源码分析

接着上一篇 Activity启动流程源码分析-setContentView源码阅读 的讲解，本节介绍一下Activity的生命周期函数何时被调用

要看Activity的生命周期函数何时被调用，不得不翻阅 ActivityThread 这个类

在这个类里面我们先找到 handleLaunchActivity 这个方法

/**
     * Extended implementation of activity launch. Used when server requests a launch or relaunch.
     */
    @Override
    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();
        mSomeActivitiesChanged = true;

        if (r.profilerInfo != null) {
            mProfiler.setProfiler(r.profilerInfo);
            mProfiler.startProfiling();
        }

        // Make sure we are running with the most recent config.
        handleConfigurationChanged(null, null);

        if (localLOGV) Slog.v(
            TAG, "Handling launch of " + r);

        // Initialize before creating the activity
        if (!ThreadedRenderer.sRendererDisabled) {
            GraphicsEnvironment.earlyInitEGL();
        }
        WindowManagerGlobal.initialize();
        //这个就是调用 onCreate 的方法
        final Activity a = performLaunchActivity(r, customIntent);

        if (a != null) {
            r.createdConfig = new Configuration(mConfiguration);
            reportSizeConfigurations(r);
            if (!r.activity.mFinished && pendingActions != null) {
                pendingActions.setOldState(r.state);
                pendingActions.setRestoreInstanceState(true);
                pendingActions.setCallOnPostCreate(true);
            }
        } else {
            // If there was an error, for any reason, tell the activity manager to stop us.
            try {
                ActivityManager.getService()
                        .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                                Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }

        return a;
    }

在这个代码块中，我们主要关心一下这个方法 performLaunchActivity() 我们点进去看一下这个方法的具体实现

/**  Core implementation of activity launch. */
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
                //前面省略了 亿行 代码
                activity.mCalled = false;
                if (r.isPersistable()) { 
                    // callActivityOnCreate 这个是回调 OnCreate 的主要方法
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                        " did not call through to super.onCreate()");
                }
                r.activity = activity;
            }
            r.setState(ON_CREATE);

            mActivities.put(r.token, r);

        } 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;
    }

在这个代码块中，我们主要关心一下这个方法 callActivityOnCreate () 我们点进去看一下这个方法的具体实现

/**
     * Perform calling of an activity's {@link Activity#onCreate}
     * method.  The default implementation simply calls through to that method.
     *  @param activity The activity being created.
     * @param icicle The previously frozen state (or null) to pass through to
     * @param persistentState The previously persisted state (or null)
     */
    public void callActivityOnCreate(Activity activity, Bundle icicle,
            PersistableBundle persistentState) {
        prePerformCreate(activity);
        // performCreate 是我们主要关心的方法，他在 Activity 类里面
        activity.performCreate(icicle, persistentState);
        postPerformCreate(activity);
    }

在这个代码块中，我们主要关心一下这个方法 performCreate() 我们点进去看一下这个方法的具体实现

final void performCreate(Bundle icicle) {
        performCreate(icicle, null);
    }

final void performCreate(Bundle icicle, PersistableBundle persistentState) {
        mCanEnterPictureInPicture = true;
        restoreHasCurrentPermissionRequest(icicle);
        if (persistentState != null) {
            //在这里我们调用了 onCreate 方法
            onCreate(icicle, persistentState);
        } else {
            onCreate(icicle);
        }
        writeEventLog(LOG_AM_ON_CREATE_CALLED, "performCreate");
        mActivityTransitionState.readState(icicle);

        mVisibleFromClient = !mWindow.getWindowStyle().getBoolean(
                com.android.internal.R.styleable.Window_windowNoDisplay, false);
        mFragments.dispatchActivityCreated();
        mActivityTransitionState.setEnterActivityOptions(this, getActivityOptions());
    }

分析了这么多，大家可能会有点懵，这是初看源码者的通病，想有效解决这个问题，大家就得始终牢记这最初的目的来看

我们的目的是想知道 OnCreate () 何时被调用，说简单点其实就是在 performLaunchActivity() 方法中被调用，而下面被延伸出来的哪些方法只不过是 performLaunchActivity（）方法的具体实现，被放到了各个地方而已

