什么时候getGraphics（）会返回一个有效的图形实例？ [英] When will getGraphics() return a valid graphics instance?

问题描述

我正在尝试渲染一个自定义Swing组件，我扩展了JComponent类。

I'm trying to render a custom Swing component where I've extended the JComponent class.

为了简化组件要求，我们只想总结一下组件需要渲染一些字符串，每个字符串都有自己的字体。

For the purpose of simplifying the component requirements, lets just summarize my component as needing to render a few strings, each with their own fonts.

我需要将组件的大小精确到我渲染字符串的总宽度和高度。

I need my component to be sized exactly to the summed width and height of my rendered strings.

为了确定这个大小，我使用FontMetrics来计算每个字符串的尺寸。有了这些信息，我可以弄清楚我的组件的大小，并适当调整大小。

In order to determine this size, I use the FontMetrics to calculate the dimensions of each string. Having this information I can figure out what size my component will be and resize it appropriately.

问题是，当我访问getGraphics（）时，它总是为null，所以我无法获得FontMetrics实例。如果我等待从覆盖的paintComponent（）方法计算我的组件大小，它已经太晚了（组件已经有一个大小，对吧？）。

The problem is that when I access the getGraphics() it is always null, so I can't get the FontMetrics instance. If I wait to calculate my component size from the overriden paintComponent() method, its pretty much too late (the component already has a size, right?).

文档说如果此组件当前不可显示，则此方法将返回null。那么我何时知道组件何时可以显示并且有一个Graphics对象来调整我的组件大小？

The documentation says that "This method will return null if this component is currently not displayable". So when do I know when the component is ready to be displayed and has a Graphics object for me to resize my component?

什么是渲染组件的Swing调用顺序一旦框架setVisible（true）被调用？

What is the Swing invokation order for rendering the component once the frame setVisible(true) is called?

谢谢

更新：2010年2月6日星期二23:34

Update: Tuesday, Feb 06, 2010 at 23:34

如评论中所述，GridLayout不尊重任何完全是setXxxSize（）。对于任何有兴趣的人，我发布了使用GridLayout，BoxLayout和FlowLayout的结果，使用一个简单的框架，接收5个固定大小的200宽50高的组件（通过设置min，max和preferred）。

As mentioned bellow in the comments, the GridLayout doesn't respect any setXxxSize() at all. For anyone interested, I've posted results of using the GridLayout, BoxLayout and FlowLayout using a simple frame that receives 5 fixed size components of 200 wide by 50 in height (by setting min, max and preferred).

测试结果：

GridLayout 总是沿宽度和高度调整大小（如评论中所述）

The GridLayout is always resized along the width and height (as mentioned in comments)

FlowLayout 始终尊重组件大小。

至于 BoxLayout ...

PAGE_AXIS 和 Y_AXIS 缩小组件的宽度大约一半大小（104），但没有缩小高度。

The PAGE_AXIS and Y_AXIS shrank the width of the components to about half their size (104) but did not shrink the height.

LINE_AXIS 和 X_AXIS 收缩组件的高度似乎为零，但没有触及宽度。

The LINE_AXIS and X_AXIS shrank the height of the components to what seemed zero but did not touch the width.

推荐答案

首先，您可以使用<测量字符串< a href =http://java.sun.com/j2 se / 1.4.2 / docs / guide / 2d / spec / j2d-fonts.htmlrel =noreferrer> TextLayout 类及其相关内容。例如

First, you can measure your Strings using the TextLayout class and its associated stuff. For example

public static void main(final String[] args) throws Exception
{
    final FontRenderContext frc = new FontRenderContext(null, true, true);
    final Font font = new Font("serif", Font.PLAIN, 18);
    final TextLayout layout = new TextLayout("This is a test", font, frc);
    final Rectangle2D bounds = layout.getBounds();
    System.err.println((int) (bounds.getWidth() + .5));
}

其次，当您的组件变为可见时，可以通过< a href =http://java.sun.com/docs/books/tutorial/uiswing/events/componentlistener.html\"rel =noreferrer> ComponentListener ，但这不是测量字符串所必需的提前！

Secondly, you can be informed when your component has become visible by using a ComponentListener, but that's not necessary for measuring your strings in "advance"!

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