就像上个文章说的,触摸事件的传递机制是从外层到内层的过程。
我们想来看看这个页面里面的层级关系:


以下我们就用what-how-why三部曲的方式来分析View的绘制过程。
由于篇幅很大,所以分几篇来解析这个过程。
这篇主要是自定义view/viewgroup,以及从Activity到DecorView的加载过程

1.what:怎么自定义一个View

1.1自定义View

自定义View的话,常见过程如下:

/**
 *   @author     DemanMath
 *   @date       2020-02-16
 *
 */
class CustomView : View {

    constructor(context: Context):super(context)
    constructor(context: Context,attributeSet: AttributeSet):super(context,attributeSet)
    constructor(context: Context,attributeSet: AttributeSet,def:Int):super(context,attributeSet,def)

    override fun onDraw(canvas: Canvas?) {
        super.onDraw(canvas)
        AppLog.i()
    }

    override fun onLayout(changed: Boolean, left: Int, top: Int, right: Int, bottom: Int) {
        super.onLayout(changed, left, top, right, bottom)
        AppLog.i()
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
        AppLog.i()
    }

}

三个构造方法+三个可以复写的方法。
我们先看下这3个方法的顺序:

2020-02-16 13:50:28.212 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onMeasure:  [at (CustomView.kt:32)]
2020-02-16 13:50:28.222 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onMeasure:  [at (CustomView.kt:32)]
2020-02-16 13:50:28.253 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onMeasure:  [at (CustomView.kt:32)]
2020-02-16 13:50:28.255 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onMeasure:  [at (CustomView.kt:32)]
2020-02-16 13:50:28.259 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onLayout:  [at (CustomView.kt:27)]
2020-02-16 13:50:28.403 23141-23141/com.joyfulmath.androidarchitecture I/Arch_App.CustomView: onDraw:  [at (CustomView.kt:22)]

1.2自定义ViewGroup

上代码

package com.joyfulmath.androidarchitecture.view

import android.content.Context
import android.util.AttributeSet
import android.view.ViewGroup
import com.joyfulmath.androidarchitecture.base.AppLog
import kotlin.math.PI
import kotlin.math.cos
import kotlin.math.min
import kotlin.math.sin

/**
 *   @author     DemanMath
 *   @date       2020-02-16
 *
 */
class FerrisWheel:ViewGroup {

    var count = 12
    var a = 2*PI/count
    var startA = PI/2

    constructor(context: Context):super(context){
        initViews()
    }
    constructor(context: Context,attributeSet: AttributeSet):super(context,attributeSet){
        initViews()
    }
    constructor(context: Context,attributeSet: AttributeSet,def:Int):super(context,attributeSet,def){
        initViews()
    }

    private fun initViews() {
        for(i in 0 until count){
            this.addView(CustomView(context))
        }
    }

    override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {
        var mViewWidth = measuredWidth
        var mViewHeight = measuredHeight
        AppLog.i("$mViewWidth,$mViewHeight")
        var cx = mViewWidth/2
        var cy = mViewHeight/2
        var r  = min(measuredWidth,measuredHeight)*0.5f -20
        AppLog.i("r:$r,cx:$cx")
        for(i in 0 until count){
            var view = getChildAt(i)
            var width = view.measuredWidth
            var height = view.measuredHeight
            var cx1 = r* sin(startA+a*i)
            var cy1 = -r* cos(startA+a*i)
            AppLog.i("width:$width,height:$height")
            AppLog.i("cx1:$cx1,cy1:$cy1")
            view.layout(cx+(cx1-width/2).toInt(),
                cy+(cy1-height/2).toInt(),
                cx+(cx1+width/2).toInt(),
                cy+(cy1+height/2).toInt())
        }
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        measureChildren(widthMeasureSpec,heightMeasureSpec)
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
    }
}

效果如下:

这里override了layout方法。可见View的绘制跟他的父View只有一个关系,ViewGroup指定了子View的位置。
关于View/ViewGroup绘制的机制,在下一节讨论。

2.How:View的绘制机制是什么

从上一节看出:整个绘制流程三个过程,measure,layout,draw这三个过程。
下面我们从源码的角度来分析下是不是这个过程。

final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
        .......

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

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

            if (localLOGV) Slog.v(
                TAG, "Resume " + r + " started activity: " +
                a.mStartedActivity + ", hideForNow: " + r.hideForNow
                + ", finished: " + a.mFinished);

            final int forwardBit = isForward ?
                    WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;

            // If the window hasn't yet been added to the window manager,
            // and this guy didn't finish itself or start another activity,
            // then go ahead and add the window.
            boolean willBeVisible = !a.mStartedActivity;
            if (!willBeVisible) {
                try {
                    willBeVisible = ActivityManager.getService().willActivityBeVisible(
                            a.getActivityToken());
                } catch (RemoteException e) {
                    throw e.rethrowFromSystemServer();
                }
            }
            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;
                        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();
                }
            }

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

            // Tell the activity manager we have resumed.
            if (reallyResume) {
                try {
                    ActivityManager.getService().activityResumed(token);
                } catch (RemoteException ex) {
                    throw ex.rethrowFromSystemServer();
                }
            }

        } else {
            // If an exception was thrown when trying to resume, then
            // just end this activity.
            try {
                ActivityManager.getService()
                    .finishActivity(token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

2.1 关键是页面绘制流程

整个的过程就是一开始讲的层级关系。
第一点:performResumeActivity 比wm.addView(decor, l)先执行。所以Activity是先获取焦点,才绘制view。
performResumeActivity->r.activity.performResume()->mInstrumentation.callActivityOnResume(this)->activity.onResume()
在performResume最后可以看到onPostResume

final void performResume() {
        performRestart();
        ...
        // mResumed is set by the instrumentation
        mInstrumentation.callActivityOnResume(this);
      ...
        onPostResume();
       ...
    }

    protected void onPostResume() {
        final Window win = getWindow();
        if (win != null) win.makeActive();
        if (mActionBar != null) mActionBar.setShowHideAnimationEnabled(true);
        mCalled = true;
    }

window出现了,这个就是phonewindow。
下面我们去看docorview的过程。

//2020.02.18 phonewindow在这里获取
if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow();
                //2020.02.18 docorview在这里获取
                View decor = r.window.getDecorView();
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
               ...
                if (a.mVisibleFromClient) {
                    if (!a.mWindowAdded) {
                        a.mWindowAdded = true;
                        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);
                    }
                }

我们来看下wm.addView(decor, l);这个的过程。wm的实现就是WindowManagerImpl

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

mGlobal是WindowManagerGlobal, addview的核心代码如下

    root = new ViewRootImpl(view.getContext(), display);

            view.setLayoutParams(wparams);

            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);

            root.setView(view, wparams, panelParentView);

关于从ViewGroup开始的绘制流程,请看下篇。

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