LinearLayout源码分析 -

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LinearLayout源码分析

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android几大布局研究

android线性布局 LinearLayout layout_weight

LinearLayout是android中最常用的布局之一。简单了解一下LinearLayout的源码实现过程，可以加深其各属性的用法，以及这么用的原因。比如常用的layout_weight等。

LinearLayout源码如下(以垂直布局为例，截取部分代码):

public class LinearLayout extends ViewGroup {

	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
		if (mOrientation == VERTICAL) {
			// 分析垂直布局
			measureVertical(widthMeasureSpec, heightMeasureSpec);
		} else {
			// measureHorizontal(widthMeasureSpec, heightMeasureSpec);
		}
	}

	/**
	 * Measures the children when the orientation of this LinearLayout is set to
	 * {@link #VERTICAL}.
	 * 
	 * @param widthMeasureSpec
	 *            Horizontal space requirements as imposed by the parent.
	 * @param heightMeasureSpec
	 *            Vertical space requirements as imposed by the parent.
	 * 
	 * @see #getOrientation()
	 * @see #setOrientation(int)
	 * @see #onMeasure(int, int)
	 */
	void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
        mTotalLength = 0;//子view的高度和
        int maxWidth = 0;
        //子视图的最大宽度(不包含layout_weight>0的子view)
        int alternativeMaxWidth = 0;
        //子视图的最大宽度(仅包含layout_weight>0的子view)
        int weightedMaxWidth = 0;
        //子视图的宽度是否全是fillParent的，用于后续判断是否需要重新计算
        boolean allFillParent = true;
        //所有子view的weight之和
        float totalWeight = 0;
      
        
        //子view的个数(仅包含直接子view，如LinearLayout1中
        //分别有Textview1，LinearLayout2,TextView2，
        //而LinearLayout2中又有多个子view。此时LinearLayout1的getVirtualChildCount();为3)
        final int count = getVirtualChildCount(); 
        //LinearLayout宽度模式
        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        //LinearLayout高度模式
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        //子view的宽度是否要由父确定。如父LinearLayout为layout_width=wrap_content，
        //子view为fill_parent则matchWidth =true
        boolean matchWidth = false;
      //以LinearLayout中第几个子view的baseLine作为LinearLayout的基准线       
        final int baselineChildIndex = mBaselineAlignedChildIndex;

        // See how tall everyone is. Also remember max width.
        for (int i = 0; i < count; ++i) {
            final View child = getVirtualChildAt(i);
            if (child == null) {
                mTotalLength += measureNullChild(i);
                //measureNullChild(i)默认都返回0，扩展预留，此处没用
                continue;
            }
            if (child.getVisibility() == View.GONE) {
               i += getChildrenSkipCount(child, i);
               //getChildrenSkipCount(child, i)默认都返回空，扩展预留，此处没用
               continue;
            }
            LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
            totalWeight += lp.weight;
            if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
                // Optimization: don't bother measuring children who are going to use
                // leftover space. These views will get measured again down below if
                // there is any leftover space.
            	//如果LinearLayout高度是已经确定的。并且这个子view的height=0，weight>0，
            	//则mTotalLength只需要加上margin即可，
            	//由于是weight>0；该view的具体高度等会还要计算
                mTotalLength += lp.topMargin + lp.bottomMargin;
            } else {
               int oldHeight = Integer.MIN_VALUE;

               if (lp.height == 0 && lp.weight > 0) {
                   // heightMode is either UNSPECIFIED OR AT_MOST, and this child
                   // wanted to stretch to fill available space. Translate that to
                   // WRAP_CONTENT so that it does not end up with a height of 0
                   oldHeight = 0;
                   lp.height = LayoutParams.WRAP_CONTENT;
               }

               // Determine how big this child would like to.  If this or
               // previous children have given a weight, then we allow it to
               // use all available space (and we will shrink things later
               // if needed).
               measureChildBeforeLayout(
                       child, i, widthMeasureSpec, 0, heightMeasureSpec,
                       totalWeight == 0 ? mTotalLength : 0);

               if (oldHeight != Integer.MIN_VALUE) {
                   lp.height = oldHeight;
               }

               mTotalLength += child.getMeasuredHeight() + lp.topMargin +
                       lp.bottomMargin + getNextLocationOffset(child);
            }

            /**
             * If applicable, compute the additional offset to the child's baseline
             * we'll need later when asked {@link #getBaseline}.
             */
            if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
               mBaselineChildTop = mTotalLength;
            }

