UI绘制流程

在Activity中我们通过setContentView(layout)来加载xml布局，那么这个过程是如何实现的呢？

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

AppCompatActivity中setContentView()通过AppCompatDelegateImpl来具体实现 AppCompatDelegateImpl.java

Window mWindow;

@Override
public void setContentView(int resId) {
    ensureSubDecor();
    ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content);
    contentParent.removeAllViews();
    LayoutInflater.from(mContext).inflate(resId, contentParent);
    mAppCompatWindowCallback.getWrapped().onContentChanged();
}

private void ensureSubDecor() {
    if (!mSubDecorInstalled) {
        mSubDecor = createSubDecor();
        ...
    }
}

private ViewGroup createSubDecor() {
    ...

    // Now let's make sure that the Window has installed its decor by retrieving it
    ensureWindow();
    mWindow.getDecorView();

    ...
}

private void ensureWindow() {
    // We lazily fetch the Window for Activities, to allow DayNight to apply in
    // attachBaseContext
    //将Activity中的Window赋值给mWindow
    if (mWindow == null && mHost instanceof Activity) {
        attachToWindow(((Activity) mHost).getWindow());
    }
    if (mWindow == null) {
        throw new IllegalStateException("We have not been given a Window");
    }
}

在ensureWindow()中会将Activity中的Window赋值给mWindow，然后调用Window中的getDecoreView()方法。

加载Activity视图

PhoneWindow

那么Activity中的Window又是什么呢？

The only existing implementation of this abstract class is android.view.PhoneWindow, which you should instantiate when needing a Window.

Window唯一的实现类为PhoneWindow，Activity中的Window是在attach()的时候创建的。接下来我们看看它里面的getDecoreView()方法： PhoneWindow.java

// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;

// This is the view in which the window contents are placed. It is either
// mDecor itself, or a child of mDecor where the contents go.
ViewGroup mContentParent;

@Override
public final View getDecorView() {
    if (mDecor == null || mForceDecorInstall) {
        installDecor();
    }
    return mDecor;
}
    
@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);
    }
    mContentParent.requestApplyInsets();
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        cb.onContentChanged();
    }
    mContentParentExplicitlySet = true;
}

mDecore为窗口的最顶层视图，mContentParent是我们自定义xml布局的摆放容器。

installDecor()方法负责创建mDecor和mContentParent，然后通过LayoutInflater将我们的xml布局加载到mContentParent。

创建DecorView

private void installDecor() {
    mForceDecorInstall = false;
    if (mDecor == null) {
        mDecor = generateDecor(-1);
        ...
    } else {
        mDecor.setWindow(this);
    }
    if (mContentParent == null) {
        mContentParent = generateLayout(mDecor);
        ...
    }
}

protected DecorView generateDecor(int featureId) {
    // System process doesn't have application context and in that case we need to directly use
    // the context we have. Otherwise we want the application context, so we don't cling to the
    // activity.
    Context context;
    if (mUseDecorContext) {
        Context applicationContext = getContext().getApplicationContext();
        if (applicationContext == null) {
            context = getContext();
        } else {
            context = new DecorContext(applicationContext, getContext().getResources());
            if (mTheme != -1) {
                context.setTheme(mTheme);
            }
        }
    } else {
        context = getContext();
    }
    return new DecorView(context, featureId, this, getAttributes());
}

这里直接new了一个DecorView，DecorView继承FrameLayout。

创建ContentParent

在上面的installDecor()方法中调用generateLayout(mDecor)来创建Contentparent

protected ViewGroup generateLayout(DecorView decor) {
    // Apply data from current theme.
    //获取当前主题
    TypedArray a = getWindowStyle();
    ...
    //根据主题调用setFlags和requestFeature()方法来设置窗体
    mIsFloating = a.getBoolean(R.styleable.Window_windowIsFloating, false);
    int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)
            & (~getForcedWindowFlags());
    if (mIsFloating) {
        setLayout(WRAP_CONTENT, WRAP_CONTENT);
        setFlags(0, flagsToUpdate);
    } else {
        setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate);
    }

    if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
        requestFeature(FEATURE_NO_TITLE);
    } else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
        // Don't allow an action bar if there is no title.
        requestFeature(FEATURE_ACTION_BAR);
    }
    ...
    // Inflate the window decor.
    //根据上面设置的features来加载对应系统布局文件
    int layoutResource;
    int features = getLocalFeatures();
    // System.out.println("Features: 0x" + Integer.toHexString(features));
    if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
        layoutResource = R.layout.screen_swipe_dismiss;
        setCloseOnSwipeEnabled(true);
    } else if(...) {
         ...
    } else {
        // Embedded, so no decoration is needed.
        layoutResource = R.layout.screen_simple;
        // System.out.println("Simple!");
    }

    mDecor.startChanging();
    //将上面的系统布局文件加载到DecorView
    mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);

