Android View 绘制过程

电玩女神 2022-08-10 00:57 194阅读 0赞

Android View 的绘制 的基本过程是:

1. Measure 测量, 即确定View 的大小

2. Layout 布局,即确定View 的摆放位置

3. Draw, 画View

首先看Measure

关于Measure 放方法有:

public final void measure(int widthMeasureSpec, int heightMeasureSpec)

final方法 , 不能重写

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)

可以重写, 若想影响测量 课在次方法中做处理

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight)

最终决定 View 大小的方法, 该不放不能重写, 且必须在 onMeasure 调用

在ViewGroup 中还有一些 关于Measure的方法:

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec)

循环遍历 调用 measureChild 方法

protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec)

在这个方法中 会调用子View 的 measure 方法去测量子类

ok 上面说了一些 Measure的相关方法, 用到最多的一班还只是 onMeasure 方法,自己改变View 的测量的大小

需要有一点需要注意的是 在onMeasure 中必须要调用 setMeasuredDimension 方法不然会报错

下面来看看 View 默认的onMeasure 方法:

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

只是的简单的调用了 setMeasuredDimension 方法,  我在onMeasure 方法中需要计算好大小, 再调用 setMeasuredDimension 方法

我们看到上面  哪些关于 Measure方法参数 都是int 行, 但是他们 不是一个 简单的 宽度高低, 还有有些其他的东西再里面:

例如:

widthMeasureSpec 这里有两个数据,  前3位代表, 测量模式,  后面数据代表宽度:

测量模式有:

public static class MeasureSpec {
            private static final int MODE_SHIFT = 30;// 移动的位数
            private static final int MODE_MASK  = 0x3 << MODE_SHIFT;// mask 0011 向右移动30位
    
            public static final int UNSPECIFIED = 0 << MODE_SHIFT;// 0000  向右移动30位
            public static final int EXACTLY     = 1 << MODE_SHIFT;// 0001向右移动30位
            public static final int AT_MOST     = 2 << MODE_SHIFT;// 0010向右移动30位
            // 向要知道 是什么模式可以调用:
            public static int getMode(int measureSpec) {
                return (measureSpec & MODE_MASK);
            }
           // 获取具体尺寸
           public static int getSize(int measureSpec) {
                return (measureSpec & ~MODE_MASK);
            }
    
         // 自己 做一个这个int 数据, 传入 尺寸 和 模式就好了
         public static int makeMeasureSpec(int size, int mode) {
                if (sUseBrokenMakeMeasureSpec) {
                    return size + mode;
                } else {
                    return (size & ~MODE_MASK) | (mode & MODE_MASK);
                }
          }
    }

具体的绘制过程只能很笼统的说一下:

在RootView 中, 会去调用 ViewGroup的measure方法:

然后在View Group  在onMeasure 中 又会依次调用子View 的measure 方法

ViewGroup遍历完后, 也会计算处自己的大小最后调用 setMeasuredDimension 方法

最后子View 调用 onMeasure setMeasuredDimension 方法

View 的大小 是子View 和父View共同决定, 我们在xml 中给出的width height 知识期望值, 在别的博客上看到一张表表示的还不错:

