幻灯片按钮仅在转到最后一张幻灯片时才在第二次单击上起作用

Question

我正在制作一个使用JSON数据填充其内容的幻灯片。当我来到最后一张幻灯片时，我必须单击2-3次，然后幻灯片才能在第一张幻灯片上再次开始。我要确保不会发生这种情况，只需单击一下就可以再次从第一张幻灯片开始。一个类似的错误是，当我第一次加载页面并第一次单击“后退”按钮时，我必须单击两次，此后它应能正常工作。我确定我的一个if语句中有一个问题，但我无法弄清楚出了什么问题。

index.html

<div class="slideshow-container">
    <div class="mySlides fade">
        <img id="image" style="width:100%" >
        <div id="description"></div>
    </div>
    <a id="prev">&#10094;</a>
    <a id="next">&#10095;</a>
</div>

<script>
    window.onload = startup;
    var xmlhttp;
    function startup () {
        xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange = statechange;
        xmlhttp.open("GET", "slideshow.json", true);
        xmlhttp.send();
    }
    function statechange() {
        if (xmlhttp.readyState === 4 && xmlhttp.status === 200) {
            var printout = " ";
            var doc = JSON.parse(xmlhttp.responseText);
            var amountOfElements = doc.length;

            var current = 0;

            var imagePath = "assets/stop2/";
            document.getElementById("image").src = imagePath + doc[current].image;
            document.getElementById("description").innerHTML = doc[current].text;

            console.log(current);

            prev.onclick = function() {
                if(current !== 0){
                    current = current - 1;

                    document.getElementById("image").src = imagePath + doc[current].image;
                    document.getElementById("description").innerHTML = doc[current].text;

                    if(current === 0) {
                        current = amountOfElements;
                    }
                }
                else {
                    current = amountOfElements;
                }
                console.log(current);
            };

            next.onclick = function() {
                if(current < amountOfElements) {
                    current = current + 1;

                    document.getElementById("image").src = imagePath + doc[current].image;
                    document.getElementById("description").innerHTML = doc[current].text;
                }
                else {
                    current = -1;
                }
                console.log(current);
            };

        }
        else if (xmlhttp.readyState === 4 && xmlhttp.status === 404) {
            alert("Something went wrong");
        }
    }
</script> 

slideshow.json

  {
    "name": "First image",
    "text": "blablabla",
    "image": "img1.png",
    "audio": "audio1.mp3"
  },
  {
    "name": "Second image",
    "text": "Lorem ipsum..",
    "image": "img2.png",
    "audio": "audio2.mp3"
  },
  {
    "name": "Third image",
    "text": "blablabla",
    "image": "img3.png",
    "audio": "audio3.mp3"
  },
  {
    "name": "Last image",
    "text": "blablablaaaaaa",
    "image": "img4.png",
    "audio": "audio4.mp3"
  }
] ```

Answer 1

如果您想要无限循环行为，请尝试以下操作：

var updateUI = function() {
    document.getElementById("image").src = imagePath + doc[current].image;
    document.getElementById("description").innerHTML = doc[current].text;
}

prev.onclick = function() {

    if(current <= 0) {
        current = amountOfElements - 1; // index of last element
    }
    else {
        current--;  // just decrement
    }

    console.log(current);
    updateUI();
};

next.onclick = function() {

    if(current < amountOfElements - 1) {
        current++; // just increment
    }
    else {
        current = 0; // start at first element
    }

    console.log(current);
    updateUI();
};

问题是，current = -1和current = amountOfElements超出范围，因此您需要递增或递减一到三次，直到索引变为“有效”。

Answer 2

您仅在if块而不是else块中进行更新。我将更新代码放在if / else语句之后。而且，您使用了元素数量，但是从0迭代，这是错误的。这就是为什么我将其更改为元素数量-1的原因。在3-> 0的情况下，您将步长更改为-1（不知道为什么？），而不是0。

prev.onclick = function() {
      if (current !== 0) {
        current = current - 1;
      } else {
        current = amountOfElements - 1;
      }

      document.getElementById("image").src = imagePath + doc[current].image;
      document.getElementById("description").innerHTML = doc[current].text;
    };

    next.onclick = function() {
      if (current < amountOfElements - 1) {
        current = current + 1;
      } else {
        current = 0;
      }

      document.getElementById("image").src = imagePath + doc[current].image;
      document.getElementById("description").innerHTML = doc[current].text;
    };