activity&view启动（绘制）流程

Activity启动流程:

ActivityThread

1、ActivityThread中调用了handleLaunchActivity()->performLaunchActivity() 通过反射创建Activity调用callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState)->activity.performCreate()->

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调用了onCreate()方法->onCreate调用setContentView->ensureSubDecor->createSubDecor->PhoneWindow中的getDecorView->installDecor->generateDecor最后这个方法中创建了DecorView；
onCreate中并没有调用View的Measure方法所以无法在onCreate中获取控件的宽高

2、onResume->ActivityThread中调用了
handleResumeActivity()
->先调用performResumeActivity()->performResume()->activity.performResume()->callActivityOnResume(this)->

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并没有为View测量所以在onResume也无法获取控件的宽高，同理在onStart也无法获取控件宽高

->WindowManager->addView()将DecorView添加进去 之后才开始measure layout draw的方法

View绘制流程

ViewRootImpl
ViewGroup默认不走onDraw 设置背景后会走onDraw
测量：setView(View LayoutParams View)
->requestLayout()
->scheduleTraversals()
->TraversalRunnable()的run方法
->doTraversal
->performTraversals
->performMeasure
->View.measure
->onMeasure
->performDraw
->draw(boolean fullRedrawNeeded)
->drawSoftware
->draw(canvas)

->接着就是

             //draw 背景
            drawBackground(canvas);

            // Step 3, draw the content draw自己
            onDraw(canvas);

            // Step 4, draw the children draw子View
            dispatchDraw(canvas);