我们都知道Android的界面是通过xml编写的 然后通过Activity 加载xml的layout文件进行显示，那么怎么从xml加载到我们的Activity呢？
下面我们一步步了解view的加载流程 ，看看到底view是怎么创建出来的

一、Android的界面显示层次

image.png

二、源码分析-View的加载

1、故事要从setContentView开始

在我们的Activity点进setContentView方法 我们看到 里面实际上会调用window的setContentView 方法进行layout的解析

    /**
     * Set the activity content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the activity.
     *
     * @param layoutResID Resource ID to be inflated.
     *
     * @see #setContentView(android.view.View)
     * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
     */
    public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

2、我们再看看window的setContentView干了什么

1.PhoneWindow，可以看到，当mContentParent ==null时，调用installDecor();

 @Override
    public void setContentView(int layoutResID) {
        if (mContentParent == null) {
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (mDecor == null) {
            mDecor = generateDecor(-1);
        } else {
            mDecor.setWindow(this);
        }

generateDecor帮我们new了一个DecorView

    protected DecorView generateDecor(int featureId) {
        return new DecorView(context, featureId, this, getAttributes());
    }

再往下 mDecor创建好了以后 通过 generateLayout 创建了一个mContentParent

 if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);
}

generateLayout 中通过我们配置的各种参数进行判断加载哪个根布局，然后从ID_ANDROID_CONTENT 找到contentParent 并返回
而ID_ANDROID_CONTENT 就是android.R.id.content，也就是上图中的最里面一层是一个FrameLayout

    /**
     * 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) {
       // ......省略部分代码
       // xxxx
       if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
            layoutResource = R.layout.screen_simple_overlay_action_mode;
        } else {
            layoutResource = R.layout.screen_simple;
        
        }
        mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
        ViewGroup contentParent = (ViewGroup) findViewById(ID_ANDROID_CONTENT);
        if (contentParent == null) {
            throw new RuntimeException("Window couldn't find content container view");
        }
     }

接着我们看我们传入的layoutId在哪里使用，mContentParent在上面已经创建完成，而我们的layout就添加到mContentParent上面，最后调用LayoutInflater .inflate 添加我们的layout

    @Override
    public void setContentView(int layoutResID) {
      
        if (mContentParent == null) {
            installDecor();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
          //xxx
        } else {
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        ...

3.LayoutInflater ，接着来到LayoutInflater 中的inflate方法 我们发现最终都会来到这个方法：

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) 

并且解析属性

            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            View result = root;

然后通过createViewFromTag 创建view

  if (TAG_MERGE.equals(name)) {
        rInflate(parser, root, inflaterContext, attrs, false);
   } else {                  
        final View temp = createViewFromTag(root, name, inflaterContext, attrs);
        ViewGroup.LayoutParams params = null;       
   }

createViewFromTag 中经过条件判断 ，最终会调用createView() 反射创建最终我们得到的view

    public final View createView(@NonNull Context viewContext, @NonNull String name,
                                 @Nullable String prefix, @Nullable AttributeSet attrs)
            throws ClassNotFoundException, InflateException {
        Constructor<? extends View> constructor = sConstructorMap.get(name);
        Class<? extends View> clazz = null;
        if (constructor == null) {
                  clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                    mContext.getClassLoader()).asSubclass(View.class);
            constructor = clazz.getConstructor(mConstructorSignature);
            constructor.setAccessible(true);
            sConstructorMap.put(name, constructor);
        }
        Object lastContext = mConstructorArgs[0];
        mConstructorArgs[0] = viewContext;
        Object[] args = mConstructorArgs;
        args[1] = attrs;
        try {
            final View view = constructor.newInstance(args);
            return view;
        } finally {
            mConstructorArgs[0] = lastContext;
        }
    }

到这里我们的view就创建出来了

三、源码分析-资源的加载

1、application的创建

 private void handleBindApplication(AppBindData data) {
        mInstrumentation = new Instrumentation();
    Application app = data.info.makeApplication(data.restrictedBackupMode, null);
}

先创建了个appContext 然后通过mInstrumentation 把application创建出来

 public Application makeApplication(boolean forceDefaultAppClass,
            Instrumentation instrumentation) {
                ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);

            NetworkSecurityConfigProvider.handleNewApplication(appContext);
            app = mActivityThread.mInstrumentation.newApplication(
                    cl, appClass, appContext);
}

2、context的创建

进入createAppContext(), 创建了个ContextImpl，给它设置recourse这个recourse就是要加载的资源

 static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo,
            String opPackageName) {
     ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, null, null,
                0, null, opPackageName);
        context.setResources(packageInfo.getResources());
     return context;
}

