Relativelayout和LinearLayout对比分析

分析之前先了解下View的绘制流程

首先view在windows中的布局样式如下图：

view绘制在windows,windows与DecoverView的交互在VIewRoot中进行。

view绘制的入口函数是ViewRootImpl中的performTraversals， performTraversals钟会一次调用performMeasure、performLayout、performDraw，三个函数中会分别调用view.Measure、view.Layout、view.Draw，然后分别调用需要子类实现的onMeasure、onLayout、onDraw函数。

而继承自view的自定义view或者系统写好的textview或者LinearLayout等实例化的onMeasure、onLayout、onDraw函数分别用于测量view大小，摆放view、以及将view画出来。

LinearLayout继承自viewGroup，在onMeasure中，会分别调用子控件的view.Measure, 进而在子控件的onMeasur中完成测量，如果子控件仍然是一个viewGroup则依次下去。

view绘制各重载函数的执行流程：

1、android:visibility=visible

创建
I/TestView: TestView(Context context, AttributeSet attrs)
I/TestView: onFinishInflate()
I/TestView: onVisibilityChanged(View changedView, int visibility) changedView = com.android.internal.policy.impl.PhoneWindow$DecorView{2192bad9 I.E..... R.....ID 0,0-0,0} visibility = 4
I/TestView: onVisibilityChanged(View changedView, int visibility) changedView = com.android.internal.policy.impl.PhoneWindow$DecorView{2192bad9 V.E..... R.....ID 0,0-0,0} visibility = 0
I/TestView: onAttachedToWindow()
I/TestView: onWindowVisibilityChanged(int visibility) visibility = 0
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743848
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743848
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073744016
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073744016
I/TestView: onSizeChanged(int w, int h, int oldw, int oldh) w = 1328 h = 2192 oldw = 0 oldh0
I/TestView: onLayout(boolean changed, int left, int top, int right, int bottom) changed = true left = 56 top = 56 right = 1384 bottom = 2248
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743820
I/TestView: onSizeChanged(int w, int h, int oldw, int oldh) w = 1328 h = 1996 oldw = 1328 oldh2192
I/TestView: onLayout(boolean changed, int left, int top, int right, int bottom) changed = true left = 56 top = 56 right = 1384 bottom = 2052
I/TestView: onDraw(Canvas canvas)
I/TestView: onWindowFocusChanged(boolean hasWindowFocus) hasWindowFocus = true
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743820
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743820
I/TestView: onLayout(boolean changed, int left, int top, int right, int bottom) changed = false left = 56 top = 56 right = 1384 bottom = 2052
I/TestView: onDraw(Canvas canvas)

销毁
I/TestView: onWindowFocusChanged(boolean hasWindowFocus) hasWindowFocus = false
I/TestView: onWindowVisibilityChanged(int visibility) visibility = 8
I/TestView: onDetachedFromWindow()

2、android:visibility=invisible

创建
I/TestView: onVisibilityChanged(View changedView, int visibility) changedView = com.example.junyizhou.rxjavademo.TestView{3ead3d52 I.ED.... ........ 0,0-0,0} visibility = 4
I/TestView: TestView(Context context, AttributeSet attrs)
I/TestView: onFinishInflate()
I/TestView: onVisibilityChanged(View changedView, int visibility) changedView = com.android.internal.policy.impl.PhoneWindow$DecorView{3aeb2b95 I.E..... R.....ID 0,0-0,0} visibility = 4
I/TestView: onVisibilityChanged(View changedView, int visibility) changedView = com.android.internal.policy.impl.PhoneWindow$DecorView{3aeb2b95 V.E..... R.....ID 0,0-0,0} visibility = 0
I/TestView: onAttachedToWindow()
I/TestView: onWindowVisibilityChanged(int visibility) visibility = 0
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743848
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743848
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073744016
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073744016
I/TestView: onSizeChanged(int w, int h, int oldw, int oldh) w = 1328 h = 2192 oldw = 0 oldh0
I/TestView: onLayout(boolean changed, int left, int top, int right, int bottom) changed = true left = 56 top = 56 right = 1384 bottom = 2248
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743820
I/TestView: onSizeChanged(int w, int h, int oldw, int oldh) w = 1328 h = 1996 oldw = 1328 oldh2192
I/TestView: onLayout(boolean changed, int left, int top, int right, int bottom) changed = true left = 56 top = 56 right = 1384 bottom = 2052
I/TestView: onWindowFocusChanged(boolean hasWindowFocus) hasWindowFocus = true
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743820
I/TestView: onMeasure(int widthMeasureSpec, int heightMeasureSpec) widthMeasureSpec = 1073743152 heightMeasureSpec = 1073743820
I/TestView: onLayout(boolean changed, int left, int top, int right, int bottom) changed = false left = 56 top = 56 right = 1384 bottom = 2052