    //在系统布局文件找到id为content的控件
    ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
    ...

    return contentParent;
}

DecorView.java

void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
    ...
    final View root = inflater.inflate(layoutResource, null);
    ...

        // Put it below the color views.
        addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    ...
}

根据设置的主题设置feature，决定加载何种系统布局文件，在系统布局文件中有一个id为content的控件即为ContentParent。

screen_simple.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    android:orientation="vertical">
    <ViewStub android:id="@+id/action_mode_bar_stub"
              android:inflatedId="@+id/action_mode_bar"
              android:layout="@layout/action_mode_bar"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:theme="?attr/actionBarTheme" />
    <FrameLayout
         android:id="@android:id/content"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:foregroundInsidePadding="false"
         android:foregroundGravity="fill_horizontal|top"
         android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

ContentParent即为上面的FrameLayout

加载传入的布局文件

我们再回到AppCompatDelegateImpl来看下setContentView()的实现

@Override
public void setContentView(int resId) {
    ensureSubDecor();
    ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content);
    contentParent.removeAllViews();
    LayoutInflater.from(mContext).inflate(resId, contentParent);
    mAppCompatWindowCallback.getWrapped().onContentChanged();
}

通过上面的分析可知ensureSubDecor()为PhoneWindow创建了DecorView以及ContentParent。 接下来通过LayoutInflater将传入的布局加载到ContentParent布局中。

小结

将视图添加至Window

上面只创建了视图，但还没有将视图添加至窗口。

我们从Activity启动开始分析

Activity启动流程

Activity启动是通过ActivityThread的handleLaunchActivity()方法实现

ActivityThread.java

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
    ...

    Activity a = performLaunchActivity(r, customIntent);

    if (a != null) {
        r.createdConfig = new Configuration(mConfiguration);
        reportSizeConfigurations(r);
        Bundle oldState = r.state;
        handleResumeActivity(r.token, false, r.isForward,
                !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

        ...
    } else {
        ...
    }
}

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ...
    Activity activity = null;
    try {
        java.lang.ClassLoader cl = appContext.getClassLoader();
        //反射创建Activity
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        StrictMode.incrementExpectedActivityCount(activity.getClass());
        r.intent.setExtrasClassLoader(cl);
        r.intent.prepareToEnterProcess();
        if (r.state != null) {
            r.state.setClassLoader(cl);
        }
    } catch (Exception e) {
        if (!mInstrumentation.onException(activity, e)) {
            throw new RuntimeException(
                "Unable to instantiate activity " + component
                + ": " + e.toString(), e);
        }
    }

    try {
            ...
            //调用Activity的attach()方法
            activity.attach(appContext, this, getInstrumentation(), r.token,
                    r.ident, app, r.intent, r.activityInfo, title, r.parent,
                    r.embeddedID, r.lastNonConfigurationInstances, config,
                    r.referrer, r.voiceInteractor, window, r.configCallback);

            ...
            if (r.isPersistable()) {
                //调用Activity的onCreate()方法
                mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
            } else {
                mInstrumentation.callActivityOnCreate(activity, r.state);
            }
            ...
            if (!r.activity.mFinished) {
                //调用Activity的onStart()方法
                activity.performStart();
                r.stopped = false;
            }
            if (!r.activity.mFinished) {
                if (r.isPersistable()) {
                    if (r.state != null || r.persistentState != null) {
                        //调用Activity的onRestoreInstanceState()方法
                        mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                                r.persistentState);
                    }
                } else if (r.state != null) {
                    mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
                }
            }
            ...
        }
        ...

