前几天在通过LayoutInflater渲染出子布局，并添加进入父容器的时候，出现了子布局的宽高属性不生效的情况，为此，总结一下和LayoutInflater相关的知识。
一、获得LayoutInflater
在Android当中，如果想要获得LayoutInflater实例，一共有以下3种方法：
1.1 LayoutInflater inflater = getLayoutInflater();
这种在Activity里面使用，它其实是调用了
/**
* Convenience for calling
* {@link android.view.Window#getLayoutInflater}.
*/
@NonNull
public LayoutInflater getLayoutInflater() {
return getWindow().getLayoutInflater();
}
下面我们再来看一下Window的实现类PhoneWindow.java
public PhoneWindow(Context context) {
super(context);
mLayoutInflater = LayoutInflater.from(context);
}
它其实就是在构造函数中调用了下面1.2的方法。
而如果是调用了Fragment中也有和其同名的方法，但是是隐藏的，它的理由是：
/**
* @hide Hack so that DialogFragment can make its Dialog before creating
* its views, and the view construction can use the dialog's context for
* inflation. Maybe this should become a public API. Note sure.
*/
public LayoutInflater getLayoutInflater(Bundle savedInstanceState) {
final LayoutInflater result = mHost.onGetLayoutInflater();
if (mHost.onUseFragmentManagerInflaterFactory()) {
getChildFragmentManager(); // Init if needed; use raw implementation below.
result.setPrivateFactory(mChildFragmentManager.getLayoutInflaterFactory());
}
return result;
}
1.2 LayoutInflater inflater = LayoutInflater.from(this);
public static LayoutInflater from(Context context) {
LayoutInflater LayoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (LayoutInflater == null) {
throw new AssertionError("LayoutInflater not found.");
}
return LayoutInflater;
}
可以看到，它其实是调用了1.3，但是加上了判空处理，也就是说我们从1.1当中的Activity和1.2方法中获取的LayoutInflater不可能为空。
1.3 LayoutInflater LayoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
这三种实现，默认最终都是调用了最后一种方式。
二、LayoutInflater#inflate
其inflater一共有四个重载方法，最终都是调用了最后一种实现。
2.1 (@LayoutRes int resource, @Nullable ViewGroup root)
/**
* 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(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
该方法，接收两个参数，一个是需要加载的xml文件的id，一个是该xml需要添加的布局，根据root的情况，返回值分为两种：
- 如果
root == null，那么返回这个root - 如果
root != null，那么返回传入xml的根View
2.2 (XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
/**
* Inflate a new view hierarchy from the specified xml node. Throws
* {@link InflateException} if there is an error. *
* <p>
* <em><strong>Important</strong></em> 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, @Nullable ViewGroup root) {
return inflate(parser, root, root != null);
}
它的返回值情况和2.1类似，不过它提供的是不是xml的id，而是XmlPullParser，但是由于View的渲染依赖于xml在编译时的预处理，因此，这个方法并不合适。
2.3 public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
/**
* 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();
}
}
如果我们需要渲染的xml是id类型的，那么会先把它解析为XmlResourceParser，然后调用2.4的方法。
2.4 public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
/**
* Inflate a new view hierarchy from the specified XML node. Throws
* {@link InflateException} if there is an error.
* <p>
* <em><strong>Important</strong></em> 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
* <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(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root;
try {.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
//1.如果根节点的元素不是START_TAG，那么抛出异常。
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
final String name = parser.getName();
//2.如果根节点的标签是<merge>，那么必须要提供一个root，并且该root要被作为xml的父容器。
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");
}
//2.1递归地调用它所有的孩子.
rInflate(parser, root, inflaterContext, attrs, false);
} else {
//temp表示传入的xml的根View
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
//如果提供了root并且不需要把xml的布局加入到其中，那么仅仅需要给它设置参数就好。
//如果提供了root并且需要加入，那么不会设置参数，而是调用addView方法。
if (root != null) {
//如果提供了root，那么产生参数。
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
temp.setLayoutParams(params);
}
}
//递归地遍历孩子.
rInflateChildren(parser, temp, attrs, true);
if (root != null && attachToRoot) {
//addView时需要加上前面产生的参数。
root.addView(temp, params);
}
//如果没有提供root，或者即使提供root但是不用将root作为parent，那么返回的是渲染的xml，在`root != null && attachToRoot`时，才会返回root。
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
InflateException ex = new InflateException(e.getMessage());
ex.initCause(e);
throw ex;
} catch (Exception e) {
InflateException ex = new InflateException(
parser.getPositionDescription()
+ ": " + e.getMessage());
ex.initCause(e);
throw ex;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
return result;
}
}
我们简单总结一下英文注释当中的说明，具体的流程可以看上面的中文注释。
- 作用：从特定的
xml节点渲染出一个新的view层级。 - 提示：为了性能考虑，不应当在运行时使用
XmlPullParser来渲染布局。 - 参数
parser：包含有描述xml布局层级的parser xml dom。 - 参数
root，可以是渲染的xml的parent（attachToRoot == true），或者仅仅是为了给渲染的xml层级提供LayoutParams。 - 参数
attachToRoot：渲染的View层级是否被添加到root中，如果不是，那么仅仅为xml的根布局生成正确的LayoutParams。 - 返回值：如果
attachToRoot为真，那么返回root，否则返回渲染的xml的根布局。
三、不指定root的情况
由前面的分析可知，当我们没有传入root的时候，LayoutInflater不会调用temp.setLayoutParams(params)，也就是像之前我遇到问题时的使用方式一样：
LinearLayout linearLayout = (LinearLayout) mLayoutInflater.inflate(R.layout.linear_layout, null);
mContentGroup.addView(linearLayout);
当没有调用上面的方法时，linearLayout内部的mLayoutParams参数是没有被赋值的，下面我们再来看一下，通过这个返回的temp参数，把它通过不带参数的addView方法添加进去，会发生什么。
调用addView后，如果没有指定index，那么会把index设为-1，按前面的分析，那么下面这段逻辑中的getLayoutParams()必然是返回空的。
public void addView(View child, int index) {
if (child == null) {
throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
}
LayoutParams params = child.getLayoutParams();
if (params == null) {
params = generateDefaultLayoutParams();
if (params == null) {
throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
}
}
addView(child, index, params);
}
在此情况下，为它提供了默认的参数，那么，这个默认的参数是什么呢？
protected LayoutParams generateDefaultLayoutParams() {
return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
}
也就是说，当我们通过上面的方法得到一个View树之后，将它添加到某个布局中，这个View数所指定的根布局中的宽高属性其实是不生效的，而是变为了LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT。
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