Android使用LayoutInflater来进行布局加载,通常获取方式有两种:
第一种:
LayoutInflater layoutInflater = LayoutInflater.from(context);
第二种:
LayoutInflater layoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
从源码中可以看出第一种是第二种的封装简化,便于使用:
1 public static LayoutInflater from(Context context) { 2 LayoutInflater LayoutInflater = 3 (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); 4 if (LayoutInflater == null) { 5 throw new AssertionError("LayoutInflater not found."); 6 } 7 return LayoutInflater; 8 }
我们通过调用inflate方法便可以完成对布局的加载:
layoutInflater.inflate(resource, root, true);
LayoutInflater中的inflate方法有若干种重载方式,最终都调用了如下代码:
1 public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) { 2 synchronized (mConstructorArgs) { 3 //获取xml中属性信息 4 final AttributeSet attrs = Xml.asAttributeSet(parser); 5 Context lastContext = (Context)mConstructorArgs[0]; 6 mConstructorArgs[0] = mContext; 7 View result = root; 8 9 try { 10 // 查找根节点. 11 int type; 12 while ((type = parser.next()) != XmlPullParser.START_TAG && 13 type != XmlPullParser.END_DOCUMENT) { 14 // Empty 15 } 16 17 if (type != XmlPullParser.START_TAG) { 18 throw new InflateException(parser.getPositionDescription() 19 + ": No start tag found!"); 20 } 21 //获取根节点名称 22 final String name = parser.getName(); 23 24 if (DEBUG) { 25 System.out.println("**************************"); 26 System.out.println("Creating root view: " 27 + name); 28 System.out.println("**************************"); 29 } 30 //如果是merge标签,必须保证父节点不为null且attachToRoot为true 31 if (TAG_MERGE.equals(name)) { 32 if (root == null || !attachToRoot) { 33 throw new InflateException("<merge /> can be used only with a valid " 34 + "ViewGroup root and attachToRoot=true"); 35 } 36 37 rInflate(parser, root, attrs, false); 38 } else { 39 //代表布局文件中根节点的view 40 View temp; 41 if (TAG_1995.equals(name)) { 42 temp = new BlinkLayout(mContext, attrs); 43 } else { 44 //利用反射,通过root名称创建view 45 temp = createViewFromTag(root, name, attrs); 46 } 47 48 ViewGroup.LayoutParams params = null; 49 50 if (root != null) { 51 if (DEBUG) { 52 System.out.println("Creating params from root: " + 53 root); 54 } 55 // Create layout params that match root, if supplied 56 //当提供了父容器时,由父容器根据属性值创建布局参数 57 params = root.generateLayoutParams(attrs); 58 if (!attachToRoot) { 59 // Set the layout params for temp if we are not 60 // attaching. (If we are, we use addView, below) 61 //当不把当前view附加到父容器中,则设置获取到的布局参数 62 //否则使用下面的addView方法设置 63 temp.setLayoutParams(params); 64 } 65 } 66 67 if (DEBUG) { 68 System.out.println("-----> start inflating children"); 69 } 70 // Inflate all children under temp 71 //递归调用此方法加载子布局 72 rInflate(parser, temp, attrs, true); 73 if (DEBUG) { 74 System.out.println("-----> done inflating children"); 75 } 76 77 // We are supposed to attach all the views we found (int temp) 78 // to root. Do that now. 79 if (root != null && attachToRoot) { 80 root.addView(temp, params); 81 } 82 83 // Decide whether to return the root that was passed in or the 84 // top view found in xml. 85 if (root == null || !attachToRoot) { 86 result = temp; 87 } 88 } 89 90 } catch (XmlPullParserException e) { 91 InflateException ex = new InflateException(e.getMessage()); 92 ex.initCause(e); 93 throw ex; 94 } catch (IOException e) { 95 InflateException ex = new InflateException( 96 parser.getPositionDescription() 97 + ": " + e.getMessage()); 98 ex.initCause(e); 99 throw ex; 100 } finally { 101 // Don't retain static reference on context. 102 mConstructorArgs[0] = lastContext; 103 mConstructorArgs[1] = null; 104 } 105 106 return result; 107 } 108 }
