至于为什么写这个标题呢？也许是想把这些年那些被开发者忽略了的秘密吧。话不多说，开始我们这个文集的开篇文章你不知道的layoutInflater。
一、源码中的它
public abstract class LayoutInflater {
}
从这里可以看出这就是一个抽象类，其具体的实现由其执行者实现，那么LayoutInflater是从何而来，在系统中是如何去调用的呢？这里不做详细解释，具体可参考源码中的ContextImpl.java及Policy.java。
从源码中不难看出其执行者，也就是LayoutInflater具体实现类是PhoneLayoutInflater。这个类继承了LayoutInflater这个抽象类。有意思的是这个类主要的功能就是复写了onCreateView这个方法，这里大家都不陌生对于这个方法。
其函数原型为：
onCreateView(String name,AttributeSet attires);
我们应用层一般用这个方法要么传入一个布局id,要么就是一个view,但是这里
有意思的是这个方法的第一个是一个name，这个name起到了什么作用。通常我们在使用一个控件的时候都会先去引用这个控件的包，也就是：
import android.widget.Button;
这里的Button的完整的包名也就是上面所示的，它代表了这个控件的路径也是这个控件的包名。而这里PhoneLayoutInflater定义了一个String[]的数组，就惨了两个字符串，分别是：android.widget、android.webkit。这两个字符串就是分别是各个控件包名前端的前缀字符串，而这个name呢就是控件名，如Button，将其组合成了完整的控件包名，这样就可以解析构造与之相对应的控件对象了。
二、LayoutInflater中的主要方法
//省略...
public interface Filter {
boolean onLoadClass(Class clazz);
}
public interface Factory {
public View onCreateView(String name, Context context, AttributeSet attrs);
}
public interface Factory2 extends Factory {
public View onCreateView(View parent, String name, Context context, AttributeSet attrs);
}
private static class FactoryMerger implements Factory2 {
private final Factory mF1, mF2;
private final Factory2 mF12, mF22;
FactoryMerger(Factory f1, Factory2 f12, Factory f2, Factory2 f22) {
mF1 = f1;
mF2 = f2;
mF12 = f12;
mF22 = f22;
}
public View onCreateView(String name, Context context, AttributeSet attrs) {
View v = mF1.onCreateView(name, context, attrs);
if (v != null) return v;
return mF2.onCreateView(name, context, attrs);
}
public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
View v = mF12 != null ? mF12.onCreateView(parent, name, context, attrs)
: mF1.onCreateView(name, context, attrs);
if (v != null) return v;
return mF22 != null ? mF22.onCreateView(parent, name, context, attrs)
: mF2.onCreateView(name, context, attrs);
}
}
protected LayoutInflater(Context context) {
mContext = context;
}
protected LayoutInflater(LayoutInflater original, Context newContext) {
mContext = newContext;
mFactory = original.mFactory;
mFactory2 = original.mFactory2;
mPrivateFactory = original.mPrivateFactory;
setFilter(original.mFilter);
}
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;
}
不难看出这些方法就是在构建一层一层的布局供应用层去绘制，而具体每个控件怎么摆放，绘制的大小又是怎么去实现的呢？看下面的代码：
protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException {
return createView(name, "android.view.", attrs);
}
protected View onCreateView(View parent, String name, AttributeSet attrs)
throws ClassNotFoundException {
return onCreateView(name, attrs);
}
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);
}
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
// Apply a theme wrapper, if allowed and one is specified.
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)) {
// Let's party like it's 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, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
}
}
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;
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)) {
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> 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 (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}
/**
* Parses a <code><tag></code> element and sets a keyed tag on the
* containing View.
