深刻理解view的measure

view的measure过程，是在viewGroup传递而来，

protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//此处代码决定measureSpec
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);
}

再来看看getChildMeasureSpec，

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

可知：
**子view的MeasureSpec取决于父view的MesureSpec和Layparams **
梳理getChildMeasuredSpec,可得到如下表格：

| childLayparams/parentSpecMode| EXACTLY| AT_MOST | UNSPECIFIED|
| :-------- : | : --------: | :–: |
| dp/px | EXACTLY/chilSize | EXACTLY/childSize |EXACTLY/childSize
| match_parent | EXACTLY/parentSize| AT_MOST/parentSize|UNSPECIFIED/0
| wrap_content | AT_MOST/parentSize | AT_MOST/parentSize|UNSPECIFIED/0

子View的宽高，在onMeasure方法中 调用setMeasureDimension， 取决于自己的Layparams和MeasureSpec中的MeasureMode。

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

再来看

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:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}

典型的例子，自定义View需要重新onMeasure方法，否则布局中的wrap_content就相当于使用match_parent，为什么呢？

如果布局中Layoutparams为wrap_contet,那么，View的specMode是AT_MOST,这种情况下view的的大小specSize就是parentSize,就是父容器目前可使用的大小，就是父容器当前剩余空间的大小，这种效果与match_parent完全一致。

怎么解决
在onMeasure方法中如果widthMode == MeasureSpec.AT_MOST，灵活设置大小即可

弘扬大神的例子，textview

	@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
{
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int width;
int height ;
if (widthMode == MeasureSpec.EXACTLY)
{
width = widthSize;
} else
{
mPaint.setTextSize(mTitleTextSize);
mPaint.getTextBounds(mTitle, 0, mTitle.length(), mBounds);
float textWidth = mBounds.width();
int desired = (int) (getPaddingLeft() + textWidth + getPaddingRight());
width = desired;
}
if (heightMode == MeasureSpec.EXACTLY)
{
height = heightSize;
} else
{
mPaint.setTextSize(mTitleTextSize);
mPaint.getTextBounds(mTitle, 0, mTitle.length(), mBounds);
float textHeight = mBounds.height();
int desired = (int) (getPaddingTop() + textHeight + getPaddingBottom());
height = desired;
}
setMeasuredDimension(width, height);
}

