android适配器，子布局不能撑满父容器

来源：互联网发布：fcc 网络中立编辑：程序博客网时间：2024/05/20 23:29

最近在看View绘制的源码，以及根据xml布局生成view的源码。然后今天刚好赶上在适配器实例化ViewHoldr根据item.xml ，以前写这块代码时没注意，结果死活子布局不能撑满父容器。然后我去看源码弄明白了。

起初实例化viewholder是如下代码:

只贴部分代码:

LayoutInflater.from(mContext).inflate(viewType,null)

无论如何都不能撑满父容器，我仔细检查了xml 中宽的属性如下，丝毫没有错误。

android:layout_width="match_parent"

后来我把ViewGroup作为参数传入了inflate(viewType,parent)中然后报了一堆错我大致看了一下意思就是已经有父容器了。后来没办法了逼的我去看这句代码的源码，如下

看它的注释没看太明白接着追到了红色inflate()的重载方法。如下：

/**
* Inflate a new view hierarchy from the specified xml resource. Throws
* {@link InflateException} if there is an error.
*
* @param resource ID for an XML layout resource to load (e.g.,
* <code>R.layout.main_page</code>)
* @param root Optional view to be the parent of the generated hierarchy (if
* <em>attachToRoot</em> is true), or else simply an object that
* provides a set of LayoutParams values for root of the returned
* hierarchy (if <em>attachToRoot</em> is false.)
* @param attachToRoot Whether the inflated hierarchy should be attached to
* the root parameter? If false, root is only used to create the
* correct subclass of LayoutParams for the root view in the XML.
* @return The root View of the inflated hierarchy. If root was supplied and
* attachToRoot is true, this is root; otherwise it is the root of
* the inflated XML file.
*/
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
if (DEBUG) {
Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
+ Integer.toHexString(resource) + ")");
}

final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}

看到这里还是不能解决问题，接着往下看，还找红色部分代码那个方法的实现。如下：

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root;

try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}

if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}

final String name = parser.getName();

if (DEBUG) {
System.out.println("**************************");
System.out.println("Creating root view: "
+ name);
System.out.println("**************************");
}

if (TAG_MERGE.equals(name)) {
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}

rInflate(parser, root, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);

ViewGroup.LayoutParams params = null;

if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}

if (DEBUG) {
System.out.println("-----> start inflating children");
}

// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);

if (DEBUG) {
System.out.println("-----> done inflating children");
}

// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
if (root != null && attachToRoot) {
root.addView(temp, params);
}

// Decide whether to return the root that was passed in or the
// top view found in xml.
if (root == null || !attachToRoot) {
result = temp;
}
}

} catch (XmlPullParserException e) {
final InflateException ie = new InflateException(e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(parser.getPositionDescription()
+ ": " + e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;

Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}

return result;
}
}

代码比较多不需全部都看懂找关键的就行。我标注了一些代码有红色的有绿色的，先看红色的。

红色部分代码表达的首先是根据xml 布局生成一个view,然后就是 root !=null 并且 attachToRoot ==false 就给view设置 LayoutParams ，这下总算明白了原来是我xml布局中的参数没有被设置上去。使用inflater()的另一个重载方法可以解决这个问题。如下：

LayoutInflater.from(mContext).inflate(viewType,parent,false)

显示的问题解决了，宽能够撑满父容器了，但是那个报错的问题是怎么回事得搞明白，和绿色那行代码有关系，于是我就顺着方法的调用找到了ViewGroup中一直跟着方法的调用追到了这个方法，如下：

private void addViewInner(View child, int index, LayoutParams params,
boolean preventRequestLayout) {
if (mTransition != null) {
// Don't prevent other add transitions from completing, but cancel remove
// transitions to let them complete the process before we add to the container
mTransition.cancel(LayoutTransition.DISAPPEARING);
}

if (child.getParent() != null) {
throw new IllegalStateException("The specified child already has a parent. " +
"You must call removeView() on the child's parent first.");
}
}

由于方法的内容太多我删掉了其它部分。红色代码行表达的很清楚，它检查childview中是否已经有了父容器，如果有了就抛异常。其实在根据xml创建view的时候就已经把父容器指定了，所以调用root.addView(temp, params);会抛异常。

现在来总结一下：

1.item视图没有撑满父容器是传参的问题。解决：

LayoutInflater.from(mContext).inflate(viewType,parent,false)

2.抛 The specified child already has a parent 是因为addView()造成的。直接来说就是传参有问题。

调用 inflate(viewType,parent) 会间接使 attachToRoot变为true从而调用 root.addview()这个方法从而会抛出异常。

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