源码分析android 系统framework（二）之view的布局加载流程
16lz
2021-01-23
源码分析android 系统framework（二）之view的布局加载流程
我们基本写android UI都是采用xml的方式来写的，那么我们写的xml到底是怎么展示到屏幕上的，今天我们就来看下这个流程。
在上一篇文章中介绍了window，view和activity的关系。知道最后我们在activity中的setContentView最终是调用到PhoneWindow 的setContentView。我们今天就从这看起：
1. PhoneWindow.java
@Override 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); // 这里这个方法就是将资源文件，转换为view，这个方法大家应该比较熟悉，在fragment中应该经常用到。 } mContentParent.requestApplyInsets(); final Callback cb = getCallback(); if (cb != null && !isDestroyed()) { cb.onContentChanged(); } mContentParentExplicitlySet = true; }2. LayoutInflater.java
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) { return inflate(resource, root, root != null); } public View inflate(@LayoutRes int resource, @Nullable 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); // 1 try { return inflate(parser, root, attachToRoot);// 2 } finally { parser.close(); } }以上1处，我们看到是在解析xml，2处才是真正加载创建加载view
3. Resource.java
@NonNull public XmlResourceParser getLayout(@LayoutRes int id) throws NotFoundException { return loadXmlResourceParser(id, "layout"); } @NonNull XmlResourceParser loadXmlResourceParser(@AnyRes int id, @NonNull String type) throws NotFoundException { final TypedValue value = obtainTempTypedValue(); try { final ResourcesImpl impl = mResourcesImpl; impl.getValue(id, value, true); if (value.type == TypedValue.TYPE_STRING) { return impl.loadXmlResourceParser(value.string.toString(), id, value.assetCookie, type); } throw new NotFoundException("Resource ID #0x" + Integer.toHexString(id) + " type #0x" + Integer.toHexString(value.type) + " is not valid"); } finally { releaseTempTypedValue(value); } }4.回到LayoutInflater.java
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) { synchronized (mConstructorArgs) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");............... if (TAG_MERGE.equals(name)) { if (root == null || !attachToRoot) { throw new InflateException(" private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) { return createViewFromTag(parent, name, context, attrs, false); } View createViewFromTag(View parent, String name, Context context, AttributeSet attrs, boolean ignoreThemeAttr) { if (name.equals("view")) { name = attrs.getAttributeValue(null, "class"); } // Apply a theme wrapper, if allowed and one is specified. if (!ignoreThemeAttr) { final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME); final int themeResId = ta.getResourceId(0, 0); if (themeResId != 0) { context = new ContextThemeWrapper(context, themeResId); } ta.recycle(); } if (name.equals(TAG_1995)) { // Let's party like it's 1995! return new BlinkLayout(context, attrs); } try { View view; if (mFactory2 != null) { view = mFactory2.onCreateView(parent, name, context, attrs); } else if (mFactory != null) { view = mFactory.onCreateView(name, context, attrs); } else { view = null; }// 这里这两个mFactory2和Factory比较重要，可以看到加载时首先是会通过它们两来创建view，//如果他们两都会空的话，则才会执行下面的onCreateView。这两个比较重要，//我们可以通过他两来搞一些事情，这个后面再说 if (view == null && mPrivateFactory != null) { view = mPrivateFactory.onCreateView(parent, name, context, attrs); } if (view == null) { final Object lastContext = mConstructorArgs[0]; mConstructorArgs[0] = context; try { if (-1 == name.indexOf('.')) { view = onCreateView(parent, name, attrs); } else { view = createView(name, null, attrs); } } finally { mConstructorArgs[0] = lastContext; } } public interface Factory { /** * Hook you can supply that is called when inflating from a LayoutInflater. * You can use this to customize the tag names available in your XML * layout files. *这里的注释，hook这个单词已经道破了所有 * * Note that it is good practice to prefix these custom names with your * package (i.e., com.coolcompany.apps) to avoid conflicts with system * names. * * @param name Tag name to be inflated. * @param context The context the view is being created in. * @param attrs Inflation attributes as specified in XML file. * * @return View Newly created view. Return null for the default * behavior. */