这里,Android使用了PULL来解析xml布局文件,并通过反射来创建出当前view:
temp = createViewFromTag(root, name, attrs);
我们查看一下源码:
1 View createViewFromTag(View parent, String name, AttributeSet attrs) { 2 if (name.equals("view")) { 3 name = attrs.getAttributeValue(null, "class"); 4 } 5 6 if (DEBUG) System.out.println("******** Creating view: " + name); 7 8 try { 9 View view; 10 if (mFactory2 != null) view = mFactory2.onCreateView(parent, name, mContext, attrs); 11 else if (mFactory != null) view = mFactory.onCreateView(name, mContext, attrs); 12 else view = null; 13 14 if (view == null && mPrivateFactory != null) { 15 view = mPrivateFactory.onCreateView(parent, name, mContext, attrs); 16 } 17 18 if (view == null) { 19 if (-1 == name.indexOf('.')) { 20 view = onCreateView(parent, name, attrs); 21 } else { 22 view = createView(name, null, attrs); 23 } 24 } 25 26 if (DEBUG) System.out.println("Created view is: " + view); 27 return view; 28 29 } catch (InflateException e) { 30 throw e; 31 32 } catch (ClassNotFoundException e) { 33 InflateException ie = new InflateException(attrs.getPositionDescription() 34 + ": Error inflating class " + name); 35 ie.initCause(e); 36 throw ie; 37 38 } catch (Exception e) { 39 InflateException ie = new InflateException(attrs.getPositionDescription() 40 + ": Error inflating class " + name); 41 ie.initCause(e); 42 throw ie; 43 } 44 }
里面根据不同情况,调用了onCreateView方法,利用反射来创建view。其中可以使用指定的factory来创建view,这样的钩子设计使得inflate方法变得十分灵活。
然后调用rInflate(parser, temp, attrs, true)方法来递归查找temp中的子view,并添加到上层view中:
1 void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs, 2 boolean finishInflate) throws XmlPullParserException, IOException { 3 4 final int depth = parser.getDepth(); 5 int type; 6 7 while (((type = parser.next()) != XmlPullParser.END_TAG || 8 parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) { 9 10 if (type != XmlPullParser.START_TAG) { 11 continue; 12 } 13 14 final String name = parser.getName(); 15 16 if (TAG_REQUEST_FOCUS.equals(name)) { 17 parseRequestFocus(parser, parent); 18 } else if (TAG_INCLUDE.equals(name)) { 19 if (parser.getDepth() == 0) { 20 throw new InflateException("<include /> cannot be the root element"); 21 } 22 parseInclude(parser, parent, attrs); 23 } else if (TAG_MERGE.equals(name)) { 24 throw new InflateException("<merge /> must be the root element"); 25 } else if (TAG_1995.equals(name)) { 26 final View view = new BlinkLayout(mContext, attrs); 27 final ViewGroup viewGroup = (ViewGroup) parent; 28 final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); 29 rInflate(parser, view, attrs, true); 30 viewGroup.addView(view, params); 31 } else { 32 final View view = createViewFromTag(parent, name, attrs); 33 final ViewGroup viewGroup = (ViewGroup) parent; 34 final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); 35 rInflate(parser, view, attrs, true); 36 viewGroup.addView(view, params); 37 } 38 } 39 40 if (finishInflate) parent.onFinishInflate(); 41 }
里面也用到onCreateView方法创建子view,然后将其加入到父view中返回。
通过查看上面的源码,我们可以发现inflate方法中的三个参数int resource, ViewGroup root, boolean attachToRoot的作用如下:
resource指定了要加载的view,root作为view外面一层的父容器,attachToRoot表示是否将view加入到父容器。
当指定了父容器,并且attachToRoot为true,则将view加入到父容器中。
如果指定了父容器,却将attachToRoot设置为false,那么只是从父容器中生成了view布局的参数并设置给view
当未指定父容器时,直接返回view本身。
总结
通过研究LayoutInflater源码的设计,我们了解到代码的执行细节的同时,也可以发现:
LayoutInflater创建view对象时候使用了简单工厂模式,并通过加入钩子方法,利用抽象工厂模式让coder可以使用自定义的工厂方法来创建view。