下面这个这个例子综合了自定义view,滑动，事件处理，滑动冲突，等多种知识，注意

public class HorizontalScrollViewEx extends ViewGroup {
private static final String TAG = "HorizontalScrollViewEx";
private int mChildrenSize;
private int mChildWidth;
private int mChildIndex;
// 分别记录上次滑动的坐标
private int mLastX = 0;
private int mLastY = 0;
// 分别记录上次滑动的坐标(onInterceptTouchEvent)
private int mLastXIntercept = 0;
private int mLastYIntercept = 0;
private Scroller mScroller;
private VelocityTracker mVelocityTracker;
public HorizontalScrollViewEx(Context context) {
super(context);
init();
}
public HorizontalScrollViewEx(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}
public HorizontalScrollViewEx(Context context, AttributeSet attrs,
int defStyle) {
super(context, attrs, defStyle);
init();
}
private void init() {
if (mScroller == null) {
mScroller = new Scroller(getContext());
mVelocityTracker = VelocityTracker.obtain();
}
}
@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
boolean intercepted = false;
int x = (int) event.getX();
int y = (int) event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN: {
intercepted = false;
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
intercepted = true;
}
break;
}
case MotionEvent.ACTION_MOVE: {
int deltaX = x - mLastXIntercept;
int deltaY = y - mLastYIntercept;
if (Math.abs(deltaX) > Math.abs(deltaY)) {
intercepted = true;
} else {
intercepted = false;
}
break;
}
case MotionEvent.ACTION_UP: {
intercepted = false;
break;
}
default:
break;
}
Log.d(TAG, "intercepted=" + intercepted);
mLastX = x;
mLastY = y;
mLastXIntercept = x;
mLastYIntercept = y;
return intercepted;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
mVelocityTracker.addMovement(event);
int x = (int) event.getX();
int y = (int) event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN: {
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
}
break;
}
case MotionEvent.ACTION_MOVE: {
int deltaX = x - mLastX;
int deltaY = y - mLastY;
scrollBy(-deltaX, 0);
break;
}
case MotionEvent.ACTION_UP: {
int scrollX = getScrollX();
mVelocityTracker.computeCurrentVelocity(1000);
float xVelocity = mVelocityTracker.getXVelocity();
if (Math.abs(xVelocity) >= 50) {
mChildIndex = xVelocity > 0 ? mChildIndex - 1 : mChildIndex + 1;
} else {
mChildIndex = (scrollX + mChildWidth / 2) / mChildWidth;
}
mChildIndex = Math.max(0, Math.min(mChildIndex, mChildrenSize - 1));
int dx = mChildIndex * mChildWidth - scrollX;
smoothScrollBy(dx, 0);
mVelocityTracker.clear();
break;
}
default:
break;
}
mLastX = x;
mLastY = y;
return true;
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int measuredWidth = 0;
int measuredHeight = 0;
final int childCount = getChildCount();
measureChildren(widthMeasureSpec, heightMeasureSpec);
int widthSpaceSize = MeasureSpec.getSize(widthMeasureSpec);
int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int heightSpaceSize = MeasureSpec.getSize(heightMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
if (childCount == 0) {
setMeasuredDimension(0, 0);
} else if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
final View childView = getChildAt(0);
measuredWidth = childView.getMeasuredWidth() * childCount;
measuredHeight = childView.getMeasuredHeight();
setMeasuredDimension(measuredWidth, measuredHeight);
} else if (heightSpecMode == MeasureSpec.AT_MOST) {
final View childView = getChildAt(0);
measuredHeight = childView.getMeasuredHeight();
setMeasuredDimension(widthSpaceSize, childView.getMeasuredHeight());
} else if (widthSpecMode == MeasureSpec.AT_MOST) {
final View childView = getChildAt(0);
measuredWidth = childView.getMeasuredWidth() * childCount;
setMeasuredDimension(measuredWidth, heightSpaceSize);
}
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
int childLeft = 0;
final int childCount = getChildCount();
mChildrenSize = childCount;
for (int i = 0; i < childCount; i++) {
final View childView = getChildAt(i);
if (childView.getVisibility() != View.GONE) {
final int childWidth = childView.getMeasuredWidth();
mChildWidth = childWidth;
childView.layout(childLeft, 0, childLeft + childWidth,
childView.getMeasuredHeight());
childLeft += childWidth;
}
}
}
private void smoothScrollBy(int dx, int dy) {
mScroller.startScroll(getScrollX(), 0, dx, 0, 500);
invalidate();
}
@Override
public void computeScroll() {
if (mScroller.computeScrollOffset()) {
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
postInvalidate();
}
}
@Override
protected void onDetachedFromWindow() {
mVelocityTracker.recycle();
super.onDetachedFromWindow();
}
}
//===================================================================
public class DemoActivity_2 extends Activity {
private static final String TAG = "DemoActivity_2";
private HorizontalScrollViewEx mListContainer;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.demo_2);
Log.d(TAG, "onCreate");
initView();
}
private void initView() {
LayoutInflater inflater = getLayoutInflater();
mListContainer = (HorizontalScrollViewEx) findViewById(R.id.container);
final int screenWidth = MyUtils.getScreenMetrics(this).widthPixels;
final int screenHeight = MyUtils.getScreenMetrics(this).heightPixels;
for (int i = 0; i < 3; i++) {
ViewGroup layout = (ViewGroup) inflater.inflate(
R.layout.content_layout, mListContainer, false);
layout.getLayoutParams().width = screenWidth;
TextView textView = (TextView) layout.findViewById(R.id.title);
textView.setText("page " + (i + 1));
layout.setBackgroundColor(Color.rgb(255 / (i + 1), 255 / (i + 1), 0));
createList(layout);
mListContainer.addView(layout);
}
}
private void createList(ViewGroup layout) {
ListView listView = (ListView) layout.findViewById(R.id.list);
ArrayList<String> datas = new ArrayList<String>();
for (int i = 0; i < 50; i++) {
datas.add("name " + i);
}
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
R.layout.content_list_item, R.id.name, datas);
listView.setAdapter(adapter);
listView.setOnItemClickListener(new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
Toast.makeText(DemoActivity_2.this, "click item",
Toast.LENGTH_SHORT).show();
}
});
}
}