public View onCreateView(String name, Context context, AttributeSet attrs); } public interface Factory2 extends Factory { /** * Version of {@link #onCreateView(String, Context, AttributeSet)} * that also supplies the parent that the view created view will be * placed in. * * @param parent The parent that the created view will be placed * in; note that this may be null. * @param name Tag name to be inflated. * @param context The context the view is being created in. * @param attrs Inflation attributes as specified in XML file. * * @return View Newly created view. Return null for the default * behavior. */ public View onCreateView(View parent, String name, Context context, AttributeSet attrs); } */ protected View onCreateView(View parent, String name, AttributeSet attrs) throws ClassNotFoundException { return onCreateView(name, attrs); } protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException { return createView(name, "android.view.", attrs); }public final View createView(String name, String prefix, AttributeSet attrs) throws ClassNotFoundException, InflateException { Constructor<? extends View> constructor = sConstructorMap.get(name); if (constructor != null && !verifyClassLoader(constructor)) { constructor = null; sConstructorMap.remove(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 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); } } constructor = clazz.getConstructor(mConstructorSignature); constructor.setAccessible(true); 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 lastContext = mConstructorArgs[0]; if (mConstructorArgs[0] == null) { // Fill in the context if not already within inflation. mConstructorArgs[0] = mContext; } Object[] args = mConstructorArgs; args[1] = attrs; 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])); } mConstructorArgs[0] = lastContext; return view; .................. }构造函数一路往下。我们从上面的createView来看，最后的 view的由来是通过反射的方法拿到的，然后拿到具体的view后，就是view自己ondraw ,onlayout来画了。这部分就不说了。到此view的加载流程我们就清楚了。这里有几点思考：
性能优化的思考:
我们从上面看到加载xml布局文件时，有几个耗时操作：
-
解析xml IO耗时
-
反射获取view 对象耗时
故如果想要在view的加载上提升下性能，可以使用直接new 的方式，例如，new TextView()。我们公司的大部分UI代码都是new 出来，我起初看到一个android 工程中没有xml 布局文件。差点都要崩溃了。
虽然我们追求了性能，但是对于代码的可读性，以及我们的维护来说又反而太差了。所以就又了下面牛逼的一个 库，我们通过xml写布局，然后这个库默认帮我们转成new 的方式。真是叼炸天了。
推荐x2c框架
全局替换UI使用
如下，还是上面中那个代码。我们可以看到，我们布局中的每一个view，都需要先经过Factory2，然后再经过factory，最后再是默认的CreatView，但是前两个默认是空的。所以呢，我们就可以自己去实现这个方法。然后每次当加载系统默认的ui 组件的时候，都会先走我们自己的creatView.
View view; if (mFactory2 != null) { view = mFactory2.onCreateView(parent, name, context, attrs); } else if (mFactory != null) { view = mFactory.onCreateView(name, context, attrs); } else { view = null; }所以我们可以如下：
public class TestActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { //这里说一下，我们为什么要用LayoutInflaterCompat，因为后面加Compat的基本都是兼容包，考虑到兼容，我们就用这个，然后我们这里用Factory2。另外，一定要在super.onCreate() 上面 LayoutInflaterCompat.setFactory2(getLayoutInflater(), new LayoutInflater.Factory2() { @Override public View onCreateView(View parent, String name, Context context, AttributeSet attrs) { Log.d("niubi", "s:" + name); int n = attrs.getAttributeCount(); for (int i=0;i<n;i++){ Log.d("niubi",attrs.getAttributeName(i)+attrs.getAttributeValue(i)); } return null; } @Override public View onCreateView(String name, Context context, AttributeSet attrs) { return null; } }); super.onCreate(savedInstanceState); setContentView(R.layout.activity_appcompart); }}我们随便写了个布局文件activity_appcompart.xml。然后从从上面的打印如下：
D/niubi: s:LinearLayout2020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: orientation12020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: fitsSystemWindowstrue2020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: layout_width-12020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: layout_height-12020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: s:ViewStub2020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: theme?168438252020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: id@169092882020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: layout@173670702020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: