一起Talk Android吧（第一百二十一回：Android中View的内部流程四)

最新推荐文章于 2024-04-28 11:20:16 发布

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#view绘制

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本文深入剖析了Android中View的绘制流程，详细介绍了performDraw()函数如何通过draw()来完成组件的绘制工作，为开发者理解View绘制提供了重要参考。

各位看官们，大家好，上一回中咱们说的是Android中View内部流程的例子，这一回咱们继续说该例子。闲话休提，言归正转。让我们一起Talk Android吧！

看官们，我们在上一章回中介绍了Viwe内部流程中的规划，这一回是我们介绍它的第三个内部流程：绘制(draw)。

绘制主要是对各个组件进行绘制，只有在组件被绘制完成后才能被我们看到。绘制功能主要是通过performDraw()函数实现的，下面是它的源代码：

private void performDraw() {
    if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
        return;
    } else if (mView == null) {
        return;
    }

    final boolean fullRedrawNeeded = mFullRedrawNeeded || mReportNextDraw;
    mFullRedrawNeeded = false;

    mIsDrawing = true;
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");

    boolean usingAsyncReport = false;
    if (mReportNextDraw && mAttachInfo.mThreadedRenderer != null
            && mAttachInfo.mThreadedRenderer.isEnabled()) {
        usingAsyncReport = true;
        mAttachInfo.mThreadedRenderer.setFrameCompleteCallback((long frameNr) -> {
            // TODO: Use the frame number
            pendingDrawFinished();
        });
    }

    try {
        boolean canUseAsync = draw(fullRedrawNeeded);
        if (usingAsyncReport && !canUseAsync) {
            mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
            usingAsyncReport = false;
        }
    } finally {
        mIsDrawing = false;
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

    // For whatever reason we didn't create a HardwareRenderer, end any
    // hardware animations that are now dangling
    if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
        final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
        for (int i = 0; i < count; i++) {
            mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
        }
        mAttachInfo.mPendingAnimatingRenderNodes.clear();
    }

    if (mReportNextDraw) {
        mReportNextDraw = false;

        // if we're using multi-thread renderer, wait for the window frame draws
        if (mWindowDrawCountDown != null) {
            try {
                mWindowDrawCountDown.await();
            } catch (InterruptedException e) {
                Log.e(mTag, "Window redraw count down interrupted!");
            }
            mWindowDrawCountDown = null;
        }

        if (mAttachInfo.mThreadedRenderer != null) {
            mAttachInfo.mThreadedRenderer.setStopped(mStopped);
        }

        if (LOCAL_LOGV) {
            Log.v(mTag, "FINISHED DRAWING: " + mWindowAttributes.getTitle());
        }

        if (mSurfaceHolder != null && mSurface.isValid()) {
            SurfaceCallbackHelper sch = new SurfaceCallbackHelper(this::postDrawFinished);
            SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();

            sch.dispatchSurfaceRedrawNeededAsync(mSurfaceHolder, callbacks);
        } else if (!usingAsyncReport) {
            if (mAttachInfo.mThreadedRenderer != null) {
                mAttachInfo.mThreadedRenderer.fence();
            }
            pendingDrawFinished();
        }
    }
}