# BetterScroll's "diagnosis"

# [Question 1] Why can't my BetterScroll work?

The problem basically lies in the Height Calculation Error. First of all, you must have a clear understanding of the scrolling principle of BetterScroll. For vertical scrolling, simply the height of the wrapper container is greater than the height of the content content, and the translateY is modified to achieve the purpose of scrolling. The principle of horizontal scrolling is similar. Then the calculation Scrollable Height is the logic necessary for BetterScroll. The general logic is:

1. Pictures with uncertain sizes

Reason

When js performs a calculation of the scrollable height, the image has not been rendered.
Solution

Call bs.refresh() inside the callback function of the image's onload to ensure that the correct height of the image is calculated before calculating the Scrollable Height.

2. Vue's keep-alive component

Scenes

Suppose there are two components of A and B wrapped by keep-alive, A component uses BetterScroll, does some operation in A component, pops up input keyboard, then enters B component, then returns to A component, bs is unable to scroll.
Reason

Because Vue's keep-alive's cache and the input keyboard pops up, it compresses the height of the viewable area, causing the previously calculated scrollable height to be incorrect.
Solution

You can call bs.refresh() on Vue's activated hook to recalculate the height or re-instantiate bs.

# [Question 2] Why do brower's vertical scrolling failed after I use BetterScroll to do horizontal scrolling?

BetterScroll provides a feature of slide. If you implement a horizontal scrollin, such as slide. do vertical scrolling in the slide area, you can't bubble to the browser, so you can't manipulate the scroll bar of the native browser.

Reason

The internal scrolling calculations of BetterScroll exist in the user's interaction. For example, the mobile terminal is the touchstart/touchmove/touchend event. The listeners of these events generally have the line e.preventDefault(), which will block the browser's default behavior so that the browser's scrollbar cannot be scrolled.

Solution

Configure the eventPassthrough attribute.

  Let bs = new BScroll('.wrapper', {
    eventPassthrough: 'vertical' // keep vertical native scrolling
  })

# [Question 3] Why can't I pop up a pop-up window after using BetterScroll.

Reason

question 2 has been mentioned, it is caused by e.preventDefault() in touchstart.
Solution

Option 1: Configure the preventDefaultException property.
```
let bs = new BScroll('.wrapper', {
  preventDefaultException: {
    className: /(^|\s)test(\s|$)/
  }
})
```
eventDefaultException can be used to control the e.preventDefault() of the touchstart and touchmove events. If the above regular expression is used to check if the class name of the currently touched target element contains test, if passed, Then e.preventDefault() will not be called.

Option 2: Configure the preventDefault property.
```
let bs = new BScroll('.wrapper', {
  preventDefault: false
})
```
preventDefault is set to false, there are some side effects, it is generally recommended to use Option one.

WARNING

The side effect is that the touch event may bubble up to the document, causing the document to be dragged. At this point you need to listen for the parent or ancestor element of the wrapper element, bind them to the touchmove event, and call e.preventDefault().

# [Question 4] Why are the listeners for all click events inside BetterScroll content not triggered?

Reason

Still caused by e.preventDefault(). On the mobile side, if you call e.preventDefault() inside the logic of touchstart/touchmove/touchend, it will prevent the execution of the click event of it and its child elements. Therefore, BetterScroll internally manages the dispatch of the click event, you only need the click configuration item.

Solution

Configure the click attribute.

  Let bs = new BScroll('.wrapper', {
    click: true
  })

# [Question 5] Why is the click event dispatched twice when Nesting BetterScroll?

Reason

As stated in Question 4, the BetterScroll dispatches a click event internally, and nested scenes must have two or more bs.
Solution

You can manage the bubbling of events by instantiating inner BetterScroll's stopPropagation configuration item, or by instantiating inner BetterScroll's click configuration item to prevent multiple triggers of clicks.
```
let innerBS = new BScroll('.wrapper', {
  stopPropagation: true
})

// or
let innerBS = new BScroll('.wrapper', {
  click: false
})
```

# [Question 6] Why do I listen to the scroll event of bs, why not execute the callback?

Reason

BetterScroll does not dispatch the scroll event at any time because there is a performance penalty for getting the scroll position of bs. As for whether or not to distribute, it depends on the probeType configuration item.

Solution

  Let bs = new BScroll('.div', {
    probeType: 3 // real-time dispatch
  })

# [Question 7] In two vertically nested bs scenes, why move the inner bs will cause the outer layer to also be scrolled.

Reason

The internal logic of BetterScroll is in the body of the listener function of the touch event. Since the touch event of the internal bs is triggered, it will naturally bubble to the outer bs.
Solution

Since you know the reason, there are corresponding solutions. For example, when you scroll the inner bs, listen for the scroll event and call the outer bs.disable() to disable the outer bs. When the inner bs scrolls to the bottom, it means that you need to scroll the outer bs at this time. At this time, call the outer bs.enable() to activate the outer layer and call the inner bs.disable(). to forbid inner scrolling. In fact, think about it, this interaction is consistent with the nested scrolling behavior of the Web browser, except that the browser handles the various scrolling nesting logic for you, and the BetterScroll requires your own dispatched events and exposed APIs to fulfill.

The scroll (opens new window) component of cube-ui gives a solution to this scenario. Code is here (opens new window)

# [Question 8] In the vertical bs nesting horizontal bs scene, why does the vertical movement of the horizontal bs area do not cause vertical scrolling of the outer vertical bs?

Reason

The reason is similar to Question 2, because e.preventDefault() affects the default scrolling behavior, causing the outer bs to not trigger the touch event.

Solution

The solution is to configure the eventPassthrough property of the inner bs to keep the default native vertical scrolling.

  Let innerBS = new BScroll('.wrapper', {
    eventPassthrough: 'vertical' // keep vertical native scrolling
  })

← FAQ