本文主要从如下几点学习LayoutInflater
- LayoutInflater是啥
- LayoutInflater的获取
- LayoutInflater的inflate方法
- 总结
LayoutInflater是啥
源码定义
Instantiates a layout XML file into its corresponding {@link android.view.View}
objects. It is never used directly. Instead, use
{@link android.app.Activity#getLayoutInflater()} or
{@link Context#getSystemService} 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.
// 说的意思就是
LayoutInflater是将XML布局文件实例化为View的对象。不要单独使用它，需要使用
Activity.getLayoutInflater 或者使用 Context.getSystemService()来获取于
当前Context相关联并且正确配置的LayoutInflater
- 一般我们叫它布局加载器
LayutInflater的获取
- Activity.getLayoutInflater()
- Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)
- LayoutInflater.from(Context)
这三种方式的对比
- Activity.getLayoutInflater()方法最终是调用了PhoneWindow中的getLayoutInflater的方法，PhoneWindow的构造方法中，是使用方式三来进行LayoutInflater的获取，所以方式一是调用了方式三来获取LayoutInflater对象的。
- 我们点开LayoutInflater.from方法我们会看见又调用了方式二获取到了LayoutInflater对象。
LayoutInflater的inflate方法
LayoutInflater类中有多个重载的inflate方法，但是这些方法最终都会调用下面这个方法
-
代码如下
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) { synchronized (mConstructorArgs) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate"); final Context inflaterContext = mContext; //获取到Xml文件中属性信息集合 final AttributeSet attrs = Xml.asAttributeSet(parser); Context lastContext = (Context) mConstructorArgs[0]; mConstructorArgs[0] = inflaterContext; View result = root; try { // 用来检查xml文件的根布局是否合理 advanceToRootNode(parser); final String name = parser.getName(); if (DEBUG) { System.out.println("**************************"); System.out.println("Creating root view: " + name); System.out.println("**************************"); } // 如果是Merge标签的话，必须依附一个RootView，否则的话就抛出异常 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 //如果不是Merger标签，就通过标签的名字创建一个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 //如果Root不为null，那么根据标签的参数生成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) //如果不是attachToRoot 那么就将这个Tag创建出来的View设置LayoutParams 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. //如果root不为null并且是attachToRoot那么就将创建好的View添加到Root中 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) { final InflateException ie = new InflateException(e.getMessage(), e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } catch (Exception e) { final InflateException ie = new InflateException( getParserStateDescription(inflaterContext, attrs) + ": " + e.getMessage(), e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } finally { // Don't retain static reference on context. mConstructorArgs[0] = lastContext; mConstructorArgs[1] = null; Trace.traceEnd(Trace.TRACE_TAG_VIEW); } return result; } } -
infalte方法总结
- 如果是Merge标签，必须要依附于一个RootView，之后调用了rInflate方法
- 否则的话就调用了createViewFromTag()创建View的对象。
rInflate方法
-
代码如下
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; // 循环去解析View下面的子View while (((type = parser.next()) != XmlPullParser.END_TAG || parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) { // XmlPullParse.START_DOCUMENT 也符合 while循环的条件 // 也符合下面代码的条件 if (type != XmlPullParser.START_TAG) { continue; } //获取到标签的名字 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)) { // include标签不能作为布局文件的根布局 if (parser.getDepth() == 0) { throw new InflateException("<include /> cannot be the root element"); } parseInclude(parser, context, parent, attrs); } else if (TAG_MERGE.equals(name)) { //Merge标签必须放到布局文件的根节点上 throw new InflateException("<merge /> must be the root element"); } else { //调用createViewFromTag 创建View对象 final View view = createViewFromTag(parent, name, context, attrs); final ViewGroup viewGroup = (ViewGroup) parent; final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); //循环遍历创建View下的子View，该方法最终会调用到 rInflate()方法 rInflateChildren(parser, view, attrs, true); //将每次创建的View添加到父布局中，这样就形成了完整的DOM树 viewGroup.addView(view, params); } } if (pendingRequestFocus) { parent.restoreDefaultFocus(); } if (finishInflate) { //如果inflate结束，那么回调parent的onFinishInflate方法 parent.onFinishInflate(); } } rInflate方法总结：该方法内主要就是循环View下面的子View，拿到表标签的名字
如果是include标签，不能把这个标签作为布局文件的根布局
如果是merge标签必须放到布局文件的根布局上
其他标签通过createViewFromTag()方法创建View，然后调用rInflateChildren()继续循环当前获取到View下面的子View重复rInflate方法所做的事情，之后把获取到的View添加到父布局中，这样就形成了成型的DOM树了。
createViewFromTag方法
-
代码如下
