从源码角度深入理解LayoutInflater
【摘要】 关于LayoutInflater，在开发中经常会遇到，特别是在使用ListView的时候，这个几乎是必不可少。今天我们就一起来探讨LayoutInflater的工作原理。
一般情况下，有两种方式获得一个LayoutInflater实例：
LayoutInflater inflater1, inflater2; inflater1 = LayoutInflater....
关于LayoutInflater，在开发中经常会遇到，特别是在使用ListView的时候，这个几乎是必不可少。今天我们就一起来探讨LayoutInflater的工作原理。
一般情况下，有两种方式获得一个LayoutInflater实例：
LayoutInflater inflater1, inflater2; inflater1 = LayoutInflater.from(this); inflater2 = (LayoutInflater) this .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
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但是当我们查看源码的时候，却发现这两种其实是一种，只不过第一种将第二种封装了一下，我们看看from这个方法的源码：
/** * 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; }
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获得LayoutInflater对象之后，我们就可以调用inflate来获得View对象了，inflate方法的源码如下：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) { return inflate(resource, root, root != null); }
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这里调用了一个inflate的一个重载方法，这个重载方法的最后一个参数和root有关，如果我们的root为空，那么最后一个参数默认为false。我们看看这个重载方法：
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(); } }
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可以看出，先是拿到布局的xml资源，然后，获得一个XmlResourceParser 对象，最后inflate(parser, root, attachToRoot);又是调用一个重载方法：
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("<merge /> 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) { InflateException ex = new InflateException(e.getMessage()); ex.initCause(e); throw ex; } catch (Exception 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; } }
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这个方法有点长，前面都是一些简单的判断，一般情况下（特殊情况大家可以按相应的分支走），我们的程序会执行到final View temp = createViewFromTag(root, name, inflaterContext, attrs);这行代码，这里创建了一个名为temp的view，如果我们传进来的根布局为null的话，那么这里拿到的就是一个根布局。我们看看这个方法的源代码：
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) { return createViewFromTag(parent, name, context, attrs, false); }
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好啊，这里又调用了一个它的重载方法，那我们就看看这个重载方法，注意最后一个参数恒为false。
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) { view = mFactory2.onCreateView(parent, name, context, attrs); } else if (mFactory != null) { view = mFactory.onCreateView(name, context, attrs); } else { view = null; } if (view == null && mPrivateFactory != null) { view = mPrivateFactory.onCreateView(parent, name, context, attrs); } if (view == null) { final Object lastContext = mConstructorArgs[0]; mConstructorArgs[0] = context; try { 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) { final InflateException ie = new InflateException(attrs.getPositionDescription() + ": Error inflating class " + name); ie.initCause(e); throw ie; } catch (Exception e) { final InflateException ie = new InflateException(attrs.getPositionDescription() + ": Error inflating class " + name); ie.initCause(e); throw ie; } }
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仔细分析一下这个重载方法，发现里边的mFactory2和mFactory都为null，那么程序最终其实执行了这个方法里边的这一段：
if (view == null) { final Object lastContext = mConstructorArgs[0]; mConstructorArgs[0] = context; try { if (-1 == name.indexOf('.')) { view = onCreateView(parent, name, attrs); } else { view = createView(name, null, attrs); } } finally { mConstructorArgs[0] = lastContext; } }
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前面的判断不用说，后面的判断，如果name中包含.，说明我们用的不是普通的view，有可能是自定义View等等，这一条大家可以自行去研究，如果name中不包含.,那么程序会执行view = onCreateView(parent, name, attrs);，那么我们就去看看这个方法：
protected View onCreateView(View parent, String name, AttributeSet attrs) throws ClassNotFoundException { return onCreateView(name, attrs); } 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; 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); } } constructor = clazz.getConstructor(mConstructorSignature); constructor.setAccessible(true); 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; 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; } 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); } }
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经过了前两个方法的互相扯皮，最后我们来到了第三个方法上，这是最后创建View的地方，代码虽然很长，但是大家不用怕，这里的代码我们主要分析下面这几行，因为大部分不会被执行到。
public final View createView(String name, String prefix, AttributeSet attrs) throws ClassNotFoundException, InflateException { ..... ..... 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); ..... ..... constructor = clazz.getConstructor(mConstructorSignature); constructor.setAccessible(true); sConstructorMap.put(name, constructor); } Object[] args = mConstructorArgs; args[1] = attrs; 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; }traceEnd(Trace.TRACE_TAG_VIEW); } }
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我把这个方法稍微精简一下，可以看到，先是通过Java的反射机制拿到这个name所表示的布局对应的那个Java类，然后是拿到构造方法，最后通过构造方法拿到一个View实例，逻辑还是比较清楚的。
好的，到这里，我们就已经拿到根View了，现在我们再回到上面说的那个inflate(…)方法中，在该方法中创建完temp这个View之后，接着就会执行到rInflateChildren(parser, temp, attrs, true);，里边最终会执行到一个递归方法，这个方法是这样的：
void rInflate(XmlPullParser parser, View parent, Context context, 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_TAG.equals(name)) { parseViewTag(parser, parent, attrs); } else if (TAG_INCLUDE.equals(name)) { if (parser.getDepth() == 0) { throw new InflateException("<include /> cannot be the root element"); } parseInclude(parser, context, parent, attrs); } else if (TAG_MERGE.equals(name)) { throw new InflateException("<merge /> 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 (finishInflate) { parent.onFinishInflate(); } }
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这个方法还不算长，进入while循环以后，在if分支里会走到最后一个else里，这里还是先调用我们前文说的那个createViewFromTag方法获得一个布局，然后递归，如果获得的这个View是个ViewGroup，那么会把它的子View添加到这个ViewGroup中，如果是普通View，那么接着循环就是了。做完这些之后，下面就和一开始提到的root是否为null有关了，如果root为null，那么attachToRoot这个参数为false，这个时候会执行：
if (root == null || !attachToRoot) { result = temp; }
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然后我们解析过得到的View就会被返回，如果root不为null,那么attachToRoot这个参数默认为true，那么系统会执行
if (root != null && attachToRoot) { root.addView(temp, params); }
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也就是会把root套在我们解析得到的View之外，然后返回。
好了，到这里我们的LayoutInflater基本上就分析完了.
如果大家还有什么问题，欢迎留言讨论
文章来源: wangsong.blog.csdn.net，作者：_江南一点雨，版权归原作者所有，如需转载，请联系作者。
原文链接：wangsong.blog.csdn.net/article/details/49314327
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