Android从xml加载到View对象过程解析
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我们从Activity的setContentView()入手，开始源码解析，
//Activity.setContentView public void setContentView(int layoutResID) { getWindow().setContentView(layoutResID); initActionBar(); } //PhoneWindow.setContentView public void setContentView(int layoutResID) { if (mContentParent == null) { installDecor(); } else { mContentParent.removeAllViews(); } mLayoutInflater.inflate(layoutResID, mContentParent); final Callback cb = getCallback(); if (cb != null && !isDestroyed()) { cb.onContentChanged(); } }
发现是使用mLayoutInflater创建View的，所以我们去LayoutInflater.inflate()里面看下，
public View inflate(int resource, ViewGroup root, boolean attachToRoot) { if (DEBUG) System.out.println("INFLATING from resource: " + resource); XmlResourceParser parser = getContext().getResources().getLayout(resource); try { return inflate(parser, root, attachToRoot); } finally { parser.close(); } }
先根据resource id 获取到XmlResourceParseer，意如其名，就是xml的解析器，继续往下，进入到inflate的核心方法，有些长，我们只分析关键部分：
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) { ...... 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); } else { // Temp is the root view that was found in the xml View temp; if (TAG_1995.equals(name)) { temp = new BlinkLayout(mContext, attrs); } else { temp = createViewFromTag(root, name, attrs); } ...... } 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; } return result; } }
如果tag的名字不是TAG_1995（名字是个梗），就调用函数createViewFromTag()创建View，进去看看，
View createViewFromTag(View parent, String name, AttributeSet attrs) { if (name.equals("view")) { name = attrs.getAttributeValue(null, "class"); } ...... View view; if (mFactory2 != null) view = mFactory2.onCreateView(parent, name, mContext, attrs); else if (mFactory != null) view = mFactory.onCreateView(name, mContext, attrs); else view = null; if (view == null && mPrivateFactory != null) { view = mPrivateFactory.onCreateView(parent, name, mContext, attrs); } if (view == null) { if (-1 == name.indexOf(‘.‘)) { view = onCreateView(parent, name, attrs); } else { view = createView(name, null, attrs); } } if (DEBUG) System.out.println("Created view is: " + view); return view; ...... }
首先尝试用3个Fractory创建View，如果成功就直接返回了。注意，我们可以利用这个机制，创建自己的Factory来控制View的创建过程。
如果没有Factory或创建失败，那么走默认逻辑。
先判断name中是否有‘.‘字符，如果没有，则认为使用android自己的View，此时会在name的前面加上包名"android.view."；如果有这个‘.‘，则认为使用的自定义View，这时无需添加任何前缀，认为name已经包含全包名了。
最终，使用这个全包名的name来创建实例，
private static final HashMap<String, Constructor<? extends View>> sConstructorMap = new HashMap<String, Constructor<? extends View>>(); 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 { Constructor<? extends View> constructor = sConstructorMap.get(name); Class<? extends View> clazz = null; ...... if (constructor == null) { // Class not found in the cache, see if it‘s real, and try to add it 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); 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; return constructor.newInstance(args); ...... }
从源码中看到，在创建实例前，会先从一个静态Map中获取缓存，
Constructor<? extends View> constructor = sConstructorMap.get(name);
缓存的是Constructor对象，目的是用于创建实例，这里要注意sConstructorMap是静态的，并且通过Constructor创建的实例，是使用和Constructor对象同一个ClassLoader来创建的，换句话说，在同一个进程中，同一个自定义View对象，是无法用不同ClassLoader加载的，如果想解决这个问题，就不要让系统使用createView()接口创建View，做法就是自定义Factory或Factory2来自行创建View。
继续往下看，如果缓存里没有，则创建View的Class对象clazz，并缓存到sConstructorMap中，
if (constructor == null) { // Class not found in the cache, see if it‘s real, and try to add it 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); sConstructorMap.put(name, constructor); }
然后就是newInstance了，至此这个View便从xml中变成了java对象，我们继续返回到inflate函数中，看看这个View返回之后做了什么，
...... // Temp is the root view that was found in the xml View temp; if (TAG_1995.equals(name)) { temp = new BlinkLayout(mContext, attrs); } else { temp = createViewFromTag(root, name, attrs); } ViewGroup.LayoutParams params = null; if (root != null) { // 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); } } // Inflate all children under temp rInflate(parser, temp, attrs, true); // 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; } ...... return result;
从createViewFromTag返回后，会调用个rInflate()，其中parent参数就是刚才创建出的View，应该能猜到里面做了什么，
void rInflate(XmlPullParser parser, View parent, final 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_INCLUDE.equals(name)) { if (parser.getDepth() == 0) { throw new InflateException("<include /> cannot be the root element"); } parseInclude(parser, parent, attrs); } else if (TAG_MERGE.equals(name)) { throw new InflateException("<merge /> must be the root element"); } else if (TAG_1995.equals(name)) { final View view = new BlinkLayout(mContext, attrs); final ViewGroup viewGroup = (ViewGroup) parent; final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); rInflate(parser, view, attrs, true); viewGroup.addView(view, params); } else { final View view = createViewFromTag(parent, name, attrs); final ViewGroup viewGroup = (ViewGroup) parent; final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); rInflate(parser, view, attrs, true); viewGroup.addView(view, params); } } if (finishInflate) parent.onFinishInflate(); }
没错，就是递归的使用createViewFromTag()创建子View，并通过ViewGroup.addView添加到parent view中。
之后，这个View树上的所有View都创建完毕。然后会根据inflate()的参数（root和attachToRoot）判断是否将新创建的View添加到root view中，
if (root != null && attachToRoot) { root.addView(temp, params); }
然后，inflate()就将View返回了。
整个分析到此结束。
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