自定义ViewGroup考虑padding，margin

在View.java中：

public final void measure(int widthMeasureSpec,int heightMeasureSpec){

...

onMeasure();

...

}

protected void onMeasure(int widthMeasureSpec,int heightMeasureSpec) {

setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),

getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));

}

public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);

switch (specMode) {
case MeasureSpec.UNSPECIFIED: //未指定
result = size;
break;
case MeasureSpec.AT_MOST: //至多
case MeasureSpec.EXACTLY: //精确
result = specSize;
break;
}
return result;
}

在ViewGroup中：

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}

protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();

final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);

child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);

child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

说明：

1. measure是final修饰的方法，不可被重写。

在外部直接调用view.measure(int wSpec, int hSpec)来对view进行自身宽高的测量。

measure()内部调用onMeasure(), 所以自定义view时，重写onMeasure即可。

2.MeasureSpec 这是一个含mode和size的结合体，不需要我们来具体的关心。

当在测量时，可以调用MeasureSpec.getSize | getMode() 得到相应的size和mode。

然后使用MeasureSpec.makeMeasureSpec(size, mode); 来创建MeasureSpec对象。

mode是根据使用该自定义view时的layout_with| height参数决定的，不能随便new一个。

size可以自己指定，也可以直接使用 measureSpec.getSize()。

3.如果是一个View，重写onMeasure时要注意：

如果在使用自定义view时，用了wrap_content。那么在onMeasure中就要调用setMeasuredDimension，

来指定view的宽高。如果使用的fill_parent或者一个具体的dp值。那么直接使用super.onMeasure即可。

4.如果是一个ViewGroup，重写onMeasure时要注意：

首先，结合上面的介绍测量子View的宽高。

然后，结合子view的测量宽高来设置自身的测量宽高。

测量子view的方式有：

getChildAt(int index).可以拿到index上的子view。

通过getChildCount()得到直接子view的数目，再循环遍历出子subView。

接着，subView.measure(int wSpec, int hSpec); // 使用子view自身的测量方法

或者调用viewGroup的测量子view的方法：

// 某一个子view，宽，高, 内部加上了viewGroup的padding值

measureChild(subView, int wSpec, int hSpec);

//所有子view 宽，高, 内部调用了measureChild方法

measureChildren(int wSpec, int hSpec);

//某一个子view，宽，高, 内部加上了viewGroup的padding值、margin值和传入的宽高wUsed、hUsed

measureChildWithMargins(subView, intwSpec, int wUsed, int hSpec, int hUsed);

ViewGroup中的子View 不支持margin

 
 
  
  总结两点
  
  自定义View在onDraw里面需要处理padding的影响，widthMeasureSpec和heightMeasureSpec是包含padding大小的。
子View的margin属性是由ViewGroup处理的，ViewGroup在onMeasure和onLayout时一定要考虑 ViewGroup自己的padding和子View的margin的影响。

 
 
  
  你可能遇到过下面这样的错误。

  
  java.lang.ClassCastException: android.view.ViewGroup$LayoutParams cannot be cast to android.view.ViewGroup$MarginLayoutParams

  
  
  
  下面我们分析为什么会遇到这种错误以及解决方法。
  
  你可能见过很多人在自定义ViewGroup的
onMeasure()中使用
measureChildren(widthMeasureSpec, heightMeasureSpec); 来测量所有子View的尺寸。
  
  ViewGroup.measureChildren的源码如下：
  
  final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
    final View child = children[i];
    if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
        // ******************* 注意这里 ********************
        measureChild(child, widthMeasureSpec, heightMeasureSpec);
    }
}

  
  measureChild是不是不太合适呢，查阅了FrameLayout和LinearLayout等都没有用过这个measureChildren呢，几乎全部都重写了，我们的自定义ViewGroup的measureChildren是不是应该是改成下面这样才对。
  
  final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
    final View child = children[i];
    if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
        // ******************* 注意这里 ********************
        measureChildWithMargins(child, widthMeasureSpec, heightMeasureSpec);
    }
}

