Activity绘制流程(2)

Activity的绘制流程

PhoneWindow是window在源码中的唯一实例

Activity onCreate会走到Acyivity的setContentView

public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

接下来会走到phonewindow的setContentView方法

@Override
    public void setContentView(int layoutResID) {
       //如果没有decorview 先初始化decorView
        if (mContentParent == null) {
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        //转场动画，暂时不看
        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
        //正常的加载xml的过程
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

会执行到LayoutInflater.inflate的代码

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) + ")");
        }
        //将布局文件以xml的形式加载进来进行解析
        final XmlResourceParser parser = res.getLayout(resource);
        try {
        // 这边是真正的解析，加载
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            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("**************************");
                }
                // 特殊处理merge请求
                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, inflaterContext, attrs, false);
                } else {
                    // Temp is the root view that was found in the xml
                    // temp就是我们写在xml的跟布局
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                    ViewGroup.LayoutParams params = null;

                    if (root != null) {
                        if (DEBUG) {
                            System.out.println("Creating params from root: " +
                                    root);
                        }
                        // Create layout params that match root, if supplied
                        // 从这个地方我们可以看出childview的layoutParams其实是parent给的
                        params = root.generateLayoutParams(attrs);
                        //在这个流程里，attachToRoot为true
                        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 temp against its context.
                    // 在这边接着生成viewtree
                    rInflateChildren(parser, temp, attrs, 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) {
                final InflateException ie = new InflateException(e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } catch (Exception e) {
                final InflateException ie = new InflateException(parser.getPositionDescription()
                        + ": " + e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } finally {
                // Don't retain static reference on context.
                mConstructorArgs[0] = lastContext;
                mConstructorArgs[1] = null;

                Trace.traceEnd(Trace.TRACE_TAG_VIEW);
            }

            return result;
        }
    }

到上面这一步我们就把xml里面写的控件全部加载进来了

这样的话已经形成的界面随需要的view的树形结构，只是还没有能够触发performTraversals

ps:这边有一个地方需要注意一下,就是上面这段代码会调到addview，addview又会调到requestLayout，但是这个时候由于viewrootImpl还没有生成，所以不会触发view的绘制

@CallSuper
    public void requestLayout() {
        if (mMeasureCache != null) mMeasureCache.clear();

        if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
            // Only trigger request-during-layout logic if this is the view requesting it,
            // not the views in its parent hierarchy
            ViewRootImpl viewRoot = getViewRootImpl();
            if (viewRoot != null && viewRoot.isInLayout()) {
                if (!viewRoot.requestLayoutDuringLayout(this)) {
                    return;
                }
            }
            mAttachInfo.mViewRequestingLayout = this;
        }

        mPrivateFlags |= PFLAG_FORCE_LAYOUT;
        mPrivateFlags |= PFLAG_INVALIDATED;

        if (mParent != null && !mParent.isLayoutRequested()) {
            mParent.requestLayout();
        }
        if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
            mAttachInfo.mViewRequestingLayout = null;
        }
    }

可以看到上面的代码

if (mParent != null && !mParent.isLayoutRequested()) {
            mParent.requestLayout();
        }

这个一个典型的责任链模式，最后会遍历到ViewRootImpl，此时还没有生成，所有不会执行view的绘制流程

接下来看ViewRootImpl是什么时候生成的，简单了解一下Activity的创建过程：首先，在ActivityThread#handleLaunchActivity中启动Activity，在这里面会调用到Activity#onCreate方法，从而完成上面所述的DecorView创建动作，当onCreate()方法执行完毕，在handleLaunchActivity方法会继续调用到ActivityThread#handleResumeActivity方法：

   if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow(); // 获取activity的窗口
                View decor = r.window.getDecorView(); //获取activity的decorview
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
                if (r.mPreserveWindow) {
                    a.mWindowAdded = true;
                    r.mPreserveWindow = false;
                    // Normally the ViewRoot sets up callbacks with the Activity
                    // in addView->ViewRootImpl#setView. If we are instead reusing
                    // the decor view we have to notify the view root that the
                    // callbacks may have changed.
                    ViewRootImpl impl = decor.getViewRootImpl();
                    if (impl != null) {
                        impl.notifyChildRebuilt();
                    }
                }
                if (a.mVisibleFromClient) {
                    if (!a.mWindowAdded) {
                        a.mWindowAdded = true;
                        wm.addView(decor, l);// 会执行到这一步addView，addview的参数是decor 和 layoutParams，我们有理由相信rootView是在这时候生成的
                    } 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);
                    }
                }

可以看到这么一句代码wm.addView(decor, l);viewroot就是在这里面生成的 看到WindowManagerImpl的

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

接着查看mGlobal.addView,可以看到viewroot在这个方法里生成了

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

        ViewRootImpl root;
        View panelParentView = null;

        ......

        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;
        }
    }
}

接着看 root.setView(view, wparams, panelParentView)

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        ......
        // 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.
        //这个调用performTraverals方法
        requestLayout();
        ......
        // 这个是将decorView的parent只想viewRoot
        view.assignParent(this);
        ......
    }

在这个方法里，可以看到调用了rootViewImpl的requestLayout这个方法

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

这这边有一个经典的就是checkThread，不能在子线程中更新ui，其实是不完全对的，如果在rootView生成之前是不会出错的

接着调用

void scheduleTraversals() {
     ......
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
         ......
    }

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

    void doTraversal() {
      ......

            performTraversals();
      ......
        }
    }

可以看到performTraversals,ViewRootImpl#performTraverals方法开始View的测量、布局、绘制流程