销毁
I/TestView: onWindowFocusChanged(boolean hasWindowFocus) hasWindowFocus = false
I/TestView: onWindowVisibilityChanged(int visibility) visibility = 8
I/TestView: onDetachedFromWindow()

3、android:visibility=gone

创建

I/TestView: onVisibilityChanged(View changedView, int visibility) changedView = com.example.junyizhou.rxjavademo.TestView{3ead3d52 G.ED.... ......I. 0,0-0,0} visibility = 8
I/TestView: TestView(Context context, AttributeSet attrs)
I/TestView: onFinishInflate()
I/TestView: onVisibilityChanged(View changedView, int visibility) changedView = com.android.internal.policy.impl.PhoneWindow$DecorView{3aeb2b95 I.E..... R.....ID 0,0-0,0} visibility = 4
I/TestView: onVisibilityChanged(View changedView, int visibility) changedView = com.android.internal.policy.impl.PhoneWindow$DecorView{3aeb2b95 V.E..... R.....ID 0,0-0,0} visibility = 0
I/TestView: onAttachedToWindow()
I/TestView: onWindowVisibilityChanged(int visibility) visibility = 0
I/TestView: onWindowFocusChanged(boolean hasWindowFocus) hasWindowFocus = true

销毁

I/TestView: onWindowFocusChanged(boolean hasWindowFocus) hasWindowFocus = false I/TestView: onWindowVisibilityChanged(int visibility) visibility = 8 I/TestView: onDetachedFromWindow()

下面比较下LinearLayout和RelativeLayout的性能

一个实现垂直列表的布局测试数据如下：

LinearLayout

Measure：0.738ms
Layout：0.176ms
draw：7.655ms

RelativeLayout

Measure：2.280ms
Layout：0.153ms
draw：7.696ms

考虑到误差，两者layout和draw几乎差不多，主要区别在于Measure。

1、分析源码发现，在RelativeLayout中，由于布局可以是B横向依赖A，A横向依赖B, 所以，Measure中会被Measure两次，一次是横向的Measure，一次是纵向。

而在LinearLayout中，在测量之前就判断了是横向还是纵向布局。

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

2、但是在Linearlayout中如果设置weight会影响性能，因为LinearLayout会避开设置过weight属性的view做第一次measure，完了再对设置过weight属性的view做第二次measure。

3、view中的Measure有个优化，但是如果RelativeLayout中设置的margin就会跳过这个优化，多以尽量使用padding而不是margin。

4、

.在不影响层级深度的情况下,使用LinearLayout和FrameLayout而不是RelativeLayout。
最后再思考一下文章开头那个矛盾的问题，为什么Google给开发者默认新建了个RelativeLayout，而自己却在DecorView中用了个LinearLayout。因为DecorView的层级深度是已知而且固定的，上面一个标题栏，下面一个内容栏。采用RelativeLayout并不会降低层级深度，所以此时在根节点上用LinearLayout是效率最高的。而之所以给开发者默认新建了个RelativeLayout是希望开发者能采用尽量少的View层级来表达布局以实现性能最优，因为复杂的View嵌套对性能的影响会更大一些。

参考：
作者：尹star
链接：http://www.jianshu.com/p/8a7d059da746

作者：周君宜
链接：http://www.jianshu.com/p/08e6dab7886e