为什么我允许在非GUI线程中加载图像？

Question

众所周知,您无法使用除GUI线程之外的任何其他线程更改GUI.所以常用的一个简单技巧(我使用的)是调用:

this.Invoke((MethodInvoker)delegate { pictureBox1.Visible = false; });

我正在构建我的程序并启动它,很快就注意到我忘了把它PictureBox.Load(string url)放入调用程序中,但是没有发生错误.

所以我很好奇,为什么我不允许这样做(在非GUI线程中):

pictureBox1.Visible = false; // eg.

但我被允许这样做:

pictureBox1.Load(url); // url = link to image

Answer 1

当你加载一个新图像时,这就是内部发生的事情PictureBox(这是从Load方法中调用的):

    private void InstallNewImage(Image value,
                                 ImageInstallationType installationType)
    {
        StopAnimate();
        this.image = value;

        LayoutTransaction.DoLayoutIf(AutoSize, this, this, PropertyNames.Image); 

        Animate();
        if (installationType != ImageInstallationType.ErrorOrInitial)
        {
            AdjustSize();
        }
        this.imageInstallationType = installationType;

        Invalidate();
        CommonProperties.xClearPreferredSizeCache(this);
    }

所以你可以看到它真正做的就是设置图像,然后调用Invalidate,可以从其他线程调用.

Visible(继承自Control),你可以在这里看到,通过p/invoke做很多事情,必须在主UI线程上完成.

    protected virtual void SetVisibleCore(bool value) {
        try {
            System.Internal.HandleCollector.SuspendCollect();

            if (GetVisibleCore() != value) {
                if (!value) {
                    SelectNextIfFocused();
                }

                bool fireChange = false;

                if (GetTopLevel()) {

                    // The processing of WmShowWindow will set the visibility
                    // bit and call CreateControl()
                    //
                    if (IsHandleCreated || value) {
                            SafeNativeMethods.ShowWindow(new HandleRef(this, Handle), value ? ShowParams : NativeMethods.SW_HIDE);
                    }
                }
                else if (IsHandleCreated || value && parent != null && parent.Created) {

                    // We want to mark the control as visible so that CreateControl
                    // knows that we are going to be displayed... however in case
                    // an exception is thrown, we need to back the change out.
                    //
                    SetState(STATE_VISIBLE, value);
                    fireChange = true;
                    try {
                        if (value) CreateControl();
                        SafeNativeMethods.SetWindowPos(new HandleRef(window, Handle),
                                                       NativeMethods.NullHandleRef,
                                                       0, 0, 0, 0,
                                                       NativeMethods.SWP_NOSIZE
                                                       | NativeMethods.SWP_NOMOVE
                                                       | NativeMethods.SWP_NOZORDER
                                                       | NativeMethods.SWP_NOACTIVATE
                                                       | (value ? NativeMethods.SWP_SHOWWINDOW : NativeMethods.SWP_HIDEWINDOW));
                    }
                    catch {
                        SetState(STATE_VISIBLE, !value);
                        throw;
                    }
                }
                if (GetVisibleCore() != value) {
                    SetState(STATE_VISIBLE, value);
                    fireChange = true;
                }

                if (fireChange) {
                    // We do not do this in the OnPropertyChanged event for visible
                    // Lots of things could cause us to become visible, including a
                    // parent window.  We do not want to indescriminiately layout
                    // due to this, but we do want to layout if the user changed
                    // our visibility.
                    //

                    using (new LayoutTransaction(parent, this, PropertyNames.Visible)) {
                        OnVisibleChanged(EventArgs.Empty);
                    }
                }
                UpdateRoot();
            }
            else { // value of Visible property not changed, but raw bit may have

                if (!GetState(STATE_VISIBLE) && !value && IsHandleCreated) {
                    // PERF - setting Visible=false twice can get us into this else block
                    // which makes us process WM_WINDOWPOS* messages - make sure we've already 
                    // visible=false - if not, make it so.
                     if (!SafeNativeMethods.IsWindowVisible(new HandleRef(this,this.Handle))) {
                        // we're already invisible - bail.
                        return;
                     }
                }

                SetState(STATE_VISIBLE, value);

                // If the handle is already created, we need to update the window style.
                // This situation occurs when the parent control is not currently visible,
                // but the child control has already been created.
                //
                if (IsHandleCreated) {

                    SafeNativeMethods.SetWindowPos(
                                                      new HandleRef(window, Handle), NativeMethods.NullHandleRef, 0, 0, 0, 0, NativeMethods.SWP_NOSIZE |
                                                      NativeMethods.SWP_NOMOVE | NativeMethods.SWP_NOZORDER | NativeMethods.SWP_NOACTIVATE |
                                                      (value ? NativeMethods.SWP_SHOWWINDOW : NativeMethods.SWP_HIDEWINDOW));
                }
            }
        }
        finally {
            System.Internal.HandleCollector.ResumeCollect();
        }
    }

就像旁注一样,很多这些" 幕后发生的事情"可以通过浏览参考源,或者创建一个小应用程序并反编译来解决(我建议使用JetBrains DotPeek,或者ildasm也可以).