ViewGroup 子类 LinearLayout 的measure 流程

最新推荐文章于 2023-01-11 21:48:26 发布

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本文详细解析了LinearLayout在Android中的测量流程，重点分析了垂直布局下子View的测量过程，包括如何处理不同权重（weight）的子View。通过具体示例说明了如何避免布局比例不准确的问题。

ViewGroup 子类 LinearLayout 的measure 流程

在笔记"View和ViewGroup 的measure过程"中已经提到ViewGroup没有执行具体的测量过程,只是调用child view 的measure()方法.这是因为ViewGroup之类太多,不好统一处理.实际是各个之类去重写onMeasure来自己处理的.下面主要是分析ViewGroup 子类 LinearLayout 的measure 流程.

从onMeasure()开始

LinearLayout.java

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

我这边这里只分析垂直布局measureVertical()的流程.

具体分析见代码中的注释.

LinearLayout.java

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
	mTotalLength = 0;//子view的高度和  
	int maxWidth = 0;
	int childState = 0;
	int alternativeMaxWidth = 0;//子视图的最大宽度(不包含layout_weight>0的子view)  
	int weightedMaxWidth = 0;//子视图的最大宽度(仅包含layout_weight>0的子view)  
	boolean allFillParent = true;//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  
	float totalWeight = 0; //所有子view的weight之和  
	//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;
	boolean skippedMeasure = false;
	//以LinearLayout中第几个子view的baseLine作为LinearLayout的基准线       
	final int baselineChildIndex = mBaselineAlignedChildIndex;        
	final boolean useLargestChild = mUseLargestChild;
	int largestChildHeight = Integer.MIN_VALUE;
	if (sDebugLayout) {
		Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] + Pass.1, this=" + this);
	}
	// 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);//默认都返回0，扩展预留，此处没用  
			continue;
		}
		if (child.getVisibility() == View.GONE) {
		   i += getChildrenSkipCount(child, i);//默认都返回0，扩展预留，此处没用  
		   continue;
		}
		if (sDebugLayout) {
			Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] + Child in Pass1: " + i + ", child=" + child
					+ ", this=" + this);
		}
		if (hasDividerBeforeChildAt(i)) {
			mTotalLength += mDividerHeight;
		}
		//child 的 LayoutParams
		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.
			final int totalLength = mTotalLength;
			//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 = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
			if (sDebugLayout) {
				Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Child in Pass1-1: " + i
						+ ", child=" + child + ", this=" + this + ", "
						+ "status=Skipped, mTotalLength=" + mTotalLength
						+ ", totalWeight=" + totalWeight);
			}
			skippedMeasure = true;
	   } 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;
				if (sDebugLayout) {
					Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Child in Pass1-2-1: " + i
							+ ", child=" + child + ", this=" + this + ", "
							+ "status=Modified, LayoutParams.height change to WRAP_CONTENT");
				}
			}
			// Determine how big this child would like to be. 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;
			}
			final int childHeight = child.getMeasuredHeight();
			final int totalLength = mTotalLength;
			mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
				   lp.bottomMargin + getNextLocationOffset(child));
			if (useLargestChild) {//是否设置了android:measureWithLargestChild
				largestChildHeight = Math.max(childHeight, largestChildHeight);
			}
			if (sDebugLayout) {
				Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Child in Pass1-2-2: " + i
						+ ", child=" + child + ", this=" + this + ", "
						+ "status=Measured, mTotalLength=" + mTotalLength
						+ ", totalWeight=" + totalWeight);
			}
		}
		/**
		 * 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) {//i 不能小于android:baselineAlignedChildIndex的值的同时weight >0
			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.MATCH_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  
		childState = combineMeasuredStates(childState, child.getMeasuredState());
	   //子view宽度是否全是FILL_PARENT  
		allFillParent = allFillParent && lp.width == LayoutParams.MATCH_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);
		}
		if (sDebugLayout) {
			Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Child in Pass1: " + i + ", child=" + child
					+ ", this=" + this + ", " + "matchWidth=" + matchWidth
					+ ", matchWidthLocally=" + matchWidthLocally + ", "
					+ "weightedMaxWidth=" + weightedMaxWidth
					+ ", alternativeMaxWidth=" + alternativeMaxWidth);
		}
		i += getChildrenSkipCount(child, i);
	}
	if (sDebugLayout) {
		Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Pass.1, this=" + this + ", "
				+ "maxWidth=" + maxWidth + ", weightedMaxWidth=" + weightedMaxWidth
				+ ", alternativeMaxWidth=" + alternativeMaxWidth + ", "
				+ "mTotalLength=" + mTotalLength + ", totalWeight=" + totalWeight);
	}
	if (mTotalLength > 0 && hasDividerBeforeChildAt(count)) {
		mTotalLength += mDividerHeight;
	}
	if (useLargestChild &&
			(heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
		if (sDebugLayout) {
			Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] + Pass.1.5, this=" + this);
		}
		mTotalLength = 0;
		for (int i = 0; i < count; ++i) {
			final View child = getVirtualChildAt(i);
			if (child == null) {
				mTotalLength += measureNullChild(i);
				continue;
			}
			if (child.getVisibility() == GONE) {
				i += getChildrenSkipCount(child, i);
				continue;
			}
			final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
					child.getLayoutParams();
			// Account for negative margins
			final int totalLength = mTotalLength;
			mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
					lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
		}
		if (sDebugLayout) {
			Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Pass.1.5, this=" + this
					+ ", largestChildHeight=" + largestChildHeight
					+ ", mTotalLength=" + mTotalLength);
		}
	}
	// 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
	int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
	heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
	
