Android LayoutInflater 源码解析
在上篇文章中我们学习了setContentView的源码,还记得其中的LayoutInflater吗?本篇文章就来学习下LayoutInflater。
@Override public void setContentView(int resId) { ensureSubDecor(); ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content); contentParent.removeAllViews(); LayoutInflater.from(mContext).inflate(resId, contentParent); mOriginalWindowCallback.onContentChanged(); }
备注:本文基于 Android 8.1.0。
1、LayoutInflater 简介
Instantiates a layout XML file into its corresponding View objects. It is never used directly. Instead, use Activity.getLayoutInflater() or Context.getSystemService(Class) to retrieve a standard LayoutInflater instance that is already hooked up to the current context and correctly configured for the device you are running on.
翻译过来就是:LayoutInflater 的作用就是将XML布局文件实例化为相应的 View 对象,需要通过Activity.getLayoutInflater() 或 Context.getSystemService(Class) 来获取与当前Context已经关联且正确配置的标准LayoutInflater。
总共有三种方法来获取 LayoutInflater:
- Activity.getLayoutInflater();
- Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ;
- LayoutInflater.from(context);
事实上,这三种方法之间是有关联的:
- Activity.getLayoutInflater() 最终会调用到 PhoneWindow 的构造方法,实际上最终调用的就是方法三;
- 而方法三最终会调用到方法二 Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ;
2、inflate 方法解析
imageLayoutInflater 的 inflate 方法总共有四个,属于重载的关系,最终都会调用到 inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 方法。
备注:以下源码中有七条备注。
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; // ① 寻找布局的根节点,判断布局的合理性 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 (TAG_MERGE.equals(name)) { // ② 如果是Merge标签,则必须依附于一个RootView,否则抛出异常 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 // ③ 根据节点名来创建View对象 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 // ④ 如果设置的Root不为null,则根据当前标签的参数生成LayoutParams params = root.generateLayoutParams(attrs); if (!attachToRoot) { // Set the layout params for temp if we are not // attaching. (If we are, we use addView, below) // ⑤ 如果不是attachToRoot ,则对这个Tag和创建出来的View设置LayoutParams;注意:此处的params只有当被添加到一个Viewz中的时候才会生效; temp.setLayoutParams(params); } } if (DEBUG) { System.out.println("-----> start inflating children"); } // Inflate all children under temp against its context. // ⑥ inflate children tag rInflateChildren(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) { // ⑦ 如果Root不为null且是attachToRoot,则添加创建出来的View到Root 中 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) { ...... } return result; } }
备注:根据以上源码,我们也可以分析出来 inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 不同参数值带来的影响:
- 如果root为null,attachToRoot将失去作用,设置任何值都没有意义;
- 如果root不为null,attachToRoot设为true,则会给加载的布局文件的指定一个父布局,即root;
- 如果root不为null,attachToRoot设为false,则会将布局文件最外层的所有layout属性进行设置,当该view被添加到父view当中时,这些layout属性会自动生效;
- 在不设置attachToRoot参数的情况下,如果root不为null,attachToRoot参数默认为true;
3、rInflate 方法解析
以上代码中我们还有两个方法没有分析:rInflate 和 rInflateChildren ;而 rInflateChildren 实际上是调用了rInflate;
备注:以下源码中有六条备注。
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) { // ① 如果这里出现了include标签,就会抛出异常 throw new InflateException(" cannot be the root element"); } parseInclude(parser, context, parent, attrs); } else if (TAG_MERGE.equals(name)) { // ② 同理如果这里出现了merge标签,也会抛出异常 throw new InflateException(" must be the root element"); } else { // ③ 最重要的方法在这里,createViewFromTag final View view = createViewFromTag(parent, name, context, attrs); final ViewGroup viewGroup = (ViewGroup) parent; final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); // ④如果当前View是ViewGroup(包裹了别的View)则在此处inflate其所有的子View rInflateChildren(parser, view, attrs, true); // ⑤添加inflate出来的view到parent中 viewGroup.addView(view, params); } } if (pendingRequestFocus) { parent.restoreDefaultFocus(); } if (finishInflate) { // ⑥如果inflate结束,则回调parent的onFinishInflate方法 parent.onFinishInflate(); } }
总结:
- 首先进行View的合理性校验,include、merge等标签;
- 通过 createViewFromTag 创建出 View 对象;
- 如果是 ViewGroup,则重复以上步骤;
- add View 到相应的 parent 中;
4、createViewFromTag 方法解析
备注:以下源码中有六条备注。
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) { // ① 有mFactory2,则调用mFactory2的onCreateView方法 view = mFactory2.onCreateView(parent, name, context, attrs); } else if (mFactory != null) { // ② 有mFactory,则调用mFactory的onCreateView方法 view = mFactory.onCreateView(name, context, attrs); } else { view = null; } if (view == null && mPrivateFactory != null) { // ③ 有mPrivateFactory,则调用mPrivateFactory的onCreateView方法 view = mPrivateFactory.onCreateView(parent, name, context, attrs); } if (view == null) { // ④ 走到这步说明三个Factory都没有,则开始自己创建View final Object lastContext = mConstructorArgs[0]; mConstructorArgs[0] = context; try { if (-1 == name.indexOf('.')) { // ⑤ 如果View的name中不包含 '.' 则说明是系统控件,会在接下来的调用链在name前面加上 'android.view.' view = onCreateView(parent, name, attrs); } else { // ⑥ 如果name中包含 '.' 则直接调用createView方法,onCreateView 后续也是调用了createView view = createView(name, null, attrs); } } finally { mConstructorArgs[0] = lastContext; } } return view; } catch (InflateException e) { throw e; } }
总结:
- createViewFromTag 方法比较简单,首先尝试通过 Factory 来创建View;
- 如果没有 Factory 的话则通过 createView 来创建View;
5、createView 方法解析
备注:以下源码中有三条备注。
public final View createView(String name, String prefix, AttributeSet attrs) throws ClassNotFoundException, InflateException { 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); 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); } } // ① 反射获取这个View的构造器 constructor = clazz.getConstructor(mConstructorSignature); constructor.setAccessible(true); // ② 缓存构造器 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 lastContext = mConstructorArgs[0]; if (mConstructorArgs[0] == null) { // Fill in the context if not already within inflation. mConstructorArgs[0] = mContext; } Object[] args = mConstructorArgs; args[1] = attrs; // ③ 使用反射创建 View 对象,这样一个 View 就被创建出来了 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])); } mConstructorArgs[0] = lastContext; return view; } catch (ClassCastException e) { } }
总结:
- createView 方法也比较简单,通过反射来创建的 View 对象;
6、总结
通过本文我们学习到 LayoutInflater 创建 View的过程,也知道了 inflate 方法不同参数的意义,以及开发中遇到的一些异常在源码中的根源。可以看到从布局中 inflate 一个个具体的 View 的过程其实也很简单:
- 通过 XML 的 Pull 解析方式获取 View 的标签;
- 通过标签以反射的方式来创建 View 对象;
- 如果是 ViewGroup 的话则会对子 View 遍历并重复以上步骤,然后 add 到父 View 中;
- 与之相关的几个方法:inflate ——》 rInflate ——》 createViewFromTag ——》 createView ;
参考
- 《Android LayoutInflater原理分析,带你一步步深入了解View(一)》
- LayoutInflater
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