我们先看看XmlResourceParser 类:
/**
* The XML parsing interface returned for an XML resource. This is a standard
* XmlPullParser interface, as well as an extended AttributeSet interface and
* an additional close() method on this interface for the client to indicate
* when it is done reading the resource.
*/
public interface XmlResourceParser extends XmlPullParser, AttributeSet, AutoCloseable {
/**
* Close this interface to the resource. Calls on the interface are no
* longer value after this call.
*/
public void close();
}
该类的解释为:
为XML资源返回的XML解析接口,这是一个标准XmlPullParser接口,同时继承AttributeSet接口以及当用户完成资源读取时调用的close接口,简单来说就是一个xml资源解析器,用于解析xml资源。
那么该类的作用是什么?
我们知道Activity的setContentView()其实调用的是PhoneWindow里面的setContentView
@Override
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
@Override
public void setContentView(View view) {
setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
}
我们可以看到接口
mLayoutInflater.inflate(layoutResID, mContentParent);
将activity传递进来的layoutResID即布局ID,加载到mContentParent里面去。
接着我们进到LayoutInflater的inflate接口可以看到
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();
}
}
里面调用了一个:
final XmlResourceParser parser = res.getLayout(resource);
并且再通过
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(" 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;
}
}
返回view
这里最终会走到一个方法,在这个方法里面进行处理
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(" 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 (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}
我们知道XmlPullParser是安卓内置的一个XML解析器,用于解析XML文件,同时我们的布局文件也是一种XML格式文件,通过XmlPullParser解析整个布局文件,解析其中定义的View,例如LinearLayout和FrameLayout等等,再通过AttributeSet attrs = Xml.asAttributeSet(parser)来获取view定义的属性,并通过createViewFromTag()方法来创建View的实例,且在每次递归完成的时候将这个View添加到父布局中去。
好了,我们粗略过了一遍布局文件的加载过程了,我们由此可大概知道,XmlResourcePareser主要用于解析安卓中的资源文件,例如布局资源文件(layout.xml),动画资源文件(anim.xml)等资源文件,由于其继承XmlPullParser,AttributeSet,因此其同时具有解析xml布局文件和操作xml中定义的属性的作用。
我们打开android.content.res.Resources类查看,其中有以下方法用于获取XmlResourceParser
public XmlResourceParser getLayout(@LayoutRes int id) throws NotFoundException {
return loadXmlResourceParser(id, "layout");
}
public XmlResourceParser getAnimation(@AnimatorRes @AnimRes int id) throws NotFoundException {
return loadXmlResourceParser(id, "anim");
}
public XmlResourceParser getXml(@XmlRes int id) throws NotFoundException {
return loadXmlResourceParser(id, "xml");
}
开始写代码之前我们先了解一些相关的常量和方法:
我们主要讲解以下接口:
public XmlResourceParser getLayout(@LayoutRes int id)
顾名思义,根据提供的布局资源文件id返回一个可读取布局文件中view相关属性的XmlResourceParser。
我们先定义一个layout文件:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:background="#66000000"
android:layout_width="800dp"
android:layout_height="800dp"
tools:context=".MainActivity">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="@+id/iv3"
android:layout_width="300dp"
android:layout_height="300dp" />
RelativeLayout>
<ImageView
android:id="@+id/iv2"
android:layout_width="200dp"
android:layout_height="200dp" />
RelativeLayout>
<ImageView
android:id="@+id/iv1"
android:layout_width="100dp"
android:layout_height="100dp" />
RelativeLayout>
RelativeLayout>
主要代码为:
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
XLog.configure().setLogHeadEnable(false);
XmlResourceParser parser = getResources().getLayout(R.layout.activity_main);
try {
int type;
//过滤掉END_DOCUMENT和非START_TAG的事件
while ((type = parser.next())!= XmlPullParser.END_DOCUMENT ) {
if (type != XmlPullParser.START_TAG){
continue;
}
StringBuilder stringBuilder = new StringBuilder()
.append("current type: ")
.append(getType(type))
.append("\n")
.append("current depth: ")
.append(parser.getDepth())
.append("\n")
.append("current name: ")
.append(parser.getName())
.append("\n");
for (int i = 0; i < parser.getAttributeCount(); i++){
stringBuilder.append(String.format("第%d个属性 -> ", i+1))
.append(parser.getAttributeName(i))
.append(" : ")
.append(parser.getAttributeValue(i))
.append("\n");
}
XLog.e(stringBuilder.toString());
}
}catch (IOException e){
e.printStackTrace();
} catch (XmlPullParserException e) {
e.printStackTrace();
} finally {
parser.close();
}
}
private String getType(int type){
switch (type){
case XmlPullParser.START_DOCUMENT:
return "START_DOCUMENT";
case XmlPullParser.END_DOCUMENT:
return "END_DOCUMENT";
case XmlPullParser.START_TAG:
return "START_TAG";
case XmlPullParser.END_TAG:
return "END_TAG";
}
return String.valueOf(type);
}
}
我们可以看到,深度是从1开始的。
所有的log为:
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ current type: START_TAG
│ current depth: 1
│ current name: RelativeLayout
│ 第1个属性 -> background : #66000000
│ 第2个属性 -> layout_width : 800.0dip
│ 第3个属性 -> layout_height : 800.0dip
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ current type: START_TAG
│ current depth: 2
│ current name: RelativeLayout
│ 第1个属性 -> layout_width : -1
│ 第2个属性 -> layout_height : -1
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ current type: START_TAG
│ current depth: 3
│ current name: RelativeLayout
│ 第1个属性 -> layout_width : -1
│ 第2个属性 -> layout_height : -1
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ current type: START_TAG
│ current depth: 4
│ current name: ImageView
│ 第1个属性 -> id : @2131165256
│ 第2个属性 -> layout_width : 300.0dip
│ 第3个属性 -> layout_height : 300.0dip
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ current type: START_TAG
│ current depth: 3
│ current name: ImageView
│ 第1个属性 -> id : @2131165255
│ 第2个属性 -> layout_width : 200.0dip
│ 第3个属性 -> layout_height : 200.0dip
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ current type: START_TAG
│ current depth: 2
│ current name: ImageView
│ 第1个属性 -> id : @2131165254
│ 第2个属性 -> layout_width : 100.0dip
│ 第3个属性 -> layout_height : 100.0dip
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────
回到问题
那么该类的作用是什么?
你应该知道了吧?