View的繪製二：View的繪製流程

繪製入口：ActivityThread,咱們先來看看ActivityThread中的handleMessage()方法。handleMessage()方法是由ActivityThread中一個名爲H的繼承了Handler的子類中實現的。

public void handleMessage(Message msg) {
    if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
    switch (msg.what) {
        case LAUNCH_ACTIVITY: {
            ......
            handleLaunchActivity(r, null);
            ......
        } break;
        ......
    }
    if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
}複製代碼

在handleMessage方法中調用了handleLaunchActivity(r, null);啓動一個activity，咱們再來看看handleLaunchActivity（）方法：

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

    Activity a = performLaunchActivity(r, customIntent);

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

        ......
    } else {
        // If there was an error, for any reason, tell the activity
        // manager to stop us.
        try {
            ActivityManagerNative.getDefault()
                .finishActivity(r.token, Activity.RESULT_CANCELED, null, false);
        } catch (RemoteException ex) {
            // Ignore
        }
    }
}複製代碼

在handleLaunchActivity方法中首先調用了performLaunchActivity()方法實例化activity對象。而後調用handleResumeActivity()方法。

final void handleResumeActivity(IBinder token,
        boolean clearHide, boolean isForward, boolean reallyResume) {
   ......

    // TODO Push resumeArgs into the activity for consideration
    ActivityClientRecord r = performResumeActivity(token, clearHide);

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

        ......

        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 (a.mVisibleFromClient) {
                a.mWindowAdded = true;
                wm.addView(decor, l);  //重點是addView方法
            }

        // 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); ...... } }複製代碼

在handleResumeActivity()方法中會調用performResumeActivity()方法，performResumeActivity()方法中會回調activity生命週期的onResume()方法。handleResumeActivity()中調用了 wm.addView(decor, l)方法，wm指的是WindowManagerImpl類。全部咱們再來看看 WindowManagerImpl中的addView方法：

@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyDefaultToken(params);
    mGlobal.addView(view, params, mDisplay, mParentWindow);
}複製代碼

在此方法中又調用了 mGlobal.addView(view, params, mDisplay, mParentWindow);mGlobal是WindowManagerGlobal。咱們再去WindowManagerGlobal查看addView方法：

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

    ViewRootImpl root;
    View panelParentView = null;

    synchronized (mLock) {
        // Start watching for system property changes.
        if (mSystemPropertyUpdater == null) {
            mSystemPropertyUpdater = new Runnable() {
                @Override public void run() {
                    synchronized (mLock) {
                        for (int i = mRoots.size() - 1; i >= 0; --i) {
                            mRoots.get(i).loadSystemProperties();
                        }
                    }
                }
            };
            SystemProperties.addChangeCallback(mSystemPropertyUpdater);
        }

        int index = findViewLocked(view, false);
        if (index >= 0) {
            if (mDyingViews.contains(view)) {
                // Don't wait for MSG_DIE to make it's way through root's queue. mRoots.get(index).doDie(); } else { throw new IllegalStateException("View " + view + " has already been added to the window manager."); } // The previous removeView() had not completed executing. Now it has. } // If this is a panel window, then find the window it is being // attached to for future reference. if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW && wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) { final int count = mViews.size(); for (int i = 0; i < count; i++) { if (mRoots.get(i).mWindow.asBinder() == wparams.token) { panelParentView = mViews.get(i); } } } root = new ViewRootImpl(view.getContext(), display); view.setLayoutParams(wparams); //添加到對應的集合中 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. synchronized (mLock) { final int index = findViewLocked(view, false); if (index >= 0) { removeViewLocked(index, true); } } throw e; } }複製代碼

在此方法中實例化了一個名爲root的ViewRootImpl對象。並調用了root.setView(view, wparams, panelParentView)；咱們再來看ViewRootImpl中的setView方法的實現：

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();
           
            ......
    }
}複製代碼

咱們能夠看到在setView中調用了requestLayout()方法；在來看看requestLayout()方法的實現：

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();  // 校驗繪製過程是否在主線程執行
        mLayoutRequested = true;
        scheduleTraversals();
    }
}複製代碼

在requestLayout()方法中又調用了scheduleTraversals()方法：

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
       ......
    }
}複製代碼

在scheduleTraversals方法中調用了mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null)，這裏調用了mTraversalRunnable的線程。在這個線程中的run方法中調用了doTraversal()方法：

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

doTraversal()方法中的實現以下：

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

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

        performTraversals();

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

doTraversal()方法中主要調用了perforTraversals()方法，在這個方法中調用了View的繪製流程的三大步：

private void performTraversals() {
performMeasure();  //測量    performLayout();  //佈局
performDraw();  //繪製}複製代碼

總結：ActivityThread調用handleResumeActivity()方法，在handleResumeActivity()方法中調用WindowManagerImpl的addView()方法，進而調用WindowManagerGlobal中的addView()方法,在此方法中會建立ViewRootImpl對象，而後調用setView()方法將DecorView、佈局屬性對象關聯。關聯成功後調用會依次調用ViewRootImpl的requestLayout()-->scheduleTraversals()-->doTraversal()-->performTraversals();最終在performTraversals()方法中執行繪製流程三大步performMeasure()、performLayout()、performDraw()等方法。