            // if we are trying to use a child index for our baseline, the above
            // book keeping only works if there are no children above it with
            // weight.  fail fast to aid the developer.
            if (i < baselineChildIndex && lp.weight > 0) {
            	//为什么i < baselineChildIndex && lp.weight > 0不行。
            	//假如行的话，如果LinearLayout与其他view视图对其的话，
            	//由于weight>0的作用，会影响其他所有的view位置
            	//应该是由于效率的原因才不允许这样。
                throw new RuntimeException("A child of LinearLayout with index "
                        + "less than mBaselineAlignedChildIndex has weight > 0, which "
                        + "won't work.  Either remove the weight, or don't set "
                        + "mBaselineAlignedChildIndex.");
            }

            boolean matchWidthLocally = false;
            if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.FILL_PARENT) {
                // The width of the linear layout will scale, and at least one
                // child said it wanted to match our width. Set a flag
                // indicating that we need to remeasure at least that view when
                // we know our width
            	//如果LinearLayout宽度不是已确定的，如是wrap_content,而子view是FILL_PARENT，
            	//则做标记matchWidth=true； matchWidthLocally = true;
                matchWidth = true;
                matchWidthLocally = true;
            }

            final int margin = lp.leftMargin + lp.rightMargin;
            final int measuredWidth = child.getMeasuredWidth() + margin;
            maxWidth = Math.max(maxWidth, measuredWidth);//最大子view的宽度
            //子view宽度是否全是FILL_PARENT
            allFillParent = allFillParent && lp.width == LayoutParams.FILL_PARENT;
           
            if (lp.weight > 0) {
                /*
                 * Widths of weighted Views are bogus if we end up
                 * remeasuring, so keep them separate.
                 */
            	//如父width是wrap_content，子是fill_parent，则子的宽度需要在父确定后才能确定。这里并不是真实的宽度
                weightedMaxWidth = Math.max(weightedMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);
            } else {
                alternativeMaxWidth = Math.max(alternativeMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);
            }

            i += getChildrenSkipCount(child, i);
        }
        
        // Add in our padding
        mTotalLength += mPaddingTop + mPaddingBottom;

        int heightSize = mTotalLength;

        // Check against our minimum height
        heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
        
        // Reconcile our calculated size with the heightMeasureSpec
        heightSize = resolveSize(heightSize, heightMeasureSpec);
        
        // Either expand children with weight to take up available space or
        // shrink them if they extend beyond our current bounds
        int delta = heightSize - mTotalLength;
        if (delta != 0 && totalWeight > 0.0f) {
            float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;

            mTotalLength = 0;

            for (int i = 0; i < count; ++i) {
                final View child = getVirtualChildAt(i);
                
                if (child.getVisibility() == View.GONE) {
                    continue;
                }
                
                LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
                
                float childExtra = lp.weight;
                if (childExtra > 0) {
                    // Child said it could absorb extra space -- give him his share
                    int share = (int) (childExtra * delta / weightSum);
                    weightSum -= childExtra;
                    delta -= share;

                    final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            mPaddingLeft + mPaddingRight +
                                    lp.leftMargin + lp.rightMargin, lp.width);

                    // TODO: Use a field like lp.isMeasured to figure out if this
                    // child has been previously measured
                    if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {
                        // child was measured once already above...
                        // base new measurement on stored values
                        int childHeight = child.getMeasuredHeight() + share;
                        if (childHeight < 0) {
                            childHeight = 0;
                        }
                        
                        child.measure(childWidthMeasureSpec,
                                MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));
                    } else {
                        // child was skipped in the loop above.
                        // Measure for this first time here      
                        child.measure(childWidthMeasureSpec,
                                MeasureSpec.makeMeasureSpec(share > 0 ? share : 0,
                                        MeasureSpec.EXACTLY));
                    }
                }

                final int margin =  lp.leftMargin + lp.rightMargin;
                final int measuredWidth = child.getMeasuredWidth() + margin;
                maxWidth = Math.max(maxWidth, measuredWidth);

                boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&
                        lp.width == LayoutParams.FILL_PARENT;

                alternativeMaxWidth = Math.max(alternativeMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);

                allFillParent = allFillParent && lp.width == LayoutParams.FILL_PARENT;

                mTotalLength += child.getMeasuredHeight() + lp.topMargin +
                        lp.bottomMargin + getNextLocationOffset(child);
            }

            // Add in our padding
            mTotalLength += mPaddingTop + mPaddingBottom;            
        } else {
            alternativeMaxWidth = Math.max(alternativeMaxWidth,
                                           weightedMaxWidth);
        }

        if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
            maxWidth = alternativeMaxWidth;
        }
        
        maxWidth += mPaddingLeft + mPaddingRight;

        // Check against our minimum width
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
        
        setMeasuredDimension(resolveSize(maxWidth, widthMeasureSpec), heightSize);

        if (matchWidth) {
            forceUniformWidth(count, heightMeasureSpec);
        }
    }
}