<table style="border:1px solid silver; border-collapse:collapse; color:rgb(75,75,75); font-family:verdana,Arial,helvetica,sans-seriff; font-size:12px; padding:0pt 5.4pt; width:598px"> 
 <tbody> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border:0.5pt solid rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt; width:28.929765886287623%"> <p style="padding-top:0px; padding-bottom:0px; text-align:center"><span style="font-size:10.5pt; font-family:宋体">父视图能力尺寸</span></p> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-top-width:0.5pt; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:solid solid solid none; border-top-color:rgb(0,0,0); border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt; width:42.474916387959865%"> <p style="padding-top:0px; padding-bottom:0px; text-align:center"><span style="font-size:10.5pt; font-family:宋体">子视图期望尺寸</span></p> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-top-width:0.5pt; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:solid solid solid none; border-top-color:rgb(0,0,0); border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt; width:28.428093645484946%"> <p style="padding-top:0px; padding-bottom:0px; text-align:center"><span style="font-size:10.5pt; font-family:宋体">子视图最终允许尺寸</span></p> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">EXACTLY&nbsp;+&nbsp;Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">EXACTLY&nbsp;+&nbsp;Size2</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">EXACTLY&nbsp;+&nbsp;Size2</span> 
    </div> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">EXACTLY&nbsp;+&nbsp;Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">fill_parent/match_parent</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:'Times New Roman'">EXACTLY</span> 
     <span style="font-size:10.5pt; font-family:宋体">+Size1</span> 
    </div> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">EXACTLY&nbsp;+&nbsp;Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">wrap_content</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:'Times New Roman'">AT_MOST</span> 
     <span style="font-size:10.5pt; font-family:宋体">+Size1</span> 
    </div> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:'Times New Roman'">AT_MOST</span> 
     <span style="font-size:10.5pt; font-family:宋体">+Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">EXACTLY&nbsp;+&nbsp;Size2</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:'Times New Roman'">EXACTLY</span> 
     <span style="font-size:10.5pt; font-family:宋体">+Size2</span> 
    </div> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:'Times New Roman'">AT_MOST</span> 
     <span style="font-size:10.5pt; font-family:宋体">+Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">fill_parent/match_parent</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:'Times New Roman'">AT_MOST</span> 
     <span style="font-size:10.5pt; font-family:宋体">+Size1</span> 
    </div> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:'Times New Roman'">AT_MOST</span> 
     <span style="font-size:10.5pt; font-family:宋体">+Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">wrap_content</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:'Times New Roman'">AT_MOST</span> 
     <span style="font-size:10.5pt; font-family:宋体">+Size1</span> 
    </div> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <span style="font-family:宋体; font-size:10.5pt"><br> </span> 
    </div> 
    <div style="margin-left:40px"> 
     <span style="font-family:宋体; font-size:10.5pt">UNSPECIFIED+Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">EXACTLY&nbsp;+&nbsp;Size2</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">EXACTLY&nbsp;+&nbsp;Size2</span> 
    </div> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">UNSPECIFIED+Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">fill_parent/match_parent</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">UNSPECIFIED+0</span> 
    </div> </td> 
  </tr> 
  <tr> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-left-width:0.5pt; border-style:none solid solid; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-left-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">UNSPECIFIED+Size1</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">wrap_content</span> 
    </div> </td> 
   <td style="font-family:Verdana,Helvetica,Arial; word-break:break-all; border-right-width:0.5pt; border-bottom-width:0.5pt; border-style:none solid solid none; border-right-color:rgb(0,0,0); border-bottom-color:rgb(0,0,0); border-collapse:collapse; padding:0pt 5.4pt"> 
    <div style="margin-left:40px"> 
     <br> 
    </div> 
    <div style="padding:0px; margin-left:40px"> 
     <span style="font-size:10.5pt; font-family:宋体">UNSPECIFIED+0</span> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

ok 下面说说layout 方法:

关于Layout的方法有:

public void layout(int l, int t, int r, int b)

这个方法是View 的方法

protected abstract void onLayout(boolean changed,int l, int t, int r, int b);

ViewGroup的方法且 ViewGroup 必须实现该方法

public void layout(int l, int t, int r, int b)中得  l、t、r、b是指view的左left、上top、右right、底bottom的位置

View 的具体位置是有 ViewGroup中得 onLayout 方法决定

在ViewGroup中的onLayout方法需要依次的遍历子View 调用他们的Layout方法

下面可以看看 LinearLayout 的onLayout方法

@Override
      protected void onLayout(boolean changed, int l, int t, int r, int b) {
          if (mOrientation == VERTICAL) {
              layoutVertical();
          } else {
              layoutHorizontal();
          }
      }
    void layoutVertical() {
          final int paddingLeft = mPaddingLeft;
    
          int childTop;
          int childLeft;
    
