Android API：自定义ViewGroup

无论是出于项目功能需求还是出于界面美观，或者是因为项目中对控件进行封装等，自定义View在我们开发中是非常常见的，
Android官方DEMO中也提供了自定义View的例子，今天学习的是自定义ViewGroup.

public class CustomLayout extends ViewGroup {
/** 子控件离左边的距离 */
private int mLeftWidth;
/** 子控件离左边的距离 */
private int mRightWidth;

/** 计算子控件容器的范围 */
private final Rect mTmpContainerRect = new Rect();
/** 计算子控件的范围 */
private final Rect mTmpChildRect = new Rect();

/*
* View的三个构造函数
*/
public CustomLayout(Context context) {
super(context);
}

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

public CustomLayout(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}

/**
* 任何不滚动的布局管理器都应该像这样重写该方法
*/
@Override
public boolean shouldDelayChildPressedState() {
return false;
}

/**
* 测量（子）控件大小
*/
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// 子控件数量
int count = getChildCount();
mLeftWidth = 0;
mRightWidth = 0;

// 设置最大高度，最大宽度及状态
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
// 遍历所有子控件，分别进行测量
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
// 测量子控件：某一个子view宽及高,
// 内部加上了viewGroup的padding值、margin值和传入的宽高widthUsed、heightUsed
measureChildWithMargins(child, widthMeasureSpec, 0,
heightMeasureSpec, 0);

// 通过LayoutParams来设置大小
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
if (lp.position == LayoutParams.POSITION_LEFT) {
mLeftWidth += Math.max(maxWidth, child.getMeasuredWidth()
+ lp.leftMargin + lp.rightMargin);
} else if (lp.position == LayoutParams.POSITION_RIGHT) {
mRightWidth += Math.max(maxWidth, child.getMeasuredWidth()
+ lp.leftMargin + lp.rightMargin);
} else {
maxWidth = Math.max(maxWidth, child.getMeasuredWidth()
+ lp.leftMargin + lp.rightMargin);
}
maxHeight = Math.max(maxHeight, child.getMeasuredHeight()
+ lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState,
child.getMeasuredState());
}
}

// 总宽度是内部所有子视图的宽度加上其他约束的宽度
maxWidth += mLeftWidth + mRightWidth;

// 检查默认的最小高度和最小宽度，取最大值
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

// 设置最终的尺寸
setMeasuredDimension(
resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
}

/**
* 设置子控件在ViewGroup中的位置
*/
@Override
protected void onLayout(boolean changed, int left, int top, int right,
int bottom) {
final int count = getChildCount();

// 视图在布局中放置的左右外边
int leftPos = getPaddingLeft();
int rightPos = right - left - getPaddingRight();

// 视图流内部的中心区域
final int middleLeft = leftPos + mLeftWidth;
final int middleRight = rightPos - mRightWidth;

// 所操作的布局的顶边和底边
final int parentTop = getPaddingTop();
final int parentBottom = bottom - top - getPaddingBottom();

for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();

// 测量宽度和测量高度
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();

// 计算正在放置子视图的窗体
if (lp.position == LayoutParams.POSITION_LEFT) {
mTmpContainerRect.left = leftPos + lp.leftMargin;
mTmpContainerRect.right = leftPos + width + lp.rightMargin;
leftPos = mTmpContainerRect.right;
} else if (lp.position == LayoutParams.POSITION_RIGHT) {
mTmpContainerRect.right = rightPos - lp.rightMargin;
mTmpContainerRect.left = rightPos - width - lp.leftMargin;
rightPos = mTmpContainerRect.left;
} else {
mTmpContainerRect.left = middleLeft + lp.leftMargin;
mTmpContainerRect.right = middleRight - lp.rightMargin;
}
mTmpContainerRect.top = parentTop + lp.topMargin;
mTmpContainerRect.bottom = parentBottom - lp.bottomMargin;
// //通过子视图的重心值和尺寸决定其在容器内的最终布局
Gravity.apply(lp.gravity, width, height, mTmpContainerRect,
mTmpChildRect);

// 放置子视图
child.layout(mTmpChildRect.left, mTmpChildRect.top,
mTmpChildRect.right, mTmpChildRect.bottom);
}
}
}

/**
* 下面的实现部分是针对每个子视图的布局参数的，如果你不需要这些（比如说你写了一个布局管理器）不需要混合放置子视图，那么你可以删除这部分
*/
// ----------------------------------------------------------------------

/**
* 要自定义ViewGroup支持子控件的layout_margin参数，
* 则自定义的ViewGroup类必须重载generateLayoutParams
* ()函数，并且在该函数中返回一个ViewGroup.MarginLayoutParams派生类对象，这样才能使用margin参数。
*/
@Override
public LayoutParams generateLayoutParams(AttributeSet attrs) {
return new CustomLayout.LayoutParams(getContext(), attrs);
}

// 返回一组默认布局参数。没有设置布局参数执行addview（View）时，这些参数被请求时返回null，并抛出一个异常。
@Override
protected LayoutParams generateDefaultLayoutParams() {
return new LayoutParams(LayoutParams.MATCH_PARENT,
LayoutParams.MATCH_PARENT);
}

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

@Override
protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
return p instanceof LayoutParams;
}

/**
* 自定义子控件的LayoutParams
*/
public static class LayoutParams extends MarginLayoutParams {
// 设置居中方式
public int gravity = Gravity.TOP | Gravity.START;

public static int POSITION_MIDDLE = 0;
public static int POSITION_LEFT = 1;
public static int POSITION_RIGHT = 2;

public int position = POSITION_MIDDLE;

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

// 获取定义在XML中的自定义属性
TypedArray a = c.obtainStyledAttributes(attrs,
R.styleable.CustomLayoutLP);
gravity = a.getInt(
R.styleable.CustomLayoutLP_android_layout_gravity, gravity);
position = a.getInt(R.styleable.CustomLayoutLP_layout_position,
position);
a.recycle();
}

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

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

把定义好的ViewGroup当作我们通常使用的诸如LinearLayout等控件一样使用就可以啦。