    } catch (SuperNotCalledException e) {
        throw e;

    } catch (Exception e) {
        if (!mInstrumentation.onException(activity, e)) {
            throw new RuntimeException(
                "Unable to start activity " + component
                + ": " + e.toString(), e);
        }
    }

    return activity;
}

performLaunchActivity先通过反射创建Activity，然后调用Activity的一些生命周期方法：attach()、onCreate()、onStart()、onRestoreInstance()

在Activity的attach()方法中会创建Window：

mWindow = new PhoneWindow(this, window, activityConfigCallback);

在Activity的onCreate()方法中调用setContentView()创建DecorView视图，此时还没有将DecorView添加至Window。

final void handleResumeActivity(IBinder token,
        boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
    ActivityClientRecord r = mActivities.get(token);
    ...
    // TODO Push resumeArgs into the activity for consideration
    //执行Activity的onResume()方法
    r = performResumeActivity(token, clearHide, reason);

    if (r != null) {
        final Activity a = r.activity;

        ...
        if (r.window == null && !a.mFinished && willBeVisible) {
            //获取Activity中的Window
            r.window = r.activity.getWindow();
            //获取Window中的DecorView
            View decor = r.window.getDecorView();
            decor.setVisibility(View.INVISIBLE);
            //获取Activity的WindowManager
            ViewManager wm = a.getWindowManager();
            //获取Window的LayoutParams
            WindowManager.LayoutParams l = r.window.getAttributes();
            ...
            if (a.mVisibleFromClient) {
                if (!a.mWindowAdded) {
                    a.mWindowAdded = true;
                    //通过WindowManager将DecorView添加到Window
                    wm.addView(decor, l);
                } else {
                    // The activity will get a callback for this {@link LayoutParams} change
                    // earlier. However, at that time the decor will not be set (this is set
                    // in this method), so no action will be taken. This call ensures the
                    // callback occurs with the decor set.
                    a.onWindowAttributesChanged(l);
                }
            }

        ...

    } else {
        ...
    }
}

handleResumeActivity()会调用Activity的onResume()方法，并且将DecorView添加至Window。

接下来分析WindowManager.addView()方法，WindowManger的实现类为WindowManagerImpl WindowManagerImpl.java

private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();

@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyDefaultToken(params);
    mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}

WindowManagerGlobal.java

public void addView(View view, ViewGroup.LayoutParams params,
        Display display, Window parentWindow) {
    ...

    final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
    ...

    ViewRootImpl root;
    View panelParentView = null;

    synchronized (mLock) {
        ...
        
        root = new ViewRootImpl(view.getContext(), display);

        view.setLayoutParams(wparams);

        mViews.add(view);
        mRoots.add(root);
        mParams.add(wparams);

        // do this last because it fires off messages to start doing things
        try {
            root.setView(view, wparams, panelParentView);
        } catch (RuntimeException e) {
            // BadTokenException or InvalidDisplayException, clean up.
            if (index >= 0) {
                removeViewLocked(index, true);
            }
            throw e;
        }
    }
}

创建一个ViewRootImpl调用其setView()方法

ViewRootImpl.java

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
    synchronized (this) {
        if (mView == null) {
            mView = view;
            ....
            // Schedule the first layout -before- adding to the window
            // manager, to make sure we do the relayout before receiving
            // any other events from the system.
            requestLayout();
            ...
            //设置DecoreView的Parent为ViewRootImpl
            view.assignParent(this);
            ...
        }
    }
}

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }
}

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}

void doTraversal() {
    if (mTraversalScheduled) {
        ...
        performTraversals();
        ...
    }
}

setView()方法主要就是调用了requestLayout()方法，将DecorView的Parent设置为ViewRootImpl。而requestLayout()最终会调用performTraversals()方法

private void performTraversals() {
    ...
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    ...
    performLayout(lp, mWidth, mHeight);
    ...
    performDraw();
}

这里就开始执行视图的绘制流程

performMeasure()会调用mView的measure()方法，measure()再调用onMeasure()

performLayout()会调用mView的layout()方法,layout()再调用onLayout()

performDraw()会调用mView的draw()方法，draw()再调用onDraw()

小结

Activity通过ActivityThread启动，首先调用attach()方法该方法会创建Window，让后调用onCreate()方法在该方法中通过setContentView()创建布局视图，在Activity的resume时候通过ViewRootImpl执行布局视图的绘制流程。