3、Resources的创建

进入LoadedApk.getResources(), 经过条件判断，通过ResourcesManager.getResources()

 public Resources getResources() {
        if (mResources == null) {
            final String[] splitPaths;
               mResources = ResourcesManager.getInstance().getResources(null, mResDir,
                    splitPaths, mOverlayDirs, mApplicationInfo.sharedLibraryFiles,
                    Display.DEFAULT_DISPLAY, null, getCompatibilityInfo(),
                    getClassLoader(), null);
        }
        return mResources;
    }

继续进入getRecourse，通过findOrCreateResourcesImplForKeyLocked方法获取到资源

private @Nullable Resources createResources(@Nullable IBinder activityToken,@NonNull ResourcesKey key, @NonNull ClassLoader classLoader) synchronized (this) {
              ResourcesImpl resourcesImpl = findOrCreateResourcesImplForKeyLocked(key);           
    }

然后判断空 为空 调用createResourcesImpl 创建资源

 private @Nullable ResourcesImpl findOrCreateResourcesImplForKeyLocked(
            @NonNull ResourcesKey key) {
        ResourcesImpl impl = findResourcesImplForKeyLocked(key);
        if (impl == null) {
            impl = createResourcesImpl(key);
            if (impl != null) {
                mResourceImpls.put(key, new WeakReference<>(impl));
            }
        }
        return impl;
    }

进入createResourcesImpl 创建了个AssetManager 资源即是通过AssetManager 获取

  private @Nullable ResourcesImpl createResourcesImpl(@NonNull ResourcesKey key) {
        final DisplayAdjustments daj = new DisplayAdjustments(key.mOverrideConfiguration);
        daj.setCompatibilityInfo(key.mCompatInfo);
        final AssetManager assets = createAssetManager(key);
        if (assets == null) {
            return null;
        }
        final DisplayMetrics dm = getDisplayMetrics(key.mDisplayId, daj);
        final Configuration config = generateConfig(key, dm);
        final ResourcesImpl impl = new ResourcesImpl(assets, dm, config, daj);

        return impl;
    }

=创建出AssetManager后调用addApkAssets 添加不同的资源

 protected @Nullable AssetManager createAssetManager(@NonNull final ResourcesKey key) {
        final AssetManager.Builder builder = new AssetManager.Builder();
        // already.
        if (key.mResDir != null) {
                builder.addApkAssets(loadApkAssets(key.mResDir, false /*sharedLib*/,
        }


       if (key.mSplitResDirs != null) {
            for (final String splitResDir : key.mSplitResDirs) {
                    builder.addApkAssets(loadApkAssets(splitResDir, false /*sharedLib*/, false /*overlay*/));              
            }
        }

        if (key.mLibDirs != null) {
            for (final String libDir : key.mLibDirs) {        
                        builder.addApkAssets(loadApkAssets(libDir, true /*sharedLib*/,
                                false /*overlay*/));              
            }
        }
 return builder.build();

最后 调用 AssetManager.Builder.build把system资源、loader资源、user资源等整合在一起 通过AssetManager.nativeSetApkAssets() 加载资源

    public AssetManager build() {
        final ApkAssets[] systemApkAssets = getSystem().getApkAssets();
        final ArrayList<ApkAssets> loaderApkAssets = new ArrayList<>();
        final ArraySet<ApkAssets> uniqueLoaderApkAssets = new ArraySet<>();
        for (int i = mLoaders.size() - 1; i >= 0; i--) {
            final List<ApkAssets> currentLoaderApkAssets = mLoaders.get(i).getApkAssets();
            for (int j = currentLoaderApkAssets.size() - 1; j >= 0; j--) {
                final ApkAssets apkAssets = currentLoaderApkAssets.get(j);
                if (uniqueLoaderApkAssets.add(apkAssets)) {
                    loaderApkAssets.add(0, apkAssets);
                }
            }
        }
        final int totalApkAssetCount = systemApkAssets.length + mUserApkAssets.size()
                + loaderApkAssets.size();
        final ApkAssets[] apkAssets = new ApkAssets[totalApkAssetCount];
        System.arraycopy(systemApkAssets, 0, apkAssets, 0, systemApkAssets.length);

        for (int i = 0, n = mUserApkAssets.size(); i < n; i++) {
            apkAssets[i + systemApkAssets.length] = mUserApkAssets.get(i);
        }
        for (int i = 0, n = loaderApkAssets.size(); i < n; i++) {
            apkAssets[i + systemApkAssets.length  + mUserApkAssets.size()] =
                    loaderApkAssets.get(i);
        }
        final AssetManager assetManager = new AssetManager(false /*sentinel*/);
        assetManager.mApkAssets = apkAssets;
        AssetManager.nativeSetApkAssets(assetManager.mObject, apkAssets,

        return assetManager;
    }