸即可。因此，此时的mode是AT_MOST，size给出了父控件允许的最大尺寸。

————也就是说此时通过 MeasureSpec.getSize(widthMeasureSpec)获取的size只是给出了允许的最大尺寸。

MeasureSpec.UNSPECIFIED是未指定尺寸，这种情况不多，一般都是父控件是AdapterView，通过measure方法传入的模式。

因此，在重写onMeasure方法时要根据模式不同进行尺寸计算。下面代码就是一种比较典型的方式：

@Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        if (mOrientation == HORIZONTAL) {            setMeasuredDimension(measureLong(widthMeasureSpec), measureShort(heightMeasureSpec));        } else {            setMeasuredDimension(measureShort(widthMeasureSpec), measureLong(heightMeasureSpec));        }    }    /**     * Determines the width of this view     *     * @param measureSpec     *            A measureSpec packed into an int     * @return The width of the view, honoring constraints from measureSpec     */    private int measureLong(int measureSpec) {        int result;        int specMode = MeasureSpec.getMode(measureSpec);        int specSize = MeasureSpec.getSize(measureSpec);        Log.d("TAG", "measureLong specSize="+specSize);        //此处由于android:layout_height="fill_parent"属性        //那么，specMode值为MeasureSpec.EXACTLY，        //specSize计算出的值为精确值，可以直接使用        if ((specMode == MeasureSpec.EXACTLY) || (mViewPager == null)) {            Log.d("TAG", "measureLong MeasureSpec.EXACTLY");            //We were told how big to be            result = specSize;        } else {            Log.d("TAG", "measureLong other");            //Calculate the width according the views count            final int count = mViewPager.getAdapter().getCount();            result = (int)(getPaddingLeft() + getPaddingRight()                    + (count * 2 * mRadius) + (count - 1) * mRadius + 1);            //Respect AT_MOST value if that was what is called for by measureSpec            if (specMode == MeasureSpec.AT_MOST) {                result = Math.min(result, specSize);            }        }        return result;    }    /**     * Determines the height of this view     *     * @param measureSpec     *            A measureSpec packed into an int     * @return The height of the view, honoring constraints from measureSpec     */    private int measureShort(int measureSpec) {        int result;        int specMode = MeasureSpec.getMode(measureSpec);        int specSize = MeasureSpec.getSize(measureSpec);        Log.d("TAG", "measureShort specMode="+specMode);        Log.d("TAG", "measureShort specSize="+specSize);        //此处由于android:layout_height="wrap_content"属性        //那么，specMode值为MeasureSpec.AT_MOST，        //specSize计算出的值为控件允许的最大值，记住，仅仅是最大允许的值，而非实际值。        if (specMode == MeasureSpec.EXACTLY) {            Log.d("TAG", "measureShort MeasureSpec.EXACTLY");            //We were told how big to be            result = specSize;        } else {            Log.d("TAG", "measureShort other");            //Measure the height            result = (int)(2 * mRadius + getPaddingTop() + getPaddingBottom() + 1);            //Respect AT_MOST value if that was what is called for by measureSpec            if (specMode == MeasureSpec.AT_MOST) {                result = Math.min(result, specSize);            }        }        return result;    }

该自定义View在xml中的布局如下：

    <com.viewpagerindicator.CirclePageIndicator        android:id="@+id/indicator"        android:padding="10dip"        android:layout_height="wrap_content"        android:layout_width="fill_parent"        />