  
  你应该看到了区别，measureChild和measureChildWithMargins区别就是
测量child尺寸时，保证child的 最大可用尺寸，感觉这个with前缀起的不太好。
  
  measureChild减去了 ViewGroup的padding 保证child最大可用空间
measureChildWithMargins减去了ViewGroup的padding和子View的margin 保证child最大可用空间
  
  至于 measureChild和measureChildWithMargins中是如何**生成child的MeasureSpec，并最终调用child.measure() -- > child.onMeasure()的，这里就不贴源码了。
  
  
   
   总结 : ViewGroup中测量child一定要用measureChildWithMargins而不是measureChild
  
  
  
  使用measureChildWithMargins后却产生异常
终于改成measureChildWithMargins了，却突然产生了异常，这是为什么？
找到异常产生的位置，追踪到ViewGroup.addView()方法，源码如下：
  
  public void addView(View child, int index) {
    if (child == null) {
        throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
    }
    LayoutParams params = child.getLayoutParams();
    if (params == null) {
        // **************** 注意这里 ****************
        params = generateDefaultLayoutParams();
        if (params == null) {
            throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
        }
    }
    addView(child, index, params);
}

  
  异常信息是 ClassCastException
cannot be cast to android.view.ViewGroup$MarginLayoutParams
而addView中,如果child.getLayoutParams();获取不到，则默认生成一个
generateDefaultLayoutParams();
  
  protected LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }

  
  这个默认生成的肯定不能强制转换为MarginLayoutParams了。
  
  再来看addView中的其他方法
  
  private void addViewInner(View child, int index, LayoutParams params,boolean preventRequestLayout) {
        if (!checkLayoutParams(params)) {
            // **************** 注意这里 ****************
            params = generateLayoutParams(params);
        }

        if (preventRequestLayout) {
            child.mLayoutParams = params;
        } else {
            child.setLayoutParams(params);
        }

        if (index < 0) {
            index = mChildrenCount;
        }

        addInArray(child, index);
        ................
        ................
}


  
  里面还有检测这个child的LayoutParams 是不是为空的，干脆全部重写得了。
  
  在你的自定义ViewGroup中加入如下代码即可令 子View 的margin生效。
  
  public class MyViewGroup extends ViewGroup {
    // ..................... 其他代码省略 .....................
    
    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new MyLayoutParams(getContext(), attrs);
    }

    @Override
    protected ViewGroup.LayoutParams generateLayoutParams(ViewGroup.LayoutParams lp) {
        return new MyLayoutParams(lp);
    }

    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        return new MyLayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }

    public static class MyLayoutParams extends MarginLayoutParams {

        public MyLayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
        }

        public MyLayoutParams(int width, int height) {
            super(width, height);
        }

        public MyLayoutParams(LayoutParams lp) {
            super(lp);
        }
    }
}

  
  另外在ViewGroup.onLayout()时中千万别忘记根据 ViewGroup的padding和子View的margin 灵活给子View布局。

ViewGroupX.java

public class ViewGroupX extends ViewGroup {
private int mPaddingLeft;
private int mPaddingTop;
private int mPaddingRight;
private int mPaddingBottom;

public ViewGroupX(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}

public ViewGroupX(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
/**
* getPaddingLeft() 当前容器的paddingLeft
* widthMeasureSpec.size 当前容器的宽度(parentWidth - parentPadding - this.margin)
* view.getMeasuredHeight() (widthMeasureSpec.size - this.padding - viewMargin)
*/
mPaddingLeft = getPaddingLeft();
mPaddingTop = getPaddingTop();
mPaddingRight = getPaddingRight();
mPaddingBottom = getPaddingBottom();

int desireWidth = 0, desireHeight = 0;
int childCount = getChildCount(); // 直接子元素的个数
for (int i = 0; i < childCount; i++) {
View view = getChildAt(i);
if (view.getVisibility() != GONE) {
LayoutParams params = (LayoutParams) view.getLayoutParams();
measureChildWithMargins(view, widthMeasureSpec, 0, heightMeasureSpec, 0);
desireWidth = Math.max(desireWidth, view.getMeasuredWidth());
desireHeight += view.getMeasuredHeight() + (params.bottomMargin + params.topMargin);
}
}
// count with padding
desireWidth += mPaddingLeft + mPaddingRight;
desireHeight += mPaddingTop + mPaddingBottom;

