RelativeLayout 和 LinearLayout 性能分析-CFANZ编程社区

在写布局文件中 RelativeLayout 和 LinearLayout 用的是较多的两种布局；而这两种哪一种方式性能更高？

1.首先了解 View 的绘制流程。

不管简单的 TextView 还是复杂的 RelativeLayout ，他们的共同基类都是 View ；所以它们都有三个重要的绘制方法（不详细介绍）：Measure（测量），Layout（布局）和 Draw（绘制）;

2.对比 RelativeLayout 和 LinearLayout 性能。

View 的绘制流程会走 measure()，layout() 和 draw() 这三个重要的方法，RelativeLayout 和 LinearLayout 也不例外，下面是耗时数据：

LinearLayout

Measure：0.738ms
Layout：0.176ms
Draw：7.655ms

RelativeLayout

Measure：2.280ms
Layout：0.153ms
Draw：7.696ms

从上面的数据不难看出来， RelativeLayout 和 LinearLayout 的后两个方法 Layout（布局）和 Draw（绘制）耗时相差无几，而第一个方法 Measure （测量）RelativeLayout 比 LinearLayout 慢了很多。下面是针对 RelativeLayout 和 LinearLayout 中的 Measure 方法分析和对比。

RelativeLayout 的 onMeasure() 方法（代码太多，截取了一部分）：

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        //此处省略...
        for (int i = 0; i < count; i++) {
            View child = views[i];
            if (child.getVisibility() != GONE) {
                LayoutParams params = (LayoutParams) child.getLayoutParams();
                int[] rules = params.getRules(layoutDirection);
                applyHorizontalSizeRules(params, myWidth, rules);
                measureChildHorizontal(child, params, myWidth, myHeight);
                if (positionChildHorizontal(child, params, myWidth, isWrapContentWidth)) {
                    offsetHorizontalAxis = true;
                }
            }
        }

        views = mSortedVerticalChildren;
        count = views.length;
        final int targetSdkVersion = getContext().getApplicationInfo().targetSdkVersion;

        for (int i = 0; i < count; i++) {
            final View child = views[i];
            if (child.getVisibility() != GONE) {
                final LayoutParams params = (LayoutParams) child.getLayoutParams();
                applyVerticalSizeRules(params, myHeight, child.getBaseline());
                measureChild(child, params, myWidth, myHeight);
                if (positionChildVertical(child, params, myHeight, isWrapContentHeight)) {
                    offsetVerticalAxis = true;
                }
                //此处省略...
            }
        }

        //此处省略...
        setMeasuredDimension(width, height);
    }

根据源码发现 RelativeLayout 会对子 View 做两次 measure。因为 RelativeLayout 中子 View 的排列方式是基于彼此的依赖关系，而这个依赖关系可能和布局中 View 的顺序并不相同，在确定每个子 View 的位置的时候，就需要先给所有的子 View 排序一下。所以需要横向纵向分别测量。

LinearLayout的 onMeasure() 方法（代码太多，截取了一部分）：

@Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    if (mOrientation == VERTICAL) {
      measureVertical(widthMeasureSpec, heightMeasureSpec);
    } else {
      measureHorizontal(widthMeasureSpec, heightMeasureSpec);
    }
  }

从上面的代码可以看出， LinearLayout 先进行判断为横向还是纵向，然后再进行一次测量。

for (int i = 0; i < count; ++i) {
      final View child = getVirtualChildAt(i);

      if (child == null) {
        mTotalLength += measureNullChild(i);
        continue;
      }

      if (child.getVisibility() == View.GONE) {
       i += getChildrenSkipCount(child, i);
       continue;
      }

      if (hasDividerBeforeChildAt(i)) {
        mTotalLength += mDividerHeight;
      }

      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;
        mTotalLength = Math.max(totalLength, totalLength + 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 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) {
          largestChildHeight = Math.max(childHeight, largestChildHeight);
        }
      }

3.RelativeLayout 和 LinearLayout 性能总结。

总结： RelativeLayout会让子View调用2次onMeasure，而 LinearLayout 在没有 weight 时，只会调用1次onMeasure（如果 LinearLayout 有 weight 属性时，也会调用两次 onMeasure ）；因此LinearLayout 性能要高于 RelativeLayout 的性能，在实际操作时可以用 LinearLayout 完成，尽量用 LinearLayout 不去用 RelativeLayout 。