从源码的角度说说Activity的setContentView的原理

我们在Activity开发的时候天天会用到这个方法，有时候还需要根据需求在setContentView调用的时候做一些动作，因此我们就需要知道它内部是如何工作的，我们来一起看一下：

setContentView有三个重载方法：

    public void setContentView(int layoutResID) {getWindow().setContentView(layoutResID);initWindowDecorActionBar();}public void setContentView(View view) {getWindow().setContentView(view);initWindowDecorActionBar();}public void setContentView(View view, ViewGroup.LayoutParams params) {getWindow().setContentView(view, params);initWindowDecorActionBar();}

他们实际都在调用getWindow方法返回的Window对象的setContentView方法，那getWindow对象返回的是谁呢？

    public Window getWindow() {return mWindow;}

我们可以看到getWindow返回的是一个mWindows的对象，那么它在哪被赋值的呢？通过查找我们可以找到在Activity的attach方法中有这么一段：

mWindow = PolicyManager.makeNewWindow(this);

也就是说继续往下看，发现PolicyManager.makeNewWindow内部确实这样的：

    // The static methods to spawn new policy-specific objectspublic static Window makeNewWindow(Context context) {return sPolicy.makeNewWindow(context);}

好，那我们找找sPolicy又是谁：

我们可以在PolicyManager方法内看到静态代码块以及它的一些属性：

    private static final String POLICY_IMPL_CLASS_NAME ="com.android.internal.policy.impl.Policy";private static final IPolicy sPolicy;static {// Pull in the actual implementation of the policy at run-timetry {Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);sPolicy = (IPolicy)policyClass.newInstance();} catch (ClassNotFoundException ex) {throw new RuntimeException(POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);} catch (InstantiationException ex) {throw new RuntimeException(POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);} catch (IllegalAccessException ex) {throw new RuntimeException(POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);}}

也就是说，sPolicy对象是类com.android.internal.policy.impl.Policy的一个实例，OK，我们进入这个类看一下它的makeNewWindow方法，噢噢，是这样啊：

    public Window makeNewWindow(Context context) {return new PhoneWindow(context);}

原来那个getWindow返回的是PhoneWindow对象，setContentView调用的则是PhoneWindow的setContentView，OK，说了这么多重点就是看下PhoneWindow的setContentView方法是如何工作的：

PhoneWindow的setContentView的重载方法内部有些不同，我们先看参数为int的setContentView：

    public void setContentView(int layoutResID) {// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window// decor, when theme attributes and the like are crystalized. Do not check the feature// before this happens.if (mContentParent == null) {installDecor();} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {mContentParent.removeAllViews();}if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,getContext());transitionTo(newScene);} else {mLayoutInflater.inflate(layoutResID, mContentParent);}final Callback cb = getCallback();if (cb != null && !isDestroyed()) {cb.onContentChanged();}}

我们只看重点部分，都是通过LayoutInflater的inflate的方法加载的，是不是很熟悉呢？inflate方法的第二个参数是mContentParent，就是说要把这个layoutResID的这个布局添加到mContentParent中，那么mContentParent就是我们的主布局了，从代码中也可以看出来：

    /*** The ID that the main layout in the XML layout file should have.*/public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;

    protected ViewGroup generateLayout(DecorView decor) {// Apply data from current theme....ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);if (contentParent == null) {throw new RuntimeException("Window couldn't find content container view");}...return contentParent;}

            mContentParent = generateLayout(mDecor);

好，入口的东西都分析的差不多了，着重看一下LayoutInflater的inflate的工作原理，我们常用的方法是这3个：

    public View inflate(int resource, ViewGroup root) {return inflate(resource, root, root != null);}public View inflate(XmlPullParser parser, ViewGroup root) {return inflate(parser, root, root != null);}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();}}

其实它们都没做什么，主要做工作的是它：

    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, false);} else {// Temp is the root view that was found in the xmlfinal 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 suppliedparams = 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 temprInflate(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.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;}}

这段代码比较多，我们挑一下重点看：

                    // Temp is the root view that was found in the xmlfinal View temp = createViewFromTag(root, name, attrs, false);ViewGroup.LayoutParams params = null;if (root != null) {// Create layout params that match root, if suppliedparams = 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);}}// 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;}

这段代码就是说通过createViewFromTag方法生成临时View对象，然后如果View生成成功，给它产生一个默认的LayoutParams对象附在上面，最后判断root是否为空，决定是否要将这个临时View添加到root之上，好，接下来，我们的重点是createViewFromTag方法：

    /*** Creates a view from a tag name using the supplied attribute set.* <p>* If {@code inheritContext} is true and the parent is non-null, the view* will be inflated in parent view's context. If the view specifies a* <theme> attribute, the inflation context will be wrapped with the* specified theme.* <p>* Note: Default visibility so the BridgeInflater can override it.*/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;}}

这么多代码，其实我们的重点是这部分：

            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;}}

我们可以看到，刚开始会交给mFactory2、mFactory、mPrivateFactory这三个View工厂来进行处理，这三个对象都是通过构造方法设置进来的，如下：

    protected LayoutInflater(LayoutInflater original, Context newContext) {mContext = newContext;mFactory = original.mFactory;mFactory2 = original.mFactory2;mPrivateFactory = original.mPrivateFactory;setFilter(original.mFilter);}

这个构造方法在哪里被调用我们先不去看，我们可以继续我们的重点：

                    if (-1 == name.indexOf('.')) {view = onCreateView(parent, name, attrs);} else {view = createView(name, null, attrs);}

这里的意思是，如果这个Tag是没有( . )的话，那就是说这个tag是android提供的默认的控件，像View,TextView,Button等等。否则，就是其它情况了，我们先看默认没有( . )的情况：

    protected View onCreateView(String name, AttributeSet attrs)throws ClassNotFoundException {return createView(name, "android.view.", attrs);}

请注意这个方法的第二个参数android.view.，然后我们进入方法内部：

    public final View createView(String name, String prefix, AttributeSet attrs)throws ClassNotFoundException, InflateException {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 itclazz = 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);}}constructor = clazz.getConstructor(mConstructorSignature);sConstructorMap.put(name, constructor);} else {// If we have a filter, apply it to cached constructorif (mFilter != null) {// Have we seen this name before?Boolean allowedState = mFilterMap.get(name);if (allowedState == null) {// New class -- remember whether it is allowedclazz = 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);final View view = constructor.newInstance(args);if (view instanceof ViewStub) {// Use the same context when inflating ViewStub later.final ViewStub viewStub = (ViewStub) view;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 subclassInflateException 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);}}

这段代码其实重点就是通过ClassLoader的loadClass方法将类加载进来，然后通过反射的方式获取它的构造方法进行实例化，然后基本功能就完成了。有的同学可能会注意到这里有个mFilter属性，这个属性是用来定义这个类是否允许被加载的。

好，以上就是最基本的Inflate加载过程，其实实际过程不是这样的，我们将在下一章查看详细的加载过程。

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