通過上篇的LayoutInflater 分析,我們知道了LayoutInflater服務的註冊流程,最終是通過PhoneLayoutInflater物件的onCreateView來建立對應的View物件的。那麼具體的View的建立過程是怎麼樣的呢,今天我們來一起分析一下。
通常情況下,一個Activity的介面的建立是通過setContentView來引入佈局。
mWindow = new PhoneWindow(this, window);
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
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而Activity的setContentView方法是通過呼叫Window的setContentView方法,而Window是一個抽象物件,它的具體實現類就是PhoneWindow。在PhoneWindow中找到setContentView方法:
@Override
public void setContentView(int layoutResID) {
// 如果mContentParent為空時先構建
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 {
// 通過LayoutInflater解析佈局
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
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可以發現,setContentView也是通過LayoutInflater載入佈局載入到mContentParent中。我們再看inflate方法:
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
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();
}
}
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物件
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
// 父檢視
View result = root;
try {
// Look for the root node.
int type;
// 找到root元素
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();
// 解析merge標籤
if (TAG_MERGE.equals(name)) {
// 如果是merge標籤呼叫新方法,將merge標籤內的元素全部載入到父檢視中
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// 通過xml的tag來解析根檢視
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
// 不是merge標籤,直接解析佈局中的檢視
ViewGroup.LayoutParams params = null;
if (root != null) {
// 生成佈局引數
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
temp.setLayoutParams(params);
}
}
// Inflate all children under temp against its context.
// 解析temp檢視下的所有view
rInflateChildren(parser, temp, attrs, true);
// 如果root不為空並且attachToRoot為true,將temp加入到父檢視中
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// 如果root為空 或者 attachToRoot為false,返回的結果就是temp
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
throw ie;
} catch (Exception e) {
throw ie;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
return result;
}
}
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最終呼叫public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
方法,其中第一個引數是xml解析器,第二個引數是要解析佈局的父檢視,第三個引數標識是否需要加入到父檢視中。
上面的inflate方法所做的操作主要有以下幾步:
- 解析xml的根標籤
- 如果根標籤是merge,那麼呼叫rInflate解析,將merge標籤下的所有子View直接新增到根標籤中
- 如果不是merge,呼叫createViewFromTag解析該元素
- 呼叫rInflate解析temp中的子View,並將這些子View新增到temp中
- 通過attachToRoot,返回對應解析的根檢視
我們先看createViewFromTag方法:
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
try {
View view;
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
// 通過.來判斷是自定義View還是內建View
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) {
throw ie;
}
}
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我們可以發現,解析View的時候是通過.
來判斷是內建的View還是自定義的View的,那麼我們就能知道為什麼在寫佈局檔案中自定義的View需要完整路徑了。
在解析內建View的時候就是通過類似於PhoneLayoutInflater的onCreateView的解析方式,通過在name前加上android.view.
最終也是呼叫createView來解析。
而自定義view則是在呼叫createView(name, null, attrs)時,第二個引數的字首傳遞null。
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
// 從快取中獲取view的建構函式
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> clazz = null;
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
// 如果沒有快取
if (constructor == null) {
// 如果字首不為空構造完整的View路徑並載入該類
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
// 獲取該類的建構函式
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
// 將建構函式加入快取中
sConstructorMap.put(name, constructor);
} else {
}
Object[] args = mConstructorArgs;
args[1] = attrs;
// 通過反射構建View
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
return view;
}
}
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createView相對簡單,通過判斷字首,來構建View的完整路徑,並將該類載入到虛擬機器中,獲取建構函式並快取,再通過建構函式建立該View物件,並返回。這個時候我們就獲得了根檢視。接著呼叫rInflateChildren方法解析子View,並最終呼叫rInflate方法:
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
// 獲取樹的深度,通過深度優先遍歷
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)) {// 解析tag標籤
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {// 解析include標籤
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {// 解析到merge標籤,並報錯
throw new InflateException("<merge /> must be the root element");
} else {
// 解析到普通的子View,並呼叫createViewFromTag獲得View物件
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
// 遞迴解析
rInflateChildren(parser, view, attrs, true);
// 將View加入到父檢視中
viewGroup.addView(view, params);
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}
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rInflate方法通過深度優先遍歷的方式來構造檢視樹,當解析到一個View的時候就再次呼叫rInflate方法,直到將路徑下的最後一個元素,並最終將View加入到父檢視中。
這就是完整的View的建立流程。