[Android] 转-LayoutInflater丢失View的LayoutParams _LayoutParams

愿君学长松，慎勿作桃李。这篇文章主要讲述[Android] 转-LayoutInflater丢失View的LayoutParams相关的知识，希望能为你提供帮助。
原文地址：http://lmbj.net/blog/layoutinflater-and-layoutparams/
【[Android] 转-LayoutInflater丢失View的LayoutParams】
View view = inflater.inflate(R.layout.item, null);
在使用类似以上方法获取view时会遇到的一个问题就是布局文件中定义的LayoutParams被忽略了。以下三个stackoverflow问题就是这样:
http://stackoverflow.com/questions/5288435/layout-params-of-loaded-view-are-ignored
http://stackoverflow.com/questions/5026926/making-sense-of-layoutinflater
http://stackoverflow.com/questions/2738670/layoutinflater-ignoring-parameters
都说这样使用就可以了：

View view = inflater.inflate( R.layout.item /* resource id */, parent /* parent */,false /*attachToRoot*/);

至此问题已经解决了，但是三个问题都没有提到为什么要这样调用才行。
感兴趣的我们到源码里来找答案，LayoutInflater的inflate方法，最后都是调用的同一个方法：

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)

原因自然在这个方法里，大概意思就是只有ViewGroup root不为空才会读取View的LayoutParams。attachToRoot = true的时候，会把View添加到root中。而大多情况不希望被addView到root中，自然要赋值为flase，这样就是上面的解决方案了。
可以参看LayoutInflater的inflate方法相关的源码：

/** * 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. * @return The root View of the inflated hierarchy. If root was supplied, *this is the root View; otherwise it is the root of the inflated *XML file. */ public View inflate(int resource, ViewGroup root) { return inflate(resource, root, root != null); }/** * Inflate a new view hierarchy from the specified xml node. Throws * {@link InflateException} if there is an error. * * * Important & nbsp; & nbsp; & nbsp; For performance * reasons, view inflation relies heavily on pre-processing of XML files * that is done at build time. Therefore, it is not currently possible to * use LayoutInflater with an XmlPullParser over a plain XML file at runtime. * * @param parser XML dom node containing the description of the view *hierarchy. * @param root Optional view to be the parent of the generated hierarchy. * @return The root View of the inflated hierarchy. If root was supplied, *this is the root View; otherwise it is the root of the inflated *XML file. */ public View inflate(XmlPullParser parser, ViewGroup root) { return inflate(parser, root, root != null); }/** * 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 * attachToRoot is true), or else simply an object that *provides a set of LayoutParams values for root of the returned *hierarchy (if attachToRoot 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(int resource, ViewGroup root, boolean attachToRoot) { if (DEBUG) System.out.println("INFLATING from resource: " + resource); XmlResourceParser parser = getContext().getResources().getLayout(resource); try { return inflate(parser, root, attachToRoot); } finally { parser.close(); } }/** * Inflate a new view hierarchy from the specified XML node. Throws * {@link InflateException} if there is an error. * * Important & nbsp; & nbsp; & nbsp; For performance * reasons, view inflation relies heavily on pre-processing of XML files * that is done at build time. Therefore, it is not currently possible to * use LayoutInflater with an XmlPullParser over a plain XML file at runtime. * * @param parser XML dom node containing the description of the view *hierarchy. * @param root Optional view to be the parent of the generated hierarchy (if * attachToRoot is true), or else simply an object that *provides a set of LayoutParams values for root of the returned *hierarchy (if attachToRoot 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(XmlPullParser parser, ViewGroup root, boolean attachToRoot) { synchronized (mConstructorArgs) { final AttributeSet attrs = Xml.asAttributeSet(parser); mConstructorArgs[0] = mContext; 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, attrs); } else { // Temp is the root view that was found in the xml View temp = createViewFromTag(name, 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 rInflate(parser, temp, attrs); 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) { InflateException ex = new InflateException(e.getMessage()); ex.initCause(e); throw ex; } catch (IOException e) { InflateException ex = new InflateException( parser.getPositionDescription() + ": " + e.getMessage()); ex.initCause(e); throw ex; }return result; } }