了解drawRect或图形坐标在Android中的实际工作方式

我正试图在画布上画一个矩形，我在理解Android矩形绘制的深度方面遇到了困难。我已经阅读了教程和所有可能的内容，但我被卡住了

在图中，红色矩形是我的目标。

不管矩形的大小，我都需要画出红色的矩形位在基座和矩形的中间。我在这里面临的最糟糕的噩梦是理解X，Y宽度和高度坐标

有人能解释一下数学原理吗，有时候我们往上走，Y很小，但相同宽度的坐标更高。我永远也无法正确地证明红色的内部矩形。在某些屏幕上，它工作得很好，在另一些屏幕上，它失败了。红色矩形有时会出现在父矩形之外

议程是了解坐标如何工作，并确保内部红色矩形的完整性

如果能从一个例子中得到解释，那就太好了。我正在使用-

void drawRect(float left, float top, float right, float bottom, Paint paint)

---

要绘制矩形，请从左到右水平运行X。Y从上到下垂直运行。它与图形上的完全相同。所以（0/0）在左上角

当你往上走的时候，Y当然会变小，因为它会从上到下增长

---

您必须注意布局元素，如ListView，这些元素将为您绘制的视图提供部分（或新的，您不能说）画布。这些视图在其自身的上/左位置将有0x0。如果需要绝对值，则必须随后调用

View.getLocationOnScreen（）

并自己计算偏移量。

canvas.drawRect（左、上、右、下、绘制）

在这个

左侧：矩形的左侧与矩形的左侧之间的距离
帆布

顶部：矩形顶部与矩形顶部之间的距离
帆布

右侧：矩形的右侧与矩形的左侧之间的距离
帆布
底部：矩形底面到画布顶面的距离
这是有道理的

float left = 100, top = 100; // basically (X1, Y1)

float right = left + 100; // width (distance from X1 to X2)
float bottom = top + 100; // height (distance from Y1 to Y2)

因此

希望我下面的笔记能帮助你理解直线、画布和视图的相对性

/**
 * Rect holds four integer coordinates for a rectangle.
 * The rectangle is represented by the coordinates of its 4 edges (left, top, right bottom).
 * These fields can be accessed directly. Use width() and height() to retrieve the rectangle's width and height.
 *
 * Note that the right and bottom coordinates are exclusive.
 * This means a Rect being drawn untransformed onto a Canvas will draw into the column and row described by its left and top coordinates
 * , but not those of its bottom and right.
 *
 * With regard to calling to Canvas#drawRect(left,top,right,bottom,paint)
 *
 * left: Distance of the left side of rectangle from left side of canvas.
 * top: Distance of top side of rectangle from the top side of canvas
 * right: Distance of the right side of rectangle from left side of canvas.
 * bottom: Distance of the bottom side of rectangle from top side of canvas.
 * __________________________________
 *|
 *|
 *|   __l_______________________r__
 *|  |         view group A        |
 *| t|  0______________________w   |
 *|  |  | **** view group B *** |  |
 *|  |  | **** canvas of B **** |  |
 *|  |  | ********************* |  |
 *|  |  | ********************* |  |
 *|  |  | ********************* |  |
 *|  |  | ***** __________ **** |  |
 *|  |  | *****|## rect ##|**** |  |
 *|  |  | *****|##########|**** |  |
 *|  |  | *****|##########|**** |  |
 *|  |  | *****|##########|**** |  |
 *|  |  | *****|##########|**** |  |
 *|  |  | ***** ---------- **** |  |
 *|  |  | ********************* |  |
 *| b|  h-----------------------   |
 *|  |                             |
 *|  |                             |
 *|   -----------------------------
 *|
 * -----------------------------------
 *
 * 1. l, t, r, b are coordinates of view group B (PastryChart) relative to view group A (parent of PastryChart).
 * 2. The size of canvas of B is same as the size of the view group B
 *    , which means canvas of B is a canvas which the view group B is rendered to.
 * 3. The coordinates of rect is relative to a canvas, here is the canvas of B
 *    , which means the coordinates of rect going to represent child of view group B are relative to the canvas of B.
 *    ex. for a rect holding left = 0, the position of its left is located on the same position of the left of view group B
 *    ex. for a rect holding right = w, the position of its right is located on the same position of the right of view group B
 *    ex. for a rect holding top = 0, the position of its top is located on the same position of the top of view group B
 *    ex. for a rect holding bottom = h, the position of its bottom is located on the same position of the bottom of view group B
 * 4. The rect is used to stored the child measurement computed in measure pass
 *    for forward positioning child view (PastryView) in the layout pass taken by parent view (PastryChart).
 * 5. All of them are in pixels (px)
 */

以下是我的方法简单易行

            int x = 100;  //position coordinate from left
            int y = 100;  //position coordinate from top
            int w = 100; //width of the rectangle
            int h = 100; //height of the rectangle
            drawRectangle(x, y, w, h, canvas, paint);

这是我的函数

    public void drawRectangle(int left, int top, int right, int bottom, Canvas canvas, Paint paint) {
    right = left + right; // width is the distance from left to right
    bottom = top + bottom; // height is the distance from top to bottom
    canvas.drawRect(left, top, right, bottom, paint);
}

它工作得很好
Rect（topleftx，toplefty，bottomrightx，bottomrighty）我知道这些参数，但我无法理解X，Y值，以及如何设置它。。任何例子的解释都是正确的help@JRowan在我发布的矩形中，这些参数值在画布的左上角设置了x=0和y=0，画布的右下角是x=canvas.getWidth（）和y=canvas.getHeight（）这正是矩形在角点坐标处的外观，这是书中的问题还是什么我认为2和4是不正确的。它们必须是：2。顶部：矩形顶部与画布顶部之间的距离4。底部：矩形底部与画布顶部之间的距离。萨文：这些值是以像素为单位的吗？简单而完美的解释。这正是我真正理解这一点所需要的。这就是我需要的梅雷德里卡：这些值以像素为单位？我认为矩形变量名是我在StackOverflow上见过的最有趣的事情。除息的
    public void drawRectangle(int left, int top, int right, int bottom, Canvas canvas, Paint paint) {
    right = left + right; // width is the distance from left to right
    bottom = top + bottom; // height is the distance from top to bottom
    canvas.drawRect(left, top, right, bottom, paint);
}