自定义属性能够做到配置的灵活运用，比如在TextView中text配置什么就显示什么，textColor指定什么颜色就显示什么颜色等等，这些都是自定义属性。首先在res下的values目录下新建一个attrs.xml文件，其他名称可以吗？是可以的如attr.xml或者zzz.xml但是原则上一看attrs就知道是自定义属性。一切规范化。
<resources>
// 自定义TextView
<declare-styleable name="TextView">
// name 是名称，format是格式 color（颜色），string（文本），dimension（sp,dp）...
<attr name="textColor" format="color"/>
<attr name="text" format="string"/>
<attr name="textSize" format="dimension"/>
</declare-styleable>
</resources>
在布局文件中使用
<com.darren.view.TextView
// app: 自定义属性
app:text="自定义文本"
app:textColor="@color/colorAccent"
app:textSize="18sp"
// android: 系统自带的属性
android:layout_width="wrap_content"
android:layout_height="match_parent" />
代码中获取，我们可以去仿照TextView的源码写
public TextView(Context context) {
this(context,null);
}
public TextView(Context context, AttributeSet attrs) {
this(context, attrs,0);
}
public TextView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
// 获取TypedArray
TypedArray typedArray = context.obtainStyledAttributes(attrs,R.styleable.TextView);
// 获取文本
mText = typedArray.getText(R.styleable.TextView_text);
// 获取文字颜色
mTextColor = typedArray.getColorStateList(R.styleable.TextView_textColor);
// 获取文字大小
mTextSize = typedArray.getDimensionPixelSize(R.styleable.TextView_textSize,mTextSize);
// 回收
typedArray.recycle();
}
ScrollView嵌套ListView会出现显示不全的现象，怎么解决这个问题的？
在网上有很多解决办法，一搜就有，但是确不知道为啥，在面试中也经常问道，下面做下总结
解决问题的代码：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// TODO Auto-generated method stub
int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2,
MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, expandSpec);
}
但是为啥呢，别急下面一一道来，没有看过我上篇文章的建议先看一下，才不至于懵逼
从ScrollView源码分析点进去（onMeasure -->measureChildWithMargins）你会发现ScrollView的measureChildWithMargins方法覆盖了父类的measureChildWithMargins方法，ScrollView的measureChildWithMargins方法如下
@Override
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 = MeasureSpec.makeSafeMeasureSpec(
MeasureSpec.getSize(parentHeightMeasureSpec), MeasureSpec.UNSPECIFIED);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
况且这句代码：
final int childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
MeasureSpec.getSize(parentHeightMeasureSpec), MeasureSpec.UNSPECIFIED);
发现设计模式是：MeasureSpec.UNSPECIFIED
我们再看LIstView的onMeasure 方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// Sets up mListPadding
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int childWidth = 0;
int childHeight = 0;
int childState = 0;
mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED
|| heightMode == MeasureSpec.UNSPECIFIED)) {
final View child = obtainView(0, mIsScrap);
// Lay out child directly against the parent measure spec so that
// we can obtain exected minimum width and height.
