大家都很熟悉LayoutInflater来添加View。我们看一下它的实现。
public abstract class LayoutInflater {}
首先LayoutInflater是个抽象类,也没有继承其他类.
1.我们可以通过下面两种方式获得LayoutInflater;
LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
或者Activity.getLayoutInflater();
2.一般使用LayoutInflater的方式是以下几种
public View inflate(int resource, ViewGroup root, boolean attachToRoot)
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)
public View inflate(int resource, ViewGroup root)
public View inflate(XmlPullParser parser, ViewGroup root)
其中加载xml分R.id.xx和直接载入.xml,但是由于解析xml需要时间，这时如果xml不是预先被编译就不能显示视图报错(如在加载assets文件中的xml文件).使用R.id.xx实际上也是通过XmlPullParser来加载视图的.
ViewGroup相当于父视图,如果有root视图，默认attachToRoot为false,这时会messure父视图的layout来加载子视图.如果root视图为空,会创建视图来加载到viewtree的根部.其实创建的是内部类BlinkLayout，它是继承帧视图，方便上下图层的叠加.在加载BlinkLayout之前还是先创建了
ViewStub，来占空视图.
private static class BlinkLayout extends FrameLayout
下面我们看下最关键的inflate
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. 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, 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); } 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 rInflate(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 (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; } }
它在解析xml时候实际是调用了rInflate,来递归xml解析.
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(); }
解析的是一下xml的标签
private static final String TAG_MERGE = "merge"; private static final String TAG_INCLUDE = "include"; private static final String TAG_1995 = "blink"; private static final String TAG_REQUEST_FOCUS = "requestFocus";
我们在xml中引用其他视图的时候会用到<include>来减少重复视图的编写.
我们在parseInclude中可以看到解析不同的tag执行不同的操作.其中requestFocus标签目前还没有做任何事情.
private void parseInclude(XmlPullParser parser, View parent, AttributeSet attrs) throws XmlPullParserException, IOException { int type; if (parent instanceof ViewGroup) { final int layout = attrs.getAttributeResourceValue(null, "layout", 0); if (layout == 0) { final String value = attrs.getAttributeValue(null, "layout"); if (value == null) { throw new InflateException("You must specifiy a layout in the" + " include tag: <include layout="@layout/layoutID" />"); } else { throw new InflateException("You must specifiy a valid layout " + "reference. The layout ID " + value + " is not valid."); } } else { final XmlResourceParser childParser = getContext().getResources().getLayout(layout); try { final AttributeSet childAttrs = Xml.asAttributeSet(childParser); while ((type = childParser.next()) != XmlPullParser.START_TAG && type != XmlPullParser.END_DOCUMENT) { // Empty. } if (type != XmlPullParser.START_TAG) { throw new InflateException(childParser.getPositionDescription() + ": No start tag found!"); } final String childName = childParser.getName(); if (TAG_MERGE.equals(childName)) { // Inflate all children. rInflate(childParser, parent, childAttrs, false); } else { final View view = createViewFromTag(parent, childName, childAttrs); final ViewGroup group = (ViewGroup) parent; // We try to load the layout params set in the <include /> tag. If // they don't exist, we will rely on the layout params set in the // included XML file. // During a layoutparams generation, a runtime exception is thrown // if either layout_width or layout_height is missing. We catch // this exception and set localParams accordingly: true means we // successfully loaded layout params from the <include /> tag, // false means we need to rely on the included layout params. ViewGroup.LayoutParams params = null; try { params = group.generateLayoutParams(attrs); } catch (RuntimeException e) { params = group.generateLayoutParams(childAttrs); } finally { if (params != null) { view.setLayoutParams(params); } } // Inflate all children. rInflate(childParser, view, childAttrs, true); // Attempt to override the included layout's android:id with the // one set on the <include /> tag itself. TypedArray a = mContext.obtainStyledAttributes(attrs, com.android.internal.R.styleable.View, 0, 0); int id = a.getResourceId(com.android.internal.R.styleable.View_id, View.NO_ID); // While we're at it, let's try to override android:visibility. int visibility = a.getInt(com.android.internal.R.styleable.View_visibility, -1); a.recycle(); if (id != View.NO_ID) { view.setId(id); } switch (visibility) { case 0: view.setVisibility(View.VISIBLE); break; case 1: view.setVisibility(View.INVISIBLE); break; case 2: view.setVisibility(View.GONE); break; } group.addView(view); } } finally { childParser.close(); } } } else { throw new InflateException("<include /> can only be used inside of a ViewGroup"); } final int currentDepth = parser.getDepth(); while (((type = parser.next()) != XmlPullParser.END_TAG || parser.getDepth() > currentDepth) && type != XmlPullParser.END_DOCUMENT) { // Empty } }