如何在Gtk +中使用Cairo進行離屏繪製？

Question

我想在開羅圖像上下文中做我的繪圖東西。有什麼方法可以在開啓事件時將圖片上下文的內容加載到開羅語境中？

例如，我想繪製一系列基於繪圖區域上光標移動的點，如果我想保留所有的點，我應該使用一個屏幕外緩衝區，所以我使用圖像上下文，但是我不能找到一種方法將其繪製到揭露事件的開羅上下文...

任何解決方案？

Answer 1

我自己找到了解決方案！

那就是：

using Cairo; 
using Gtk; 

public class Canvas : Gtk.DrawingArea{ 
public Canvas(MainWindow mw){ 
    stdout.printf("-> Canvas\n"); 

    main_window = mw; 
    is_pressed_down = false; 

    add_events(Gdk.EventMask.BUTTON_PRESS_MASK | 
       Gdk.EventMask.BUTTON_RELEASE_MASK | 
         Gdk.EventMask.POINTER_MOTION_MASK); 

      set_size_request(400, 300); 
} 

~Canvas(){ 
    stdout.printf("<- Canvas\n"); 
} 

public override void realize(){ 
    base.realize(); 
    stdout.printf("realize\n"); 
} 

protected override bool configure_event(Gdk.EventConfigure event){ 
    int x, y; 

    window.get_size(out x, out y); 
    offscreen_surface = new Cairo.ImageSurface(Cairo.Format.RGB24, x, y); 
    gc = new Cairo.Context(offscreen_surface); 
    gc.set_antialias(Antialias.NONE); 
    gc.set_line_width(1); 
    gc.set_source_rgb(1, 1, 1); 
    gc.paint(); // it will make trouble if user resize the window 

    string msg = "x: " + x.to_string() + ", y: " + y.to_string(); 
    main_window.set_statusbar(msg); 

    return true; 
} 

protected override bool expose_event(Gdk.EventExpose event){ 

    var tgc = Gdk.cairo_create(window); //!!! 
    tgc.set_source_rgb(1, 1, 1); 
    tgc.paint(); 

    tgc.set_source_surface(offscreen_surface, 0, 0); 
    tgc.paint(); 

    return true; 
} 

public override bool motion_notify_event(Gdk.EventMotion event) 
{ 
    string msg = "x: " + event.x.to_string() + ", y: " + event.y.to_string(); 
    main_window.set_statusbar(msg); 

    if(is_pressed_down){ 
     //gc.set_source_rgb(1, 1, 1); 
     //gc.paint(); 
     gc.set_source_rgb(1, 0.5, 0); 

     //gc.move_to(event.x, event.x); 
     gc.arc(event.x, event.y, 1, 0, 360);   
     gc.stroke(); 

     weak Gdk.Region region = this.window.get_clip_region(); 
     this.window.invalidate_region(region, true); 
     this.window.process_updates(true); 
    } 

    return true; 
} 

public override bool button_press_event(Gdk.EventButton event) 
{ 
    stdout.printf("Canvas.button_press_event\n"); 
    is_pressed_down = true; 
    return true; 
} 

public override bool button_release_event(Gdk.EventButton event) 
{ 
    stdout.printf("Canvas.button_release_event\n"); 
    is_pressed_down = false; 
    return true; 
} 

public Cairo.Context get_context(){ 
    return gc; 
} 

private Cairo.Context  gc; 
private weak MainWindow main_window; 
private Cairo.ImageSurface offscreen_surface; 
private bool    is_pressed_down; 
}