理解MeasureSpec

Posted 2023-01-11 呼啸

MesaureSpec是View的内部类。它封装了一个View的规格尺寸。包括View的宽和高的信息。它的作用就是，在Measure的过程中，系统会将View的LayoutParams根据父容器所施加的规则转换成对应的MeasureSpec，然后在onMeasure方法中根据这个MeasureSpec来确定View的宽和高。

我们来看下MeasureSpec类的代码：

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 

        /**
         * 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(@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);
            
        

        /**
         * 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
         */
        @UnsupportedAppUsage
        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
         */
        @MeasureSpecMode
        public static int getMode(int measureSpec) 
            //noinspection ResourceType
            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();
        
    

从MeasureSpec的常量可以看出，它代表了32位的int值，其中高2位代表了specMode,低30位则代表了specSize。specMode指的是测量模式，specSize指的是测量大小。

specMode有3种模式，如下：

UNSPECIFIED:未指定模式，View想多大就多大。父容器不做限制。一般用于系统内部的测量。

AT_MOST:最大模式，对应于wrap_content属性，子View的最终大小是父View指定的specSize值，并且子View的大小不能大于这个值。

EXACTLY:精确模式。对应于match_parent属性和具体的数值，父容器测量出View所需要的大小，也就是specSize的值。

对于每一个View,都持有一个MeasureSpec,而该MeasureSpec则保存了该View的尺寸规格。在View的测量流程中，通过makeMeasureSpec来保存宽和高的信息。通过getMode或getSize得到模式和宽、高。MesaureSpec是受自身LayoutParams和父容器的MeasureSpect共同影响。那么作为顶层View的DecorView来说，它并没有父容器，那么它的MeasureSpec是如何得来的呢？我们再回头看ViewRootImpl的performTraversals方法：

private void performTraversals()
	...
    if(!mStopped)
		int childWidthMeasureSpec = getRootMeasureSpec(mWidth,lp.width);
		int childHeightMeasureSpec = getRootMeasureSpec(mHeight,lp.height);
		performLayout(lp,desiredWindowWidth,desiredWindowHeight);
    


   if(didLayout) 
	 performLayout(lp,desiredWindowWidth,desiredWindowHeight);
	 ..
   
   if(!cancelDraw && !newSureface) 
		if(!skipDraw || mReportNextDraw) 
			if(mPendingTransition != null && mPendingTransitions.size > 0) 
				for(int i=0; i< mPendingTransitions.size;++i) 
					mPendingTransitions.get(i).startChangingAimations();
				
				mPendingTransitions.clear();
			
			performDraw();
		
   
   ...

看里面这句：

int childWidthMeasureSpec = getRootMeasureSpec(mWidth,lp.width);

这里调用了getRootMearueSpec方法。下面来勘察这个方法做了什么：

 private static int getRootMeasureSpec(int windowSize, int rootDimension) 
        int measureSpec;
        switch (rootDimension) 

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        
        return measureSpec;
    

第一个参数windowSize指的是窗口的尺寸。所以对于DecorView来说，它的MeasureSpec由自身的LayoutParams和窗口的尺寸决定。这一点和普通的View不同。还记得普通View是怎么决定的吗？普通View是由自身的LayoutParams和父容器的MeasureSpec共同决定。我们接着看下面的代码就会看到DecorView根据自身的layoutParams来得到不同的MeasureSpec。所以，我们前面说的performMeasure方法中需要传入两个参数，即childWidthMeasureSpec和childHeightMeasureSpec所代表的意思就是chid的MesaureSpec的宽和高。接着回到performTraversals方法，看看perfromMeasure方法内部做了什么：

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) 
        if (mView == null) 
            return;
        
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try 
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
         finally 
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        
    

这里可以很明显的看到，调用了View的measure方法。