Using Metal to Draw a View’s Contents

时间：2021-9-25
分类：Xcode，Metal开发，
针对人群：初学者，具备初级英语能力
来源：XCode帮助文件
Create a MetalKit view and a render pass to draw the view’s contents.
创建一个MetalKit视图和一个渲染通道来绘制视图内容。

Overview

In this sample, you’ll learn the basics of rendering graphics content with Metal.
You’ll use the MetalKit framework框架 to create a view that uses Metal to draw the contents of the view. Then, you’ll encode commands for a render pass that erases the view to a background color.
在这个例子中，你将学习用Metal渲染图形内容的基本操作。你会用MetalKit框架来创建一个视图，用Metal来绘制视图的内容。然后，你编码命令给一个渲染通道来“擦除”掉视图的背景颜色。

Note: MetalKit automates windowing system tasks, loads textures, and handles 3D model data. See [MetalKit][MetalKit] for more information.

Prepare a MetalKit View to Draw

MetalKit provides a class called [MTKView][MTKView], which is a subclass of [NSView][NSView] (in macOS) or [UIView][UIView] (in iOS and tvOS).
MTKView handles many of the details related相关联的细节 to getting the content you draw with Metal onto the screen.

An MTKView needs a reference引用（就是 C++中的指针） to a Metal device object in order to create resources internally, so your first step is to set the view’s device property to an existing [MTLDevice][MTLDevice].

_view.device = MTLCreateSystemDefaultDevice();

Other properties属性 on MTKView allow you to control its behavior行为. To erase the contents of the view to a solid background color, you set its clearColor property. You create the color using the MTLClearColorMake function, specifying the red, green, blue, and alpha透明度 values.

_view.clearColor = MTLClearColorMake(0.0, 0.5, 1.0, 1.0);

Because you won’t be drawing animated content in this sample, configure the view so that it only draws when the contents need to be updated, such as when the view changes shape:
因为在这里例子中，你不会绘制动画内容，所以只要设置视图在内容需要更新的时候重新绘制，例如当视图大小改变时：

_view.enableSetNeedsDisplay = YES;

Delegate Drawing Responsibilities代理绘图职责

MTKView relies依赖 on your app to issue发给 commands to Metal to produce生成 visual视觉 content.
MTKView uses the delegate pattern代理模式 to inform通知 your app when it should draw.
To receive delegate callbacks回调, set the view’s delegate property to an object that conforms符合 to the [MTKViewDelegate][MTKViewDelegate] protocol.

_view.delegate = _renderer;

设置了视图的代理后，就可以在代理对象中来通知程序绘制窗口。
The delegate implements实现 two methods方法:

The view calls the mtkView:drawableSizeWillChange: method whenever the size of the contents changes.
This happens when the window containing the view is resized, or when the device orientation changes (on iOS).
This allows your app to adapt适应 the resolution at which it renders to the size of the view.
The view calls the drawInMTKView: method whenever每当 it’s time to update the view’s contents.
In this method, you create a command buffer, encode commands that tell the GPU what to draw and when to display it onscreen, and enqueue排列 that command buffer to be executed by the GPU. This is sometimes referred to as drawing a frame. You can think of a frame as all of the work that goes into producing a single image that gets displayed on the screen. In an interactive交互式 app, like a game, you might draw many frames per second.

In this sample, a class called AAPLRenderer implements the delegate methods and takes on the responsibility of drawing.本例中，一个称为AAPLRenderer的类实现了代理方法，并承担绘图响应。
The view controller creates an instance of this class and sets it as the view’s delegate.

Create a Render Pass Descriptor建立一个渲染通道描述符

When you draw, the GPU stores the results into textures, which are blocks of memory that contain image data and are accessible to the GPU. In this sample, the MTKView creates all of the textures you need to draw into the view. It creates multiple textures so that it can display the contents of one texture while you render into another.

To draw, you create a render pass渲染通道, which is a sequence序列 of rendering commands that draw into a set of textures. When used in a render pass, textures are also called render targets. To create a render pass, you need a render pass descriptor, an instance of [MTLRenderPassDescriptor][MTLRenderPassDescriptor]. In this sample, rather than configuring your own render pass descriptor, ask the MetalKit view to create one for you.在本例中，相比你自己定义一个渲染通道描述符，更好的方法是让MetalKit视图来建立一个。

MTLRenderPassDescriptor *renderPassDescriptor = view.currentRenderPassDescriptor;
if (renderPassDescriptor == nil)
{
    return;
}

A render pass descriptor 描述了 the set of render targets, and how they should be processed at the start and end of the render pass. Render passes also define some other aspects面貌 of rendering that are not part of this sample. The view returns a render pass descriptor with a single color attachment附着 that points to one of the view’s textures, and otherwise configures the render pass based on the view’s properties. By default, this means that at the start of the render pass, the render target is erased to a solid color that matches the view’s clearColor property, and at the end of the render pass, all changes are stored back to the texture.

Because a view’s render pass descriptor might be nil, you should test to make sure the render pass descriptor object is non-nil before creating the render pass.

Create a Render Pass 建立一个渲染通道

You create the render pass by encoding it into the command buffer using a [MTLRenderCommandEncoder] object. Call the command buffer’s [renderCommandEncoderWithDescriptor:] method and pass in the render pass descriptor.
通过渲染命令编码器对象，调用命令缓冲的renderCommandEncoderWithDescriptor方法来传送进渲染通道描述符。

id<MTLRenderCommandEncoder> commandEncoder = [commandBuffer renderCommandEncoderWithDescriptor:renderPassDescriptor];

In this sample, you don’t encode编码 any drawing commands, so the only thing the render pass does is erase the texture. Call the encoder’s endEncoding method to indicate表明 that the pass is complete.

[commandEncoder endEncoding];

Present a Drawable to the Screen 准备一个“可绘制”到屏幕

Drawing to a texture doesn’t automatically display the new contents onscreen. In fact, only some textures can be presented onscreen. In Metal, textures that can be displayed onscreen are managed by drawable objects, and to display the content, you present the drawable. 在Metal中，在屏幕能被显示的贴图由drawable对象管理、显示内容，你（负责）准备这个可绘制。

MTKView automatically creates drawable objects to manage its textures. Read the currentDrawable property to get the drawable that owns the texture that is the render pass’s target. The view returns a [CAMetalDrawable][CAMetalDrawable] object, an object connected to Core Animation.
MTKView自动创建可绘制对象来管理它的贴图。读取视图的currentDrawable属性以获得拥有渲染通道目标纹理的drawable。视图返回一个[CAMetalDrawable]对象——一个连接到Core Animation的对象。

id<MTLDrawable> drawable = view.currentDrawable;

Call the presentDrawable: method on the command buffer, passing in the drawable.

[commandBuffer presentDrawable:drawable];

This method tells Metal that when the command buffer is scheduled for execution, Metal should coordinate协调配合 with Core Animation to display the texture after rendering completes. When Core Animation presents the texture, it becomes the view’s new contents. In this sample, this means that the erased texture becomes the new background for the view. The change happens alongside与…一起 any other visual updates that Core Animation makes for onscreen user interface elements.