// see if the size is big enough
desireWidth = Math.max(desireWidth, getSuggestedMinimumWidth());
desireHeight = Math.max(desireHeight, getSuggestedMinimumHeight());

setMeasuredDimension(resolveSize(desireWidth, widthMeasureSpec), resolveSize(desireHeight, heightMeasureSpec));
}

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
/**
* l this(相对父布局)left
* r this(相对父布局)right
*/
int top = mPaddingTop;
int childCount = getChildCount(); // 直接子元素的个数
for (int i = 0; i < childCount; i++) {
View view = getChildAt(i);
if (view.getVisibility() != GONE) {
LayoutParams params = (LayoutParams) view.getLayoutParams();

int viewLeft = mPaddingLeft + params.leftMargin;
int viewTop = top + params.topMargin;

view.layout(viewLeft, viewTop, viewLeft + view.getMeasuredWidth(), viewTop + view.getMeasuredHeight());
top += params.topMargin + view.getMeasuredHeight() + params.bottomMargin;
}
}
}

@Override
protected android.view.ViewGroup.LayoutParams generateDefaultLayoutParams() {
return new LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
ViewGroup.LayoutParams.MATCH_PARENT);
}

@Override
public android.view.ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) {
return new LayoutParams(getContext(), attrs);
}

@Override
protected android.view.ViewGroup.LayoutParams generateLayoutParams(
android.view.ViewGroup.LayoutParams p) {
return new LayoutParams(p);
}

public static class LayoutParams extends MarginLayoutParams {

public LayoutParams(Context c, AttributeSet attrs) {
super(c, attrs);
}

public LayoutParams(int width, int height) {
super(width, height);
}

public LayoutParams(android.view.ViewGroup.LayoutParams source) {
super(source);
}
}
}

LineViewGroup.java

public class LineViewGroup extends ViewGroup {

private int mPaddingLeft;
private int mPaddingTop;
private int mPaddingRight;
private int mPaddingBottom;

public LineViewGroup(Context context, AttributeSet attrs) {
super(context, attrs);
}

public LineViewGroup(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int desireWidth = 0, desireHeight = 0;

mPaddingLeft = getPaddingLeft();
mPaddingTop = getPaddingTop();
mPaddingRight = getPaddingRight();
mPaddingBottom = getPaddingBottom();

int childCount = getChildCount(); // 直接子元素的个数
for (int i = 0; i < childCount; i++) {
View view = getChildAt(i);
if (view.getVisibility() != GONE) {
ViewGroupX.LayoutParams params = (ViewGroupX.LayoutParams) view.getLayoutParams();
measureChildWithMargins(view, widthMeasureSpec, 0, heightMeasureSpec, 0);
desireWidth = Math.max(desireWidth, view.getMeasuredWidth());
desireHeight += view.getMeasuredHeight() + params.bottomMargin + params.topMargin;
}
}
// count with padding
desireWidth += mPaddingLeft + mPaddingRight;
desireHeight += mPaddingTop + mPaddingBottom;

// see if the size is big enough
desireWidth = Math.max(desireWidth, getSuggestedMinimumWidth());
desireHeight = Math.max(desireHeight, getSuggestedMinimumHeight());

setMeasuredDimension(resolveSize(desireWidth, widthMeasureSpec), resolveSize(desireHeight, heightMeasureSpec));
}