inflatedId@169092892020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: layout_width-12020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: layout_height-22020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: s:FrameLayout2020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: id@169082902020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: layout_width-12020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: layout_height-12020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: foreground?168428412020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: foregroundGravity0x372020-05-29 22:50:40.566 2170-2170/com.xct.codebase D/niubi: foregroundInsidePaddingfalse2020-05-29 22:50:40.569 2170-2170/com.xct.codebase D/niubi: s:android.support.v7.widget.ActionBarOverlayLayout2020-05-29 22:50:40.569 2170-2170/com.xct.codebase D/niubi: id@21311652432020-05-29 22:50:40.569 2170-2170/com.xct.codebase D/niubi: fitsSystemWindowstrue2020-05-29 22:50:40.569 2170-2170/com.xct.codebase D/niubi: layout_width-12020-05-29 22:50:40.569 2170-2170/com.xct.codebase D/niubi: layout_height-12020-05-29 22:50:40.570 2170-2170/com.xct.codebase D/niubi: s:android.support.v7.widget.ContentFrameLayout2020-05-29 22:50:40.570 2170-2170/com.xct.codebase D/niubi: id@21311651912020-05-29 22:50:40.570 2170-2170/com.xct.codebase D/niubi: layout_width-12020-05-29 22:50:40.570 2170-2170/com.xct.codebase D/niubi: layout_height-12020-05-29 22:50:40.570 2170-2170/com.xct.codebase D/niubi: foreground?168428412020-05-29 22:50:40.570 2170-2170/com.xct.codebase D/niubi: foregroundGravity0x372020-05-29 22:50:40.571 2170-2170/com.xct.codebase D/niubi: s:android.support.v7.widget.ActionBarContainer2020-05-29 22:50:40.571 2170-2170/com.xct.codebase D/niubi: gravity0x302020-05-29 22:50:40.571 2170-2170/com.xct.codebase D/niubi: id@21311651922020-05-29 22:50:40.571 2170-2170/com.xct.codebase D/niubi: layout_width-12020-05-29 22:50:40.571 2170-2170/com.xct.codebase D/niubi: layout_height-22020-05-29 22:50:40.571 2170-2170/com.xct.codebase D/niubi: layout_alignParentToptrue2020-05-29 22:50:40.571 2170-2170/com.xct.codebase D/niubi: touchscreenBlocksFocustrue2020-05-29 22:50:40.571 2170-2170/com.xct.codebase D/niubi: style?21308375092020-05-29 22:50:40.572 2170-2170/com.xct.codebase D/niubi: s:android.support.v7.widget.Toolbar2020-05-29 22:50:40.572 2170-2170/com.xct.codebase D/niubi: id@21311651902020-05-29 22:50:40.572 2170-2170/com.xct.codebase D/niubi: layout_width-12020-05-29 22:50:40.572 2170-2170/com.xct.codebase D/niubi: layout_height-22020-05-29 22:50:40.572 2170-2170/com.xct.codebase D/niubi: navigationContentDescription@21314273292020-05-29 22:50:40.572 2170-2170/com.xct.codebase D/niubi: style?21308378132020-05-29 22:50:40.573 2170-2170/com.xct.codebase D/niubi: s:android.support.v7.widget.ActionBarContextView2020-05-29 22:50:40.573 2170-2170/com.xct.codebase D/niubi: theme?21308375132020-05-29 22:50:40.573 2170-2170/com.xct.codebase D/niubi: id@21311651982020-05-29 22:50:40.573 2170-2170/com.xct.codebase D/niubi: visibility22020-05-29 22:50:40.573 2170-2170/com.xct.codebase D/niubi: layout_width-12020-05-29 22:50:40.573 2170-2170/com.xct.codebase D/niubi: layout_height-22020-05-29 22:50:40.573 2170-2170/com.xct.codebase D/niubi: style?21308375312020-05-29 22:50:40.574 2170-2170/com.xct.codebase D/niubi: s:LinearLayout2020-05-29 22:50:40.574 2170-2170/com.xct.codebase D/niubi: layout_width-1可以看到每一个view 控件都会传到这个回调方法里来。所以我们可以在这个方法写去重写想要替换的view，例如，如果想要将自己UI中所有的textView 全局替换成自己的自定义view。则可以直接在这个方法里面去new 自己定义的view。
public class TestActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { LayoutInflaterCompat.setFactory2(getLayoutInflater(), new LayoutInflater.Factory2() { @Override public View onCreateView(View parent, String name, Context context, AttributeSet attrs) { switch(name){case "TextView" :return new CustomTextView(context,attrs)} } }); super.onCreate(savedInstanceState); setContentView(R.layout.activity_appcompart); }}更多相关文章
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