measureScrapChild(child, 0, widthMeasureSpec, heightSize);
childWidth = child.getMeasuredWidth();
childHeight = child.getMeasuredHeight();
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (recycleOnMeasure() && mRecycler.shouldRecycleViewType(
((LayoutParams) child.getLayoutParams()).viewType)) {
mRecycler.addScrapView(child, 0);
}
}
if (widthMode == MeasureSpec.UNSPECIFIED) {
widthSize = mListPadding.left + mListPadding.right + childWidth +
getVerticalScrollbarWidth();
} else {
widthSize |= (childState & MEASURED_STATE_MASK);
}
if (heightMode == MeasureSpec.UNSPECIFIED) {
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
}
if (heightMode == MeasureSpec.AT_MOST) {
// TODO: after first layout we should maybe start at the first visible position, not 0
heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
setMeasuredDimension(widthSize, heightSize);
mWidthMeasureSpec = widthMeasureSpec;
}
方法 final int heightMode = MeasureSpec.getMode(heightMeasureSpec);获取高度的设计模式，为ScrollView传进来的
MeasureSpec.UNSPECIFIED会走下面的方法
if (heightMode == MeasureSpec.UNSPECIFIED) {
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;//表示高度为一个item高度
}
为了让它走
if (heightMode == MeasureSpec.AT_MOST) {
// TODO: after first layout we should maybe start at the first visible position, not 0
heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
方法所以要自定义ListVIew,重写onMeasure 添加
int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2,
MeasureSpec.AT_MOST);
那么为什么要这样写呢，下面讲解一下
重写之后进入该方法：
if (heightMode == MeasureSpec.AT_MOST) {
// TODO: after first layout we should maybe start at the first visible position, not 0
heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
之后进入measureHeightOfChildren（）方法
final int measureHeightOfChildren(int widthMeasureSpec, int startPosition, int endPosition,
int maxHeight, int disallowPartialChildPosition) {
final ListAdapter adapter = mAdapter;
if (adapter == null) {
return mListPadding.top + mListPadding.bottom;
}
// Include the padding of the list
int returnedHeight = mListPadding.top + mListPadding.bottom;
final int dividerHeight = ((mDividerHeight > 0) && mDivider != null) ? mDividerHeight : 0;
// The previous height value that was less than maxHeight and contained
// no partial children
int prevHeightWithoutPartialChild = 0;
int i;
View child;
// mItemCount - 1 since endPosition parameter is inclusive
endPosition = (endPosition == NO_POSITION) ? adapter.getCount() - 1 : endPosition;
final AbsListView.RecycleBin recycleBin = mRecycler;
final boolean recyle = recycleOnMeasure();
final boolean[] isScrap = mIsScrap;
for (i = startPosition; i <= endPosition; ++i) {
child = obtainView(i, isScrap);
measureScrapChild(child, i, widthMeasureSpec, maxHeight);
if (i > 0) {
// Count the divider for all but one child
returnedHeight += dividerHeight;
}
// Recycle the view before we possibly return from the method
if (recyle && recycleBin.shouldRecycleViewType(
((LayoutParams) child.getLayoutParams()).viewType)) {
recycleBin.addScrapView(child, -1);
}
returnedHeight += child.getMeasuredHeight();
if (returnedHeight >= maxHeight) {
// We went over, figure out which height to return. If returnedHeight > maxHeight,
// then the i'th position did not fit completely.
return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
&& (i > disallowPartialChildPosition) // We've past the min pos
&& (prevHeightWithoutPartialChild > 0) // We have a prev height
&& (returnedHeight != maxHeight) // i'th child did not fit completely
? prevHeightWithoutPartialChild
: maxHeight;
}
if ((disallowPartialChildPosition >= 0) && (i >= disallowPartialChildPosition)) {
prevHeightWithoutPartialChild = returnedHeight;
}
}
// At this point, we went through the range of children, and they each
// completely fit, so return the returnedHeight
return returnedHeight;
}
我们传的为maxHeight参数：传的最大值使它进入不到方法
if (returnedHeight >= maxHeight) {
// We went over, figure out which height to return. If returnedHeight > maxHeight,
// then the i'th position did not fit completely.
return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
&& (i > disallowPartialChildPosition) // We've past the min pos
&& (prevHeightWithoutPartialChild > 0) // We have a prev height
&& (returnedHeight != maxHeight) // i'th child did not fit completely
? prevHeightWithoutPartialChild
: maxHeight;
}
正常返回returnedHeight；
heightMeasureSpec：包含两个信息 一个是模式;2位，一个是值：30位
为什么要右移两位Integer.MAX_VALUE >> 2，因为源码size加mode
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
Integer.MAX_VALUE 为32位
/**
* Constant for the maximum {@code int} value, 2<sup>31</sup>-1.
*/
public static final int MAX_VALUE = 0x7FFFFFFF;
所以右移2位拼接位32位
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