根据源码，对layout_weight的用法进行说明，

以下均是对第二个LinearLayout 即linear1的调试过程

如下布局1：在垂直线性布局中放三个textview，第三个高度90，余下的高度由第一个占2/3，第二个占1/3

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
	android:layout_width="fill_parent" android:layout_height="480dip">
	<LinearLayout android:id="@+id/linear1" xmlns:android="http://schemas.android.com/apk/res/android"
		android:orientation="vertical" android:layout_width="fill_parent"
		android:background="#ff888888" android:layout_height="fill_parent">
		<TextView android:id="@+id/id1" android:layout_width="fill_parent"
			android:layout_height="0dip" android:layout_weight="2"
			android:background="#ff765423" android:text="11111" />
		<TextView android:id="@+id/id2" android:layout_width="fill_parent"
			android:layout_height="0dip" android:layout_weight="1"
			android:background="#ffff0000" android:text="aaaaa" />
		<TextView android:id="@+id/id3"  android:layout_width="fill_parent"
			android:background="#ff234532" android:layout_height="90dip"
			android:text="2222222" />
	</LinearLayout >
</LinearLayout>

运行结果，高度分别为260、130、90符合预期。

在linear1运行到源码171行时，heightSize =480(480由mTotalLength以及linear1的高度确定)，mTotalLength=90(90=0+0+90)；此时剩余的高度int delta = heightSize - mTotalLength=390;

由于totalWeight=2；由189行计算得tv1的附加高度为260；同理tv2高度130；

布局2：当将tv1与tv2的layout_height改成fill_parent结果又如何呢？

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
	android:layout_width="fill_parent" android:layout_height="480dip">
	<LinearLayout
android:id="@+id/linear"
xmlns:android="http://schemas.android.com/apk/res/android"
		android:orientation="vertical" android:layout_width="fill_parent"
		android:background="#ff888888" android:layout_height="fill_parent">
		<TextView android:id="@+id/id1" android:layout_width="fill_parent"
			android:layout_height="fill_parent" android:layout_weight="2"
			android:background="#ff765423" android:text="11111" />
		<TextView android:id="@+id/id2" android:layout_width="fill_parent"
			android:layout_height="fill_parent" android:layout_weight="1"
			android:background="#ffff0000" android:text="aaaaa" />
		<TextView android:id="@+id/id3"  android:layout_width="fill_parent"
			android:background="#ff234532" android:layout_height="90dip"
			android:text="2222222" />
	</LinearLayout>
</LinearLayout>

运行结果，三者高度分别为100，290，90；结果为什么会是这样？tv1与tv2比例为什么不是2:1或者1：2？

linear运行到第171行时，heightSize=480，mTotalLength=1050（tv1与tv2的两个fill_parent都为480，tv3高度90；1050=480+480+90）

因此剩余的高度delta = heightSize - mTotalLength=-570;

因此在189行， int share = (int) (childExtra * delta / weightSum);

对tv1来说share=2*（-570）/3=-380 对tv2来说share=1*(-190)/1=-190;

由行202，int childHeight = child.getMeasuredHeight() + share;

计算的tv1高度为100，tv2高度为290；

总结：

其他模式如tv1为fill_parent,tv2为wrap_content时，这两个的layout_weight又起到什么作用？都可以从源码中分析出来。以上说的高度均是指mMeasuredHeight，因此有可能为负数，总和也可能大于手机屏幕高度。

在线性布局中，需要用到layout_weight的地方，均需要将对应的layout_height(垂直时)或者layout_width(水平时)

设置为0dip，这样他们的高度或者宽度就是把剩余的长度按layout_weight成正比分配给对应的view。如果不是设置为0dip，剩余空间如何按比例分配是不确定的。根据计算，有可能刚好按layout_weight的比例分配(如子view的layout_weight都设置为1，并且设置为fill_parent,这种情况下，各子view刚好平分。)。

一般地， layout_weight>0时，将layout_height或者layout_width设置为0dip，不但能正确的按比例分配剩余空间，而且还会提高效率(由源码67行可见）。

layout_weight的作用是把剩余的部分，按比例分。

如布局2，总共是480的高度，tv1与tv2 由于layout_height="fill_parent"则高度都为480，tv3高度为90.剩余的高度为-570.将-570按比例2比1分配给tv1与tv2，则tv1的高度为100，tv2高度为290.

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图形处理 | view 继承图

2011-07-05 12:31
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评论(2)
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2 楼北极光之吻 2015-11-30

很好的分析

1 楼 internetfox 2012-05-22

谢谢分享，分析的很透彻，为我指引了一点方向。
不知道楼主对于Android源代码的学习有什么指导意见？还有我最近在研究ListView, Gallery等控件在源代码中的实现，可以给我点建议吗?谢谢

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