天天记录 - Android requestLayout源码简单分析

最新推荐文章于 2025-10-20 13:31:57 发布

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本文通过对View.requestLayout和ViewRootImpl的相关源码分析，揭示了Android中布局更新的内部工作原理，探讨了requestLayout方法如何触发视图的重新测量和绘制流程。

先来看以下View.requestLayout源码

    /**
     * Call this when something has changed which has invalidated the
     * layout of this view. This will schedule a layout pass of the view
     * tree.
     */
    public void requestLayout() {
        mPrivateFlags |= PFLAG_FORCE_LAYOUT;
        mPrivateFlags |= PFLAG_INVALIDATED;

        if (mParent != null && !mParent.isLayoutRequested()) {
            // 先上传递，最总到达顶层View即RootViewImpl
            mParent.requestLayout();
        }
    }

查看下ViewRootImpl.requestLayout源码

    public void requestLayout() {
        // 检测是否在UI线程
        checkThread();
        mLayoutRequested = true;
        // 重点
        scheduleTraversals();
    }

    public void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;

            //noinspection ConstantConditions
            if (ViewDebug.DEBUG_LATENCY && mLastTraversalFinishedTimeNanos != 0) {
                final long now = System.nanoTime();
                Log.d(TAG, "Latency: Scheduled traversal, it has been "
                        + ((now - mLastTraversalFinishedTimeNanos) * 0.000001f)
                        + "ms since the last traversal finished.");
            }
            // 发出一个遍历的Handler信息
            sendEmptyMessage(DO_TRAVERSAL);
        }
    }

RootViewImpl.handleMessage方法源码：

    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
        ......
        case DO_TRAVERSAL:
            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }

            final long traversalStartTime;
            if (ViewDebug.DEBUG_LATENCY) {
                traversalStartTime = System.nanoTime();
                mLastDrawDurationNanos = 0;
            }
            // 接收到消息之后，开始执行遍历,具体measure，layout，draw操作
            performTraversals();

            if (ViewDebug.DEBUG_LATENCY) {
                long now = System.nanoTime();
                Log.d(TAG, "Latency: Spent "
                        + ((now - traversalStartTime) * 0.000001f)
                        + "ms in performTraversals(), with "
                        + (mLastDrawDurationNanos * 0.000001f)
                        + "ms of that time in draw()");
                mLastTraversalFinishedTimeNanos = now;
            }

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