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今天就跟大家聊聊有關LayoutInflater怎么在Android 應用中使用，可能很多人都不太了解，為了讓大家更加了解，小編給大家總結了以下內容，希望大家根據這篇文章可以有所收獲。
LayoutInflater解析
前言：
在Android中，如果是初級玩家，很可能對LayoutInflater不太熟悉，或許只是在Fragment的onCreateView()中模式化的使用過而已。但如果稍微有些工作經驗的人就知道，這個類有多么重要，它是連接布局XMl和Java代碼的橋梁，我們常常疑惑，為什么Android支持在XML書寫布局？
我們想到的必然是Android內部幫我們解析xml文件，LayoutInflater就是幫我們做了這個工作。
首先LayoutInflater是一個系統服務，這個我們可以從from方法看出來
/**
* 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;
}通常我們拿到LayoutInflater對象之后就會調用其inflate方法進行加載布局，inflate是一個重載方法
public View inflate(int resource, ViewGroup root) {
return inflate(resource, root, root != null);
}可以看到，我們調用2個參數的方法時候其默認是添加到父布局中的（父布局一般不為空）
public View inflate(int resource, 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();
}
}這個方法中，其實是使用Resources將資源ID還原為XMlResoourceParser對象，然后調用inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)方法，解析布局的具體步驟都是在這個方法中實現
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
View result = root;
try {
// Look for the root node.
//1.循環尋找根節點，其實就是節點指針遍歷的過程
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!");
}
//2.得到節點的名字，用于判斷該節點
final String name = parser.getName();
if (DEBUG) {
System.out.println("**************************");
System.out.println("Creating root view: "
+ name);
System.out.println("**************************");
}
//3.對節點名字進行判斷，然后是merge就將其添加到父布局中（依據Merge的特性必須添加到父布局中）
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, false, false);
} else {
//4.創建根據節點創建View
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, attrs, false);
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
//5.根據attrs生成布局參數
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
//6.如果View不添加到父布局中，那就給其本身設置布局參數
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp
// 7.將該節點下的子View全部加載
rInflate(parser, temp, attrs, true, 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.
//8.如果添加到父布局中，直接addView
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.
//9.如果不添加到父布局，那么將自己返回
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;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
return result;
}
}重點的步驟我已經加上注釋了，核心
1.找到根布局標簽
2.創建根節點對應的View
3.創建其子View
我們從這里面可以看出來，子View的解析其實都是rInflate方法，如果xml中有根布局，那么就調用createViewFromTag創建布局中的根View。我們也可以明白merge的原來，因為它直接調用rInflate添加到父View中，看到rInflate(parser, root, attrs, false, false)和rInflate(parser, temp, attrs, true, true)第二個參數區別我們就明白了。
接下來我們看下rInflate如何創建多個布局
void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
boolean finishInflate, boolean inheritContext) throws XmlPullParserException,
IOException {
//獲取當前解析器指針所在節點處于布局層次
final int depth = parser.getDepth();
int type;
//進行樹的深度優先遍歷（如果一個節點有子節點將會再次進入rInflate，否則繼續循環）
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();
//如果其中有request_focus標簽，那就給這個節點View設置焦點
if (TAG_REQUEST_FOCUS.equals(name)) {
parseRequestFocus(parser, parent);
//如果其中有tag標簽，那就給這個節點View設置tag(key,value)
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
//如果其中是include標簽，如果include標簽
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, parent, attrs, inheritContext);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
//創建該節點代表的View并添加到父view中，此外遍歷子節點
final View view = createViewFromTag(parent, name, attrs, inheritContext);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflate(parser, view, attrs, true, true);
viewGroup.addView(view, params);
}
}
//代表著一個節點含其子節點遍歷結束
if (finishInflate) parent.onFinishInflate();
}從上面可以看到，所以創建View都將會交給createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext)中，我們可以看下該方法如何創建View
View createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
Context viewContext;
if (parent != null && inheritContext) {
viewContext = parent.getContext();
} else {
viewContext = mContext;
}
// Apply a theme wrapper, if requested.
final TypedArray ta = viewContext.obtainStyledAttributes(attrs, ATTRS_THEME);
final int themeResId = ta.getResourceId(0, 0);
if (themeResId != 0) {
viewContext = new ContextThemeWrapper(viewContext, themeResId);
}
ta.recycle();
if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(viewContext, attrs);
}
if (DEBUG) System.out.println("******** Creating view: " + name);
try {
View view;
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, viewContext, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, viewContext, attrs);
} else {
view = null;
}
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, viewContext, attrs);
}
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = viewContext;
try {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
if (DEBUG) System.out.println("Created view is: " + view);
return view;
} catch (InflateException e) {
throw e;
} catch (ClassNotFoundException e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;
} catch (Exception e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;
}
}其實很簡單，就是4個降級處理
if(factory2!=null){
factory2.onCreateView();
}else if(factory!=null){
factory.onCreateView();
}else if(mPrivateFactory!=null){
mPrivateFactory.onCreateView();
}else{
onCreateView()
}其他的onCreateView我們不去設置的話為null，我們看下自己的onCreateView(),其實這個方法會調用createView()
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
//從構造器Map（緩存）中獲取需要的構造器
Constructor<? extends View> constructor = sConstructorMap.get(name);
Class<? extends View> clazz = null;
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
if (constructor == null) {
// Class not found in the cache, see if it's real, and try to add it
//如果緩存中沒有需要的構造器，那就通過ClassLoader加載需要的類
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);
}
}
//將使用過的構造器緩存
constructor = clazz.getConstructor(mConstructorSignature);
sConstructorMap.put(name, constructor);
} else {
// If we have a filter, apply it to cached constructor
if (mFilter != null) {
// Have we seen this name before?
Boolean allowedState = mFilterMap.get(name);
if (allowedState == null) {
// New class -- remember whether it is allowed
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;
constructor.setAccessible(true);
//通過反射獲取需要的實例對象
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
//ViewStub將創建一個屬于自己的LayoutInflater，因為它需要在不同的時機去inflate
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) {
// If loaded class is not a View subclass
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Class is not a View "
+ (prefix != null ? (prefix + name) : name));
ie.initCause(e);
throw ie;
} catch (ClassNotFoundException e) {
// If loadClass fails, we should propagate the exception.
throw e;
} catch (Exception e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class "
+ (clazz == null ? "<unknown>" : clazz.getName()));
ie.initCause(e);
throw ie;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}大體步驟就是，
1.從緩存中獲取特定View構造器，如果沒有，則加載對應的類，并緩存該構造器，
2.利用構造器反射構造對應的View
3.如果是ViewStub則復制一個LayoutInflater對象傳遞給它
看完上述內容，你們對LayoutInflater怎么在Android 應用中使用有進一步的了解嗎？如果還想了解更多知識或者相關內容，請關注億速云行業資訊頻道，感謝大家的支持。
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