然后，我们会调用 handleResumeActivity 

@Override
    public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
            String reason) {
        //前面省略亿行代码
        if (r.window == null && !a.mFinished && willBeVisible) {
            r.window = r.activity.getWindow();
            View decor = r.window.getDecorView();
            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;
                    //关键所在
                    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);
                }
            }

            // If the window has already been added, but during resume
            // we started another activity, then don't yet make the
            // window visible.
        } else if (!willBeVisible) {
            if (localLOGV) Slog.v(TAG, "Launch " + r + " mStartedActivity set");
            r.hideForNow = true;
        }

        // Get rid of anything left hanging around.
        cleanUpPendingRemoveWindows(r, false /* force */);

        // The window is now visible if it has been added, we are not
        // simply finishing, and we are not starting another activity.
        if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {
            if (r.newConfig != null) {
                performConfigurationChangedForActivity(r, r.newConfig);
                if (DEBUG_CONFIGURATION) {
                    Slog.v(TAG, "Resuming activity " + r.activityInfo.name + " with newConfig "
                            + r.activity.mCurrentConfig);
                }
                r.newConfig = null;
            }
            if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward=" + isForward);
            WindowManager.LayoutParams l = r.window.getAttributes();
            if ((l.softInputMode
                    & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
                    != forwardBit) {
                l.softInputMode = (l.softInputMode
                        & (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
                        | forwardBit;
                if (r.activity.mVisibleFromClient) {
                    ViewManager wm = a.getWindowManager();
                    View decor = r.window.getDecorView();
                    wm.updateViewLayout(decor, l);
                }
            }

            r.activity.mVisibleFromServer = true;
            mNumVisibleActivities++;
            if (r.activity.mVisibleFromClient) {
                r.activity.makeVisible();
            }
        }

        r.nextIdle = mNewActivities;
        mNewActivities = r;
        if (localLOGV) Slog.v(TAG, "Scheduling idle handler for " + r);
        Looper.myQueue().addIdleHandler(new Idler());
    }

在这个代码块里我们需要关注的是 addView()这个方法，我们的主要目的是它调用了那个类里面的 addView()

初次看是 vm 的，于是我们得知道 vm 到底是一个什么东西

ViewManager wm = a.getWindowManager();  //a 代表 activity 实例，所以我们得去 activity 里找getWindowManager() 方法，看他到底返回的是什么

 /** Retrieve the window manager for showing custom windows. */
    public WindowManager getWindowManager() {
        return mWindowManager;
    }

从上面可以看出给我们返回的是 mWindowManager ，很明显，这不是最终结果

我们还得深究 给我们返回的 mWindowManager  是个什么东西

mWindowManager = mWindow.getWindowManager();

从上面可以看到，他又调用了 Window 类里面的 getWindowManager() 

mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);

从上面这行代码 可以看出 mWindowManager  是由 createLocalWindowManager() 创建的

public WindowManagerImpl createLocalWindowManager(Window parentWindow) {
        return new WindowManagerImpl(mContext, parentWindow);
    }

即他返回了一个 WindowManagerImpl 对象

从而 可以调用 WindowManagerImpl实例里的 addView 方法，即

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

下面我们再来看 mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow); 这个方法干了啥

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

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

其中有一个关键语句 root.setView(view, wparams, panelParentView);  root是 ViewRootImpl 的一个实例 他调用了 setView() 方法

/**
     * We have one child
     */
    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        synchronized (this) {
               // 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(); //准备开始画布局了
            }
    }

 requestLayout()方法的具体实现如下

@Override
    public void requestLayout() {
        if (!mHandlingLayoutInLayoutRequest) {
            checkThread();  //这里就是为什么只能在主线程更改 UI 了
            mLayoutRequested = true;
            scheduleTraversals();  //绘制布局的关键方法
        }
    }

scheduleTraversals() 方法的具体实现

void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
            mChoreographer.postCallback( //mTraversalRunnable是我们需要关心的，点进去看看
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            if (!mUnbufferedInputDispatch) {
                scheduleConsumeBatchedInput();
            }
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }

mTraversalRunnable

final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

我们再来看看 TraversalRunnable 类

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

doTraversal()具体实现

void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }

            performTraversals(); //关键方法

            if (mProfile) {
                Debug.stopMethodTracing();
                mProfile = false;
            }
        }
    }

大家在代码中可以看到 performTraversals() 就代表你越来越接近布局的绘制了，下节我们继续分解 performTraversals()