LayoutInflater解析
前言:
在Android中,如果是初级玩家,很可能对LayoutInflater不太熟悉,或许只是在Fragment的onCreateView()中模式化的使用过而已。但如果稍微有些工作经验的人就知道,这个类有多么重要,它是连接布局XMl和Java代码的桥梁,我们常常疑惑,为什么Android支持在XML书写布局?
我们想到的必然是Android内部帮我们解析xml文件,LayoutInflater就是帮我们做了这个工作。
首先LayoutInflater是一个系统服务,这个我们可以从from方法看出来
/** * 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; }
通常我们拿到LayoutInflater对象之后就会调用其inflate方法进行加载布局,inflate是一个重载方法
public View inflate(int resource, ViewGroup root) { return inflate(resource, root, root != null); }
可以看到,我们调用2个参数的方法时候其默认是添加到父布局中的(父布局一般不为空)
public View inflate(int resource, 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(); } }
这个方法中,其实是使用Resources将资源ID还原为XMlResoourceParser对象,然后调用inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)方法,解析布局的具体步骤都是在这个方法中实现
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) { synchronized (mConstructorArgs) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate"); final AttributeSet attrs = Xml.asAttributeSet(parser); Context lastContext = (Context)mConstructorArgs[0]; mConstructorArgs[0] = mContext; View result = root; try { // Look for the root node. //1.循环寻找根节点,其实就是节点指针遍历的过程 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!"); } //2.得到节点的名字,用于判断该节点 final String name = parser.getName(); if (DEBUG) { System.out.println("**************************"); System.out.println("Creating root view: " + name); System.out.println("**************************"); } //3.对节点名字进行判断,然后是merge就将其添加到父布局中(依据Merge的特性必须添加到父布局中) 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, false); } else { //4.创建根据节点创建View // Temp is the root view that was found in the xml final View temp = createViewFromTag(root, name, attrs, false); 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 //5.根据attrs生成布局参数 params = root.generateLayoutParams(attrs); if (!attachToRoot) { // Set the layout params for temp if we are not // attaching. (If we are, we use addView, below) //6.如果View不添加到父布局中,那就给其本身设置布局参数 temp.setLayoutParams(params); } } if (DEBUG) { System.out.println("-----> start inflating children"); } // Inflate all children under temp // 7.将该节点下的子View全部加载 rInflate(parser, temp, attrs, true, 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. //8.如果添加到父布局中,直接addView 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. //9.如果不添加到父布局,那么将自己返回 if (root == null || !attachToRoot) { result = temp; } } } 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; } Trace.traceEnd(Trace.TRACE_TAG_VIEW); return result; } }
重点的步骤我已经加上注释了,核心
1.找到根布局标签
2.创建根节点对应的View
3.创建其子View
我们从这里面可以看出来,子View的解析其实都是rInflate方法,如果xml中有根布局,那么就调用createViewFromTag创建布局中的根View。我们也可以明白merge的原来,因为它直接调用rInflate添加到父View中,看到rInflate(parser, root, attrs, false, false)和rInflate(parser, temp, attrs, true, true)第二个参数区别我们就明白了。
接下来我们看下rInflate如何创建多个布局
void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs, boolean finishInflate, boolean inheritContext) throws XmlPullParserException, IOException { //获取当前解析器指针所在节点处于布局层次 final int depth = parser.getDepth(); int type; //进行树的深度优先遍历(如果一个节点有子节点将会再次进入rInflate,否则继续循环) 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(); //如果其中有request_focus标签,那就给这个节点View设置焦点 if (TAG_REQUEST_FOCUS.equals(name)) { parseRequestFocus(parser, parent); //如果其中有tag标签,那就给这个节点View设置tag(key,value) } else if (TAG_TAG.equals(name)) { parseViewTag(parser, parent, attrs); } else if (TAG_INCLUDE.equals(name)) { //如果其中是include标签,如果include标签 if (parser.getDepth() == 0) { throw new InflateException("<include /> cannot be the root element"); } parseInclude(parser, parent, attrs, inheritContext); } else if (TAG_MERGE.equals(name)) { throw new InflateException("<merge /> must be the root element"); } else { //创建该节点代表的View并添加到父view中,此外遍历子节点 final View view = createViewFromTag(parent, name, attrs, inheritContext); final ViewGroup viewGroup = (ViewGroup) parent; final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); rInflate(parser, view, attrs, true, true); viewGroup.addView(view, params); } } //代表着一个节点含其子节点遍历结束 if (finishInflate) parent.onFinishInflate(); }
从上面可以看到,所以创建View都将会交给createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext)中,我们可以看下该方法如何创建View
View createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext) { if (name.equals("view")) { name = attrs.getAttributeValue(null, "class"); } Context viewContext; if (parent != null && inheritContext) { viewContext = parent.getContext(); } else { viewContext = mContext; } // Apply a theme wrapper, if requested. final TypedArray ta = viewContext.obtainStyledAttributes(attrs, ATTRS_THEME); final int themeResId = ta.getResourceId(0, 0); if (themeResId != 0) { viewContext = new ContextThemeWrapper(viewContext, themeResId); } ta.recycle(); if (name.equals(TAG_1995)) { // Let's party like it's 1995! return new BlinkLayout(viewContext, attrs); } if (DEBUG) System.out.println("******** Creating view: " + name); try { View view; if (mFactory2 != null) { view = mFactory2.onCreateView(parent, name, viewContext, attrs); } else if (mFactory != null) { view = mFactory.onCreateView(name, viewContext, attrs); } else { view = null; } if (view == null && mPrivateFactory != null) { view = mPrivateFactory.onCreateView(parent, name, viewContext, attrs); } if (view == null) { final Object lastContext = mConstructorArgs[0]; mConstructorArgs[0] = viewContext; try { if (-1 == name.indexOf('.')) { view = onCreateView(parent, name, attrs); } else { view = createView(name, null, attrs); } } finally { mConstructorArgs[0] = lastContext; } } if (DEBUG) System.out.println("Created view is: " + view); return view; } catch (InflateException e) { throw e; } catch (ClassNotFoundException e) { InflateException ie = new InflateException(attrs.getPositionDescription() + ": Error inflating class " + name); ie.initCause(e); throw ie; } catch (Exception e) { InflateException ie = new InflateException(attrs.getPositionDescription() + ": Error inflating class " + name); ie.initCause(e); throw ie; } }
其实很简单,就是4个降级处理
if(factory2!=null){ factory2.onCreateView(); }else if(factory!=null){ factory.onCreateView(); }else if(mPrivateFactory!=null){ mPrivateFactory.onCreateView(); }else{ onCreateView() }
其他的onCreateView我们不去设置的话为null,我们看下自己的onCreateView(),其实这个方法会调用createView()
public final View createView(String name, String prefix, AttributeSet attrs) throws ClassNotFoundException, InflateException { //从构造器Map(缓存)中获取需要的构造器 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 //如果缓存中没有需要的构造器,那就通过ClassLoader加载需要的类 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); } } //将使用过的构造器缓存 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; constructor.setAccessible(true); //通过反射获取需要的实例对象 final View view = constructor.newInstance(args); if (view instanceof ViewStub) { // Use the same context when inflating ViewStub later. final ViewStub viewStub = (ViewStub) view; //ViewStub将创建一个属于自己的LayoutInflater,因为它需要在不同的时机去inflate 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); } }
大体步骤就是,
1.从缓存中获取特定View构造器,如果没有,则加载对应的类,并缓存该构造器,
2.利用构造器反射构造对应的View
3.如果是ViewStub则复制一个LayoutInflater对象传递给它
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