关于LayoutInflater，在开发中常常会遇到，特别是在使用ListView的时候，这个几近是必不可少。今天我们就1起来探讨LayoutInflater的工作原理。
1般情况下，有两种方式取得1个LayoutInflater实例：
LayoutInflater inflater1, inflater2;
inflater1 = LayoutInflater.from(this);
inflater2 = (LayoutInflater) this .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
但是当我们查看源码的时候，却发现这两种实际上是1种，只不过第1种将第2种封装了1下，我们看看from这个方法的源码：
/**
* Obtains the LayoutInflater from the given context.
*/ 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;
}
取得LayoutInflater对象以后，我们就能够调用inflate来取得View对象了，inflate方法的源码以下：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) { return inflate(resource, root, root != null);
}
这里调用了1个inflate的1个重载方法，这个重载方法的最后1个参数和root有关，如果我们的root为空，那末最后1个参数默许为false。我们看看这个重载方法：
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();
}
}
可以看出，先是拿到布局的xml资源，然后，取得1个XmlResourceParser 对象，最后inflate(parser, root, attachToRoot);又是调用1个重载方法：
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 { int type; while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) { } 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("can be used only with a valid " + "ViewGroup root and attachToRoot=true");
}
rInflate(parser, root, inflaterContext, attrs, false);
} else { 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);
} params = root.generateLayoutParams(attrs); if (!attachToRoot) { temp.setLayoutParams(params);
}
} if (DEBUG) {
System.out.println("-----> start inflating children");
} rInflateChildren(parser, temp, attrs, true); if (DEBUG) {
System.out.println("-----> done inflating children");
} if (root != null && attachToRoot) {
root.addView(temp, params);
} 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 { mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW); return result;
}
}
这个方法有点长，前面都是1些简单的判断，1般情况下（特殊情况大家可以按相应的分支走），我们的程序会履行到final View temp = createViewFromTag(root, name, inflaterContext, attrs);这行代码，这里创建了1个名为temp的view，如果我们传进来的根布局为null的话，那末这里拿到的就是1个根布局。我们看看这个方法的源代码：
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) { return createViewFromTag(parent, name, context, attrs, false);
}
好啊，这里又调用了1个它的重载方法，那我们就看看这个重载方法，注意最后1个参数恒为false。
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) { if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
} if (!ignoreThemeAttr) { final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME); final int themeResId = ta.getResourceId(0, 0); if (themeResId != 0) {
context = new ContextThemeWrapper(context, themeResId);
}
ta.recycle();
} if (name.equals(TAG_1995)) { return new BlinkLayout(context, attrs);
} try {
View view; if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
} if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
} if (view == null) { final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context; try { if (-1 == name.indexOf(.)) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
} return view;
} catch (InflateException e) { throw e;
} catch (ClassNotFoundException e) { final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e); throw ie;
} catch (Exception e) { final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e); throw ie;
}
}
仔细分析1下这个重载方法，发现里边的mFactory2和mFactory都为null，那末程序终究其实履行了这个方法里边的这1段：
if (view == null) { final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context; try { if (-1 == name.indexOf(.)) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
前面的判断不用说，后面的判断，如果name中包括.，说明我们用的不是普通的view，有多是自定义View等等，这1条大家可以自行去研究，如果name中不包括.,那末程序会履行view = onCreateView(parent, name, attrs);，那末我们就去看看这个方法：
protected View onCreateView(View parent, String name, AttributeSet attrs)
throws ClassNotFoundException { return onCreateView(name, attrs);
} protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException { return createView(name, "android.view.", attrs);
} public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructorextends View> clazz = null; try { Trace.traceBegin(Trace.TRACE_TAG_VIEW, name); if (constructor == null) { clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class); if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz); if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
} else { if (mFilter != null) { Boolean allowedState = mFilterMap.get(name); if (allowedState == null) { clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
mFilterMap.put(name, allowed); if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
} else if (allowedState.equals(Boolean.FALSE)) {
failNotAllowed(name, prefix, attrs);
}
}
}
Object[] args = mConstructorArgs;
args[1] = attrs; final View view = constructor.newInstance(args); if (view instanceof ViewStub) { final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
} return view;
} catch (NoSuchMethodException e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + (prefix != null ? (prefix + name) : name));
ie.initCause(e); throw ie;
} catch (ClassCastException e) { InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Class is not a View " + (prefix != null ? (prefix + name) : name));
ie.initCause(e); throw ie;
} catch (ClassNotFoundException e) { throw e;
} catch (Exception e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + (clazz == null ? "" : clazz.getName()));
ie.initCause(e); throw ie;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
经过了前两个方法的相互扯皮，最后我们来到了第3个方法上，这是最后创建View的地方，代码虽然很长，但是大家不用怕，这里的代码我们主要分析下面这几行，由于大部份不会被履行到。
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException Object[] args = mConstructorArgs;
args[1] = attrs;
final View view = constructor.newInstance(args); if (view instanceof ViewStub) return view;
}traceEnd(Trace.TRACE_TAG_VIEW);
}
}
我把这个方法略微精简1下，可以看到，先是通过Java的反射机制拿到这个name所表示的布局对应的那个Java类，然后是拿到构造方法，最后通过构造方法拿到1个View实例，逻辑还是比较清楚的。
好的，到这里，我们就已拿到根View了，现在我们再回到上面说的那个inflate(…)方法中，在该方法中创建完temp这个View以后，接着就会履行到rInflateChildren(parser, temp, attrs, true);，里边终究会履行到1个递归方法，这个方法是这样的：
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth(); int type; while (((type = parser.next()) != XmlPullParser.END_TAG || parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) { if (type != XmlPullParser.START_TAG) {
continue;
}
final String name = parser.getName(); if (TAG_REQUEST_FOCUS.equals(name)) {
parseRequestFocus(parser, parent);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) { if (parser.getDepth() == 0) {
throw new InflateException("cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("must be the root element");
} else {
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);
}
} if (finishInflate) {
parent.onFinishInflate();
}
}
这个方法还不算长，进入while循环以后，在if分支里会走到最后1个else里，这里还是先调用我们前文说的那个createViewFromTag方法取得1个布局，然后递归，如果取得的这个View是个ViewGroup，那末会把它的子View添加到这个ViewGroup中，如果是普通View，那末接着循环就是了。做完这些以后，下面就和1开始提到的root是不是为null有关了，如果root为null，那末attachToRoot这个参数为false，这个时候会履行：
if (root == null || !attachToRoot) {
result = temp;
}
然后我们解析过得到的View就会被返回，如果root不为null,那末attachToRoot这个参数默许为true，那末系统会履行
if (root != null && attachToRoot) {
root.addView(temp, params);
}
也就是会把root套在我们解析得到的View以外，然后返回。
好了，到这里我们的LayoutInflater基本上就分析完了.
如果大家还有甚么问题，欢迎留言讨论
