血淚教訓：由於不夠了解MeasureSpec而引起的生產事故

夭壽啦，出生產事故了

那天，風和日麗，菜雞Android開發工程師小張開心的寫着 bug 代碼 ，忽然，測試經理老石跑過來講客戶報上來一個線上bug，小張忽然菊花一緊，說不可能，個人代碼是不可能有問題的。而後老石從胯下 口袋 掏出來手機，三下五除二將bug復現出來了：

情景再現：

從圖中咱們明顯能看到，底部的服務彈框被裁切擋住了，顯示的不完整。

分析緣由：

看到復原的犯罪現場，小張內心一個放鬆：「還好還好，不是我寫的代碼。」當着老石的面，小張認可是代碼出了問題，答應了老石下個版本修復，老石纔不依不撓的走開了。打發走了老石，小張開始分析起來問題了。開始，小張簡單的認爲是底部的彈框(Dialog)的高度過小了，形成了dialog的內容顯示不完整。然鵝，看到出問題的代碼，小張開始發現事情並不簡單，涉事代碼以下：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <!-- 省略無關代碼 -->
    <com.xx.ListViewForScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginLeft="20dp"
        android:layout_marginTop="18dip"
        android:layout_marginRight="20dp"
        android:layout_marginBottom="48dp"
        android:divider="@null">
    </com.xx.ListViewForScrollView>
</RelativeLayout>
複製代碼

看到上面Dialog的佈局xml文件中的ListViewForScrollView小張內心不由泛起了嘀咕：「怎麼又是自定義的View，又要看一大堆邏輯了，腦袋疼，腦袋疼...」轉念一想，不對啊，這個View看名字一看就是繼承自ListView，就算是Dialog的高度不夠，這個ListViewForScrollView也應該能滑動纔對啊，如今不能滑動了是怎麼一回事，有問題有問題...

不看代碼光靠猜是解決不了問題的，小張下定決心去研究研究這個自定義View，打開這個ListViewForScrollView的代碼，小張鬆了一口氣：還好還好，代碼量並很少，看起來邏輯並不複雜：

public class ListViewForScrollView extends ListView {

	public ListViewForScrollView(Context context) {
		super(context);
	}

	public ListViewForScrollView(Context context, AttributeSet attrs) {
		super(context, attrs);
	}

	public ListViewForScrollView(Context context, AttributeSet attrs,
			int defStyle) {
		super(context, attrs, defStyle);
	}

	@Override
	/**
	 * 重寫該方法，達到使ListView適應ScrollView的效果
	 */
	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
		int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2,
				MeasureSpec.AT_MOST);
		super.onMeasure(widthMeasureSpec, expandSpec);
	}
}
複製代碼

看到這段代碼，小張更疑惑了：「這不就是繼承的ListView麼，也沒什麼特殊的邏輯啊，爲啥它就是不能滑動呢？」
爲了解決內心的疑惑，小張決定從這個ListViewForScrollView惟一重寫的方法onMeasure(int widthMeasureSpec, int heightMeasureSpec)下手，看能不能發現問題。
剛開始看第一段代碼，小張就犯難了，這個MeasureSpec是個什麼東東？看起來不理解這個MeasureSpec是解決不了問題了，小張決定了解一下這個MeasureSpec：

關於MeasureSpec，小張想說

咱們都知道，View展現在手機上，須要經歷測量-繪製-佈局三個流程，對應到View的代碼中就是onMeasure()、onDraw()、onLayout()三個方法。在onMeasure()過程當中，咱們就須要藉助MeasureSpec對View進行測量。
onMeasure()方法接受兩個參數：widthMeasureSpec和heightMeasureSpec，這兩個參數都是32位的int類型，由View的父控件傳過來。重點來了：View的onMeasure()方法，實際上是由父控件調用，那View的大小難道就只能有父控件決定麼？其實否則。咱們剛纔也說了，widthMeasureSpec和heightMeasureSpec都是32位的int類型，他們的高兩位表示測量模式mode，低30位表示測量大小size。經過MeasureSpec這個靜態類，咱們很容易獲取View的測量模式和測量大小，其中測量模式有3種：

• UNSPECIFIED
  父控件不對你有任何限制，你想要多大給你多大，想上天就上天。這種狀況通常用於系統內部，表示一種測量狀態。（這個模式主要用於系統內部屢次Measure的情形，並非真的說你想要多大最後就真有多大）

• EXACTLY
  父控件已經知道你所需的精確大小，你的最終大小應該就是這麼大。

• AT_MOST
  你的大小不能大於父控件給你指定的size，但具體是多少，得看你本身的實現。

父控件經過ViewGroup中的靜態方法getChildMeasureSpec來獲取子控件的測量規格。下面是getChildMeasureSpec()的代碼(因爲小張terrible的英語水平，就不給你們翻譯註釋了，你們能夠自行有道～)：

/**
     * 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.
     *
     * 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);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        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;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent has imposed a maximum size on us 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; 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;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        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);
    }
複製代碼

該方法經過將父控件的測量規格和childview的佈局參數LayoutParams相結合，獲得一個最可能符合條件的child view的測量規格。
經過以上代碼，你們能夠有一個概念：就是一個View的大小是由它的父控件加上自身的LayoutParams決定的。詳情以下(圖片來源於任玉剛博客)：

