ViewGroup是View的容器类，里面会包含多个View。经常用的LinearLayout，RelativeLayout等都是ViewGroup的子类。
还是从方法开始说明ViewGroup，Android 自定义View（二）函数分析 中已经有说明了一下方法函数的意思，ViewGroup的实现方法有必要的两个 onMeasure 和 onLayout 和自定义View的不同的是：
onDraw在自定义ViewGroup是，一般是调用了子类的onDraw方法，ViewGroup是View的容器，本身一般不需要draw额外的修饰，所以往往在onDraw方法里面，只需要调用ViewGroup的onDraw默认实现方法即可。（自定义View时onLayout是空方法，ViewGroup是onLayout却是必须实现的）
onMeasure
Measure过程还是测量ViewGroup的大小，如果layout_widht和layout_height是match_parent或具体的dp大小，直接调用setMeasuredDimension()方法，设置ViewGroup的宽高即可，如果是wrap_content，我们需要遍历所有的子View，然后对每个子View进行测量，然后根据子View的排列规则，计算出最终ViewGroup的大小。onLayout
layout过程其实就是对子View的位置进行排列，onLayout方法给我一个机会，来按照我们想要的规则自定义子View排列。
一个简单的栗子（ViewGroup 中的 View 排成一列）看下 onMeasure 和 onLayout 的代码实现：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec,heightMeasureSpec);
measureChildren(widthMeasureSpec,heightMeasureSpec);
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
int height = 0;
View child;
for(int i = 0,size = getChildCount();i < size;i++) {
child = getChildAt(i);
child.layout(0, height, child.getMeasuredWidth(),height + child.getMeasuredHeight());
height += child.getMeasuredHeight();
}
}
要实现View一列显示，然后每个子View的宽度是一样的，并且每个子View的left和right是一样的。所以每个子View只有top和bottom不一样。我们首先定义个高度height初始为0，然后得到所有子View的个数，依次设置每个子View的top和bottom。top就是定义的height,bottom则为height加上子View的高度。设置完后height累加。
LayoutParams
LayoutParams存储了子View在加入ViewGroup中时的一些参数信息，在继承ViewGroup类时，一般也需要新建一个新的LayoutParams类，就像SDK中我们熟悉的LinearLayout.LayoutParams，RelativeLayout.LayoutParams类等一样，那么可以这样做，在你定义的ViewGroup子类中，新建一个LayoutParams类继承与ViewGroup.LayoutParams。实际使用：
@Override
protected void onLayout(boolean arg0, int arg1, int arg2, int arg3, int arg4) {
int childCount = this.getChildCount();
for (int i = 0; i < childCount; i++) {
View child = this.getChildAt(i);
LayoutParams lParams = (LayoutParams) child.getLayoutParams();
child.layout(lParams.left, lParams.top, lParams.left + childWidth,
lParams.top + childHeight);
}
}
实例应用
自定义一个根据屏幕宽度自动换行的ViewGroup：
public class LineBreakLayout extends ViewGroup implements View.OnClickListener {
private final static int VIEW_MARGIN = 2;
private int widthMargin = VIEW_MARGIN;//view width space
private int heightMargin = VIEW_MARGIN;//view height space
private LineLayoutItemListener mListener;
public LineBreakLayout(Context context) {
super(context);
}
public LineBreakLayout(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
public LineBreakLayout(Context context, AttributeSet attrs) {
super(context, attrs);
}
public void setViewMargin(int widthMargin, int heightMargin) {
this.widthMargin = widthMargin;
this.heightMargin = heightMargin;
}
public void setOnLineLayoutItemListener(LineLayoutItemListener listener) {
mListener = listener;
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//得到ViewGroup的初始宽高
final int width = MeasureSpec.getSize(widthMeasureSpec);
int height = MeasureSpec.getSize(heightMeasureSpec) + getPaddingBottom()+getPaddingTop();
int line_height = 0;
//获取第一个子View的起始点位置
int xpos = getPaddingLeft();
int ypos = getPaddingTop();
//计算每一个子View的尺寸,并算出ViewGroup的高度
for (int index = 0; index < getChildCount(); index++) {
final View child = getChildAt(index);
if (child.getVisibility() == GONE) {
continue;
}
child.setId(index);
child.setOnClickListener(this);
final LayoutParams lp = child.getLayoutParams();
//算出子View宽的MeasureSpec值
int wSpec = MeasureSpec.makeMeasureSpec(lp.width, MeasureSpec.UNSPECIFIED);
//算出子View高的MeasureSpec值
int hSpec = MeasureSpec.makeMeasureSpec(lp.height, MeasureSpec.UNSPECIFIED);
//让子View记住自己宽高的MeasureSpec值,子View的
//函数传入的就是这里算出来的这两个值
child.measure(wSpec, hSpec);
//设置完MeasureSpec值后调用View.getMeasuredWidth()函数算出View的宽度
final int childw = child.getMeasuredWidth();
//记录最大行高(子View的高度有可能不一样,行高取最大高度)
line_height = Math.max(line_height, child.getMeasuredHeight() + heightMargin);
if (xpos + childw + widthMargin > width) {
//初始坐标的x偏移值+子View宽度>ViewGroup宽度 就换行
xpos = getPaddingLeft();//坐标x偏移值归零
ypos += line_height; //坐标y偏移值再加上本行的行高也就是换行
}
//算出下一个子View的起始点x偏移值
xpos += childw + widthMargin;
}
if (MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.UNSPECIFIED) {
//对高度期望值没有限制
height = ypos + line_height + heightMargin;
} else if (MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.AT_MOST) {
//达不到指定高度则缩小高度
if (ypos + line_height < height) {
height = ypos + line_height + heightMargin;
}
} else {
height = ypos + line_height + heightMargin;
}
//设置ViewGroup宽高值
setMeasuredDimension(width, height);
}
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
final int count = getChildCount();
int parentWidth = right - left;
top = 0;//clear top distance
int row = 0;// which row lay you view relative to parent
int lengthX = 0; // right position of child relative to parent
int lengthY = top; // bottom position of child relative to parent
//计算每一个子View的尺寸,并算出ViewGroup的高度
for (int i = 0; i < count; i++) {
final View child = this.getChildAt(i);
int width = child.getMeasuredWidth();
int height = child.getMeasuredHeight();
lengthX += width + widthMargin;
lengthY = row * (height + heightMargin) + heightMargin + height + top;
// if it can't drawing on a same line , skip to next line
if (lengthX > parentWidth) {
lengthX = width + widthMargin;
row ++;
lengthY = row * (height + heightMargin) + heightMargin + height + top;
}
child.layout(lengthX - width, lengthY - height, lengthX, lengthY);
}
}
@Override
public void onClick(View v) {
if (mListener != null) {
int position = v.getId();
mListener.onLineItemClick(this, v, position);
}
}
public interface LineLayoutItemListener {
void onLineItemClick(ViewGroup parent, View view, int position);
}
}