Introducing the precautions for canvas development of WeChat mini program

小program development tutorialThe column introduces the precautions for canvas development

## Relevant learning recommendations:

Mini Program Development Tutorial

WeChat Mini Program wxcanvas

The test phone is IPHONE6, Developer tools version 0.10.102800. The same goes for the developer tools version 0.11.112301

The canvas in the WeChat applet is different from the h5 canvas. The canvas of the WeChat applet is called wxcanvas in the following.

The following These are all the useful information that I tested little by little, please read patiently:

1.wxcanvas does not have width and height attributes and width and height styles like h5canvas. It only has a style, which can be understood as a box;

2. Don’t think of wxcanvas as a real H5canvas, just think of it as a p. There are also things that draw ranges. Change the width and height. It is displayed, or there is redrawing here, but I don’t know how to implement it;

3. Changing the width and height of the wxcanvas style does not change the size of the things on the original canvas;

4. The enlargement and reduction in the css transform transformation cannot change the size of things on the original canvas

5. Official statement: context is just a container that records method calls, used to generate and record drawing behaviors. actions array. There is no corresponding relationship between context and . The drawing action array of a context-generated canvas can be applied to multiple .

In fact, after context.getActions(), the information in the context will be cleared. If you want to reuse it, you need var temp=context.getActions() to save the operation array before it can be reused in wx .drawcanvas;

6. Note that

wx.drawCanvas({
　　canvasId: 'target',
　　actions: context.getActions()
});

will clear the canvas by default. If you don’t want to clear it, you need

wx.drawCanvas({
  canvasId: 'target',
  actions: context.getActions(),
  reserve:true
});

7.《 Key points》

<canvas disable-scroll="true" catchtouchmove="ccvsMove" bindtouchmove="cvsMove" canvas-id="target" id="target" style=&#39;width:{{width}}px;height:{{height}}px&#39;></canvas>

Note that disable-scroll="true" and bindtouchmove="cvsMove" exist at the same time to prevent the page from moving. Catchtouchmove does not work

Above The way of writing, ccvsMove and cvsMove will both trigger.

ccvsMove returns a normal touch event object, including pageX, clientX, etc.

cvsMove returns a canvasTouch event object, without pageX, clientX, Only x, y

8.

wx.drawCanvas({
  canvasId: 'target',
  actions: [],
  reserve:false
});

can clear the canvas and the status of the canvas

9. The scale tranlate rotate and other states of the canvas will continue when reserve:true The status of times

10. The array returned by context.getActions() is very useful. When you print it out, you will find that you can understand the contents inside. You can directly modify the array to change the drawing actions

11. When wx.drawCanvas, context.drawImage can be drawn on the mobile phone, but cannot be drawn on the computer development tool

12.wx.canvasToTempFilePath

There is only one line in the official document, originally

The wx.canvasToTempFilePath parameter is an object including canvasID, success, fail, and complete, which is similar to wx.saveFile;

      wx.canvasToTempFilePath({
       canvasId: 'target',
       success: function success(res) {
        wx.saveFile({
         tempFilePath: res.tempFilePath,
         success: function success(res) {
          console.log('saved::' + res.savedFilePath);
         },
         complete: function fail(e) {
          console.log(e.errMsg);
         }
        });
       },
       complete: function complete(e) {
        console.log(e.errMsg);
       }
      });

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