解決問題

經過了解View的Measure過程，小張知道了，在ListViewForScrollView的onMeasure()方法裏，強行將ListView的測量模式改成了AT_MOST，並將測量大小改成了MAX_VALUE >> 2,接着調用父類ListView的onMeasure()方法，咱們來看看ListView的onMeasure()方法：

/** 源碼來自8.0.0_r4,不一樣版本可能略有不一樣 **/
@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        /** 此處省略不相關代碼 **/
        //......
        // 測量模式爲AT_MOST，測量大小爲MAX_VALUE >> 2,也就是536870911
        if (heightMode == MeasureSpec.AT_MOST) {
            heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
        }

        setMeasuredDimension(widthSize, heightSize);
        /** 繼續省略 **/
        //......
    }
複製代碼

經過上面的代碼咱們能夠看到，最終這個自定義的ListViewForScrollView的高度是由measureHeightOfChildren這個方法肯定的，小張接着查看這個方法：

/**
     * Measures the height of the given range of children (inclusive) and
     * returns the height with this ListView's padding and divider heights * included. If maxHeight is provided, the measuring will stop when the * current height reaches maxHeight. * * @param widthMeasureSpec The width measure spec to be given to a child's
     *            {@link View#measure(int, int)}.
     * @param startPosition The position of the first child to be shown.
     * @param endPosition The (inclusive) position of the last child to be
     *            shown. Specify {@link #NO_POSITION} if the last child should be
     *            the last available child from the adapter.
     * @param maxHeight The maximum height that will be returned (if all the
     *            children don't fit in this value, this value will be * returned). * @param disallowPartialChildPosition In general, whether the returned * height should only contain entire children. This is more * powerful--it is the first inclusive position at which partial * children will not be allowed. Example: it looks nice to have * at least 3 completely visible children, and in portrait this * will most likely fit; but in landscape there could be times * when even 2 children can not be completely shown, so a value * of 2 (remember, inclusive) would be good (assuming * startPosition is 0). * @return The height of this ListView with the given children. */ final int measureHeightOfChildren(int widthMeasureSpec, int startPosition, int endPosition, int maxHeight, int disallowPartialChildPosition) { final ListAdapter adapter = mAdapter; if (adapter == null) { return mListPadding.top + mListPadding.bottom; } // Include the padding of the list int returnedHeight = mListPadding.top + mListPadding.bottom; final int dividerHeight = mDividerHeight; // The previous height value that was less than maxHeight and contained // no partial children int prevHeightWithoutPartialChild = 0; int i; View child; // mItemCount - 1 since endPosition parameter is inclusive endPosition = (endPosition == NO_POSITION) ? adapter.getCount() - 1 : endPosition; final AbsListView.RecycleBin recycleBin = mRecycler; final boolean recyle = recycleOnMeasure(); final boolean[] isScrap = mIsScrap; for (i = startPosition; i <= endPosition; ++i) { child = obtainView(i, isScrap); measureScrapChild(child, i, widthMeasureSpec, maxHeight); if (i > 0) { // Count the divider for all but one child returnedHeight += dividerHeight; } // Recycle the view before we possibly return from the method if (recyle && recycleBin.shouldRecycleViewType( ((LayoutParams) child.getLayoutParams()).viewType)) { recycleBin.addScrapView(child, -1); } returnedHeight += child.getMeasuredHeight(); if (returnedHeight >= maxHeight) { // We went over, figure out which height to return. If returnedHeight > maxHeight, // then the i'th position did not fit completely.
                return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
                            && (i > disallowPartialChildPosition) // We've past the min pos && (prevHeightWithoutPartialChild > 0) // We have a prev height && (returnedHeight != maxHeight) // i'th child did not fit completely
                        ? prevHeightWithoutPartialChild
                        : maxHeight;
            }

            if ((disallowPartialChildPosition >= 0) && (i >= disallowPartialChildPosition)) {
                prevHeightWithoutPartialChild = returnedHeight;
            }
        }

        // At this point, we went through the range of children, and they each
        // completely fit, so return the returnedHeight
        return returnedHeight;
    }
複製代碼

經過查看該方法，小張知道了這個方法的意義就是測量給定範圍的高度，並返回包含此ListView的填充和分隔符高度的高度。若是提供了maxHeight，噹噹前高度達到maxHeight時，測量將中止。而對於ListViewForScrollView，maxHeight就是上面onMeasure()中咱們傳過來的536870911(即MAX_VALUE >> 2)，若是ListView的itemView的總高度不超過這個值，那麼ListView的高度就是全部itemView高度加上分隔符加上padding的高度，不然就爲536870911。因此問題的緣由就很清楚了，ListViewForScrollView的高度在大多數狀況下都爲咱們指定的536870911，而這種狀況下ListView是不能滑動的。

解決問題

經過研究ListViewForScrollView，小張發現這個自定義View是用來解決ScrollView嵌套ListView帶來的滑動衝突等問題的，而出問題的Dialog根佈局是RelativeLayout，並非ScrollView，因此小張將佈局文件中的ListViewForScrollView換成了ListView，問題圓滿解決了。

吃一塹長一智

經過此次的血淚教訓，小張明白了：

1. View的繪製流程
2. MeasureSpec的做用
3. 使用自定義View的時候要充分理解其做用