	// Either expand children with weight to take up available space or
	// shrink them if they extend beyond our current bounds. If we skipped
	// measurement on any children, we need to measure them now.
	int delta = heightSize - mTotalLength;
	if (sDebugLayout) {
		Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] + Pass.2, this=" + this + ", "
				+ "delta=" + delta + ", " + "heightSize=" + heightSize
				+ ", mTotalLength=" + mTotalLength);
	}
	//skippedMeasure=true:如果LinearLayout高度是已经确定的,并且有子view的height=0，weight>0，  
	if (skippedMeasure || 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;
			}
			
			if (sDebugLayout) {
				Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] + Child in Pass2: " + i + ", "
						+ "child=" + child + ", this=" + this);
			}
			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));
					if (sDebugLayout) {
						Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Child in Pass2-1-1-1: " + i
								+ ", child=" + child + ", this=" + this + ", "
								+ "status=Measured, share=" + share
								+ ", childHeight=" + childHeight
								+ ", mTotalLength=" + mTotalLength);
					}
				} 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));
					if (sDebugLayout) {
						Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Child in Pass2-1-1-2: " + i
								+ ", child=" + child + ", this=" + this + ", "
								+ "status=Measured, share=" + share
								+ ", mTotalLength=" + mTotalLength);
					}
				}
				// Child may now not fit in vertical dimension.
				childState = combineMeasuredStates(childState, child.getMeasuredState()
						& (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
			} else {
				if (sDebugLayout) {
					Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Child in Pass2-1-2: " + i
							+ ", child=" + child + ", this=" + this + ", "
							+ "status=Skipped, ChildExtra=" + childExtra
							+ ", mTotalLength=" + mTotalLength);
				}
			}
			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.MATCH_PARENT;
			alternativeMaxWidth = Math.max(alternativeMaxWidth,
					matchWidthLocally ? margin : measuredWidth);
			allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
			final int totalLength = mTotalLength;
			mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +
					lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
		}
		// Add in our padding
		mTotalLength += mPaddingTop + mPaddingBottom;
		if (sDebugLayout) {
			Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Pass.2-1, this=" + this + ", "
					+ "maxWidth=" + maxWidth + ", weightedMaxWidth=" + weightedMaxWidth
					+ ", alternativeMaxWidth=" + alternativeMaxWidth + ", "
					+ "delta=" + delta + ", mTotalLength=" + mTotalLength);
		}
		// TODO: Should we recompute the heightSpec based on the new total length?
   } else {
		alternativeMaxWidth = Math.max(alternativeMaxWidth,
									   weightedMaxWidth);
		// We have no limit, so make all weighted views as tall as the largest child.
		// Children will have already been measured once.
		if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {
			for (int i = 0; i < count; i++) {
				final View child = getVirtualChildAt(i);
				if (child == null || child.getVisibility() == View.GONE) {
					continue;
				}
				final LinearLayout.LayoutParams lp =
						(LinearLayout.LayoutParams) child.getLayoutParams();
				float childExtra = lp.weight;
				if (childExtra > 0) {
					child.measure(
							MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),
									MeasureSpec.EXACTLY),
							MeasureSpec.makeMeasureSpec(largestChildHeight,
									MeasureSpec.EXACTLY));
				}
				if (sDebugLayout) {
					String measureString = (childExtra > 0)
							? "status=Measured" : "status=Skipped";
					Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Child in Pass2-2: " + i
							+ ", child=" + child + ", this=" + this + ", " + measureString);
				}
			}
		}
		if (sDebugLayout) {
			Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Pass.2-2, this=" + this + ", "
					+ "maxWidth=" + maxWidth + ", weightedMaxWidth=" + weightedMaxWidth
					+ ", alternativeMaxWidth=" + alternativeMaxWidth + ", " + "delta=" + delta
					+ ", mTotalLength=" + mTotalLength
					+ ", useLargestChild=" + useLargestChild);
		}
	}
	if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
		maxWidth = alternativeMaxWidth;
	}
	