          // Where right end of child should go
          final int width = mRight - mLeft;
          int childRight = width - mPaddingRight;
    
          // Space available for child
          int childSpace = width - paddingLeft - mPaddingRight;
    
          final int count = getVirtualChildCount();
    
          final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
          final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
    
          switch (majorGravity) {
             case Gravity.BOTTOM:
                 // mTotalLength contains the padding already
                 childTop = mPaddingTop + mBottom - mTop - mTotalLength;
                 break;
    
                 // mTotalLength contains the padding already
             case Gravity.CENTER_VERTICAL:
                 childTop = mPaddingTop + (mBottom - mTop - mTotalLength) / 2;
                 break;
    
             case Gravity.TOP:
             default:
                 childTop = mPaddingTop;
                 break;
          }
    
          for (int i = 0; i < count; i++) {
              final View child = getVirtualChildAt(i);
              if (child == null) {
                  childTop += measureNullChild(i);
              } else if (child.getVisibility() != GONE) {
                  final int childWidth = child.getMeasuredWidth();
                  final int childHeight = child.getMeasuredHeight();
    
                  final LinearLayout.LayoutParams lp =
                          (LinearLayout.LayoutParams) child.getLayoutParams();
    
                  int gravity = lp.gravity;
                  if (gravity < 0) {
                      gravity = minorGravity;
                  }
                  final int layoutDirection = getLayoutDirection();
                  final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                  switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                      case Gravity.CENTER_HORIZONTAL:
                          childLeft = paddingLeft + ((childSpace - childWidth) / 2)
                                  + lp.leftMargin - lp.rightMargin;
                          break;
    
                      case Gravity.RIGHT:
                          childLeft = childRight - childWidth - lp.rightMargin;
                          break;
    
                      case Gravity.LEFT:
                      default:
                          childLeft = paddingLeft + lp.leftMargin;
                          break;
                  }
    
                  if (hasDividerBeforeChildAt(i)) {
                      childTop += mDividerHeight;
                  }
    
                  childTop += lp.topMargin;
                  setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                          childWidth, childHeight);
                  childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
    
                  i += getChildrenSkipCount(child, i);
              }
          }
      }
    private void setChildFrame(View child, int left, int top, int width, int height) {
        child.layout(left, top, left + width, top + height);
    }

在Layout中做的就是 计算好位置, 然后调用子ViewLayout方法

下面在来看看Draw 的过程,

当测量完了, 宽高弄好了, 位置也确定了, 最后就是 画View 了

draw 相关的方法有;

public void draw(Canvas canvas)

protected void onDraw(Canvas canvas)

View 的Draw 方法中有6个步骤

/*
            * Draw traversal performs several drawing steps which must be executed
            * in the appropriate order:
            *
            *      1. Draw the background
            *      2. If necessary, save the canvas' layers to prepare for fading
            *      3. Draw view's content
            *      4. Draw children
            *      5. If necessary, draw the fading edges and restore layers
            *      6. Draw decorations (scrollbars for instance)
            */

在第三步 Draw view's content 调用onDraw方法，子类中实现onDraw方法。

在第四步，Draw children步骤使用的dispatchDraw方法，这个方法在ViewGroup中有实现。

View或ViewGroup的子类不用再重载ViewGroup中该方法，因为它已经有了默认而且标准的view系统流程。dispatchDraw()内部for循环调用drawChild()分别绘制每一个子视图，而drawChild()内部又会调用draw（）函数完成子视图的内部绘制工作

ok 上面基本就是 绘制View 的一个大概的粗擦的流程

写的不是很好, 但是谢了总归会加深写印象,  一个对源码有更深的探究时, 还有在重新写一遍

产考一下文章:

[http://blog.csdn.net/xyz\_lmn/article/details/20385049][http_blog.csdn.net_xyz_lmn_article_details_20385049]

[http_blog.csdn.net_xyz_lmn_article_details_20385049]: http://blog.csdn.net/xyz_lmn/article/details/20385049