我正在尝试实施动态滚动(无论这个好主意与否都不是问题)并且经历了一些"奇怪的"行为。
function scroll(timestamp){
var deltaTime = timestamp - this.scrollLastTime;
this.scrollLastTime = timestamp;
console.log(deltaTime);
var newPosition = this.scrollTop + this.scrollSpeed*deltaTime;
if(newPosition <= 0){
this.scrollTop = 0;
this.scrolling = false;
return;
}else if(newPosition > this.scrollHeight-this.clientHeight){
this.scrollTop = this.scrollHeight-this.clientHeight;
this.scrolling = false;
}else{
this.scrollTop = newPosition;
var newSpeed = this.scrollSpeed + Math.sign(this.scrollSpeed) * this.scrollAcceleration*deltaTime;
if(this.scrollSpeed < 0 && newSpeed >= 0){
this.scrolling = false;
}else if(this.scrollSpeed >0 && newSpeed <= 0){
this.scrolling = false;
}else{
this.scrollSpeed = newSpeed;
window.requestAnimationFrame(this.scrollCallback);
}
}
}
document.getElementById("0").addEventListener('wheel',
function(e){
this.scrollSpeed = e.wheelDelta/100;
if(!this.scrolling){
this.scrolling = true;
this.scrollLastTime = performance.now();
this.scrollAcceleration = -0.01;
if(!this.scrollCallback)this.scrollCallback = scroll.bind(this);
window.requestAnimationFrame(this.scrollCallback);
}
e.preventDefault();
});
问题是 deltaTime 经常变为负数,我缺少什么?
编辑:如果有帮助,我正在使用Chromium版本51.0.2704.79 Ubuntu 14.04(64位)。
答案 0 :(得分:2)
正如@Whothehellist已经在他的评论中指出的那样:
rAF时间戳不是非常可靠
这是一个证明:
的小例子
document.getElementById("button").addEventListener('click', function(e){
this.valueOfPerformanceNow = performance.now();
if(!this.clickCallback)this.clickCallback = printTime.bind(this);
window.requestAnimationFrame(this.clickCallback);
});
function printTime(timestamp){
$("#perfromanceNow").val(this.valueOfPerformanceNow);
$("#delta").val(timestamp-this.valueOfPerformanceNow);
$("#timestamp").val(timestamp);
}
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<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div>
<input type="submit" id="button" value="print current time"><br>
<label>performance.now():</label>
<input type="text" id="perfromanceNow" readonly><br>
<label>timestamp:</label>
<input type="text" id="timestamp" readonly><br>
<label>delta:</label>
<input type="text" id="delta" readonly>
<div>
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有一个简单的解决方法。您可以在方法开头使用var timestamp = performance.now();
,而不是通过rAF时间获取时间戳。
这是一个有效的例子:
document.getElementById("button").addEventListener('click', function(e){
this.valueOfPerformanceNow = performance.now();
if(!this.clickCallback)this.clickCallback = printTime.bind(this);
window.requestAnimationFrame(this.clickCallback);
});
function printTime(){
var timestamp = performance.now();
$("#perfromanceNow").val(this.valueOfPerformanceNow);
$("#delta").val(timestamp-this.valueOfPerformanceNow);
$("#timestamp").val(timestamp);
}
&#13;
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div>
<input type="submit" id="button" value="print current time with new timestamp initialization"><br>
<label>performance.now():</label>
<input type="text" id="perfromanceNow" readonly><br>
<label>timestamp:</label>
<input type="text" id="timestamp" readonly><br>
<label>delta:</label>
<input type="text" id="delta" readonly>
<div>
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