Android View绘制流程一

我们在Activity的onCreate（）、onResume()方法里面获取控件的宽高都为0，如下：

public class MainActivity extends BaseSkinActivity {
    private static final String TAG = "MainActivity";
    private Button mButton;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main1);
        mButton = findViewById(R.id.button_change);
        Log.d(TAG, "onCreate>>>>>>" + "button的高度===" + mButton.getMeasuredHeight() + "----button的宽度" + mButton.getMeasuredWidth());
    }
    @Override
    protected void onResume() {
        super.onResume();
        Log.d(TAG, "onResume>>>>>>" + "button的高度===" + mButton.getMeasuredHeight() + "----button的宽度" + mButton.getMeasuredWidth());
    }
    }

  onCreate>>>>>>button的高度===0----button的宽度0
  onResume>>>>>>button的高度===0----button的宽度0

说明Activity在调用onResume方法时还没有测量控件宽高，在Activity的启动流程分析时，我们在ActivityThread的handleResumeActivity方法看到以下代码：

//   ViewManager wm = a.getWindowManager(); a是Activity，在Activity的getWindowManager()，
 if (r.activity.mVisibleFromClient) {
          a.mWindowAdded = true;
           wm.addView(decor, l);
 }

会不会是这个addView方法里面进行控件的测量呢？？我们在Activity找到了wm的赋值地方

 /**
     * Set the window manager for use by this Window to, for example,
     * display panels.  This is <em>not</em> used for displaying the
     * Window itself -- that must be done by the client.
     *
     * @param wm The window manager for adding new windows.
     */
    public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
            boolean hardwareAccelerated) {
        mAppToken = appToken;
        mAppName = appName;
        mHardwareAccelerated = hardwareAccelerated
                || SystemProperties.getBoolean(PROPERTY_HARDWARE_UI, false);
        if (wm == null) {
            wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
        }
        //这里给mWindowManager赋值，转换为WindowManagerImpl对象
        mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
    }

所以最终是在WindowManagerImpl里面调了addView方法。

  //mGlobal 是在常量赋值的  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);
    }

然后我们再看WindowManagerGlobal的 addView方法

 public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
         省略代码···············
             ViewRootImpl root;
             View panelParentView = null;
            //ViewRootImpl是在Window的最顶层，DecorView的parent是ViewRootImpl
            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 {
                //setView是将Decor加入到root里面，测量应该是在该方法里面
                root.setView(view, wparams, panelParentView);
            } catch (RuntimeException e) {
                // BadTokenException or InvalidDisplayException, clean up.
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
                throw e;
            }
        }
    }

再进入ViewRootImpl的setView方法

  /**
     * We have one child
     */
    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        synchronized (this) {
        省略代码···············
                 // 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.
               //看字面意思就知道是请求布局
                requestLayout();
          省略代码···············
          }

再进入ViewRootImpl的requestLayout方法

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

再进入ViewRootImpl的scheduleTraversals方法

//在这个方法里面有一个postCallback 线程mTraversalRunnable，这个线程就是正式开始进入测量doTraversal方法。
void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
            mChoreographer.postCallback(
            //mTraversalRunnable线程调用doTraversal方法
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            if (!mUnbufferedInputDispatch) {
                scheduleConsumeBatchedInput();
            }
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }

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

            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }
            //这个方法应该很熟悉了，我们大部分的View绘制流程文章都是从这个地方开始讲的。
            performTraversals();

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

看到performTraversals()就是进入onMeasure的入口了。