	maxWidth += mPaddingLeft + mPaddingRight;
	// Check against our minimum width
	maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
	
	setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
			heightSizeAndState);
	if (matchWidth) {
		if (sDebugLayout) {
			Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] + Pass.3, this=" + this
					+ ", uniformWidth=" + getMeasuredWidth() + ", this=" + this);
		}
		forceUniformWidth(count, heightMeasureSpec);
		if (sDebugLayout) {
			Xlog.d(DEBUG_LOG_TAG, "[LinearLayout] - Pass.3, this=" + this);
		}
	}
}

从上面可以看到,实际分配LinearLayout的各个child view 的高度的时候是先分配没有设置weight和height=0属性的控件并执行该child view的measure流程,然后给那些设置了weight>0同时height=0的子child view.这个也就是79行的skippedMeasure = true最终会在256的if判断中执行的工作.

为了验证,这里创建几个例子分析.

下面的480dip 在我测试的手机上面就是480*3= 1140px.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/doov_ll_root"
    android:layout_width="fill_parent"
    android:layout_height="480dip" >
    <LinearLayout
        android:id="@+id/doov_linear1"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:background="#ff888888"
        android:orientation="vertical" >
        <TextView
            android:id="@+id/doov_id1"
            android:layout_width="fill_parent"
            android:layout_height="0dip"
            android:layout_weight="2"
            android:background="#ff765423"
            android:text="11111" />
        <TextView
            android:id="@+id/doov_id2"
            android:layout_width="fill_parent"
            android:layout_height="0dip"
            android:layout_weight="1"
            android:background="#ffff0000"
            android:text="aaaaa" />
        <TextView
            android:id="@+id/doov_id3"
            android:layout_width="fill_parent"
            android:layout_height="90dip"
            android:background="#ff234532"
            android:text="2222222" />
    </LinearLayout>
</LinearLayout>

先说一下实际的结果:

doov_ll_root:1440

doov_linear1:1440

doov_id3:270

doov_id1:(1440-270)=1170 ;1170*2/3=760

doov_id2:380

计算过程如下:

256行的时候: delta=1170, heightSize=1440, mTotalLength=270

280行-child =0的时候: share=780, mTotalLength=0, delta=1170-780=390

280行-child =1的时候:share=390, mTotalLength=780,delat=0

child =2的时候不能进入257行的判断,不执行其他处理.

上面的布局实际效果图如下:

另外一种情况:

如果将的doov_id1和doov_id2配置改成如下,也就是高度改成match_parent.

这个时候doov_id1和doov_id2高度比例看起来的时候显然不是2:1,反而像是1:2.还是分析一下具体计算步骤:

256行的时候:delta=-1710, heightSize=1440, mTotalLength=3150(mTotalLength=1440*2+270)

300行child=0的时候:share=-1140, childHeight=300, mTotalLength=0

300行child=1的时候:share=-570, childHeight=870, mTotalLength=300

其中doov_id1的高度为:1440+(-1140)=300

doov_id2的高度为:1440+(-570 )=870

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/doov_ll_root"
    android:layout_width="fill_parent"
    android:layout_height="480dip" >
    <LinearLayout
        android:id="@+id/doov_linear1"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:background="#ff888888"
        android:orientation="vertical" >
        <TextView
            android:id="@+id/doov_id1"
            android:layout_width="fill_parent"
            android:layout_height="match_parent"
            android:layout_weight="2"
            android:background="#ff765423"
            android:text="11111" />
        <TextView
            android:id="@+id/doov_id2"
            android:layout_width="fill_parent"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="#ffff0000"
            android:text="aaaaa" />
        <TextView
            android:id="@+id/doov_id3"
            android:layout_width="fill_parent"
            android:layout_height="90dip"
            android:background="#ff234532"
            android:text="2222222" />
    </LinearLayout>
</LinearLayout>

上面的布局文件实际显示效果如下:

总结:在使用LinearLayout 的时候,如果设置了child view 的weight 属性,最好将对于的view 的height(或者width)属性设置为0dp,这样可以避免实际显示的比例不对,及时将2个child view 的weight都设置了为1,虽然显示的时候可能是1:1 的比例,但是实际却会多measure一次(具体体现在63和102行).所以想相关child view的height(或者width)属性设置为0dp不仅可以让视图正确显示,还可以提供代码的效率.