MeasureSpec

文章大部分內容引自Android開發藝術探索

MeasureSpec

1. 系統內部是通過MeasureSpec來進行view的測量，但是正常情況下我們無法使用它，但是我們可以使用LayoutParams，在view測量的時候，系統會將LayoutParams在父容器的約束下轉換成對應的MeasureSpec,然後再根據這個MeasureSpec確定view測量後的寬高。(也就是說子元素的MeasureSpec是又父容器傳遞的，是有自身的LayoutParams有父容器的MeasureSpec共同決定的)

2. 對於頂級View（DecorView）其MeasureSpec由自身的LayoutParams和窗口的尺寸共同決定；但是對於普通View(我們佈局中的view)來說，其measure過程由viewGroup傳遞而來(measureChildWithMargins())，viewGroup會在其onMeasure()方法中調用measureChildWithMargins()

   /**
    * sdk版本29
    * Ask one of the children of this view to measure itself, taking into
    * account both the MeasureSpec requirements for this view and its padding
    * and margins. The child must have MarginLayoutParams The heavy lifting is
    * done in getChildMeasureSpec. 大部分工作在getChildMeasureSpec中完成
    *
    * @param child The child to measure
    * @param parentWidthMeasureSpec The width requirements for this view
    * @param widthUsed Extra space that has been used up by the parent
    *        horizontally (possibly by other children of the parent)
    * @param parentHeightMeasureSpec The height requirements for this view
    * @param heightUsed Extra space that has been used up by the parent
    *        vertically (possibly by other children of the parent)
    */
   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);
   }
   

   可以看出上述方法會對子元素進行measure，在調用子元素的measure之前會先通過getChildMeasureSpec方法來得到子元素的MeasureSpec。從代碼上驗證又可以得出子元素的MeasureSpec的創建與父容器的MeasureSpec和子元素本身的LayoutParams有關，此外還和view的padding和margin有關。

   /**
    * 測量孩子最重要的部分，計算出子元素的MeasureSpec並傳遞給它，此方法表明了一個子view的寬或高的合適的尺寸
    * Does the hard part of measureChildren: figuring out the MeasureSpec to
    * pass to a particular child. This method figures out the right MeasureSpec
    * for one dimension (height or width) of one child view.
    *
    * 目標是從父元素自身的MeasureSpec和子元素的LayoutParams來得出最合適的結果(子元素的MeasureSpec)
    * The goal is to combine information from our MeasureSpec with the
    * LayoutParams of the child to get the best possible results. For example,
    * if the this view knows its size (because its MeasureSpec has a mode of
    * EXACTLY), and the child has indicated in its LayoutParams that it wants
    * to be the same size as the parent, the parent should ask the child to
    * layout given an exact size.
    *
    * @param spec The requirements for this view
    * @param padding The padding of this view for the current dimension and
    *        margins, if applicable
    * @param childDimension How big the child wants to be in the current
    *        dimension
    * @return a MeasureSpec integer for the child
    */
   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);//父元素中剩餘的尺寸
       int resultSize = 0;
       int resultMode = 0;
       switch (specMode) {
       // Parent has imposed an exact size on us，父元素是MATCH_PARENT或者確定的尺寸
       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;//充滿父佈局
               resultMode = MeasureSpec.EXACTLY;
           } else if (childDimension == LayoutParams.WRAP_CONTENT) {
               // Child wants to determine its own size. It can't be
               // bigger than us.  子元素是WRAP_CONTENT是，子元素的尺寸就是父元素剩餘的所有尺寸，也就是說會  			充滿父佈局，這就是爲什麼我們自定義view時需要處理WRAP_CONTENT來達到自適應尺寸
               resultSize = size;
               resultMode = MeasureSpec.AT_MOST;
           }
           break;
       // Parent has imposed a maximum size on us，父元素是WRAP_CONTENT
       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;//同上
               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;//同上
               resultMode = MeasureSpec.AT_MOST;
           }
           break;
       // Parent asked to see how big we want to be，對尺寸沒有任何限制，這種情況一般用於系統內部，表示一種	   測量的狀態
       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);
   }
   

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總結：前面已經提到，對於普通View，其MeasureSpec由父容器的MeasureSpec和自身的LayoutParams共同決定。那麼針對不同的父容器和View本身不同的LayoutParams，View就可以有多種MeasureSpec。The last：當View採用固定寬高時，不管父容器的MeasureSpec是什麼，View的MeasureSpec都是精確模式(EXACTLY)並且其大小遵循LayoutParams的大小；當View的寬/高是MATCH_PARENT時，如果父容器是精確模式EXACTLY，那麼View也是精確模式並且其大小是父容器的剩餘空間，如果父容器是最大模式AT_MOST，那麼View也是最大模式並且大小不會超過父容器的剩餘空間；當View的寬/高是WRAP_CONTENT，不管父容器的模式是精準模式還是最大模式，View的模式總是最大模式AT_MOST並且其大小不能超過---------------------------------------------------------------------------------------------------------------------精確模式EXACTLY，父容器已經檢測出View的精確大小，這個時候View的最終大小就是SpecSize(int specSize = MeasureSpec.getSize(spec);)所指定的值，它對應LayoutParams中的MATCH_PARENT和具體數值這兩種模式；最大模式AT_MOST，父容器指定了一個可用大小及SpecSize，View的大小不能大於這個值，具體是什麼要看不同View的具體實現，它對應LayoutParams中的WRAP_CONTENT