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
/**
* l this相对父布局的left
*/
int top = mPaddingTop;
int childCount = getChildCount(); // 直接子元素的个数
for (int i = 0; i < childCount; i++) {
View view = getChildAt(i);
if (view.getVisibility() != GONE) {
ViewGroupX.LayoutParams params = (ViewGroupX.LayoutParams) view.getLayoutParams();

int viewLeft = mPaddingLeft + params.leftMargin;
int viewTop = top + params.topMargin;
view.layout(viewLeft, viewTop, viewLeft + view.getMeasuredWidth(), viewTop + view.getMeasuredHeight());

top += params.topMargin + view.getMeasuredHeight() + params.bottomMargin;
}
}
}

@Override
protected android.view.ViewGroup.LayoutParams generateDefaultLayoutParams() {
return new ViewGroupX.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
ViewGroup.LayoutParams.MATCH_PARENT);
}

@Override
public android.view.ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) {
return new ViewGroupX.LayoutParams(getContext(), attrs);
}

@Override
protected android.view.ViewGroup.LayoutParams generateLayoutParams(
android.view.ViewGroup.LayoutParams p) {
return new ViewGroupX.LayoutParams(p);
}
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:orientation="vertical"
android:padding="@dimen/bottom_padding"
android:layout_width="match_parent"
android:layout_height="match_parent">
<com.lenovo.ext.views.ViewGroupX
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="#123"
android:layout_margin="@dimen/margin"
android:padding="@dimen/view_group_padding">

<LinearLayout
android:id="@+id/top_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="#234"
android:layout_margin="@dimen/layout_padding"
android:orientation="vertical">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/view_group_text"/>
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="#FFF"
android:src="@mipmap/ic_launcher"/>

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/view_group_text"/>
</LinearLayout>

<LinearLayout
android:id="@+id/middle_layout"
android:background="#749"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/view_group_middle_text"/>
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="#000"
android:src="@mipmap/ic_launcher_foreground"/>

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/view_group_middle_text"/>
</LinearLayout>

<com.lenovo.ext.views.LineViewGroup
android:layout_margin="@dimen/line_padding"
android:padding="@dimen/line_padding"
android:background="#264"
android:layout_width="match_parent"
android:layout_height="wrap_content">

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/view_group_bottom_text"/>
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="#000"
android:src="@mipmap/ic_launcher_round"/>

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/view_group_bottom_text"/>
</com.lenovo.ext.views.LineViewGroup>

</com.lenovo.ext.views.ViewGroupX>
</LinearLayout>
</ScrollView>

dimens.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
<dimen name="bottom_height">120dp</dimen>
<dimen name="layout_padding">10dp</dimen>
<dimen name="padding">5dp</dimen>
<dimen name="margin">5dp</dimen>
<dimen name="line_padding">7dp</dimen>
<dimen name="view_group_padding">15dp</dimen>
<dimen name="bottom_padding">8dp</dimen>
<dimen name="divider_height">0.5dp</dimen>
</resources>

string.xml

<resources>
<string name="app_name">DateEngine</string>

<string name="view_group_text">因此只有hashCode()的低位参加运算，发生不同的hash值，但是得到的index相同的情况的几率会大大增加，这种情况称之为hash碰撞。即，碰撞率会增大。</string>

<string name="view_group_middle_text">对于扩容导致需要新建数组存放更多元素时，除了要将老数组中的元素迁移过来，也记得将老数组中的引用置null，以便GC</string>

<string name="view_group_bottom_text">当手指MotionEvent.ACTION_DOWN，需要确定手指点击的位置是否在两个滑块中的其中一个。而滑块实际上是通过 Canvas.drawRect() 函数绘制的矩形，于是可以将手指触摸的坐标与左右滑块的 top、bottom、left 和 right 进行判断，如果触摸，将 Slider.isTouching 设置为false</string>
</resources>

自定义ViewGroup考虑padding，margin

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