View-MeasuerSepc

View-MeasuerSepc

MeasuerSepc 是View 的一个内部类,在分析View 的测量(measuer)会出现这个类,这里先记录一下对MeasuerSepc的理解.

MeasuerSepc可以理解为 一个32位的int 值,其中前2位表示mode(模式),后30位表示size(具体大小),通过将mode和size体现在一个int值里面来避免过多的对象内存分配. MeasuerSepc同时提供了来makeMeasureSpec(int size,int mode) 来打包出来一个32位的int值.当然也有根据一个MeasuerSepc值来解包获取mode和size的方法.方便是getMode(int measureSpec)和getSize(int measureSpec) .

注意: MeasuerSepc代表的int值并不是MeasuerSepc自己本身.

具体源码看下面,红色部分是注释.(Android L的源码)

   
   
public static class MeasureSpec {
	private static final int MODE_SHIFT = 30;
	/**
	 * 11 0000...(30个0)
	 */
	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.父容器不对View做限制,想要多大就多大
	 */
	public static final int UNSPECIFIED = 0 << MODE_SHIFT;//00 000...(32个0)
	/**
	 * 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.父容器已经精确的检测出view的大小,View最终的大小就是由MeasureSpec的size决定.对应LayoutParams中     * 的match_parent和具体数值2种模式
	 */
	public static final int EXACTLY     = 1 << MODE_SHIFT;//01 0000...(30个0)
	/**
	 * Measure specification mode: The child can be as large as it wants up
	 * to the specified size.父容器给了一个最大值,View的大小不能超过这个值.它对应于LayoutParams中的warp_content.
	 */
	public static final int AT_MOST     = 2 << MODE_SHIFT;//10 0000...(30个0)
	public static int makeMeasureSpec(int size, int mode) {//创建一个MeasureSpec的int值:32位(前2位表示mode,后面30位表示size)
		if (sUseBrokenMakeMeasureSpec) {//JB之前的版本
			return size + mode;
		} else {
			return (size & ~MODE_MASK) | (mode & MODE_MASK);
		}
	}
	/**
	 * 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);
		if (mode == UNSPECIFIED) {
			// No need to adjust size for UNSPECIFIED mode.
			return makeMeasureSpec(0, UNSPECIFIED);
		}
		int size = getSize(measureSpec) + 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);
	}
//.......
}