*/
private void parseViewTag(XmlPullParser parser, View view, AttributeSet attrs)
throws XmlPullParserException, IOException {
final Context context = view.getContext();
final TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.ViewTag);
final int key = ta.getResourceId(R.styleable.ViewTag_id, 0);
final CharSequence value = ta.getText(R.styleable.ViewTag_value);
view.setTag(key, value);
ta.recycle();
consumeChildElements(parser);
}
private void parseInclude(XmlPullParser parser, Context context, View parent,
AttributeSet attrs) throws XmlPullParserException, IOException {
int type;
if (parent instanceof ViewGroup) {
// Apply a theme wrapper, if requested. This is sort of a weird
// edge case, since developers think the <include> overwrites
// values in the AttributeSet of the included View. So, if the
// included View has a theme attribute, we'll need to ignore it.
final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
final int themeResId = ta.getResourceId(0, 0);
final boolean hasThemeOverride = themeResId != 0;
if (hasThemeOverride) {
context = new ContextThemeWrapper(context, themeResId);
}
ta.recycle();
// If the layout is pointing to a theme attribute, we have to
// massage the value to get a resource identifier out of it.
int layout = attrs.getAttributeResourceValue(null, ATTR_LAYOUT, 0);
if (layout == 0) {
final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
if (value == null || value.length() <= 0) {
throw new InflateException("You must specify a layout in the"
+ " include tag: <include layout=\"@layout/layoutID\" />");
}
// Attempt to resolve the "?attr/name" string to an attribute
// within the default (e.g. application) package.
layout = context.getResources().getIdentifier(
value.substring(1), "attr", context.getPackageName());
}
// The layout might be referencing a theme attribute.
if (mTempValue == null) {
mTempValue = new TypedValue();
}
if (layout != 0 && context.getTheme().resolveAttribute(layout, mTempValue, true)) {
layout = mTempValue.resourceId;
}
if (layout == 0) {
final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
throw new InflateException("You must specify a valid layout "
+ "reference. The layout ID " + value + " is not valid.");
} else {
final XmlResourceParser childParser = context.getResources().getLayout(layout);
try {
final AttributeSet childAttrs = Xml.asAttributeSet(childParser);
while ((type = childParser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty.
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(childParser.getPositionDescription() +
": No start tag found!");
}
final String childName = childParser.getName();
if (TAG_MERGE.equals(childName)) {
// The <merge> tag doesn't support android:theme, so
// nothing special to do here.
rInflate(childParser, parent, context, childAttrs, false);
} else {
final View view = createViewFromTag(parent, childName,
context, childAttrs, hasThemeOverride);
final ViewGroup group = (ViewGroup) parent;
final TypedArray a = context.obtainStyledAttributes(
attrs, R.styleable.Include);
final int id = a.getResourceId(R.styleable.Include_id, View.NO_ID);
final int visibility = a.getInt(R.styleable.Include_visibility, -1);
a.recycle();
// We try to load the layout params set in the <include /> tag.
// If the parent can't generate layout params (ex. missing width
// or height for the framework ViewGroups, though this is not
// necessarily true of all ViewGroups) then we expect it to throw
// a runtime exception.
// We catch this exception and set localParams accordingly: true
// means we successfully loaded layout params from the <include>
// tag, false means we need to rely on the included layout params.
ViewGroup.LayoutParams params = null;
try {
params = group.generateLayoutParams(attrs);
} catch (RuntimeException e) {
// Ignore, just fail over to child attrs.
}
if (params == null) {
params = group.generateLayoutParams(childAttrs);
}
view.setLayoutParams(params);
// Inflate all children.
rInflateChildren(childParser, view, childAttrs, true);
if (id != View.NO_ID) {
view.setId(id);
}
switch (visibility) {
case 0:
view.setVisibility(View.VISIBLE);
break;
case 1:
view.setVisibility(View.INVISIBLE);
break;
case 2:
view.setVisibility(View.GONE);
break;
}
group.addView(view);
}
} finally {
childParser.close();
}
}
} else {
throw new InflateException("<include /> can only be used inside of a ViewGroup");
}
LayoutInflater.consumeChildElements(parser);
}
final static void consumeChildElements(XmlPullParser parser)
throws XmlPullParserException, IOException {
int type;
final int currentDepth = parser.getDepth();
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > currentDepth) && type != XmlPullParser.END_DOCUMENT) {
// Empty
}
}
很清楚的我们看到了其实它在做了一个解析xml的动作，目的就是解析我们定义的xml文件。
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