我们都知道 MVC,在Android中,这个 V 即指View,那我们今天就来探探View的究竟。
在onCreate方法中,可以调用this.setContentView(layout_id),来设置这个Activity的视图,今天就从setContentView(…)说起吧。
先编写一个简单的Activity:
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| public class ViewDemoActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); super.setContentView(R.layout.view_demo); } }
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查看父类Activity的setContentView方法:
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| public void setContentView(int layoutResID) { getWindow().setContentView(layoutResID); }
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这个getWindow()指向哪里?我们在onCreate中打个log瞧瞧:
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| D/ViewDemoActivity(3969): com.android.internal.policy.impl.PhoneWindow
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原来是PhoneWindow,来看看它的setContentView方法:
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| @Override public void setContentView(int layoutResID) { if (mContentParent == null) { installDecor(); } else { mContentParent.removeAllViews(); } mLayoutInflater.inflate(layoutResID, mContentParent); final Callback cb = getCallback(); if (cb != null && !isDestroyed()) { cb.onContentChanged(); } }
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我们分三步来介绍这个方法:
第一步:判断父容器是否为空
为空:生成Decor; (Decor是什么?参看这篇文章)
不为空:删除 contentParent 所有的子控件。
第二步:解析layoutResID所代表的xml文件
直接看LayoutInflater.inflate(…)方法:
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| public View inflate(int resource, ViewGroup root) { return inflate(resource, root, root != null); } public View inflate(int resource, ViewGroup root, boolean attachToRoot) { XmlResourceParser parser = getContext().getResources().getLayout(resource); try { return inflate(parser, root, attachToRoot); } finally { parser.close(); } } public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) { synchronized (mConstructorArgs) { final AttributeSet attrs = Xml.asAttributeSet(parser); Context lastContext = (Context)mConstructorArgs[0]; mConstructorArgs[0] = mContext; View result = root; try { int type; while ((type = parser.next()) != XmlPullParser.START_TAG && type != XmlPullParser.END_DOCUMENT) { } if (type != XmlPullParser.START_TAG) { throw new InflateException(parser.getPositionDescription() + ": No start tag found!"); } final String name = parser.getName(); 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 { 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) { params = root.generateLayoutParams(attrs); if (!attachToRoot) { temp.setLayoutParams(params); } } rInflate(parser, temp, attrs, true); if (root != null && attachToRoot) { root.addView(temp, params); } 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 { mConstructorArgs[0] = lastContext; mConstructorArgs[1] = null; } return result; } }
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(这几个inflate方法内部的逻辑,请参看注释)
我们注意到,inflate(…)最终会调用rInflate(…),继续挖:
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| 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(); }
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rInflate(…)先特殊处理了几个xml标签merge
, include
, blink
,requestFocus
,
然后在else分支,会 递归 调用rInflate(…),将xml的子控件添加到parent中,生成完整的 contentView。
看了这里,同学们就会明白,为什么不建议在布局文件中做过多地View嵌套了吧,层层递归啊:(
第三步:通知Callback,ContentView发生改变
这个getCallback()
是什么?打个Log看看:
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| @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); super.setContentView(R.layout.view_demo); Log.d("ViewDemo", getWindow().getCallback().toString()); }
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输出结果为:
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| D/ViewDemoActivity( 7228): cn.erhu.android.view.ViewDemoActivity@6562f2c0
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这不就是ViewDemoActivity吗?
在Activity.java中,我们在attach()方法中发现了蛛丝马迹:
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| final void attach(...) { ... mWindow = PolicyManager.makeNewWindow(this); mWindow.setCallback(this); ... } public void onContentChanged() { }
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我们发现原来PhoneWindow的callback原来就是Activity,并且Activity的onContentChanged()方法是空的,所以我们可以在自己的Activity中重写这个方法,来监听ContentView发生改变的事件。
OK,setContentView()大体就是这么回事,下面再补充几个知识点。
补充知识点
1、PhoneWindows的mLayoutInflater是哪里来的?
PhoneWindow.java
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| public PhoneWindow(Context context) { super(context) mLayoutInflater = LayoutInflater.from(context) }
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LayoutInflater.java
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| 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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ContextImpl.java
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| @Override public Object getSystemService(String name) { ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name); return fetcher == null ? null : fetcher.getService(this); } private static void registerService(String serviceName, ServiceFetcher fetcher) { if (!(fetcher instanceof StaticServiceFetcher)) { fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++; } SYSTEM_SERVICE_MAP.put(serviceName, fetcher); } static { ... registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext()); }}); ... }
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原来,mLayoutInflater是ContextImpl.java在加载的时候,调用PolicyManager.makeNewLayoutInflater生成的:
PoliceManager.java
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| private static final String POLICY_IMPL_CLASS_NAME = "com.android.internal.policy.impl.Policy"; private static final IPolicy sPolicy; static { try { Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME); sPolicy = (IPolicy)policyClass.newInstance(); ... } public static LayoutInflater makeNewLayoutInflater(Context context) { return sPolicy.makeNewLayoutInflater(context); }
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最终定位到Policy.java
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| public LayoutInflater makeNewLayoutInflater(Context context) { return new PhoneLayoutInflater(context); }
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mLayoutInflater是一个PhoneLayoutInflater,这便是mInflaterLayout的由来。