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(); } try { // 该方法主要使用Factory创建View，返回创建的View View view = tryCreateView(parent, name, context, attrs); //当通过Factory或者Factory2的onCreateView方法创建的View为null if (view == null) { final Object lastContext = mConstructorArgs[0]; mConstructorArgs[0] = context; try { //就通过onCreateView方法创建View对象 // 如果View的name中没有"."说明是系统的控件 //会在接下来的调用中 在name前面加 android.view. if (-1 == name.indexOf('.')) { view = onCreateView(context, parent, name, attrs); } else { //如果name中包含了. 则直接调用了createView方法 view = createView(context, name, null, attrs); } } finally { mConstructorArgs[0] = lastContext; } } return view; } catch (InflateException e) { throw e; } catch (ClassNotFoundException e) { final InflateException ie = new InflateException( getParserStateDescription(context, attrs) + ": Error inflating class " + name, e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } catch (Exception e) { final InflateException ie = new InflateException( getParserStateDescription(context, attrs) + ": Error inflating class " + name, e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } } -
createViewFromTag方法的总结
- 先通过tryCreateView创建View，内部主要是调用Factory的onCreateView创建view
- 如果上述的方式创建的View为null，就使用createView方法创建View
- 其中onCreateView方法最终调用的是createView()方法
createView方法
-
代码如下
public final View createView(@NonNull Context viewContext, @NonNull String name, @Nullable String prefix, @Nullable AttributeSet attrs) throws ClassNotFoundException, InflateException { Objects.requireNonNull(viewContext); Objects.requireNonNull(name); 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 // 通过Class.forName() 拿到当前类的Class对象 clazz = Class.forName(prefix != null ? (prefix + name) : name, false, mContext.getClassLoader()).asSubclass(View.class); if (mFilter != null && clazz != null) { boolean allowed = mFilter.onLoadClass(clazz); if (!allowed) { failNotAllowed(name, prefix, viewContext, attrs); } } // 通过反射getConstructor方法获取到Class对象的Constructor 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 = Class.forName(prefix != null ? (prefix + name) : name, false, mContext.getClassLoader()).asSubclass(View.class); boolean allowed = clazz != null && mFilter.onLoadClass(clazz); mFilterMap.put(name, allowed); if (!allowed) { failNotAllowed(name, prefix, viewContext, attrs); } } else if (allowedState.equals(Boolean.FALSE)) { failNotAllowed(name, prefix, viewContext, attrs); } } } Object lastContext = mConstructorArgs[0]; mConstructorArgs[0] = viewContext; Object[] args = mConstructorArgs; args[1] = attrs; try { //通过newInstance获取到View的对象 final View view = constructor.newInstance(args); //如果当前View是ViewStub if (view instanceof ViewStub) { // Use the same context when inflating ViewStub later. // 强转成 ViewStub final ViewStub viewStub = (ViewStub) view; viewStub.setLayoutInflater(cloneInContext((Context) args[0])); } return view; } finally { mConstructorArgs[0] = lastContext; } } catch (NoSuchMethodException e) { final InflateException ie = new InflateException( getParserStateDescription(viewContext, attrs) + ": Error inflating class " + (prefix != null ? (prefix + name) : name), e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } catch (ClassCastException e) { // If loaded class is not a View subclass final InflateException ie = new InflateException( getParserStateDescription(viewContext, attrs) + ": Class is not a View " + (prefix != null ? (prefix + name) : name), e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } catch (ClassNotFoundException e) { // If loadClass fails, we should propagate the exception. throw e; } catch (Exception e) { final InflateException ie = new InflateException( getParserStateDescription(viewContext, attrs) + ": Error inflating class " + (clazz == null ? "<unknown>" : clazz.getName()), e); ie.setStackTrace(EMPTY_STACK_TRACE); throw ie; } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } }- createView方法的核心就是通过反射获取到View的对象
总结
infalte重载的方法
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
// 当root不为null attachToRoot为true否则为false
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) + ")");
}
View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
if (view != null) {
return view;
}
XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {
// 同样 root不为null attachToRoot为true 否则为false
return inflate(parser, root, root != null);
}
//最终都会调用的inflate方法
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
-
通过如下代码我们得出：如果root不为null 并且attachToRoot为true，那么给加载的布局文件指定一个父布局
if (root != null && attachToRoot) { // 如果root不为null 并且attachToRoot为true //那么给加载的布局文件指定一个父布局 root.addView(temp, params); }
-
通过如下代码我们得出：如果root不为null，并且attacheToRoot为false，那么就把root的LayoutParams设置给布局文件的最外层。
if (root != null) { if (DEBUG) { System.out.println("Creating params from root: " + root); } // Create layout params that match root, if supplied //如果Root不为null，那么根据标签的参数生成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) //如果不是attachToRoot 那么就将这个Tag创建出来的View设置LayoutParams temp.setLayoutParams(params); } }
- 如果调用的inflate方法中，没有设置attacheToRoot的赋值入口，那么attachToRoot的取值取决于root是否为null
- 如果root为null，那么attachToRoot为false
- 如果root不为null，那么attacheToRoot为true