再写一个例子，自定义ViewGroup最重要的就是onMeasure和onLayout

public class FlowLayout extends ViewGroup {
public FlowLayout(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
//
}
public FlowLayout(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public FlowLayout(Context context) {
this(context, null);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int sizeWidth = MeasureSpec.getSize(widthMeasureSpec);
int modeWidth = MeasureSpec.getMode(widthMeasureSpec);
int sizeHeight = MeasureSpec.getSize(heightMeasureSpec);
int modeHeight = MeasureSpec.getMode(heightMeasureSpec);
int width = 0;
int height = 0;
int lineWidth = 0;
int lineHeight = 0;
int cCount = getChildCount();
for (int i = 0; i < cCount; i++) {
View child = getChildAt(i);
measureChild(child, widthMeasureSpec, heightMeasureSpec);
MarginLayoutParams lp = (MarginLayoutParams) child
.getLayoutParams();
int childWidth = child.getMeasuredWidth() + lp.leftMargin
+ lp.rightMargin;
int childHeight = child.getMeasuredHeight() + lp.topMargin
+ lp.bottomMargin;
if (lineWidth + childWidth > sizeWidth - getPaddingLeft() - getPaddingRight()) {
width = Math.max(width, lineWidth);
lineWidth = childWidth;
height += lineHeight;
lineHeight = childHeight;
} else {
lineWidth += childWidth;
lineHeight = Math.max(lineHeight, childHeight);
}
if (i == cCount - 1) {
width = Math.max(lineWidth, width);
height += lineHeight;
}
}
setMeasuredDimension(
//
modeWidth == MeasureSpec.EXACTLY ? sizeWidth : width + getPaddingLeft() + getPaddingRight(),
modeHeight == MeasureSpec.EXACTLY ? sizeHeight : height + getPaddingTop() + getPaddingBottom()//
);
}
private List<List<View>> mAllViews = new ArrayList<List<View>>();
private List<Integer> mLineHeight = new ArrayList<Integer>();
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
mAllViews.clear();
mLineHeight.clear();
int width = getWidth();
int lineWidth = 0;
int lineHeight = 0;
List<View> lineViews = new ArrayList<View>();
int cCount = getChildCount();
for (int i = 0; i < cCount; i++) {
View child = getChildAt(i);
MarginLayoutParams lp = (MarginLayoutParams) child
.getLayoutParams();
int childWidth = child.getMeasuredWidth();
int childHeight = child.getMeasuredHeight();
if (childWidth + lineWidth + lp.leftMargin + lp.rightMargin
> width - getPaddingLeft() - getPaddingRight()) {
mLineHeight.add(lineHeight);
mAllViews.add(lineViews);
lineWidth = 0;
lineHeight = childHeight + lp.topMargin + lp.bottomMargin;
lineViews = new ArrayList<View>();
}
lineWidth += childWidth + lp.leftMargin + lp.rightMargin;
lineHeight = Math.max(lineHeight, childHeight + lp.topMargin
+ lp.bottomMargin);
lineViews.add(child);
}
mLineHeight.add(lineHeight);
mAllViews.add(lineViews);
int left = getPaddingLeft();
int top = getPaddingTop();
int lineNum = mAllViews.size();
for (int i = 0; i < lineNum; i++) {
lineViews = mAllViews.get(i);
lineHeight = mLineHeight.get(i);
for (int j = 0; j < lineViews.size(); j++) {
View child = lineViews.get(j);
if (child.getVisibility() == View.GONE) {
continue;
}
MarginLayoutParams lp = (MarginLayoutParams) child
.getLayoutParams();
int lc = left + lp.leftMargin;
int tc = top + lp.topMargin;
int rc = lc + child.getMeasuredWidth();
int bc = tc + child.getMeasuredHeight();
child.layout(lc, tc, rc, bc);
left += child.getMeasuredWidth() + lp.leftMargin
+ lp.rightMargin;
}
left = getPaddingLeft();
top += lineHeight;
}
}
@Override
public LayoutParams generateLayoutParams(AttributeSet attrs) {
return new MarginLayoutParams(getContext(), attrs);
}

最后

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